Cells, Transport & Cellular Energy — Print · Life Science & Scientific Inquiry, Grade 7
Five focused units, a three-session investigation, chapter review, and chapter quiz.
Printable chapter lessons and review.
Cell Theory & Discovering Cells
Learning goals
- I can explain cell theory & discovering cells.
- I can use evidence, a model, and stated assumptions to support a biological explanation.
Before this unit
Use the supplied vocabulary, distinguish an observation from an explanation, and identify any units or model assumptions. Revisit a connected earlier unit if needed; new terms are explained here.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
In 1665 Robert Hooke shaved a sliver of cork thin enough for light to pass through, slid it under a microscope he had built himself, and saw something nobody had described before: row after row of tiny empty boxes, packed like a honeycomb. They reminded him of the bare little rooms monks slept in, which were called cellulae, so he borrowed the word and called them cells. The name has stuck for three and a half centuries, even though the thing Hooke was looking at was dead — cork is bark, and what he saw were the stiff leftover cell walls, made of a tough material called cellulose, that plants build on the outside of their cells. Ten years later, in Delft, Antoni van Leeuwenhoek ground lenses so good that he could see things Hooke could not, and he pointed one at a drop of pond water. It was crowded. Tiny shapes darted, spun and reversed, and he called them animalcules — little animals. He had found living single cells. Neither man understood what he had hold of. It took another 170 years of grinding lenses, slicing tissue and arguing before anyone could say what a cell is: not a curiosity in bark or pond scum, but the unit that every living thing on Earth is built from.
The claim arrived in three pieces, from three people, over seventeen years. In 1838 the botanist Matthias Schleiden concluded that the plant tissues he had been sectioning — groups of similar cells working together as one material — were all built of cells. The next year Theodor Schwann argued the same for animal tissue, a bolder claim, because animal cells are softer and harder to see, and nobody had expected a nerve and a leaf to share a building block. Then in 1855 Rudolf Virchow supplied the piece that made the theory predictive rather than merely descriptive: every cell comes from a cell that already existed. He put it in Latin as omnis cellula e cellula. Those three claims together are cell theory: every living thing is made of one or more cells; the cell is the basic unit of both structure and function; and new cells come only from existing cells. An organism made of a single cell is unicellular, and one built from many is multicellular. What makes the theory powerful is how little it cares what an organism looks like. An oak, an amoeba in a ditch, a blue whale and the bacteria in your gut are built from the same kind of unit, and that single fact is why biology can be one subject instead of a thousand unrelated descriptions.
Hooke's empty boxes are also the source of the most common misunderstanding about cells, and it is worth naming it directly: a living cell is not an empty box. Every cell is wrapped in a thin cell membrane, the flexible boundary that decides what gets in and out, and inside it the cell is crowded with small working parts called organelles — so crowded that the contents behave more like a gel than like water. The largest organelle in most cells is the nucleus, the compartment that holds the cell's DNA, the long molecule carrying the instructions for building and running the cell. Instruments have limits you must know to read an image honestly. A good light microscope cannot separate two points closer than about 200 nanometres — a nanometre is a millionth of a millimetre — which is enough to show a nucleus and too coarse to show a ribosome, the tiny organelle that builds proteins, at roughly 20 nanometres. The inside of a cell came into view only with the electron microscopes of the 1930s, which use beams of electrons instead of light and resolve detail about a thousand times finer. Scale matters too. A human cheek cell is roughly 60 micrometres across — a micrometre is a thousandth of a millimetre — while a typical bacterium is 1 to 2 micrometres, so thirty to sixty bacteria would fit end to end across one of your own cells.
Build your understanding
Where this fits
Chapter 1 established how to measure carefully and how to tell an observation from an inference. Now we turn those tools on something you cannot see without them. The question driving this chapter is what living things are actually built from, and this unit answers the first part of it: what a cell is, how anyone found out, and why three separate claims are needed rather than one.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
Part 1 · Why cell theory needs three claims
Each of the three claims does a different job, which is why dropping one breaks the theory. The first claim — that every living thing is made of one or more cells — is what makes the theory universal. It is the claim that lets a biologist study yeast in order to learn something about you. It is also the claim that is tested every time someone finds an unfamiliar organism: the question asked is whether it is made of cells, not whether it looks familiar.
The second claim, that the cell is the basic unit of structure and function, says where to look for an explanation. If you want to know how a muscle contracts, how a leaf captures light, or how a kidney concentrates urine, the answer is found in what cells are doing and how their activity is combined into tissues and organs. Structure and function are named together on purpose: a cell's shape and contents are the reason it can do its particular job.
The third claim, that new cells come only from existing cells, is the one that predicts. It rules out living cells assembling spontaneously from non-living material under ordinary conditions, and it means that every cell in your body has an unbroken line of division behind it. Note the scope carefully: this is a claim about living systems and reproduction as we observe them, not a complete account of how the first life on Earth arose, which is a separate and much harder question.
Pause and explain
Does cell theory require every organism to have many cells?
Check your reasoning after answering
No. The first claim says one or more cells, so a single cell can be a complete organism — a Paramecium feeds, responds, and reproduces on its own. Many organisms are instead built from large numbers of cells that divide the work between them, and both arrangements satisfy cell theory.
Part 2 · What Hooke actually saw, and what an instrument can show
Cork, a leaf cross-section and a drop of pond water give three very different observations, and reading each one honestly means knowing what the instrument could resolve. Hooke's cork showed boundaries with nothing inside, because the cells were dead: the cellulose cell wall survived and the living contents did not. Plant and cork cells have both a wall and a membrane; animal cells have only the membrane, which is part of why animal tissue was harder to see and why Schwann's claim came second.
Every microscope has a resolution limit — the smallest separation at which two points still look like two points rather than one blur. For a light microscope that limit is around 200 nanometres, set by the wavelength of visible light itself, so no amount of polishing gets past it. That is small enough to show a nucleus, a chloroplast (the green organelle where a plant cell captures light) or a large vacuole (a fluid-filled storage sac), and too large to show a ribosome at roughly 20 nanometres.
So a labelled drawing is a record of selected features that one instrument could resolve, interpreted by the person drawing it. It is evidence, and it is partial. When an image seems to show an absence — no nucleus, nothing inside, no structure — the first question is whether the instrument could have shown that thing had it been present.
Pause and explain
Why is cork a limited model of a living cell?
Check your reasoning after answering
Cork is dead bark, so only the rigid cell walls remain and the living contents are missing. It can show cell shape, packing and the existence of boundaries, but it cannot show the nucleus, the organelles or the crowded interior of a living cell, and a light microscope could not have resolved the smallest of those structures anyway.
Part 3 · Division, growth, and why four cells are not four times the mass
Cell division is the process that turns one cell into two, and it is how Virchow's third claim shows up in practice. A multicellular organism grows in two ways at once: cells divide, increasing the number, and cells take in material and enlarge, increasing the size of each. Repair works the same way, replacing damaged cells with new ones produced by division. Both processes need raw material and energy from outside the organism.
That second point is where a common arithmetic slip happens. Suppose one cell divides into two, and each of those divides once more, giving four cells. The count has gone one to two to four, which is a fourfold increase in number. It does not follow that there is four times as much material. Immediately after a division, each daughter cell is roughly half the volume of the parent, so four small cells can hold about the same total matter as the one cell you started with. The mass only increases as those cells absorb nutrients and build new components.
To reason about mass you therefore need more than a count: you need the size of the cells and what they are made of. This is a habit worth generalising. A number that is easy to measure — how many — is often not the number the question is about, and saying which quantity you actually have is half of a good scientific answer.
Pause and explain
What other information is needed to infer mass from cell count?
Check your reasoning after answering
You need the volume or size of the cells and their composition or density. Four cells produced by two rounds of division start out at about a quarter of the parent's volume each, so the count alone cannot tell you the total mass; mass changes only as the cells take in material and grow.
Part 4 · Organisms that do not look like the diagram
The real test of cell theory is an organism that breaks the familiar picture. A Paramecium is a single cell, perhaps 200 micrometres long, and it swims with rows of beating hairs called cilia, sweeps food into a groove, digests it, pumps out excess water through a contractile vacuole, responds to obstacles and reproduces. It has no organs, because organs are an arrangement some multicellular organisms use; one cell is carrying out every interacting life process by itself.
A bacterium pushes harder. It is typically 1 to 2 micrometres long, has no membrane-enclosed nucleus, and keeps its DNA loose in the body of the cell rather than in a separate compartment. Judged against a labelled plant-cell diagram it looks like it is missing the most important part. Judged against cell theory it fits without difficulty: it is a cell, bounded by a membrane, carrying out life processes, and producing new cells by dividing.
The difference between those two judgements is the difference between recognising a picture and applying a theory. A multicellular organism divides labour among specialised cells and tissues; a unicellular organism does not; neither arrangement is more alive than the other. When something unfamiliar turns up, ask what evidence there is of cells and of life processes, and leave the diagram out of it.
