Carbon & the Macromolecules of Life · Grade 10 Science
Chapter: Investigating the Chemistry of LifeCarbon & the Macromolecules of Life
Learning goals
- I can explain why carbon is the backbone of the molecules of life.
- I can identify the four major macromolecules, their monomers, and their functions.
- I can describe how cells build polymers from monomers.
Readings for this unit
Before this unit
Recognize carbon bonds and distinguish a monomer from a polymer.
Use three flexible study sessions: core explanation and Parts 1–2; Parts 3–4 with their reasoning checks and models; then the whole-unit application, worked examples, independent practice, and exit check. Allow about 50–60 minutes per session, including diagrams, data work, and notebook explanations. Attempt each check before opening its guidance. Schedule chapter investigations, review, and exams separately; add time where prerequisites are uncertain. Trace the explained visual parts and write a first model, then attempt the illustrated and changed independent cases. In the third session explain a correction and revise the opening account using evidence.
Start with something to explain
Why can changing a molecule’s arrangement alter what a cell can do?
Illustrative evidence case. A protein’s chain remains while its fold changes; carbohydrate and lipid structures have different linkages and roles.
Notice and wonder
Record two observations using the stated quantities or relationships. Keep observations separate from possible causes. Write one question that a new measurement could help answer.
Why can changing a molecule’s arrangement alter what a cell can do?
Your first model
Draw building units, a linkage and a possible functional site rather than label every molecule simply “food.”
Build the language from the idea
- macromolecule
- A large molecule; many biological macromolecules are polymers, but not every biological molecule grouped with them is a repeating-unit polymer.
- monomer
- A small molecule that serves as a building block for a larger polymer.
- polymer
- A large molecule made of many monomers bonded together in a chain.
- carbohydrate
- A macromolecule made of sugars, used for quick energy and structure; its monomer is a monosaccharide (simple sugar).
- lipid
- A broad group of largely hydrophobic molecules, including fats, phospholipids, and steroids; they are not all repeating-unit polymers.
- protein
- A macromolecule made of amino acids that does most of the cell's work; its monomer is an amino acid.
- nucleic acid
- A macromolecule (DNA or RNA) that stores and carries genetic information; its monomer is a nucleotide.
Carbon commonly forms four covalent bonds, supporting varied chains, branches, and rings. Many biological molecules have carbon skeletons with different functional groups. Their structures influence how they interact with water, one another, and cells.
Proteins are amino-acid polymers; DNA and RNA are nucleotide polymers. Many carbohydrates are sugar polymers, but simple sugars are small molecules. Polymer-building and hydrolysis models help account for matter; actual cellular synthesis uses activated building blocks, catalysts, and energy coupling.
Lipids include fats, phospholipids, and steroids. They are a diverse group, not one family of repeating-monomer polymers. Many have largely hydrophobic regions. Fats can support energy storage; phospholipids help form membranes because they have both water-interacting and hydrophobic regions.
Protein sequence influences folding and interactions. Proteins contribute to structure, transport, signaling, and catalysis. Nucleic acids contribute to information storage and expression, and some RNA molecules have catalytic functions. The four-family model is useful, but each family includes varied structures and roles.
Carbohydrates, lipids, proteins and nucleic acids have different structures and roles. A molecule's sequence, shape and chemical groups matter as much as its broad category. Lipids are not generally long chains of repeating monomers in the same sense as proteins or nucleic acids.
Cells build and break larger molecules through reactions that exchange matter and energy. Digestion breaks food molecules into smaller components that can be absorbed; it does not simply transfer an intact food protein into a human muscle.
Track atoms when discussing growth. A plant's dry mass comes substantially from carbon dioxide incorporated into organic molecules, with water and mineral inputs also needed. Soil provides essential mineral nutrients, but is not the main source of the carbon in a tree's wood.
Build your understanding
Use three flexible study sessions: core explanation and Parts 1–2; Parts 3–4 with their reasoning checks and models; then the whole-unit application, worked examples, independent practice, and exit check. Allow about 50–60 minutes per session, including diagrams, data work, and notebook explanations. Attempt each check before opening its guidance. Schedule chapter investigations, review, and exams separately; add time where prerequisites are uncertain.
