Overview
What you’ll learn
- List the chemical elements that make up carbohydrates, fats and proteins.
- State the main roles of carbohydrates, fats and proteins in living organisms.
- Describe the tests for starch, reducing sugars, proteins and fats, and their positive results.
- State that large molecules are synthesised from smaller basic units: starch, glycogen and cellulose from glucose; polypeptides from amino acids; fats from glycerol and fatty acids.
- Explain the mode of enzyme action in terms of the active site, the enzyme–substrate complex, lowering of activation energy and the ‘lock and key’ hypothesis.
- Investigate and explain the effects of temperature and pH on the rate of enzyme-catalysed reactions.
Key points
- 1Carbohydrates (carbon, hydrogen, oxygen) are the immediate energy source; fats (C, H, O) store energy long-term and insulate; proteins (C, H, O, N) build and repair cells.
- 2Food tests: iodine turns blue-black for starch; Benedict’s (heated) turns brick-red for reducing sugars; biuret turns violet for protein; ethanol gives a cloudy emulsion for fats.
- 3Big molecules are built from small units: starch, glycogen and cellulose from glucose; polypeptides from amino acids; fats from glycerol and fatty acids.
- 4Enzymes are biological catalysts: they lower the activation energy so reactions run fast at body temperature, and they emerge unchanged.
- 5The active site has a shape complementary to one substrate — the ‘lock and key’ basis of specificity; the bound pair is the enzyme–substrate complex.
- 6Rate rises with temperature up to the optimum (more kinetic energy, more successful collisions), then falls steeply as heat denatures the active site.
- 7Each enzyme has a pH optimum; the wrong pH (like excess heat) alters the active site and denatures the enzyme.
- 8Reaction rate can be measured as 1 ÷ time (for example 1 ÷ time-to-disappearance of starch), so a faster reaction gives a bigger number.
Notes · 1 of 6
The three families and their jobs
What each class of biological molecule is made of, and what it does for the body.
| Family | Elements | Main roles | Built from |
|---|---|---|---|
| Carbohydrates | C, H, O | Immediate source of energy; cellulose also builds plant cell walls | Glucose (into starch, glycogen, cellulose) |
| Fats | C, H, O | Long-term energy storage; insulation (and a source of metabolic water) | Glycerol + fatty acids |
| Proteins | C, H, O, N | Growth and repair of cells; enzymes and many other functional molecules | Amino acids (into polypeptides) |
The syllabus keeps the roles simple: carbohydrates are the immediate fuel — glucose is what respiration burns first; fats are the long-term store, stockpiled in adipose tissue and also insulating us against heat loss; proteins are the building material, supplying the amino acids that make new cells, and forming every enzyme in this chapter.
Note the nitrogen clue: proteins are the only one of the three families that must contain nitrogen — that is why excretion (chapter 7) exists, because excess amino acids cannot be stored and their nitrogen must be removed as urea.
Notes · 2 of 6
The four food tests
Iodine, Benedict’s, biuret and ethanol — reagent, method, positive result.
| Test | Reagent | Method | Positive result |
|---|---|---|---|
| Starch | Iodine in potassium iodide | Add a few drops to the sample at room temperature | Brown → blue-black |
| Reducing sugars | Benedict’s solution | Add and heat in a water bath | Blue → green → yellow → orange → brick-red precipitate |
| Protein | Biuret solution | Add to the sample (no heating) | Blue → violet/purple |
| Fats | Ethanol | Dissolve the sample in ethanol, then pour into water and shake | Cloudy white emulsion |
Marks live in the details: Benedict’s must be heated (a water bath, not a naked flame on the tube); iodine is used cold; biuret needs no heating either; and the ethanol test depends on shaking ethanol with water so the dissolved fat is forced out as a cloudy layer. A "positive result" quoted without the colour change earns nothing.
Notes · 3 of 6
Big molecules from small units
The synthetic rule: polymers from their monomers, and the reverse with water.
Every large biological molecule is assembled from a small repeating unit: starch, glycogen and cellulose are chains of glucose molecules (starch stores energy in plants, glycogen in animals, cellulose braces cell walls); polypeptides (and the proteins they form) are chains of amino acids; and fats are assembled from glycerol joined to fatty acids.