Pause and explain
Why can a bacterium be an organism without organs?
Check your reasoning after answering
Because a single cell can carry out all the interacting processes life requires — taking in material, releasing energy, responding, and reproducing by division. Organs are a way some multicellular organisms organise specialised cells; they are not a requirement for being alive, and a bacterium's lack of a nucleus does not place it outside cell theory.
Apply the whole unit · Explain a sequence of observations
Illustrative record, not a measured experiment: a single living cell divides into two, and a short time later each of those two divides once. A student looks at the four cells and claims that there must now be four times as much living material as there was at the start.
Your task
Use cell theory to explain the change in cell number, then evaluate the claim about mass and say what measurement would settle it.
What your response should show
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Response guidance — after your attempt
Count the divisions: one cell becomes two, and two become four, so the number has doubled twice and 1 × 2 × 2 = 4 cells.
Attribute the increase to Virchow's claim — each new cell came from an existing cell dividing, so no cell appeared from nothing.
Separate number from mass: immediately after dividing, each daughter cell is about half its parent's volume, so four cells can together hold roughly the material of the original one cell.
Name the measurement that would settle it: the volume or mass of the cells at the start and at the end, since mass rises only as cells absorb nutrients and build new components.
Cell theory explains the number: 1 cell → 2 cells → 4 cells, each new cell produced by the division of a cell that already existed. It does not support the mass claim. Each division splits the parent's contents between two daughters, so each of the four cells begins at roughly a quarter of the original volume and the total living material is close to what it was. Mass increases only when the cells take in material from outside and build new components, which takes time and a supply of nutrients. To decide the question you would measure total volume or mass at the start and at the end, not the number of cells — and you would state the assumption that the cells were growing under the same conditions throughout.
Scientific references for this unit
Parent / teacher teaching notes
Ask for a because-statement and a labeled relationship, not just a memorized word. Work through one calculation step at a time, permit oral or diagram responses, and use flexible sessions for difficult reading. Compare the learner’s actual evidence with the supplied model; do not reward copying or agreement with a personal preference. No private family, genetic, or medical records are required.
The key idea
Cells are the smallest units of life. Some organisms consist of one cell; others contain many cells with specialized functions. Cell theory states that organisms are made of cells, cells are basic units of structure and function, and new cells come from existing cells. These statements connect observations across very different organisms rather than defining one animal or plant as the model for all life.
Worked examples
Example 1 · Reading an image against the instrument
- Identify the specimen: cork is dead bark tissue, so the living contents are already gone and only cellulose walls remain.
- Identify the instrument's limit: a light microscope resolves to about 200 nanometres, so even in a living cell it would show a nucleus but not a ribosome at about 20 nanometres.
- Separate what is observed from what is inferred: the image supports a claim about shape, packing and the presence of walls, not about contents.
An apparent absence in an image is only evidence of absence if the instrument could have shown the thing had it been present.
Example 2 · Comparing a count with a measurement
- Subtract the dates rather than reading the spacing: 1839 − 1665 = 174 years, and 1855 − 1839 = 16 years.
- Compare the two intervals: 174 years is about eleven times 16 years, so they are nowhere near equal.
- Interpret the drawing: equal spacing is a legitimate ordering aid if it is labelled as one, but it is not a proportional time scale.
Read dates from the numbers, not from how far apart the labels sit on the page.
Avoid these mistakes
- Mistake: a living cell is an empty box with walls. Hooke's cork was dead, so only walls remained; a living cell is crowded enough to behave like a gel.
- Mistake: an organism needs organs to be alive. A Paramecium carries out every life process in one cell, and organs are an arrangement found in some multicellular organisms.
- Mistake: four cells must hold four times the material. Each division halves the parent's contents between two daughters, so mass rises only as cells take in material and grow.
- Mistake: if a microscope shows no structure, there is none. A light microscope stops at about 200 nanometres, so absence in an image is only evidence of absence when the instrument could have resolved the feature.
Check your understanding
Does cell theory require every organism to have many cells?
Discuss the answer
No. A single cell can be an organism; many organisms contain multiple cells.
Why is cork a limited model of a living cell?
Discuss the answer
Much of the living content is absent; the observed compartments mainly show remaining cell walls.
Words to know
- cell
- The basic structural and functional unit of life, bounded by a membrane; Hooke named it in 1665 after the small rooms called cellulae.
- cell theory
- Three connected claims: every living thing is made of one or more cells, the cell is the basic unit of structure and function, and new cells come only from existing cells.
- unicellular
- Made of a single cell that carries out every life process itself, as a Paramecium or a bacterium does.
- multicellular
- Made of many cells that divide the work between them, organised into tissues and in some organisms into organs.
- cell division
- The process by which one existing cell produces new cells; it increases cell number, while growth in mass requires material taken in from outside.
Extend your scientific explanation
Compare a second supplied example or the linked educational reference. Identify a connection, a model limit, and an additional observation that would test the explanation.
In the real world
Use a clear question, reliable observations, and a biological mechanism to evaluate a claim. State what remains uncertain before applying a classroom model to a different setting.
Exit check
Why can a bacterium be an organism without organs?
Check your explanation after answering
One cell can carry out the required interacting life processes; organs are arrangements in some multicellular organisms.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.
Cell Structures & Organelles
Learning goals
- I can explain cell structures & organelles.
- I can use evidence, a model, and stated assumptions to support a biological explanation.
Before this unit
Use the supplied vocabulary, distinguish an observation from an explanation, and identify any units or model assumptions. Revisit a connected earlier unit if needed; new terms are explained here.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
Your liver cells and your red blood cells carry the same DNA, and they do completely different jobs. What separates them is not their instructions but their equipment — what is actually built inside them. A cell is not a bag of soup. It is a crowded, organised system with a boundary, an interior, a store of information, and structures that each do particular work. The boundary is the cell membrane, and it is extraordinarily thin: about 8 nanometres, so roughly nine thousand of them stacked up would match the width of one human hair. Inside it is the cytoplasm, the jelly-like material that fills the cell and holds everything in place. Suspended in that cytoplasm are the organelles — the word means "little organs", and it is a fair description, because each one does a job the whole cell depends on, the way your organs do for you.
The largest organelle in most of your cells is the nucleus, and it is where almost all the cell's DNA is kept — the long molecule that carries the instructions for building proteins. The amount is hard to believe: unwound and laid end to end, the DNA from a single human cell would stretch about two metres, and it is packed into a nucleus roughly six micrometres across. Those instructions are not used in the nucleus itself. They are copied and carried out to the ribosomes, the small machines, about 20 nanometres wide, that assemble proteins by joining amino acids in the order the instructions specify. A busy cell can hold millions of ribosomes. Proteins matter because they do nearly everything: they form structures, speed up reactions, carry signals and move materials. The division of labour is worth remembering — the nucleus stores and protects the instructions, and the ribosomes build from them.
Doing all that work costs energy, and the organelles that release it are the mitochondria (one of them is a mitochondrion). During aerobic respiration they transfer energy stored in food molecules into a form the cell can spend, using oxygen and releasing carbon dioxide. Notice the verb: they transfer energy, they do not create it, and some of it always leaves as heat, which is one reason you are warm. A cell builds as many as it needs — a liver cell may hold one to two thousand, while a cell doing less work holds far fewer. Each mitochondrion is roughly a micrometre long, about the size of the bacteria from the last unit, which is a clue biologists have thought hard about. Two cautions before you meet a textbook diagram. Plant cells have mitochondria too and respire just as you do; having chloroplasts does not remove the need to release energy from food. And no single diagram fits every cell: a mature mammalian red blood cell has no nucleus and no mitochondria at all, having discarded both to make room for carrying oxygen.
Build your understanding
Where this fits
Last unit established that every living thing is made of cells and that new cells come from existing ones. That raises an obvious question: if a cell is the unit of life, what is inside one, and how do the parts work together? This unit opens the cell up. Keep the instrument limits from Chapter 1 in mind, because they decide what anyone could see and when.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
Part 1 · The boundary that has to let things through
Start at the edge. The cell membrane is a double sheet of fat-like molecules called phospholipids, each with a water-loving head and two water-fearing tails. In water, those molecules line up automatically with the tails tucked inward and the heads facing out, which is why a membrane forms and reseals itself without anything having to assemble it. That double sheet is what makes the boundary possible, and it is only about 8 nanometres thick.
A sealed boundary would be useless. A cell has to take in glucose and oxygen and get rid of carbon dioxide and other products, so the membrane has to be selectively permeable — it lets some things through and holds others back. Small, uncharged molecules such as oxygen and carbon dioxide slip between the phospholipids on their own. Larger or charged particles, including glucose and sodium ions, cannot, and they cross through protein channels and pumps embedded in the sheet. The proteins are what make the selection specific.