Part 1 · Carbon's four bonds, and the two reactions that build life
Carbon can form four covalent bonds in many stable organic arrangements. A monomer is a small building unit; a polymer contains repeated linked units, though not all biological macromolecules are simple repeating polymers. Dehydration reactions can form a linkage with water released, while hydrolysis uses water in splitting a suitable linkage. These terms describe matter accounting, not automatically whether energy is released. Actual synthesis requires appropriate activated materials, enzymes and conditions; molecules do not join merely because water is removed nearby.
Read the visual model
Count carbon’s represented connections and keep the joining reaction separate from the drawing’s size.
Does calling a reaction dehydration prove that it supplies usable energy?
Visual reasoning — after your attempt
No. The overall reactants, products and conditions determine its energy balance; a synthesis may require a coupled energy input.
Carbon forms four covalent bonds, which lets it build chains, branches, and rings that are stable yet reactive at specific points. Those reactive points are functional groups — hydroxyl, carboxyl, amino, phosphate — and they, not the carbon skeleton, determine how a molecule behaves in water and which reactions it enters. Two molecules with identical carbon frameworks and different functional groups behave differently in a cell.
Nearly all large biological molecules are assembled and dismantled by two complementary reactions. Dehydration synthesis joins two subunits and releases a water molecule; hydrolysis adds a water molecule to split the bond again. Digestion is hydrolysis, growth and storage use dehydration synthesis, and both are catalysed by enzymes because neither happens quickly on its own at body temperature.
Pause and explain
If dehydration synthesis releases water, what does a cell need in order to break the same bond later?
Check your reasoning after answering
It needs water, which hydrolysis adds across the bond, plus an enzyme to make the reaction fast enough at body temperature. The two reactions are chemical reverses of each other.
Part 2 · Carbohydrates and lipids: fuel and structure
Carbohydrates include sugars and their linked forms, used in energy pathways and structures such as cellulose. Different linkages can yield different properties even from related building units. Lipids are a diverse group with substantial water-avoiding regions; a triglyceride has glycerol linked to three fatty-acid chains, while a membrane phospholipid has a different arrangement. Saturation concerns carbon–carbon bonding in a fatty-acid chain, not how full a storage container is. Do not describe every lipid as one polymer or every carbohydrate as only a fuel.
Read the visual model
Compare the repeated sugar-unit chain with the selected lipid’s head-and-tail arrangement.
Why can two materials built from sugar units have different biological uses?
Visual reasoning — after your attempt
Their linkages and organization can differ, producing different structures and enzyme accessibility; chemical class alone does not determine function.
Carbohydrates are built from monosaccharides such as glucose. Two joined give a disaccharide; many give a polysaccharide. The same glucose monomer produces starch for plant storage, glycogen for animal storage, and cellulose for plant cell walls, with the differences arising from how the units are linked. That is why cellulose is indigestible to humans while starch is not: the enzymes fit one linkage and not the other.
Lipids are mostly nonpolar, which is why they are excluded from water and why they store so much energy per gram — about 38 kJ/g against roughly 17 kJ/g for carbohydrate or protein. A triglyceride is glycerol joined to three fatty acids. A phospholipid replaces one fatty acid with a phosphate-containing polar head, producing a molecule with a water-attracting end and a water-excluding end, which is the structural basis of every cell membrane.
Pause and explain
Why is a phospholipid, rather than a triglyceride, the molecule that forms membranes?
Check your reasoning after answering
A phospholipid has both a polar head and nonpolar tails, so in water the heads face outward and the tails cluster inward, producing a bilayer. A triglyceride is nonpolar throughout and simply forms droplets.
Part 3 · Proteins: sequence becomes shape becomes function
A protein consists of one or more folded polypeptides, chains of amino acids joined by peptide bonds. Each amino acid has a side group whose interactions influence folding. A different sequence can change a binding site, stability or location, but some sequence changes have little detectable effect. Denaturation disrupts an important folded arrangement; it is not the same as cutting every peptide bond. Enzymes, structural components, receptors and transporters are protein roles. A correct explanation connects a particular structural change to a tested function rather than saying all proteins behave alike.
