Photosynthesis & Cellular Respiration — Print · Grade 7 Science
Chapter: Cells, Transport & Cellular EnergyPhotosynthesis & 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
Carbon dioxide taken from the surroundings supplies carbon used in organic molecules; light supplies energy rather than carbon matter.
Problem 2
Answer and explanation
It uses organic material and releases usable energy through cellular processes such as respiration.
Problem 3
Answer and explanation
No. Photosynthetic uptake can exceed respiration while both continue; the measurement is a net change.
Problem 4
Answer and explanation
Carbon atoms can be reused in cycles, while usable energy flows through processes and ultimately dissipates as thermal energy.
Problem 5
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 6
Answer and explanation
Plants respire in light and darkness. Photosynthesis can occur in suitable light at the same time, so a net gas change must account for both processes.
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 7
- A typical green plant
- Only an animal
- A rock
- A scale bar
Answer and explanation
A typical green plant
Green plants can perform both; the processes have different roles.
Problem 8
Answer and explanation
-40 units
Final minus initial: 260−300=−40 units.