Two things follow from that structure. First, what a cell can take in depends on which proteins it has built into its membrane, which is another way its equipment, not just its DNA, decides what it can do. Second, the membrane is not a decision-maker. Nothing in it chooses; molecules move according to physical and chemical rules, and the pattern we call regulation is the result. The plant cell wall you will meet in the next unit sits outside this membrane and does a different job entirely.
Pause and explain
Why would a completely sealed boundary be a poor model of a cell membrane?
Check your reasoning after answering
Because a sealed boundary would stop the exchanges the cell depends on. Oxygen and glucose could not come in and carbon dioxide could not leave, so respiration would stop. A real membrane is selectively permeable: small uncharged molecules cross it directly, and larger or charged ones pass through specific proteins.
Part 2 · Where the instructions are kept, and where proteins are built
The nucleus is a compartment with its own double membrane, pierced by pores, and it holds the cell's DNA. Keeping the instructions separated from the rest of the cell has a real advantage: the copy that leaves can be controlled, so the cell can make a lot of one protein and none of another without altering the original. The nucleus does not build anything. It stores, protects and issues.
Ribosomes do the building. A ribosome reads a copied instruction and joins amino acids in the specified order, producing a protein that then folds into a shape determined by that order. Some ribosomes float free in the cytoplasm; others sit on a folded membrane system called the endoplasmic reticulum, and those tend to make proteins destined to leave the cell. A cell that secretes a great deal, such as a cell in your pancreas, is crowded with them.
This is also where the division between the two kinds of cell matters. Eukaryotes — animals, plants, fungi, and the single-celled Paramecium from the last unit — keep their DNA inside a nucleus. Prokaryotes, the bacteria, do not: their DNA sits directly in the cytoplasm. But prokaryotes still have DNA, still have ribosomes, and still build proteins. A diagram showing a tidy nucleus is a model of one kind of cell, not an inventory of life.
Pause and explain
Does the nucleus itself assemble the cell's proteins?
Check your reasoning after answering
No. The nucleus stores the DNA instructions; ribosomes assemble proteins by joining amino acids in the order a copied instruction specifies. Separating the two lets the cell control how much of each protein it makes. Bacteria have no nucleus at all and still build proteins, because they still have DNA and ribosomes.
Part 3 · Releasing energy from food, and why 'making energy' is wrong
A mitochondrion is roughly a micrometre long and has two membranes, the inner one folded into deep ridges. Those folds matter: they pack a large working surface into a small volume, and most of the energy transfer happens on them. During aerobic respiration the mitochondrion uses oxygen to break down glucose, releasing carbon dioxide and water and transferring the energy into a molecule the cell can spend wherever it is needed.
The phrase "mitochondria make energy" is the most common error in this unit, and it is worth being precise about why it is wrong. Energy is not created; it is moved from one store to another. The chemical energy was already in the glucose, put there by a plant. The mitochondrion transfers part of it into a usable form and the rest escapes as thermal energy to the surroundings. That escaping fraction is not waste in any useless sense — it is why a mammal is warm to the touch.
Cells build the number of mitochondria their work demands. A liver cell or a heart muscle cell, both working constantly, may hold one to two thousand. A mature mammalian red blood cell holds none: it discards its nucleus and mitochondria as it matures so that more of its volume can carry oxygen, and it survives on a simpler process that needs neither. Plant cells, which make their own glucose, still respire in mitochondria to use it.
Pause and explain
Why is "mitochondria create energy" misleading?
Check your reasoning after answering
Because energy cannot be created. The chemical energy is already stored in glucose; the mitochondrion transfers part of it into a form the cell can spend and loses the rest as heat. Saying a structure creates energy hides where the energy came from, which is food, and ultimately a plant that captured sunlight.
Part 4 · Explaining a system instead of labelling a diagram
Naming organelles is the easy half. An explanation connects them: the membrane admits glucose and oxygen; the nucleus issues an instruction; a ribosome builds the protein that instruction specifies; a mitochondrion supplies the usable energy that the building costs; and the proteins produced go on to form structures and run reactions, including the channels in the membrane where the chain started. That loop is the cell as a system.
The test of whether you have an explanation rather than a list is a damage question. If a cell's mitochondria stop working, protein building slows, membrane pumps that need energy fail, and internal conditions drift — not because the mitochondria did those jobs, but because the work they paid for cannot be paid for any more. If instead the membrane is torn, the contents leak and conditions cannot be maintained, even though every organelle inside may be perfectly intact.
That second case is why a cell is not a collection of independent parts. An organelle is only as useful as the system it sits in. When you read a claim about a cell, ask which structure is doing what, what it needs from the others, and what evidence shows the connection — rather than accepting a label as an explanation.
Pause and explain
Why is labelling organelles not yet an explanation of how a cell works?
Check your reasoning after answering
Because labels name parts without saying what depends on what. An explanation states each structure's role and the connections between them: the membrane supplies materials, the nucleus issues instructions, ribosomes build proteins, mitochondria supply the energy that building costs, and the proteins made then maintain the membrane.
Apply the whole unit · Use structure and function as evidence
Illustrative cell record, not a measured experiment: a cell's membrane has been damaged and substances are leaking out of it. Under the microscope the ribosomes are still clearly visible and look normal. A student's draft concludes that because the ribosomes are intact, the cell is still functioning normally.
Your task
Explain what the membrane does, evaluate the draft's reasoning, and name one further observation that would settle the question.
What your response should show
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Response guidance — after your attempt
State the membrane's role: it is the selective boundary that admits glucose and oxygen, removes products, and maintains internal conditions different from the surroundings.
Say what intact ribosomes do and do not show. They show that one structure survived; they do not show that it has the amino acids, instructions or energy it needs to work.
Identify the reasoning error: the draft treats one intact part as evidence about the whole system, when the damaged part is the one every other part depends on.
Name a further observation: whether the cell is still making protein, still maintaining a different internal concentration from its surroundings, or still consuming oxygen — each tests function rather than appearance.
The membrane is the selective boundary. It lets oxygen and glucose in, lets carbon dioxide out, and keeps the inside of the cell chemically different from the outside; a cell that cannot do this cannot maintain the conditions its reactions need. The draft's reasoning does not hold. Visible, normal-looking ribosomes establish that one organelle is structurally intact at the moment of observation, not that it is still building proteins, and certainly not that the cell as a whole is functioning. With the boundary breached, materials leak out, concentrations equalise with the surroundings, and the energy supply that protein building depends on is interrupted — so an intact ribosome may simply be an intact ribosome in a dying cell. A single further observation would settle it: test whether the cell is still producing protein, still consuming oxygen, or still holding an internal concentration different from the fluid around it. Any of those measures function directly, whereas appearance under a microscope measures only structure, and only at one instant.
Scientific references for this unit
Parent / teacher teaching notes
Ask for a because-statement and a labeled relationship, not just a memorized word. Work through one calculation step at a time, permit oral or diagram responses, and use flexible sessions for difficult reading. Compare the learner’s actual evidence with the supplied model; do not reward copying or agreement with a personal preference. No private family, genetic, or medical records are required.
The key idea
A cell is an interacting system with boundaries, internal material, genetic information, and structures that perform tasks. The membrane surrounds the cell and helps regulate exchanges. Cytoplasm contains material and structures inside the membrane. An organelle is a specialized structure within a cell; many familiar organelles occur in eukaryotes, while prokaryotes lack a membrane-enclosed nucleus.
Worked examples
Example 1 · Read equipment, not instructions
- Separate instructions from equipment: identical DNA means the same set of possible proteins, not the same structures actually built.
- Match each cell's equipment to its job. Heart muscle contracts without pause and needs a constant supply of usable energy, so it builds mitochondria in large numbers.
- A red blood cell's job is to carry oxygen. As it matures it discards its nucleus and mitochondria, freeing volume for haemoglobin and avoiding consuming the oxygen it is meant to deliver.
Same instructions, different equipment — that is the whole reason one body can contain cells that behave nothing alike.
Example 2 · Follow a chain of dependence
- Identify what mitochondria supply: not proteins, but the usable energy that other processes spend.
- Find the processes that spend it. Building a protein means joining amino acids, which costs energy, and so does pumping materials across the membrane to collect those amino acids.
- Trace the consequence rather than assuming an organelle stops working on its own: ribosomes are intact but under-supplied, so production slows and then stops.
Ask what each structure supplies to the others, not just what it is called.
Avoid these mistakes
- Mistake: mitochondria make energy. They transfer energy already stored in glucose into a usable form and lose some as heat; the energy came from food, and before that from a plant capturing sunlight.
- Mistake: the nucleus builds proteins. It stores and issues the DNA instructions, while ribosomes do the building — which is why bacteria, with no nucleus, still make proteins.
- Mistake: every cell contains every organelle in the diagram. A mature mammalian red blood cell has neither nucleus nor mitochondria, and a bacterium has no membrane-enclosed nucleus.