Read the visual model
Follow the sequence-to-fold relationship without treating colored markers as actual atom sizes.
Does an unfolded protein necessarily have a newly rewritten amino-acid sequence?
Visual reasoning — after your attempt
No. Its interactions and folding may change while much of the covalent chain sequence remains; hydrolysis and denaturation are different processes.
Proteins are chains of amino acids joined by peptide bonds. All twenty amino acids share the same backbone and differ in their side chains, which may be charged, polar, or nonpolar. As the chain is assembled, those side chains interact with each other and with water, and the chain folds into a specific three-dimensional shape. Sequence determines folding, folding determines shape, and shape determines which molecules the protein can bind.
Because folding depends on weak interactions, protein shape is conditional. Raising temperature or shifting pH disrupts those interactions and the protein unfolds — denatures — losing function while its peptide bonds remain intact. A boiled egg white demonstrates this, and it also shows that denaturation is often irreversible once unfolded chains tangle together.
Pause and explain
A single amino-acid substitution can destroy a protein's function. Explain the chain of causes.
Check your reasoning after answering
The substituted side chain interacts differently with its neighbours and with water, so the chain folds into a different shape; if the altered region forms the binding site, the protein can no longer bind its target and the function is lost.
Part 4 · Nucleic acids, and comparing the four classes
A nucleotide contains a phosphate component, a sugar and a nitrogenous base. Nucleic acids, DNA and RNA, link nucleotides in a backbone; base sequence carries information, while molecule structure and interactions affect its use. This is distinct from the amino-acid sequence of a protein. A large molecule can contain several chemical groups, and identifying carbon or nitrogen alone does not uniquely classify it. Compare building components, linkages and biological context; a food or cell sample is usually a mixture rather than one pure macromolecule.
Read the visual model
Separate the repeating backbone components from the attached base symbols.
Does a chain of nucleotides directly mean a chain of amino acids?
Visual reasoning — after your attempt
No. Nucleotide polymers are nucleic acids; amino-acid polymers are polypeptides. Gene expression can connect their information through transcription and translation.
Nucleic acids are polymers of nucleotides, each a sugar joined to a phosphate group and a nitrogenous base. The sugar-phosphate backbone is identical along the chain; the sequence of bases is what varies, and that sequence is the information. DNA stores it in a stable double strand; RNA carries working copies and performs structural and catalytic jobs. Information capacity, not energy storage, is what this class contributes.
Set the four classes side by side and the pattern is clear: each has a characteristic monomer, a characteristic elemental signature, and a job it does better than the others. Carbohydrates supply quick fuel and structural walls, lipids store concentrated energy and build membranes, proteins catalyse and transport and signal, nucleic acids store and transmit information. Nitrogen appears in proteins and nucleic acids; phosphorus marks nucleic acids and phospholipids.
Pause and explain
You analyse an unknown molecule and find carbon, hydrogen, oxygen, nitrogen, and phosphorus. Which classes remain possible, and which are excluded?
Check your reasoning after answering
Nucleic acids and phospholipids remain possible because both contain phosphorus and nitrogen appears in nucleic-acid bases. Simple carbohydrates are excluded, since they contain only carbon, hydrogen, and oxygen, and a protein without a phosphate group would not explain the phosphorus.
Apply the whole unit · Identify four unknown samples
Illustrative test records for four samples. Sample W: contains only carbon, hydrogen, and oxygen in a roughly 1:2:1 ratio; releases glucose when hydrolysed. Sample X: nonpolar, forms a layer above water, releases 38 kJ/g on combustion. Sample Y: contains nitrogen; loses its activity when heated to 70 °C but its chain length is unchanged. Sample Z: contains nitrogen and phosphorus; a repeating sugar-phosphate backbone is detected.
Your task
Assign each sample to a macromolecule class, state the single piece of evidence that is decisive for each, and explain why sample Y's unchanged chain length is the key observation rather than a detail. Then name one further test that would distinguish a storage polysaccharide from a structural one in sample W.
What your response should show
- Choose the appropriate biological model or relationship and state the conditions it assumes.
- Show the counts, rates, ratios, or intermediate reasoning the question requires.
