2.3Structure & Functions in Living Organisms

Biological Molecules — Edexcel International GCSE Biology 4BI1 Revision Notes & Topical Questions

10 min read 8 parts Lesson 3 of 21 1 quiz

📚 Edexcel International GCSE Biology 4BI1 Hub / Topical Revision

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Lesson summary

Biological Molecules Edexcel IGCSE Biology 4BI1 notes: Identify the chemical elements and building blocks of carbohydrates, proteins and lipids.Enzymes are proteins; their shape is specific to one substrate (lock and key model).Most human enzymes have an optimum near 37 °C.Above …

Structure of biological molecules
Practical — food tests
Role of enzymes
Temperature and enzyme function
Practical — temperature and enzymes
pH and enzyme function

Part 1 of 8Key ideas

Enzymes are proteins; their shape is specific to one substrate (lock and key model).

Most human enzymes have an optimum near 37 °C.

Above the optimum, bonds holding the active site break and the enzyme denatures.

Low temperature slows enzymes (fewer collisions) but does not denature them.

Pepsin in the stomach has an optimum near pH 2; many other enzymes about pH 7.

Rate of reaction = 1 ÷ time taken.

Buffers keep pH constant in enzyme investigations.

Part 2 of 8Learning objectives

Identify the chemical elements and building blocks of carbohydrates, proteins and lipids.

Carry out food tests for starch, glucose, protein and fat.

Explain enzyme action using the active site model.

Investigate how temperature and pH affect enzyme activity.

Part 3 of 8Essential knowledge

Essential knowledge

Structure of biological molecules

Structure of biological molecules carbohydrates contain C, H and O; simple sugars such as glucose join to make starch (plants) and glycogen (animals). Proteins are chains of amino acids and contain C, H, O and N. Lipids are made from glycerol and fatty acids and contain C, H and O.

Practical — food tests

Practical — food tests iodine turns blue-black with starch.

Benedict's solution heated in a water bath changes from blue to green, yellow, orange or brick-red with glucose depending on concentration.
Biuret reagent turns purple with protein.
For fat, dissolve the food in ethanol then add water: a cloudy white emulsion shows lipid.

Role of enzymes

Role of enzymes enzymes are proteins that catalyse reactions in cells, including digestion, respiration and photosynthesis. The substrate binds to the complementary active site forming an enzyme-substrate complex, the reaction happens and products are released; the enzyme can be reused.

Temperature and enzyme function

Temperature and enzyme function as temperature rises, molecules move faster and collide more often, so rate increases up to the optimum. Above the optimum, the active site changes shape (denaturation) and the rate falls quickly.

Practical — temperature and enzymes

Practical — temperature and enzymes amylase and starch are incubated at different temperatures, sampling into iodine at regular intervals. Control pH with a buffer and use the same volumes and concentrations. The shortest time for starch to disappear shows the optimum.

pH and enzyme function

pH and enzyme function each enzyme has an optimum pH. pH values away from the optimum alter the bonds holding the active site in shape so the substrate fits less well; extreme pH denatures the enzyme.

Practical — pH and enzymes

Practical — pH and enzymes use buffer solutions of different pH with amylase and starch at a constant temperature (e.g. a 35 °C water bath), timing how long until iodine no longer turns blue-black. Alternatively measure oxygen produced by catalase from hydrogen peroxide at each pH.

Preparing a food sample

Grind solid food with a pestle and mortar, add distilled water and stir, then filter. The filtrate is used for the food tests. Powdered foods can simply be dissolved in water.

Small and large carbohydrates

Glucose is a simple sugar; two glucose units join to form maltose. Many glucose units form starch (plant store), glycogen (animal store) and cellulose (plant cell walls). These large molecules are insoluble, which makes starch and glycogen good storage molecules.

Protein shape and function

Each protein has its own sequence of amino acids, which makes the chain fold into a particular shape. The shape decides the job, for example the active site of an enzyme or the binding site of an antibody.

Different optimum pH values

Most enzymes work best near pH 7, pepsin in the stomach near pH 2, and enzymes in the duodenum near pH 8–9 because pancreatic juice and bile are alkaline.