- Mistake: a labelled diagram is an explanation. Naming parts is not saying what depends on what; an explanation connects membrane, instructions, ribosomes and energy supply into one working system.
Check your understanding
Why would a completely sealed boundary be a poor cell model?
Discuss the answer
It would prevent needed exchanges such as uptake of materials and removal of wastes.
Does a nucleus itself assemble all proteins?
Discuss the answer
No. Ribosomes build proteins using cellular instructions; the nucleus stores much genetic information in a typical eukaryotic cell.
Words to know
- organelle
- A structure inside a cell that does a particular job; the word means "little organ".
- cytoplasm
- The jelly-like material filling the cell inside its membrane, holding the organelles in place.
- nucleus
- The compartment holding almost all of a eukaryotic cell's DNA; it stores and issues instructions rather than building anything.
- ribosome
- A machine about 20 nanometres across that builds a protein by joining amino acids in the order an instruction specifies.
- mitochondrion
- The organelle, about a micrometre long, that transfers energy from food into a usable form during aerobic respiration.
Extend your scientific explanation
Compare a second supplied example or the linked educational reference. Identify a connection, a model limit, and an additional observation that would test the explanation.
In the real world
Use a clear question, reliable observations, and a biological mechanism to evaluate a claim. State what remains uncertain before applying a classroom model to a different setting.
Exit check
Why is labeling organelles not a complete explanation of cell function?
Check your explanation after answering
An explanation also connects each structure with its role and interactions in the functioning system.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.
Plant, Animal & Other Cell Types
Learning goals
- I can explain plant, animal & other cell types.
- I can use evidence, a model, and stated assumptions to support a biological explanation.
Before this unit
Use the supplied vocabulary, distinguish an observation from an explanation, and identify any units or model assumptions. Revisit a connected earlier unit if needed; new terms are explained here.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
Leave a lettuce leaf out on the counter and by evening it is limp; drop it in cold water and in an hour it is crisp again. Nothing was added to it but water, and no animal behaves this way — your arm does not go limp and firm again with the weather. That everyday difference is a clue to the real structural differences between plant and animal cells. Both are eukaryotic, so both have a cell membrane, cytoplasm, ribosomes, mitochondria and a nucleus, and both follow the same cell theory. But a plant cell has three things an animal cell does not: a rigid cell wall outside the membrane, chloroplasts that capture light, and one enormous vacuole that can fill most of the cell's volume.
The cell wall is made mostly of cellulose, the same tough material that made Hooke's dead cork keep its shape, and it sits outside the membrane rather than replacing it. Its job is mechanical: it resists pressure from within. The central vacuole is a membrane-bound sac of water and dissolved substances, and when it is full it presses the cytoplasm outward against the wall. That outward pressure is called turgor, and it is what holds a non-woody plant upright. A wilted plant has lost water, the vacuoles have shrunk, the pressure has dropped — and the crisp lettuce is the same cells with their vacuoles refilled. A plant has no skeleton; turgor and cellulose walls are the skeleton.
Chloroplasts are where photosynthesis happens, and they contain chlorophyll, the pigment that makes leaves green and absorbs the light energy the reaction needs. A single leaf cell may hold fifty or more, and they move within the cell towards better light. Two cautions. First, not every plant cell has them: root cells sit in the dark and have none, which is why roots are not green. Second, a plant cell with chloroplasts still has mitochondria and still respires — making glucose and releasing energy from it are different processes that both run. More generally, cells of one organism differ from each other as much as they differ across kingdoms. A specialized cell such as a root hair cell, a nerve cell or a red blood cell is built for one job, so a general "plant cell" diagram is a composite, not a portrait of any real cell.
Build your understanding
Where this fits
We now know what is inside a cell in general. But a leaf and your arm behave nothing alike, and cells are the reason. This unit compares plant and animal cells — first what they share, which is most of it, then the three structures a plant cell adds. It also introduces turgor, which the next unit's work on water movement will explain.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
Part 1 · What plant and animal cells share, before what divides them
It is easy to learn the differences and forget how much is the same. Plant and animal cells are both eukaryotic: each keeps its DNA inside a nucleus, builds proteins on ribosomes, releases energy from food in mitochondria, and is bounded by a selectively permeable membrane. Both are filled with cytoplasm, both divide to make new cells, and both obey all three claims of cell theory. If you were handed an unlabelled list of these features you could not tell the two apart.
That shared list is not a coincidence; it reflects shared ancestry, which is a theme later units return to. It also means the differences are additions, not replacements. A plant cell does not have a wall instead of a membrane — it has a wall as well as one, with the membrane pressed against the wall from inside. Keeping that straight prevents a common error later, because the membrane is what controls what enters, while the wall is simply strong.
So the honest comparison is: same core machinery, plus three structures in the plant cell that the animal cell does without. Everything that follows in this unit is about what those three structures let a plant do — stand up without a skeleton, feed itself from light, and store water in bulk.
Pause and explain
Name three structures a plant cell and an animal cell both have, and say why that matters.
Check your reasoning after answering
Both have a cell membrane, cytoplasm with ribosomes, and mitochondria, and both keep DNA in a nucleus. It matters because it shows the differences between them are additions to a shared design rather than two unrelated kinds of cell, and because it means a plant cell still respires in mitochondria just as an animal cell does.
Part 2 · The wall and the membrane do different jobs
The cell wall is a mesh of cellulose fibres laid down outside the membrane. It is strong and it is porous: water and dissolved substances pass through it freely, so it does no selecting at all. Everything that controls what enters the cell still happens at the membrane, pressed against the wall's inner face. Confusing the two leads to the wrong prediction every time, because the wall's permeability tempts you to think a plant cell takes in anything, when in fact it is as selective as an animal cell.
What the wall does is resist pressure. Push water into an animal cell and it swells and can burst, because nothing holds it in. Push water into a plant cell and the vacuole swells, the cytoplasm presses outward, and the wall pushes back — the cell becomes firm instead of bursting. That balance is turgor, and a plant relies on it: a stem with full vacuoles stands, a stem with empty ones wilts.
The reverse case is just as telling. Put a plant cell in very salty water and water leaves it. The vacuole shrinks, the membrane pulls away from the inside of the wall, and the cell goes slack while the wall keeps its shape. That separation is called plasmolysis, and it is good evidence that wall and membrane are genuinely separate layers — you can see the gap open up between them under a microscope.
Pause and explain
Why can a plant cell take in a great deal of water without bursting, when an animal cell cannot?
Check your reasoning after answering
Because the cellulose wall outside the membrane resists the outward pressure. As water enters, the vacuole swells and presses the cytoplasm against the wall, and the wall pushes back, so the pressure rises and inflow slows instead of the cell rupturing. An animal cell has no wall, so nothing opposes the swelling.
Part 3 · Chloroplasts, and the cells that do without them
A chloroplast is an organelle with two outer membranes and stacks of flattened sacs inside, and those stacks hold chlorophyll. Chlorophyll absorbs red and blue light strongly and reflects green, which is why a leaf looks green — you are seeing the light the plant did not use. Inside the chloroplast, that captured energy drives the reaction that builds glucose from carbon dioxide and water.
Chloroplasts are not evenly distributed. A palisade cell just under the upper surface of a leaf is packed with them and shaped like a column so that light passes down through many; a cell in the root has none at all and depends entirely on sugar delivered from the leaves. If you have ever seen a potato sprout turn green on a windowsill, you have watched cells that had no chloroplasts develop them once light reached them.
The point to hold on to is that having chloroplasts does not exempt a plant from respiration. Photosynthesis stores energy in glucose; respiration in the mitochondria releases it again for work. A plant runs both, all day, and runs respiration alone at night. Treating them as alternatives — plants photosynthesise, animals respire — is the error this unit most wants you to avoid.
Pause and explain
A root cell has no chloroplasts. Is it still a plant cell, and how does it get energy?
Check your reasoning after answering
Yes. It has a cellulose wall, a vacuole, a nucleus and mitochondria, so it is a plant cell in every other respect; chloroplasts would be useless underground. It gets its energy from glucose made in the leaves and transported to it, which it then breaks down by respiration in its mitochondria, exactly as an animal cell would.
Part 4 · Classifying a cell from more than one observation
Suppose you are handed a micrograph and asked what kind of cell it shows. One feature is rarely enough. A visible wall narrows it down but does not settle it: plants have walls, and so do fungi, and so do bacteria, built of different materials. Green colour suggests chloroplasts, but some bacteria and algae are green by other means. A nucleus tells you it is eukaryotic and rules out bacteria, but says nothing about plant or animal.
Combinations do the work. Wall plus nucleus plus chloroplasts gives a photosynthetic eukaryote, most likely a plant or an alga. Wall plus DNA with no enclosed nucleus gives a prokaryote. Membrane and nucleus with no wall gives an animal cell or another wall-less eukaryote. Each conclusion is as strong as the set of features it rests on, and each should be stated with the alternatives it has not excluded.
