2.3Structures and Functions in Living Organisms

Biological molecules — Edexcel Modular - OL Revision Notes & Topical Questions

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Biological molecules Edexcel O Level Modular Biology notes: identify the chemical elements present in carbohydrates, proteins and lipids (fats and oils)Haemoglobin is a globular protein with 4 chains and haem groups (quaternary).Collagen is fibrous3 chains in a triple helix, stro…

Biological molecules Edexcel O Level Modular Biology: what to do next
Condensation
Hydrolysis
Glycosidic bond
Ester bond
Peptide bond

Section 1 of 18Process & sequence

Primary: amino acid sequence

Secondary: α-helix / β-sheet (H-bonds)

Tertiary: 3D folding (ionic, disulfide, H, hydrophobic)

Quaternary: several chains

Heat sample with Benedict’s – no colour change: no reducing sugar.

Boil a new sample with dilute HCl (hydrolysis).

Neutralise with sodium hydrogencarbonate.

Heat with Benedict’s again.

Brick-red now → non-reducing sugar present.

Section 2 of 18Keep going

Biological molecules Edexcel O Level Modular Biology: what to do next

These Biological molecules Edexcel O Level Modular Biology notes covered the key ideas, worked examples and exam technique for this topic. Check your Biological molecules Edexcel O Level Modular Biology understanding with the practice questions below, then reread any Biological molecules Edexcel O Level Modular Biology point you were unsure about before moving on.

Practice: Biological molecules Edexcel O Level Modular Biology practice questions

Previous lesson: 2.2 Cell structure — Notes

Next lesson: 2.4 Movement of substances into and out of cells — Notes

Course: Edexcel Modular - OL — all units

Official syllabus: Pearson Edexcel International GCSEs

Section 3 of 18What enzymes are

Enzymes are proteins that act as biological catalysts: they speed up chemical reactions in living cells without being changed or used up.

They are necessary because they keep the reactions of metabolism at a rate that can sustain life; without digestive enzymes it could take 2 to 3 weeks to digest a meal instead of about 4 hours.

Each enzyme has a specific three-dimensional shape determined by its sequence of amino acids.

Active site

The part where the substrate binds is the active site.

The substrate fits the active site like a key in a lock (lock-and-key model), forming an enzyme-substrate complex, and the products are released.

Section 4 of 18Temperature and enzyme activity

Temperature	What happens	Effect on rate
Below optimum	Less kinetic energy, fewer collisions between enzyme and substrate	Slow; not denatured
Near optimum (about 37 C for human enzymes)	Maximum collisions with substrate	Fastest
Above optimum	Bonds in the protein break, active site changes shape; the substrate no longer fits	Falls rapidly; enzyme denatured

Low temperatures do not denature enzymes; they simply make them work more slowly. Denaturation is permanent. Use the word 'denatured', not 'killed' or 'dead'.

Section 5 of 18Practical: temperature and amylase

Amylase digests starch into maltose. Starch is a large molecule made of many glucose units; maltose is made of two glucose molecules. Iodine solution is yellow-brown and turns blue-black with starch.

Put a drop of iodine in each well of a spotting tile.

Warm 5 cm³ starch solution in a water bath at the chosen temperature (e.g. 20 C).

Add 2 cm³ amylase and start a timer.

Every minute transfer a drop of the mixture to a well of iodine.

Record the time when the iodine no longer turns blue-black.

Repeat at other temperatures, such as 30, 40, 50 and 60 C.

Results

at the optimum, iodine stops changing colour fastest because starch is all digested;
- at cold temperatures it takes longer because enzyme and substrate rarely collide;
- at high temperatures the iodine stays blue-black because amylase is denatured.
Rate = 1 / time.

Section 6 of 18Planning with CORMS

change temperature; O
keep the same volume and concentration of amylase and starch, and pH; R
repeat several times at each temperature; M
measure the time for the iodine to stop turning blue-black; S
safety (hot water, use a thermometer, goggles).