Food test safety

Ethanol is highly flammable, so keep it away from flames and heat it only in a water bath. Biuret reagent contains sodium hydroxide (corrosive) and copper sulfate, so wear eye protection and wash splashes off skin at once.

Improving the amylase practical

Bring starch and amylase to the water-bath temperature separately before mixing. Judging when iodine stops turning blue-black is subjective; sampling more often or using a colorimeter gives more precise results.

Part 4 of 8Process & sequence

- Set water baths at a range of temperatures (e.g. 20, 30, 40, 50, 60 °C).
- Equilibrate starch solution and amylase separately in each bath.
- Mix them and start the stopwatch.
- Every 30 seconds, place a drop of the mixture into iodine on a spotting tile.
- Record the time when iodine no longer turns blue-black (starch fully digested).
- Calculate rate = 1 ÷ time and plot rate against temperature.
How an enzyme workswrite these steps in this order; each step is a marking point.
1. Substrate collides with enzyme
2. Fits complementary active site
3. Enzyme-substrate complex forms
4. Reaction takes place
5. Products released, enzyme reused

Part 5 of 8Worked examples

At 40 °C amylase digested all the starch in 50 s. Calculate the rate of reaction.

Rate = 1 ÷ time = 1 ÷ 50 = 0.020 s^-1.

2Explain why the rate of an enzyme reaction falls rapidly at 60 °C.

The high temperature breaks bonds holding the enzyme in shape, so the active site changes shape.
- The substrate no longer fits.
  - (no enzyme-substrate complexes form)
- The enzyme is denatured, so the rate falls.

Part 6 of 8Summary

- Carbohydrates, proteins and lipids are made of smaller building blocks.
- Food tests identify starch, glucose, protein and lipid.
- Enzymes are biological catalysts with a specific active site.
- Rate rises to an optimum temperature, then the enzyme denatures.
- Extreme pH changes the active site shape.

Part 7 of 8Keep going

Biological Molecules Edexcel IGCSE Biology 4BI₁: what to do next
These Biological Molecules Edexcel IGCSE Biology 4BI₁ notes covered the key ideas, worked examples and exam technique for this topic. Check your Biological Molecules Edexcel IGCSE Biology 4BI₁ understanding with the practice questions below, then reread any Biological Molecules Edexcel IGCSE Biology 4BI₁ point you were unsure about before moving on.
Biological Molecules Edexcel IGCSE Biology 4BI₁: revision checklist
- Revise the main Biological Molecules Edexcel IGCSE Biology 4BI₁ definitions until you can write them from memory.
- Practise Biological Molecules Edexcel IGCSE Biology 4BI₁ exam questions under timed conditions.
- Link each Biological Molecules Edexcel IGCSE Biology 4BI₁ idea to a diagram or a worked example.
- Practice: Biological Molecules Edexcel IGCSE Biology 4BI1 practice questions
- Previous lesson: 2.2 Cell Structure — Notes
- Next lesson: 2.4 Movement of Substances Into & Out of Cells — Notes
- Course: Edexcel International Gcse Biology 4BI1 — all units
- Official syllabus: Pearson Edexcel International GCSEs

Part 8 of 8Compare: Compare

Food tests
Substance	Reagent / method	Positive result
Starch	Iodine solution	Orange-brown to blue-black
Glucose (reducing sugar)	Benedict's solution, heat in a water bath	Blue to green, yellow or brick-red
Protein	Biuret reagent	Blue to purple / lilac
Lipid (fat)	Shake with ethanol, then add water	Cloudy white emulsion

Building blocks of biological molecules
Large molecule	Made from	Elements
Starch / glycogen	Glucose units	C, H, O
Protein	Amino acids	C, H, O, N (sometimes S)
Lipid	Glycerol + 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 · Protein · Lipid · Enzyme · Active site · Substrate · Denatured · 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)

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

Link ideas

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

Use the information in the question

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

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

Avoid vague words

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

Do not extrapolate a graph line beyond the data or to zero unless asked.

Six-mark extended responses are marked on content AND a logical, linked line of reasoning.

Last updated: 18 October 2026