This is also where specialisation complicates things. A mature red blood cell has no nucleus, so judged by one feature it would be sorted with the bacteria, which is plainly wrong. A root cell has no chloroplasts. Classification works on what a cell has and on what organism it came from, and a careful answer says which observations it used and what further observation would narrow it further.
Pause and explain
Why is a visible cell wall not enough to call something a plant cell?
Check your reasoning after answering
Because walls are not unique to plants: fungi build walls of chitin and bacteria build them of peptidoglycan, both different materials from plant cellulose. A wall shows only that the cell has a rigid outer layer. To call it a plant cell you need further evidence, such as an enclosed nucleus together with chloroplasts or a large central vacuole.
Apply the whole unit · Compare cell records
Illustrative records, not real specimens: Cell A has a wall, an enclosed nucleus and chloroplasts. Cell B has a wall and DNA, but no enclosed nucleus. Cell C has a membrane and an enclosed nucleus, and no wall.
Your task
Say what each combination of features supports, name what each conclusion has not ruled out, and give one further observation for the least certain case.
What your response should show
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Response guidance — after your attempt
Take the features in combination rather than one at a time, since a wall alone appears in plants, fungi and bacteria.
Cell A: an enclosed nucleus makes it eukaryotic, and chloroplasts make it photosynthetic, so a plant or algal cell is well supported.
Cell B: DNA present but no enclosed nucleus makes it prokaryotic, so a bacterium is well supported, and its wall is a different material from a plant's cellulose.
Cell C: a nucleus with no wall supports an animal cell, but does not rule out other wall-less eukaryotes such as a protist, which is the least certain of the three.
Cell A is a photosynthetic eukaryote. The enclosed nucleus rules out bacteria and the chloroplasts show it captures light, so a plant or algal cell fits; the records do not distinguish between those two. Cell B is a prokaryote: it holds DNA but has no enclosed nucleus, which is the defining difference, and although it has a wall, a bacterial wall is built of different material from a plant's cellulose, so the wall itself is not what identifies it. Cell C is a eukaryote without a wall, which an animal cell fits, but so do many protists and so does a cell that has lost features through specialisation — this is the least certain of the three. The useful further observation for Cell C is whether it has a large central vacuole and whether it can move or feed on its own: a central vacuole would point back towards a plant cell that happens to lack chloroplasts, while independent swimming and feeding would point to a protist. Throughout, each conclusion rests on a combination of features, and a single feature — a wall, a colour, a nucleus — would have been enough to mislead.
Scientific references for this unit
Parent / teacher teaching notes
Ask for a because-statement and a labeled relationship, not just a memorized word. Work through one calculation step at a time, permit oral or diagram responses, and use flexible sessions for difficult reading. Compare the learner’s actual evidence with the supplied model; do not reward copying or agreement with a personal preference. No private family, genetic, or medical records are required.
The key idea
Plant and animal cells are eukaryotic and share many structures, including a membrane, cytoplasm, ribosomes, mitochondria, and typically a nucleus. Comparing their structures explains some different functions, but a general diagram should not be mistaken for every cell in every tissue. Specialized cells can differ from the typical models.
Worked examples
Example 1 · Explain the limp lettuce
- Identify the structure that provides stiffness: a non-woody plant stands by turgor, the pressure of full vacuoles against cellulose walls, not by any skeleton.
- Explain going limp: water evaporated from the leaf, the vacuoles lost volume, the pressure against the walls fell, and the tissue became floppy.
- Explain recovering: water moved back into the cells by osmosis, the vacuoles refilled, pressure rose again and the walls were pushed outwards until the leaf was firm.
An animal has no equivalent, because animal cells have no walls for pressure to push against.
Example 2 · Judge a classification from one feature
- Check whether the feature is unique. It is not: fungi build walls of chitin, bacteria build them of peptidoglycan, and plants build them of cellulose.
- State what the observation does support — that the cells have a rigid outer layer, which rules out a typical animal cell.
- Name the evidence that would decide it: an enclosed nucleus to establish a eukaryote, plus chloroplasts or a large central vacuole to establish a plant.
Name what a feature rules out as well as what it suggests; that is usually the stronger half of the answer.
Avoid these mistakes
- Mistake: a plant cell has a wall instead of a membrane. It has both, with the membrane pressed against the wall from inside — and the membrane, not the porous wall, is what selects what enters.
- Mistake: plants photosynthesise and animals respire. Plant cells have mitochondria and respire constantly, including all night; photosynthesis stores energy in glucose and respiration releases it again.
- Mistake: every plant cell is green. Root cells have no chloroplasts, which is why roots are not green, and they live on sugar sent down from the leaves.
- Mistake: one feature identifies a cell. Fungi and bacteria have walls too, so a wall alone settles nothing; combinations of features, such as a nucleus together with chloroplasts, are what support a conclusion.
Check your understanding
What shared energy process occurs in both plant and animal cells?
Discuss the answer
Both can carry out cellular respiration; plant photosynthesis does not replace it.
What provides selective exchange in a plant cell?
Discuss the answer
The membrane, not simply the rigid cell wall. The wall contributes support and protection.
Words to know
- cell wall
- A rigid cellulose layer outside a plant cell's membrane that resists pressure from within; it is porous, so it does no selecting.
- chloroplast
- The organelle holding chlorophyll, where photosynthesis captures light energy and builds glucose; absent from root cells.
- vacuole
- A membrane-bound sac; in a plant cell one large central vacuole of water and dissolved substances can fill most of the volume.
- turgor
- The outward pressure of a full vacuole against the cell wall, which holds a non-woody plant upright; losing it is wilting.
- specialized cell
- A cell built for one job, such as a root hair, guard cell or red blood cell, which no general diagram represents.
Extend your scientific explanation
Compare a second supplied example or the linked educational reference. Identify a connection, a model limit, and an additional observation that would test the explanation.
In the real world
Use a clear question, reliable observations, and a biological mechanism to evaluate a claim. State what remains uncertain before applying a classroom model to a different setting.
Exit check
Why can a wall alone not identify a plant?
Check your explanation after answering
Other organisms, including many fungi and bacteria, also have walls. Additional features and context are needed.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.
Membrane Transport, Diffusion & Osmosis
Learning goals
- I can explain membrane transport, diffusion & osmosis.
- I can use evidence, a model, and stated assumptions to support a biological explanation.
Before this unit
Use the supplied vocabulary, distinguish an observation from an explanation, and identify any units or model assumptions. Revisit a connected earlier unit if needed; new terms are explained here.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
Drop a tea bag into still water and do not stir it. The colour spreads anyway, slowly and in every direction, until the whole cup is the same shade. Nothing pushed it and nothing pulled it. Every molecule in that cup is in constant random motion, bumping and rebounding millions of times a second, and that aimless jostling is enough on its own to spread the tea through the water. This is diffusion: the net movement of particles from where they are crowded to where they are sparse, produced by random motion and nothing else. The word that matters is net. Individual particles go both ways the whole time; there are simply more of them starting on the crowded side, so more happen to cross outward than inward, and the imbalance shows up as spreading.
Concentration is how crowded the particles are — how much of a substance there is in a given volume. It is always a comparison, so "a high concentration" means nothing until you say high compared with what. When two regions differ, we speak of a concentration gradient between them, and diffusion runs down that gradient. It stops being one-sided when the crowding evens out, and that state is equilibrium. Equilibrium is not stillness: the particles keep moving and keep crossing in both directions, but the two flows now match, so nothing changes overall. A cell exploits this constantly. Oxygen is used up inside a respiring cell, so its concentration inside stays low, so oxygen keeps diffusing in without the cell spending anything to fetch it.
Cells are wrapped in a selectively permeable membrane, which lets some substances through and blocks others, and that turns a simple spreading process into something a cell can live on. Water crosses freely; many dissolved substances do not. When water moves across such a membrane from the side with more water to the side with less — that is, from the weaker solution to the stronger one — the movement has its own name, osmosis. It is just diffusion of water, but it is worth a separate name because the consequences for a cell are dramatic, from a wilting lettuce leaf to a red blood cell bursting in pure water. Diffusion and osmosis cost the cell nothing, which is why they are called passive. Moving a substance the other way, against its gradient, requires energy, and that is active transport.
Build your understanding
Where this fits
The last two units named the membrane as the cell's selective boundary and left the mechanism unexplained. How does anything actually cross it, and why does a plant cell go firm in water while an animal cell bursts? This unit answers both with one idea — random motion — and separates what a cell gets for free from what it has to pay for.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
Part 1 · Concentration only means something as a comparison
Concentration is an amount of substance in an amount of space: grams of salt per litre of water, particles per unit volume, molecules per cubic micrometre. Written on its own, a figure tells you very little. Saying "the solution is concentrated" invites the question: compared with what? A fluid that is concentrated relative to the inside of a cell may be dilute relative to seawater, and the cell's behaviour depends entirely on which comparison you mean.