Digestion simply runs the tape backwards: starch is broken to maltose and then glucose, proteins to polypeptides and then amino acids, fats to glycerol and fatty acids. That is why the end-products of digestion (chapter 4) are exactly the small units listed here — the body cannot absorb the giant versions.
Notes · 4 of 6
How enzymes work
Active site, enzyme–substrate complex, lower activation energy, lock and key.
An enzyme is a biological catalyst — a protein that speeds up a specific reaction without being used up. The reaction happens on a small region of its surface called the active site, whose shape is complementary to one particular substrate. Substrate binding forms the enzyme–substrate complex; the reaction converts substrate into products, which are released, and the unchanged enzyme takes the next substrate.
The ‘lock and key’ hypothesis captures the specificity: as one key opens one lock, one substrate fits one active site. Amylase’s active site accepts starch, not protein; protease accepts protein, not starch. The fit is what makes each enzyme a precision tool.
Why does a catalyst speed things up? Every reaction needs an energy kick-over — the activation energy. Enzymes lower that barrier, so at body temperature (37 °C), where molecules carry only modest energy, a huge share of collisions still succeeds. Without enzymes, the chemistry of life would crawl far too slowly to keep anything alive.
Notes · 5 of 6
Temperature and pH
Rising rate to the optimum, then a cliff: what heat and the wrong pH do.
Warm molecules move faster, so raising temperature raises the reaction rate — at first. Around 37–40 °C for human enzymes the rate peaks at the optimum; beyond it the rate collapses toward zero. The heat has denatured the enzyme: weak bonds holding the protein’s shape break, the active site loses its complementary shape, and the substrate no longer fits. Denaturation is permanent — cooling the enzyme back down does not restore the rate.
Each enzyme also has a pH optimum, set by the environment where it works: pepsin digests protein in the stomach at around pH 2; amylase in the mouth prefers close to neutral. Either side of the optimum the rate falls, and extreme pH denatures the enzyme the same way excess heat does — the active site’s shape depends on the charges the pH maintains.
Notes · 6 of 6
Investigating enzymes: amylase and starch
The classic practical: buffers, water baths, iodine end-points and 1 ÷ time rates.
- 1Set up
Mix equal volumes of amylase solution and starch suspension with a fixed pH buffer in a tube.
- 2Keep temperature constant
Place the tube in a water bath at the temperature being tested and start timing.
- 3Test for starch
Every 30 seconds, transfer a drop to a spotting tile with iodine. Note the time when iodine stops turning blue-black — starch is gone.
- 4Calculate
Rate = 1 ÷ time-to-end-point. Compare rates across temperatures or pH values.
For a fair test, control everything except the variable studied: same amylase concentration and volume, same starch concentration and volume, same pH (when testing temperature) or same temperature (when testing pH). The iodine colour end-point is the clock: the sooner it stops turning blue-black, the faster the enzyme worked.
Revise
Key terms
The words examiners expect you to use precisely.
- carbohydrate
- A molecule of carbon, hydrogen and oxygen — such as starch, glycogen, cellulose or glucose — serving as an immediate energy source.Built from glucose units.
- fat
- A molecule of carbon, hydrogen and oxygen formed from glycerol and fatty acids, used for long-term energy storage and insulation.
- protein
- A nitrogen-containing molecule built from amino acids, used for growth and repair of cells — including all enzymes.
- reducing sugar
- A sugar (such as glucose or maltose) that turns heated Benedict’s solution from blue through green and orange to brick-red.
- emulsion test
- The fat test: dissolve the sample in ethanol, pour into water and shake; fats give a cloudy white layer.
- enzyme
- A protein catalyst that speeds up a specific biochemical reaction without being used up.
Show 6 more key termsShow fewer key terms
- active site
- The region of an enzyme whose complementary shape binds exactly one substrate.
- enzyme–substrate complex
- The temporary structure formed while the substrate sits in the enzyme’s active site.
- activation energy
- The minimum energy a reaction needs to proceed; enzymes lower this barrier.
- lock and key hypothesis
- The model of enzyme specificity: one active site fits one substrate, as one key fits one lock.
- denaturation
- The permanent loss of an enzyme’s active-site shape caused by excess heat or extreme pH, stopping the reaction.
- optimum
- The temperature or pH at which an enzyme works fastest.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Giving proteins the elements C, H and O only.
