Biological molecules and enzymes — Edexcel International GCSE Science (Double Award)
The elements and building blocks of carbohydrates, proteins and lipids, food tests, and how temperature and pH affect enzymes.
Revision notes
5 short notes, in the order of the specification. Each one in short:
Carbohydrates, proteins and lipids are the three main groups of biological molecules. All contain carbon, hydrogen and oxygen, and proteins also contain nitrogen. They are large molecules built from smaller units: starch and glycogen from simple sugars (glucose), proteins from amino acids, and lipids from one glycerol and three fatty acids.
Practical: food tests
PracticalFood tests use chemical reagents to identify biological molecules in a food. Benedict's solution, heated in a water bath, turns from blue to brick-red with glucose. Iodine solution turns from orange-brown to blue-black with starch. Biuret reagent turns from blue to purple with protein. Fat shaken with ethanol and poured into water forms a cloudy white emulsion.
Enzymes are proteins that act as biological catalysts, speeding up metabolic reactions without being used up. The substrate has a shape complementary to the enzyme's active site, so it fits like a key in a lock and forms an enzyme-substrate complex. Enzymes are specific: each catalyses only one type of reaction.
Enzymes work fastest at their optimum temperature and optimum pH. Raising the temperature increases collisions between enzyme and substrate, so the rate rises, but above the optimum the active site changes shape, the substrate no longer fits and the enzyme is denatured. Extreme pH also denatures enzymes. Many human enzymes work best at about 37 °C.
The effect of temperature or pH on enzyme activity can be investigated using amylase and starch. Drops of the mixture are added to iodine solution every 30 seconds, and the time is recorded when the iodine stays orange-brown, showing all the starch is broken down. The shortest time, and so the fastest rate, is at the optimum.
Exam questions
6 exam-style questions (23 marks), each with its mark scheme.
Answer the questionsFlashcards
15 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of structure and functions in living organisms on one page, so you can see where this subtopic fits.
Open the mind mapPractical sheets
Method, variables, risks and a results table, with questions to answer.
Print it
Free PDFs to print or save.
Quick check questions
Which elements are present in proteins but not in carbohydrates or lipids?
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Nitrogen (and sometimes sulfur).
What are the smaller units that make up a protein?
Show answer
Amino acids.
What colour does Benedict's solution turn with glucose when heated?
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From blue to green, yellow, orange or brick-red.
What is meant by a biological catalyst?
Show answer
An enzyme that speeds up a reaction in a living thing without being used up.
Why does a very high temperature stop an enzyme working?
Show answer
The shape of the active site changes, so the substrate no longer fits and the enzyme is denatured.
Frequently asked questions
How do enzymes speed up reactions?
Enzymes speed up reactions by acting as biological catalysts. The substrate fits into the enzyme's active site because their shapes are complementary, forming an enzyme-substrate complex. The reaction takes place at the active site, then the products leave and the enzyme is free to be used again, because it is not used up.
Why do enzymes denature at high temperatures?
Enzymes denature at high temperatures because the heat damages the enzyme's structure, so the shape of the active site changes. The substrate is no longer complementary to the active site and cannot fit, so the rate of reaction falls quickly towards zero. Denaturing is permanent, and enzymes are not alive, so they are not 'killed'.
What happens to enzymes at low temperatures?
At low temperatures enzymes work slowly but are not denatured. The enzyme and substrate particles have less kinetic energy and move more slowly, so they collide less often and fewer enzyme-substrate complexes form. As the temperature rises towards the optimum, around 37 °C for many human enzymes, collisions increase and the rate rises.
What happens to enzymes at high pH?
If the pH is higher than an enzyme's optimum, the shape of the active site is altered, so the substrate fits less well and the rate decreases. At extreme pH the active site changes so much that the substrate cannot fit and the enzyme is denatured. Different enzymes have different optimum pH values, such as amylase at about pH 7.
Why are enzymes specific?
Enzymes are specific because each has an active site with a particular shape. Only a substrate whose shape is complementary to the active site can bind to it, like a key fitting a lock. This means each enzyme catalyses only one type of reaction, so amylase breaks down starch but cannot break down protein.
Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026