So always name both sides. Inside this cell there are ten particles per unit volume; outside there are five. That sentence supports a prediction, and "the outside is dilute" does not. The difference between two concentrations is a concentration gradient, and the size of the gradient affects how fast diffusion goes: a steep difference spreads faster than a shallow one, which is why the first minute of the tea bag changes the water more than the tenth.
One more distinction worth fixing now. A concentration can change because the amount of substance changed, or because the volume changed. If water leaves a cell, the substances left behind become more concentrated without a single extra particle arriving. When you read a rising concentration, ask which of the two happened, because the explanations are different.
Pause and explain
Why must a concentration always be stated as a comparison?
Check your reasoning after answering
Because concentration is an amount in a volume, and whether that counts as high or low depends on what it is being compared with. The same fluid can be concentrated relative to a cell's interior and dilute relative to seawater. Only a stated comparison — ten particles per unit volume inside against five outside — supports a prediction about which way material moves.
Part 2 · Net movement happens without anything intending it
It is tempting to say particles "want" to spread out, or that they "move towards" the empty space. They do neither. Each particle moves randomly, changing direction every time it collides, with no information about where the crowding is. Put a hundred particles on the left of a container and ten on the right, and in the next instant each one sets off in a random direction. More set off from the left simply because there are more of them there, so more arrive on the right than return. The one-way appearance is a consequence of the numbers, not of any tendency.
That is why the correct phrase is net movement. Traffic crosses in both directions throughout, and the net is the difference between the two flows. When the numbers on each side even out, both flows continue at the same rate and the net becomes zero. Nothing has stopped. If you could watch individual particles at equilibrium, you would see them crossing constantly in both directions.
Thinking this way prevents two errors. The first is expecting diffusion to stop once something arrives — it does not stop, it balances. The second is imagining that a cell has to do something to make diffusion happen. It does not: oxygen enters a respiring cell for free, because respiration keeps the inside concentration low and the gradient does the rest.
Pause and explain
At equilibrium, have the particles stopped moving?
Check your reasoning after answering
No. They keep moving randomly and keep crossing in both directions at the same rate, so the net movement is zero while the motion continues. Equilibrium describes a balance between two flows, not stillness, and warming the system would speed both flows without changing the balance.
Part 3 · Water across a selective membrane, and what it does to cells
Now make the barrier selective: water can cross it, a dissolved substance cannot. Water still diffuses in both directions, but the side holding more dissolved particles has proportionally less water, so the net flow of water is towards the stronger solution. That is osmosis. Note what it is not: the dissolved substance is not pulling the water, and the substance is not moving at all. Only the water moves, and only because its own concentration differs across the barrier.
The consequences for cells are immediate. An animal cell in pure water takes in water with nothing to resist the swelling, and can burst; a red blood cell does exactly that, which is why a drip must be matched to blood. The same cell in strong salt solution loses water and shrivels. A plant cell in pure water also takes in water, but its cellulose wall pushes back and the cell becomes firm instead — the turgor from the last unit. In strong salt solution the plant cell loses water, and the membrane pulls away from the wall.
A sealed model makes the logic clear. Two compartments of equal volume, separated by a membrane water can cross and solute cannot: compartment A holds 5 solute particles per unit volume, compartment B holds 10. Water moves net from A to B, because A is the weaker solution. B gains volume, which dilutes it, while A loses volume, which concentrates it, and the movement slows as the two approach each other.
Pause and explain
In the model above, which way does water move, and what actually crosses the membrane?
Check your reasoning after answering
Water moves net from compartment A, with 5 particles per unit volume, into compartment B, with 10, because A is the weaker solution and so holds proportionally more water. Only water crosses; the solute cannot pass the membrane. The flow slows as B is diluted and A is concentrated by the volume change.
Part 4 · When a cell has to pay: active transport
Diffusion and osmosis are passive: they run down a gradient and cost the cell nothing. But cells constantly need the opposite. A root hair cell takes up mineral ions from soil water where those ions are scarcer than inside the root. A nerve cell holds sodium out and potassium in against both gradients, and resets that difference after every signal. Neither can happen by diffusion, because both run uphill.
Active transport is the uphill case. A protein pump in the membrane binds the substance and moves it across using energy, which the cell supplies from respiration. Three predictions follow and all are testable: active transport stops when a cell is starved of oxygen or poisoned, while diffusion carries on; it is specific, because each pump handles particular substances; and it can go on raising a concentration difference long after diffusion would have evened things out.
The practical way to tell them apart is to ask which way the substance is going relative to its gradient, and whether stopping the cell's energy supply stops the movement. Down the gradient and unaffected by energy means passive. Up the gradient and dependent on energy means active. A cell uses both at once, all the time, and the fact that it never has to spend anything on taking up oxygen is part of why it can afford the pumps.
Pause and explain
How would you test whether a cell is taking up a substance by diffusion or by active transport?
Check your reasoning after answering
Compare the concentrations inside and outside, and remove the cell's energy supply. If the substance moves from high concentration to low and continues when respiration is blocked, it is diffusion. If it moves from low concentration to high and stops when oxygen is withheld or respiration is poisoned, it is active transport, because the pumps have nothing to spend.
Apply the whole unit · Read a selective-membrane record
Illustrative model, not a measured experiment: two compartments of equal volume at equal pressure are separated by a membrane that water can cross and solute cannot. Compartment A holds 5 solute particles per unit volume; compartment B holds 10.
Your task
Predict the initial net movement of water, state exactly what crosses the membrane, and explain why reaching equilibrium does not mean the movement has stopped.
What your response should show
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Response guidance — after your attempt
Compare the two solutions: B at 10 particles per unit volume is twice as concentrated as A at 5, so A is the weaker solution and holds proportionally more water.
Predict the net flow: water moves net from A into B, down its own concentration gradient, while the solute stays where it is because the membrane will not pass it.
Follow the consequence: B gains volume and is diluted, A loses volume and is concentrated, so the gap narrows from both ends and the net flow slows.
Describe equilibrium correctly: water keeps crossing in both directions at matched rates, so the net is zero while the motion continues.
Water moves net from A to B. B holds 10 solute particles per unit volume against A's 5, so B is the stronger solution and therefore the one with proportionally less water; water diffuses down its own gradient towards it. Only water crosses — the solute cannot pass this membrane, so its particles stay in the compartments they started in, and their number never changes. What changes is volume: B swells and is diluted, A shrinks and is concentrated, so the difference closes from both directions and the net flow slows as it does. At equilibrium the two concentrations match and the net movement is zero, but nothing has stopped: water molecules continue to cross in both directions at the same rate, and the balance between the two flows is what makes the net zero. Two assumptions are worth stating. The model holds the pressure equal on both sides, so nothing opposes the volume change the way a plant cell wall would. And it assumes the membrane passes water freely and the solute not at all, which is an idealisation — a real membrane is selective by degree, not absolutely.
Scientific references for this unit
Parent / teacher teaching notes
Ask for a because-statement and a labeled relationship, not just a memorized word. Work through one calculation step at a time, permit oral or diagram responses, and use flexible sessions for difficult reading. Compare the learner’s actual evidence with the supplied model; do not reward copying or agreement with a personal preference. No private family, genetic, or medical records are required.
The key idea
Cells exchange materials across selective membranes. Diffusion is the net movement of particles from a region of higher concentration toward lower concentration through their random motion. Particles do not intentionally seek an even arrangement, and individual particles can move either way. The net pattern depends on concentrations and conditions.
Worked examples
Example 1 · Predict what happens to two cells in pure water
- Compare concentrations: both cells hold dissolved substances, so pure water outside is the weaker solution, and water moves net into both cells by osmosis.
- The red blood cell has only a membrane. Nothing resists the swelling, so it stretches and can burst.
- The plant cell has a cellulose wall outside the membrane. As the vacuole swells, the wall pushes back and pressure rises, which opposes further inflow and leaves the cell firm.
Same direction of water movement, opposite outcome — the difference is the wall, not the osmosis.
Example 2 · Tell diffusion from active transport
- Check the direction against the gradient: nitrate is more concentrated inside than outside, so uptake is moving it from low concentration to high — uphill.
- Diffusion cannot do that, since it only ever produces net movement down a gradient.
- Check the energy dependence: withholding oxygen stops aerobic respiration, which is the cell's energy supply, and uptake stops with it.
Two tests together — direction relative to the gradient, and whether cutting the energy supply stops it.
Avoid these mistakes
- Mistake: particles want to spread out. Each particle moves at random with no information about where the crowding is; net movement is just the result of more particles starting on the crowded side.
- Mistake: at equilibrium everything stops. Particles keep crossing in both directions at matched rates, so the net is zero while the motion continues.