All proteins contain nitrogen — the element that sets them apart and creates the need for excretion.
Instead: Proteins: C, H, O and N (sometimes S). Carbohydrates and fats: C, H, O only.
Mistake: Calling fats the immediate energy source.
Fats are the long-term store; glucose is what the body burns first.
Instead: Carbohydrates = immediate energy; fats = storage and insulation; proteins = growth and repair.
Show 11 more mistakesShow fewer mistakes
Mistake: Forgetting to heat Benedict’s solution.
Without heating, reducing sugars give no colour change even when present.
Instead: Benedict’s needs a hot water bath; iodine and biuret are cold tests.
Mistake: Saying iodine turns red for starch.
Red-brown is iodine’s own colour; the positive result is blue-black.
Instead: Brown iodine → blue-black in the presence of starch.
Mistake: Writing "starch is made of amino acids".
Amino acids build proteins; starch is a glucose polymer.
Instead: Starch, glycogen and cellulose ← glucose; polypeptides ← amino acids; fats ← glycerol + fatty acids.
Mistake: Saying enzymes are used up in reactions.
The enzyme emerges unchanged after releasing products and catalyses again.
Instead: Catalysts are not consumed; the same enzyme molecules work millions of times.
Mistake: Describing the active site as "anywhere on the enzyme".
Only the specific complementary region binds the substrate.
Instead: The active site is a small, precisely shaped region where the substrate binds.
Mistake: Claiming denaturation reverses on cooling.
Denaturation permanently destroys the active site’s shape.
Instead: Below the optimum, cooling slows molecules (reversible rate change); denaturation above the optimum is permanent.
Mistake: Saying higher temperature always means faster enzyme reactions.
Beyond the optimum the rate falls sharply as the enzyme denatures.
Instead: Rate rises to the optimum, then collapses.
Mistake: Quoting rate as "80 seconds" when asked for rate.
Time is not rate; a bigger time means a slower reaction.
Instead: Rate = 1 ÷ time (e.g. 1 ÷ 80 = 0.0125 per second), so faster reactions score bigger numbers.
Mistake: Giving amylase a pH 2 optimum.
That is pepsin, working in stomach acid; amylase works near neutral in the mouth.
Instead: Match enzyme to environment: pepsin ≈ pH 2 (stomach); amylase ≈ pH 7 (mouth).
Mistake: Confusing the enzyme–substrate complex with the product.
The complex is the enzyme and unreacted substrate joined together, before the reaction.
Instead: Substrate binds → complex → reaction → products released → enzyme unchanged.
Mistake: Explaining denaturation as "the enzyme dies".
Enzymes are not alive; they are molecules with shapes.
Instead: Denaturation alters the active site’s three-dimensional shape so the substrate cannot fit.
Check yourself
Quick quiz
30 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
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- Question 3: not answered
List the chemical elements that make up carbohydrates.
Which elements make up carbohydrates?
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5 multiple-choice questions and structured questions with answer lines. Chemical elements and roles of carbohydrates, fats and proteins; food tests; synthesis of large molecules from smaller units; enzyme mode of action, specificity, and the effects of temperature and pH.
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Frequently asked questions
Which food tests need heating?
Only Benedict’s (for reducing sugars) — heat the tube in a water bath after adding the solution. Iodine, biuret and the ethanol emulsion test are all performed without heating.
How do I calculate reaction rate in enzyme experiments?
Use rate = 1 ÷ time. A reaction finishing in 40 seconds has a rate of 1 ÷ 40 = 0.025 per second. Bigger rate values mean faster reactions, so you can compare temperatures or pH values directly.
What is the difference between denaturation and just being cold?
Cold enzymes work slowly because their molecules move slowly — warm them and the rate returns (reversible). Denatured enzymes have permanently lost their active-site shape through excess heat or extreme pH, so the rate stays near zero even when conditions normalise.
Why do only proteins contain nitrogen?
Carbohydrates and fats are built from carbon, hydrogen and oxygen alone. Amino acids — the units of proteins — carry an amino group containing nitrogen, which is why excess protein in the diet produces urea for excretion.
How specific is an enzyme, really?
The active site fits one complementary substrate — that is the ‘lock and key’ model. Amylase cannot digest protein and protease cannot digest starch; changing the substrate changes nothing until the shape matches.