2.3.1 Structure of Biological Molecules — Edexcel International GCSE Science Double Award (Modular) - Biology Unit 1 Revision Notes & Topical Questions

Part 1 of 8Key ideas

Key ideas

Rate of reaction = 1 ÷ time taken (unit s⁻¹), or = amount of product formed (or substrate used) ÷ time.

Starch, glycogen and cellulose are all made of glucose.

Proteins are made of amino acids; lipids of glycerol and three fatty acids.

Enzymes are proteins acting as biological catalysts.

The active site is complementary to the substrate shape (lock-and-key).

As temperature rises towards the optimum, molecules gain kinetic energy and collide more often, so more enzyme–substrate complexes form per second.

Above the optimum the active site changes shape (denatures) and rate falls quickly.

Each enzyme has an optimum pH

Each enzyme has an optimum pH e.g. pepsin ~pH 2, amylase ~pH 7; extreme pH denatures enzymes.

Benedict's needs heating; biuret and iodine do not.

Part 2 of 8Learning objectives

1

Recall the chemical elements in carbohydrates, proteins and lipids.

2

Describe the structure of carbohydrates, proteins and lipids as large molecules made from smaller units.

3

Describe food tests for glucose, starch, protein and fat.

4

Understand the role of enzymes as biological catalysts and how temperature and pH affect them.

5

Investigate the effect of temperature and pH on enzyme activity.

Part 3 of 8Essential knowledge

1

Carbohydrates contain carbon, hydrogen and oxygen; proteins contain C, H, O and nitrogen; lipids contain C, H and O (but much less oxygen than carbohydrates).

2

Simple sugars such as glucose join into large molecules: starch (plant store), glycogen (animal store) and cellulose (plant cell walls).

3

Proteins are chains of amino acids; the sequence of amino acids determines how the chain folds, giving each protein its specific shape.

4

Lipids (fats and oils) are made of glycerol and fatty acids; they are an energy store, provide insulation and form part of membranes.

5Glucose test

Glucose test add Benedict's solution and heat in a water bath; blue → brick-red precipitate if glucose present. Starch: iodine turns blue-black. Protein: biuret turns purple. Fat: ethanol emulsion test gives a cloudy white layer.

6Enzymes are biological catalysts

Enzymes are biological catalysts they speed up reactions in cells (e.g. respiration, digestion, protein synthesis) without being used up, so metabolism happens fast enough at body temperature.

7

Each enzyme has an active site whose shape is complementary to its substrate; the substrate binds to form an enzyme–substrate complex, the reaction occurs and the products are released. This makes enzymes specific.

8

Increasing temperature increases the kinetic energy of enzyme and substrate, so there are more successful collisions and more enzyme–substrate complexes; rate increases up to the optimum.

9

Above the optimum, bonds holding the enzyme's shape break, the active site changes shape (denatures), the substrate no longer fits and the rate drops; denaturation is irreversible.

10

pH affects the charges holding the enzyme's shape; away from the optimum pH the active site changes shape and the enzyme may denature. Protease pepsin in the stomach works best near pH 2; most other enzymes near pH 7.

11

In practicals, control variables include enzyme and substrate concentration and volume, and pH (buffer) when temperature is varied (or temperature using a water bath when pH is varied).

12Preparing food samples

Before testing a solid food, grind it with a pestle and mortar, mix it with distilled water and filter; the filtrate is then used for the Benedict's, iodine and biuret tests. Powdered foods can be dissolved directly.

13Food test safety

Wear eye protection

biuret contains copper(II) sulfate and corrosive sodium hydroxide, and iodine irritates eyes.
Ethanol is highly flammable, so keep it away from flames;
- use a water bath rather than a naked flame when heating Benedict's tests.
Never taste samples.

14Fats and oils

Lipids that are solid at room temperature are called fats; those that are liquid are called oils. Both are built from glycerol and fatty acids.

15Maltose

Amylase breaks starch into maltose, a sugar made of two glucose units joined together. Starch, glycogen and cellulose are insoluble, which makes starch and glycogen good storage molecules.

16Low temperature

Cold does not denature enzymes. Molecules simply have less kinetic energy, so there are fewer enzyme–substrate collisions and the rate is slow; activity returns if the temperature rises again.

17Alkaline optimum

Not all enzymes work best at pH 7: those secreted into the duodenum and small intestine have an optimum around pH 8–9, while stomach protease works best near pH 2.

18Improving the amylase practical

Judging when iodine stops turning blue-black is subjective, and heating water with a Bunsen gives unstable temperatures. Use thermostatic water baths, let solutions reach temperature before mixing, and a colorimeter can give an objective reading of the end-point.

Part 4 of 8Process & sequence

- Set up test tubes of enzyme solution (e.g. amylase) and substrate (starch), each in a water bath at the chosen temperature, and leave to equilibrate.
- Place one drop of iodine solution in each well of a spotting tile.
- Mix the amylase and starch, add buffer to fix the pH, and start a stopwatch.
- Every 30 s, transfer a drop of the mixture to a fresh iodine well using a pipette.
- Record the time when iodine no longer turns blue-black (stays orange-brown) – starch has been fully digested.
- Repeat at other temperatures (e.g. 20, 30, 40, 50, 60 °C) and repeat each three times to calculate a mean.
- Calculate rate = 1 ÷ time (s⁻¹) and plot rate against temperature to find the optimum.
How an enzyme works (lock and key)write these steps in this order; each step is a marking point.
1. Substrate collides with the enzyme's active site.
2. Substrate fits the complementary active site → enzyme–substrate complex.
3. Reaction is catalysed; bonds broken or formed.
4. Products leave the active site.
5. Unchanged enzyme is free to bind another substrate.