- Use the supplied evidence to justify the conclusion and separate observation from inference.
- Identify a meaningful limitation, uncertainty, or next measurement.
Response guidance — after your attempt
W: the 1:2:1 element ratio and glucose on hydrolysis identify a polysaccharide — a carbohydrate.
X: nonpolarity, separation from water, and about 38 kJ/g, roughly double carbohydrate's energy density, identify a lipid.
Y: nitrogen points to protein or nucleic acid; loss of activity at 70 °C with chain length intact means weak folding interactions broke while peptide bonds held, which is denaturation of a protein.
Z: nitrogen plus phosphorus plus a sugar-phosphate backbone identifies a nucleic acid.
Further test for W: offer the sample to a starch-digesting enzyme; hydrolysis to glucose indicates a storage polysaccharide such as starch, while resistance indicates a structural one such as cellulose, since the linkage geometry differs.
W is a carbohydrate, X a lipid, Y a protein, and Z a nucleic acid. Sample Y's unchanged chain length is decisive because it separates two different kinds of damage: breaking peptide bonds would shorten the chain, whereas losing function with the chain intact means only the folded shape was lost. That is denaturation, and it shows that protein function depends on a shape maintained by weak interactions rather than on the covalent backbone alone. The element tests are suggestive rather than conclusive on their own — nitrogen alone does not separate protein from nucleic acid — so each assignment rests on combining composition with behaviour. Distinguishing starch from cellulose in W requires an enzyme test, because both have the same formula and differ only in linkage.
Scientific references and adapted source figures
Parent / teacher teaching notes
The sequence-shape-function chain is the idea this unit exists to establish, and it returns in enzymes, membranes, protein synthesis, and mutation. Ask for it in that order every time rather than accepting "the protein stopped working." The denaturation case is worth dwelling on, because students routinely say heat "broke the protein apart" when the peptide bonds are intact; the distinction is what makes enzyme temperature curves intelligible in the next unit. Ask for an explanation of each label, arrow and axis before calculation. Keep first and revised models; attempt the changed case before consulting guidance. Distinguish invented instructional records from named historical sources. Source chromosome symbols are allele-category models, not DNA sequences. These independently authored high-school supplements are not the full OpenSciEd teacher editions or an endorsement.
The key idea
Structure supports function: proteins, nucleic acids, carbohydrates, and lipids have different components and roles; lipids are not a single repeating-monomer polymer family.
Worked examples
Example 1
- Amino acids are the monomers of proteins.
- Enzymes are a well-known job carried out by proteins.
- So a folded chain of amino acids acting as an enzyme must be a protein.
Example 2 · Worked application
- Starch is assembled from glucose units and can store carbohydrate energy.
- Proteins are assembled from amino acids and can serve roles such as enzymes or structural support.
- Different sequences and arrangements affect function.
Do not infer that every carbon-containing molecule has the same biological role.
Example 3
- To join a chain of monomers, the number of bonds is always one less than the number of monomers.
- 8 amino acids in a chain need 8 − 1 = 7 bonds.
- Each bond releases one water molecule, so 7 bonds release 7 water molecules.
Example 4
- Photosynthesis incorporates carbon from atmospheric CO₂.
- That carbon can be rearranged into carbohydrates and other organic molecules.
Example 5 · Find and fix the mistake
- Test the claim against the stated conditions: Do not infer that every carbon-containing molecule has the same biological role.
- Proteins use amino acids; nucleic acids use nucleotides and many carbohydrates use sugar units. Lipids are a distinct class.
Do not infer that every carbon-containing molecule has the same biological role.
Example 6 · Reason from the visual evidence
- A protein consists of one or more folded polypeptides, chains of amino acids joined by peptide bonds. Each amino acid has a side group whose interactions influence folding. A different sequence can change a binding site, stability or location, but some sequence changes have little detectable effect. Denaturation disrupts an important folded arrangement; it is not the same as cutting every peptide bond. Enzymes, structural components, receptors and transporters are protein roles. A correct explanation connects a particular structural change to a tested function rather than saying all proteins behave alike.
- No. Its interactions and folding may change while much of the covalent chain sequence remains; hydrolysis and denaturation are different processes.