Section 7 of 18Interpreting enzyme graphs and calculating rates

Rate of reaction can be calculated from the time taken: rate = 1 / time (s^-1), or from the amount of product formed divided by time.

Interpreting enzyme graphs and calculating rates

For example if iodine stopped turning blue-black after 60 s at 30 C and after 30 s at 40 C, the rates are 1/60 = 0.017 s^-1 and 1/30 = 0.033 s^-1,

so the rate doubled over a 10 degree rise.

On a graph of rate against temperature, the steep rise at low temperature is explained by increased kinetic energy; the peak is the optimum; the steep fall after is denaturation.

The temperature coefficient (Q10) for many enzymes is about 2 between 10 and 30 C.

Other enzyme factsthey are specific because only a substrate with a complementary shape fits the active site, they are not used up so a small amount can catalyse many reactions, and they are involved in digestion, respiration, photosynthesis and protein synthesis.

Interpreting enzyme graphs and calculating rates

Examples of use: biological washing powders contain proteases and lipases that work at low temperatures,

and lactase is used to make lactose-free milk.

Enzymes made inside cells (intracellular) and those secreted (extracellular, such as digestive enzymes) can both be affected by temperature and pH.

Remember that the substrate concentration and enzyme concentration also affect the rate: increasing substrate raises the rate until all active sites are occupied, after which the rate levels off.

Section 8 of 18Compare

Biological molecules compared
Molecule	Monomer	Bond	Structure	Function	Test
Starch (amylose)	α-glucose	α-1,4 glycosidic	Unbranched helix	Energy store in plants – compact	Iodine → blue-black
Amylopectin / glycogen	α-glucose	α-1,4 and α-1,6	Branched (glycogen more)	Rapid glucose release	Iodine
Cellulose	β-glucose	β-1,4 glycosidic	Straight chains, H-bonds → microfibrils	Cell wall strength	—
Triglyceride	Glycerol + 3 fatty acids	Ester	Hydrophobic	Energy store, insulation	Emulsion test
Phospholipid	Glycerol + 2 fatty acids + phosphate	Ester	Hydrophilic head, hydrophobic tails	Membrane bilayer	—
Protein	Amino acids	Peptide	Primary → quaternary	Enzymes, haemoglobin, collagen	Biuret → purple
Water	—	Hydrogen bonds	Polar molecule	Solvent, coolant, high SHC	—

Section 9 of 18Worked examples

Explain why an enzyme is denatured at 70 C. [3]3 marks
Describe the results expected at 5 C. [2]2 marks
Why is iodine used? [1]1 mark

Section 10 of 18Effect of pH on enzyme activity

Every enzyme has an optimum pH at which it works fastest.

Effect of pH on enzyme activity

Enzymes made in acidic conditions, such as pepsin in the stomach (optimum about pH 2), work best in acid.

Enzymes made in alkaline conditions, such as those in the duodenum, have a higher optimum, around pH 8 to 9.

If the pH is too high or too low, the bonds holding the amino acid chain together are disrupted or destroyed, the shape of the active site changes and the enzyme becomes denatured,
so the substrate can no longer fit.

Enzyme	Where it works	Optimum pH
Pepsin (protease)	Stomach	About 2
Salivary amylase	Mouth	About 7
Pancreatic lipase and protease	Small intestine	About 8 to 9

On a graph of rate against pH the curve rises to a peak at the optimum and falls on both sides. Describe it, then explain each part: rate increases towards the peak as more enzyme molecules have a correctly shaped active site; after the peak, the enzyme is denatured.

Section 11 of 18Practical: effect of pH on amylase

Place a drop of iodine in each well of a spotting tile.

Mix 2 cm³ amylase with 2 cm³ buffer solution of a chosen pH (for example pH 5.0, 7.0, 9.0 and 11.0) and add 2 cm³ starch solution; start a timer.

After each minute transfer a drop of the mixture into a well of iodine.

Record the time (or number of wells) taken for the iodine to stop turning blue-black.