- Mistake: in osmosis the solute pulls the water across. The solute does not move at all; water diffuses down its own concentration gradient towards the stronger solution.
- Mistake: all membrane transport is free. Diffusion and osmosis cost nothing, but moving a substance against its gradient needs pumps and energy from respiration, which is why it stops when oxygen is withheld.
Check your understanding
Why can particle count alone fail to show concentration?
Discuss the answer
Concentration depends on both amount and volume; counts in unequal volumes are not directly comparable.
Does diffusion equilibrium mean all particles stop?
Discuss the answer
No. Motion continues; opposing movements can balance so there is no net concentration change.
Words to know
- diffusion
- The net movement of particles from a crowded region to a sparse one, produced by random motion alone and costing the cell nothing.
- concentration
- The amount of a substance in a given volume; it only supports a prediction when stated as a comparison between two places.
- osmosis
- The diffusion of water across a selectively permeable membrane, from the weaker solution towards the stronger one.
- selectively permeable
- Letting some substances cross while holding others back, which is what turns simple diffusion into something a cell can live on.
- equilibrium
- The state where two opposite flows match, so the net movement is zero; the particles are still moving and still crossing.
Extend your scientific explanation
Compare a second supplied example or the linked educational reference. Identify a connection, a model limit, and an additional observation that would test the explanation.
In the real world
Use a clear question, reliable observations, and a biological mechanism to evaluate a claim. State what remains uncertain before applying a classroom model to a different setting.
Exit check
Is movement of a dissolved particle always osmosis?
Check your explanation after answering
No. Osmosis describes net water movement across a selective membrane; other particles and processes require their own explanation.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.
Photosynthesis & Cellular Respiration
Learning goals
- I can explain photosynthesis & cellular respiration.
- I can use evidence, a model, and stated assumptions to support a biological explanation.
Before this unit
Use the supplied vocabulary, distinguish an observation from an explanation, and identify any units or model assumptions. Revisit a connected earlier unit if needed; new terms are explained here.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
Almost every bit of the energy running through living things on Earth arrived here as sunlight, and almost all of it entered life through one reaction. In photosynthesis, a plant uses light energy captured by chlorophyll to build glucose out of two very unpromising ingredients: carbon dioxide from the air and water from the soil. Oxygen is released as a by-product — the oxygen you are breathing right now is waste from a plant. The reactants, the substances that go in, are carbon dioxide and water; the products, the substances that come out, are glucose and oxygen. Written as a summary, six carbon dioxide and six water molecules yield one glucose and six oxygen molecules, with light energy driving the reaction. The mass of a tree is worth pausing on: most of the wood in an oak was assembled from a gas. It came out of the air, not out of the ground.
Releasing that stored energy again is cellular respiration, and it runs the reaction in reverse. Glucose and oxygen go in; carbon dioxide and water come out; and the energy released is transferred into a form the cell can spend on building, moving and pumping. This happens in mitochondria, in every living cell that respires aerobically — in you, in a mushroom, in a bacterium, and in every cell of the plant that made the glucose in the first place. A plant does not photosynthesise in place of respiring. It does both, all day, and respires alone at night when there is no light to drive the other reaction.
Because the two reactions use each other's products, it is easy to picture a tidy closed loop and conclude that energy goes round and round. It does not. Matter cycles — the same carbon atoms move between air, sugar, and air again, over and over. Energy does not cycle. It enters as sunlight, is stored briefly in chemical bonds, and leaves as heat at every transfer, never to be reused. That is why a sealed jar of plants cannot run forever without light, and it is also why measuring gases tells you only so much. A sealed chamber reading records a net change: the difference between what photosynthesis removed and what respiration added. Falling carbon dioxide under light means photosynthesis outpaced respiration, not that respiration stopped.
Build your understanding
Where this fits
We have followed materials into the cell and found the mitochondria that release energy from food. The question left over is where that food comes from in the first place. This unit closes the chapter by connecting the reaction that stores energy with the one that releases it, and by showing why a sealed-chamber reading never measures just one of them.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
Part 1 · What goes in, what comes out, and where the energy is from
Begin with the bookkeeping, because most confusion here is bookkeeping confusion. Photosynthesis takes in carbon dioxide and water and gives out glucose and oxygen, with light as the energy input. Those are the reactants and the products: reactants are consumed, products are made. Light is not a reactant — it is not a substance and it is not built into the glucose. What it does is supply the energy the reaction needs to run, and that energy ends up stored in the chemical bonds of the sugar.
The site of the reaction is the chloroplast, and the molecule that captures the light is chlorophyll. Only cells with chloroplasts can do this, which is why a root cannot and a leaf can. The light a leaf uses is mostly red and blue, which is why it reflects green and why plants grown under green light alone grow poorly.
Now the fact that makes the reaction feel real. A large tree weighs many tonnes, and the great majority of that dry mass is carbon that came out of the air as carbon dioxide, assembled molecule by molecule into sugar and then into wood. The soil supplies water and mineral ions; it does not supply the bulk. People have tested this since van Helmont weighed a willow and its pot in the seventeenth century and found the soil had barely lost any mass at all.
Pause and explain
Is light a reactant in photosynthesis?
Check your reasoning after answering
No. The reactants are carbon dioxide and water, the substances consumed and rebuilt into products. Light is the energy input: chlorophyll captures it and it drives the reaction, ending up stored in the chemical bonds of the glucose rather than becoming part of the molecule.
Part 2 · Respiration happens in every living cell, including the plant's
Cellular respiration takes glucose and oxygen and releases carbon dioxide and water, transferring the stored energy into a usable form. In eukaryotes most of this happens in the mitochondria. The word respiration is unfortunate, because in everyday speech it means breathing. Breathing moves air; respiration is a chemical process inside cells. An animal breathes in order to supply its cells with oxygen for respiration, and a plant, which does not breathe, still respires.
This is the step most often got wrong, so state it plainly: plant cells respire. They have mitochondria, they consume oxygen, and they release carbon dioxide — continuously, day and night. During the day in bright light, photosynthesis in the same plant usually runs faster, so the plant takes in more carbon dioxide than it gives out and the respiration is hidden by the larger flow. At night, with no light, only respiration continues, and a plant releases carbon dioxide and consumes oxygen like any animal.
Not all respiration needs oxygen. When oxygen runs short, many cells switch to anaerobic routes that release far less energy from the same glucose — which is why a sprinter's legs ache and why yeast can make bread rise in a sealed dough. The aerobic route is the one this unit measures, and it is much the more productive of the two.
Pause and explain
A student says plants respire only at night. What is right and what is wrong in that claim?
Check your reasoning after answering
What is right is that at night respiration is the only one of the two processes running, so the plant releases carbon dioxide and takes in oxygen. What is wrong is the word only: plants respire continuously, day and night. In daylight photosynthesis usually runs faster, so the net flow of gases is the other way and respiration is hidden rather than absent.
Part 3 · Reading a gas measurement as a net change
Put a plant in a sealed chamber with a carbon dioxide sensor and you do not measure photosynthesis. You measure the difference between two processes happening at once: photosynthesis removing carbon dioxide and respiration adding it. The number the sensor reports is the net change, and the same net figure could arise from fast photosynthesis with fast respiration, or from slow with slow.
Work an example. The reading starts at 400 units, falls to 350 under light, then rises to 370 during a later dark interval. The change in the light is 350 − 400 = −50 units, a net removal. The change in the dark is 370 − 350 = +20 units, a net addition. In the light, photosynthesis exceeded respiration by an amount corresponding to 50 units. In the dark, with no photosynthesis, the 20-unit rise is respiration alone over that interval.
Now the limits of what those two numbers support. They do not give the rate of photosynthesis, because respiration was subtracting from it the whole time — the true photosynthetic uptake during the light period was more than 50 units. They do not establish that the intervals were the same length, unless that is stated. And a sensor reading in units on one calibration is not a mass of carbon. State the net change, say which processes could produce it, and name what you would need to go further.
Pause and explain
A sealed chamber's carbon dioxide falls by 50 units under light. Why is that not the rate of photosynthesis?
Check your reasoning after answering
Because respiration was adding carbon dioxide throughout the same period, so the reading records only the difference between the two. The true photosynthetic uptake was larger than 50 units by whatever amount respiration released. To separate them you would need the respiration rate measured over a comparable interval, such as from the dark period.
Part 4 · Matter cycles, energy does not
Draw the two reactions facing each other and a loop appears: the products of one are the reactants of the other. For matter, the loop is real. A carbon atom in the air can be fixed into glucose by a leaf, eaten, released again by respiration, and fixed by another leaf, round and round indefinitely. The same atoms have been cycling for a very long time.
For energy the picture is wrong, and this is the error the unit is built to prevent. Energy enters as sunlight, is stored in chemical bonds, and at every transfer some leaves the system as heat that cannot be recovered to do work. It travels one way: in as light, out as heat. Nothing recycles it, which is why life on Earth needs a continuous supply of sunlight rather than a stock of energy that keeps going round.