Original symbolic amino-acid sequence and folded-chain model. It illustrates an explanatory relationship, not a solved structure of a named protein.
Avoid these mistakes
- The carbon in plant biomass does not mainly come from soil minerals.
Check your understanding
A growing plant gains dry mass while soil mass changes little. Explain a major source of its carbon.
Discuss the answer
Atmospheric carbon dioxide is a major carbon source; water and mineral nutrients are also required.
Why does molecule shape matter when comparing proteins with the same broad chemical category?
Discuss the answer
Protein sequence, folding, interactions and cellular conditions determine function; sharing a chemical class does not make two proteins identical in shape or activity.
Return to the phenomenon
Return to the initial case. Why can changing a molecule’s arrangement alter what a cell can do? Revise your drawing or explanation using two named ideas and one stated limitation.
Compare explanations after your attempt
Composition, sequence, linkages and folding influence function. Structural changes need suitable functional measurements; chemical category alone does not specify one biological job.
What should we investigate next?
Which comparison would test a remaining uncertainty? State what you would change, keep comparable and measure.
Investigate, explain and revise
Complete these tasks after the teaching parts. Preserve your first model and a supported revision; use guidance after a real attempt.
Activity 1 · Return to the opening case
Use the opening question, defined terms, Parts 1–4 and your notebook. The supplied-data route needs no equipment or external account.
Your task
Return to the initial case. Why can changing a molecule’s arrangement alter what a cell can do? Revise your drawing or explanation using two named ideas and one stated limitation.
Success criteria
- Use relevant stated observations.
- Explain the mechanism with a labeled model.
- Distinguish quantities and assumptions.
- Describe a revision and a useful next test.
Hint
Follow objects, materials and information separately. Check what each measured quantity means.
Response guidance — discuss after your attempt
Composition, sequence, linkages and folding influence function. Structural changes need suitable functional measurements; chemical category alone does not specify one biological job.
Activity 2 · Transfer to a changed case
Cover the worked guidance and use the unit explanations/figures to solve a changed problem.
Your task
A treatment unfolds an enzyme but leaves its amino-acid chain largely intact. Explain what may change, what need not change, and what evidence would test retained function.
Success criteria
- Use the stated measurements or conditions, with correct units.
- Connect the result to the biological mechanism or evidence relationship.
- Identify the assumption, model limit or further evidence requested.
Hint
Name the stated conditions, select the relevant mechanism and show intermediate reasoning.
Response guidance — discuss after your attempt
Folding and active-site arrangement may change while amino-acid sequence remains largely intact. Compare catalytic rates under matched conditions and examine whether the fold/function can recover; unfolding is not automatically complete peptide hydrolysis.
Words to know
- macromolecule
- A large molecule; many biological macromolecules are polymers, but not every biological molecule grouped with them is a repeating-unit polymer.
- monomer
- A small molecule that serves as a building block for a larger polymer.
- polymer
- A large molecule made of many monomers bonded together in a chain.
- carbohydrate
- A macromolecule made of sugars, used for quick energy and structure; its monomer is a monosaccharide (simple sugar).
- lipid
- A broad group of largely hydrophobic molecules, including fats, phospholipids, and steroids; they are not all repeating-unit polymers.
- protein
- A macromolecule made of amino acids that does most of the cell's work; its monomer is an amino acid.
- nucleic acid
- A macromolecule (DNA or RNA) that stores and carries genetic information; its monomer is a nucleotide.
Use the model’s boundary
A chain model counts links between units but omits activated intermediates and enzyme mechanisms. Distinguish net matter accounting from the actual steps used in a cell.
In the real world
The 'carbs, fats, and protein' on every nutrition label are literally three of these macromolecule families — your body digests them into monomers and rebuilds them into you. Understanding lipids also drives modern medicine: the mRNA in some vaccines is wrapped in tiny lipid bubbles so it can slip through the fatty cell membrane.
Exit check
Compare information and function in nucleic acids and proteins.
Check your explanation after answering
Nucleic acids store or transmit genetic information; proteins carry out many structural, catalytic, signaling, and transport functions. Their different structures support different roles.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.