Repeat the whole investigation at each pH and several times for reliability.

Interpretation

at the optimum pH the iodine stops turning blue-black soonest, because amylase works at its fastest rate and digests all the starch.
At extreme pH values the iodine stays blue-black throughout, because the enzyme has been denatured and starch remains.

Section 12 of 18Using CORMS

C - change the pH using buffers.

O - other variables kept the same: temperature, volume and concentration of amylase and starch.

R - repeat several times to ensure reliability.

M - measure the time taken (or count wells) for the iodine to stop turning blue-black.

S - safety: wear eye protection, avoid contact with alkaline buffers.

Exam questions will often ask you to apply CORMS to an unfamiliar practical, so practise with different enzymes.

Terminology

write that an enzyme is 'denatured', never 'dead'.
Describe AND explain each effect for full marks.

Section 13 of 18Further examples and graph interpretation

Graph examplean enzyme from the duodenum with an optimum at pH 8 has a bell-shaped curve.

Further examples and graph interpretation

At pH 4 the activity is nearly zero because the acid disrupts the ionic bonds that maintain the shape of the active site; at pH 8 the active site is perfectly shaped; at pH 12 activity falls again because of the alkaline conditions.

When describing a graph, quote data: 'the rate increases from 0 at pH 3 to a maximum of 10 units at pH 8 and then falls to 2 units at pH 11'.

When explaining, use the active site idea: 'the shape of the active site changes so the substrate cannot fit'.

Why do different enzymes have different optimum pH values?

Because their amino acid sequences produce different three-dimensional shapes and charge distributions,

so they are adapted to the conditions where they work.

Pepsin is unusual because it is stable in strong acid; if it passed into the small intestine it would be denatured by the alkaline conditions.

This is why the pancreas also releases bicarbonate and bile to neutralise acid entering the duodenum.

Investigations of enzymes using other substrates are possible: catalase breaks down hydrogen peroxide to water and oxygen,

and the volume of oxygen collected over time can be used to measure the rate at different pH values or temperatures.

Section 14 of 18Worked examples

Describe and explain the effect on pepsin of raising the pH from 2 to 8. [3]3 marks
Which variable is changed and which is measured in the pH practical? [2]2 marks
Why must temperature be kept constant? [1]1 mark

Section 15 of 18Key terms

Term	Meaning
Enzyme	A protein that acts as a biological catalyst.
Active site	The part of an enzyme where the substrate binds.
Substrate	The substance an enzyme acts on.
Denaturation	Permanent change in enzyme shape caused by heat or pH.
Optimum temperature	The temperature at which an enzyme works fastest.

Term	Meaning
Optimum pH	The pH at which an enzyme works fastest.
Buffer	A solution that maintains a constant pH.
Pepsin	A protease working in the stomach.
CORMS	Change, Organism, Repeat, Measure, Same.
Denatured	Having lost its normal shape irreversibly.

Section 16 of 18Syllabus checklist

identify the chemical elements present in carbohydrates, proteins and lipids (fats and oils)

describe the structure of carbohydrates, proteins and lipids as large molecules made up from smaller basic units

starch and glycogen from simple sugars
protein from amino acids
and lipid from fatty acids and glycerol

practical: investigate food samples for the presence of glucose, starch, protein and fat

understand the role of enzymes as biological catalysts in metabolic reactions

understand how temperature changes can affect enzyme function, including changes to the shape of active site

practical: investigate how enzyme activity can be affected by changes in temperature

understand how enzyme function can be affected by changes in pH altering the active site 2.14B practical: investigate how enzyme activity can be affected by changes in pH

Section 17 of 18Command 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.

Section 18 of 18Exam technique

Saying cold temperatures denature enzymes.

Saying enzymes are used up.

Saying the enzyme dies.

Saying the substrate changes shape.

Saying all enzymes work best at pH 7.

Saying a high pH 'kills' enzymes.

Forgetting buffers keep the pH constant.

Not repeating results.

Last updated: 18 October 2026