Part 5 of 8Worked examples

Worked examples

Starch was fully digested by amylase in 40 s at 40 °C and 160 s at 20 °C. Calculate the rate at each temperature and compare.

Rate = 1 ÷ time. 40 °C: 1 ÷ 40 = 0.025 s⁻¹. 20 °C: 1 ÷ 160 = 0.00625 s⁻¹ (0.0063 s⁻¹).
- Rate at 40 °C is 0.025 ÷ 0.00625 = 4 times faster, because molecules have more kinetic energy and collide more often.

An enzyme produced 18 cm³ of oxygen in 2 minutes. Calculate the rate in cm³/min and cm³/s.

Rate = volume ÷ time = 18 ÷ 2 = 9.0 cm³/min; 18 ÷ 120 s = 0.15 cm³/s.

Explain why the rate of an amylase reaction is zero at 80 °C.

At 80 °C the enzyme has been denatured: bonds holding its shape break, the active site changes shape,
- so starch can no longer fit/bind and no enzyme–substrate complexes form.

Part 6 of 8Summary

- Carbohydrates, proteins and lipids contain C, H and O; proteins also contain N.
- Starch and glycogen are made of glucose; proteins of amino acids; lipids of fatty acids and glycerol.
- Enzymes are biological catalysts with a specific active site.
- Rate increases up to the optimum, then enzymes denature.
- Rate = 1 ÷ time.

Part 7 of 8Keep going

Biological Molecules Edexcel O Level Biology: what to do next
These Biological Molecules Edexcel O Level Biology notes covered the key ideas, worked examples and exam technique for this topic. Check your Biological Molecules Edexcel O Level Biology understanding with the practice questions below, then reread any Biological Molecules Edexcel O Level Biology point you were unsure about before moving on.
Biological Molecules Edexcel O Level Biology: revision checklist
- Revise the main Biological Molecules Edexcel O Level Biology definitions until you can write them from memory.
- Practise Biological Molecules Edexcel O Level Biology exam questions under timed conditions.
- Link each Biological Molecules Edexcel O Level Biology idea to a diagram or a worked example.
- List the common Biological Molecules Edexcel O Level Biology mistakes and how to avoid them.
- Explain one Biological Molecules Edexcel O Level Biology idea out loud as if teaching a friend.
- Summarise Biological Molecules Edexcel O Level Biology in five key points on a revision card.
- Practice: Biological Molecules Edexcel O Level Biology practice questions
- Previous lesson: 2.2 Cell Structure — O Level Notes
- Next lesson: 2.4 Movement of Substances Into & Out of Cells — O Level Notes
- Course: Edexcel O Level Biology — all units
- Official syllabus: Pearson Edexcel International GCSEs

Part 8 of 8Compare: Compare

Food tests
Molecule	Reagent	Method	Positive result	Negative result
Glucose (reducing sugar)	Benedict's solution	Add equal volume, heat in water bath ~80 °C for a few minutes	Green → yellow → orange → brick-red precipitate (more sugar = further along)	Stays blue
Starch	Iodine solution	Add a few drops to sample	Blue-black	Stays orange-brown
Protein	Biuret reagent (NaOH + copper sulfate)	Add to sample, shake, no heating	Purple / lilac	Stays blue
Lipid	Ethanol (emulsion test)	Shake sample with ethanol, pour into water	Cloudy white emulsion	Stays clear

Large molecules and their building blocks
Large molecule	Built from	Elements
Starch / glycogen / cellulose	Many glucose molecules	C, H, O
Protein	Amino acids	C, H, O, N (S)
Lipid (fat/oil)	Glycerol + 3 fatty acids	C, H, O

Key terms

Command terms

Give / State / Name

Recall a fact; one idea per mark.

Define

Give the exact meaning using the specification keywords.

Describe

Give an account of what happens / the pattern in data, quoting values; no reasons needed.

Explain

Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.

Suggest

Use knowledge in a new context; more than one answer may be accepted.

Compare

Similarities and differences; each point must refer to both items.

Calculate

Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.

Discuss

Explore the issue from different angles and link the points.

Evaluate / Assess

Weigh strengths and weaknesses and reach a supported judgement.

Deduce

Draw a conclusion from the information given.

Justify

Support a statement/choice with evidence.

Exam technique

Keywords the mark scheme looks for in this lesson

Carbohydrate · Glucose · Starch · Glycogen · Lipid · Protein · Amino acid · Enzyme · Active site · Substrate · Denaturation · Optimum · Catalyst

Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1

Use specification keywords exactly (e.g. enzyme–substrate complex, active site, complementary).

2Link ideas

Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.

3Use the information in the question

“discuss/evaluate” answers must use the data provided and match the number of marks.

4

Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.

5Avoid vague words

“it”, “stuff”, “goes up”, “energy is made”. Name the substance and the process.

6

Extended (6-mark) responses are marked on content AND a logical, linked line of reasoning.

Words to avoid

1

the enzyme is killed at high temperaturethe enzyme is denatured; its active site changes shape

2

the substrate and active site are the same shapethe active site has a shape complementary to the substrate

3

Benedict's turns redBenedict's solution gives a brick-red precipitate after heating

4

proteins are made of glucoseproteins are made of amino acids

5

at low temperatures enzymes are denaturedat low temperatures enzymes are inactive because molecules have less kinetic energy and collide less often

6

starch test gives blackiodine turns blue-black in the presence of starch