The practical test is a sealed jar. Put a plant in a sealed, transparent jar in the light and it can continue for a long time, because light keeps entering even though matter cannot. Put the same jar in a cupboard and it dies, with all its matter still present and intact. Matter was never the limit; the energy supply was.
Pause and explain
Why can a sealed jar of plants survive in sunlight but not in a dark cupboard?
Check your reasoning after answering
Because the seal stops matter entering and leaving, but light still passes through the glass. Matter cycles within the jar — carbon moves between air and sugar and back — while energy does not cycle. It enters as light and leaves as heat, so without a continuous light supply the energy is never replaced, even though every atom is still there.
Apply the whole unit · Explain a sealed-chamber record
Illustrative sealed-chamber data for a short study, not a measured experiment: a carbon dioxide reading begins at 400 units, falls to 350 while the chamber is lit, and rises to 370 during a later dark interval. All readings share one calibration, and the chamber holds living plant material throughout.
Your task
Calculate both changes, explain each through the processes that could produce it, and state what the record does not establish.
What your response should show
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Response guidance — after your attempt
Calculate the lit interval: 350 − 400 = −50 units, a net removal of carbon dioxide.
Calculate the dark interval: 370 − 350 = +20 units, a net addition of carbon dioxide.
Attribute each correctly: in the light both processes ran and photosynthesis exceeded respiration; in the dark only respiration ran, so the 20-unit rise is respiration over that interval.
State the limits: the 50-unit fall understates photosynthesis because respiration opposed it throughout, and the record gives no interval lengths and no conversion from units to mass.
The lit interval shows a change of 350 − 400 = −50 units and the dark interval a change of 370 − 350 = +20 units. In the light, both photosynthesis and respiration were running in the same chamber: photosynthesis removed carbon dioxide while respiration added it, and the net removal of 50 units means photosynthesis exceeded respiration by that amount. It does not mean respiration stopped, and it does not give the rate of photosynthesis, which must have been larger than 50 units by however much respiration released over the same period. In the dark there is no photosynthesis, so the 20-unit rise is respiration alone, and that figure is the better estimate of the respiration rate. Combining them, photosynthesis during the lit interval removed something in the order of 70 units if the two intervals were equal in length and conditions were comparable — and that condition is exactly what the record does not state. Three things remain unestablished: the length of each interval, the temperature, which affects both processes, and what the units correspond to in mass of carbon, since a sensor calibrated in arbitrary units is not a balance.
Scientific references for this unit
Parent / teacher teaching notes
Ask for a because-statement and a labeled relationship, not just a memorized word. Work through one calculation step at a time, permit oral or diagram responses, and use flexible sessions for difficult reading. Compare the learner’s actual evidence with the supplied model; do not reward copying or agreement with a personal preference. No private family, genetic, or medical records are required.
The key idea
Photosynthesis stores energy by using light to support the production of sugar from carbon dioxide and water. In the plant model, chloroplasts perform this process and oxygen is released. Light is an energy input, not matter that becomes the carbon atoms in sugar. Carbon enters mainly from carbon dioxide, while water and other resources also matter.
Worked examples
Example 1 · Where does the mass of a tree come from?
- Identify the reactants of photosynthesis: carbon dioxide from the air and water from the soil, assembled into glucose using light energy.
- Follow the carbon: glucose is built into cellulose and the other materials of wood, so most of the tree's dry mass is carbon that arrived as a gas.
- Check the claim against evidence: van Helmont grew a willow in weighed soil and found the soil had lost almost no mass while the tree gained a great deal.
It is counterintuitive because a gas feels like nothing. The carbon is there; it is simply not visible.
Example 2 · Separate two processes from one measurement
- Recognise that the lit figure is a net change: photosynthesis removed carbon dioxide while respiration added it at the same time.
- Use the dark interval to estimate respiration alone, since no photosynthesis occurs without light: 25 units added over that interval.
- Add the respiration back to the net removal: 60 + 25 = 85 units taken up by photosynthesis, assuming respiration ran at the same rate in the light.
A net change is never a rate on its own; you need the opposing process measured separately.
Avoid these mistakes
- Mistake: plants photosynthesise and animals respire. Plant cells have mitochondria and respire day and night; in daylight photosynthesis usually runs faster and hides it.
- Mistake: energy cycles between the two reactions. Matter cycles, energy does not — it enters as light and leaves as heat at every transfer, which is why a sealed jar in a dark cupboard dies with all its atoms intact.
- Mistake: light is a reactant. The reactants are carbon dioxide and water; light supplies the energy that drives the reaction and ends up stored in the bonds of the glucose.
- Mistake: a falling carbon dioxide reading gives the rate of photosynthesis. It gives a net change, and respiration was adding carbon dioxide throughout, so the true uptake was larger.
Check your understanding
Where does much of a growing plant’s carbon come from?
Discuss the answer
Carbon dioxide taken from the surroundings supplies carbon used in organic molecules; light supplies energy rather than carbon matter.
Why does a root without chloroplasts still need sugar?
Discuss the answer
It uses organic material and releases usable energy through cellular processes such as respiration.
Words to know
- photosynthesis
- The reaction in chloroplasts that uses light energy to build glucose from carbon dioxide and water, releasing oxygen as a by-product.
- cellular respiration
- The release of energy from glucose using oxygen, in mitochondria, giving out carbon dioxide and water; it runs in plant cells as well as animal cells.
- reactant
- A substance consumed by a reaction. Light is not one, because it supplies energy rather than matter.
- product
- A substance made by a reaction; the oxygen you breathe is a product of photosynthesis.
- net change
- The difference between two opposing processes, which is all a sealed-chamber gas reading can report.
Extend your scientific explanation
Compare a second supplied example or the linked educational reference. Identify a connection, a model limit, and an additional observation that would test the explanation.
In the real world
Use a clear question, reliable observations, and a biological mechanism to evaluate a claim. State what remains uncertain before applying a classroom model to a different setting.
Exit check
Why is “energy cycles just like carbon” misleading?
Check your explanation after answering
Carbon atoms can be reused in cycles, while usable energy flows through processes and ultimately dissipates as thermal energy.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.
Problem 1
Answer and explanation
The sequence fits new cells arising from existing cells and gives four cells after the two division rounds. The mass claim is unsupported: cells can differ in size, and growth requires material uptake. Cell count alone does not establish four times the starting mass. Additional mass or size observations would be needed.
Count one to two to four cells.
Connect new cells with previous cells dividing.
Distinguish cell production from total mass.
Response criteria
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Problem 2
Answer and explanation
No. A single cell can be an organism; many organisms contain multiple cells.
Problem 3
Answer and explanation
The membrane helps maintain regulated exchanges and internal conditions. Leakage suggests disruption even though ribosomes remain visible. Presence of one structure does not prove normal system function. Measurements of exchange, internal conditions, and cell activity would help assess the extent of damage.
Locate the exchange boundary.
Connect uncontrolled leakage with disrupted internal conditions.
Separate visible presence from measured function.
Response criteria
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Problem 4
Answer and explanation
It would prevent needed exchanges such as uptake of materials and removal of wastes.
Problem 5
Answer and explanation
A fits the supplied photosynthetic plant model; B fits a prokaryotic cell; C fits a typical animal model. These are supported model comparisons, not a complete identification of species. Other organisms and specialized cells can share selected features, and microscope resolution and sample context matter.
A’s combination fits a photosynthetic plant-cell model.
B’s DNA location fits a prokaryotic model.
C fits a typical animal-cell model but needs sample context.
Response criteria
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Problem 6
Answer and explanation
Both can carry out cellular respiration; plant photosynthesis does not replace it.
Problem 7
Answer and explanation
Initial net water movement tends from A toward B under these simplified conditions. Water crosses; the stipulated solute does not. This can change concentrations and pressures, limiting later flow. At equilibrium water molecules can still move both ways even when there is no net change. The conclusion depends on the stated permeability and pressure assumptions.
Compare nonpenetrating solute concentrations.
Apply the stipulated water permeability.
Separate net movement from individual crossings.
Response criteria
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Problem 8
Answer and explanation
Concentration depends on both amount and volume; counts in unequal volumes are not directly comparable.
Problem 9
Answer and explanation
The light interval shows a 50-unit decrease, consistent with net photosynthetic uptake exceeding respiratory release. The dark interval shows a 20-unit increase, consistent with respiration without the same light-driven uptake. These net gas measurements do not separately measure each process; leaks, other organisms, and instrument conditions would need checking in an actual investigation.
Light change: 350−400=−50 units.
Dark change: 370−350=+20 units.
Compare photosynthetic uptake and respiratory release under the model.
Response criteria
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Problem 10
Answer and explanation
Carbon dioxide taken from the surroundings supplies carbon used in organic molecules; light supplies energy rather than carbon matter.