Biological Molecules & Enzymes

The main biological molecules

Living things are built from a small number of biological molecules. The three groups you must know for Edexcel IGCSE Biology are carbohydrates, proteins and lipids. Each is made of smaller building-block units joined together, and each contains a characteristic set of chemical elements.

Key terms

Biological molecule — a molecule made by living organisms, built from smaller repeating sub-units.

Monomer — a single small sub-unit (e.g. a simple sugar or an amino acid).

Polymer — a large molecule made of many monomers joined in a chain (e.g. starch).

Carbohydrates

Carbohydrates are made from simple sugars such as glucose. A single sugar unit is a monosaccharide; two joined together form a disaccharide (for example maltose). Many sugar units joined into long chains form large polysaccharide molecules:

Starch — the storage carbohydrate in plants.
Glycogen — the storage carbohydrate in animals (stored in liver and muscle).
Cellulose — a structural carbohydrate that makes up plant cell walls, giving them strength.

Carbohydrates contain only the elements carbon, hydrogen and oxygen (C, H, O), with the hydrogen and oxygen usually in a 2:1 ratio (the same ratio as in water).

Proteins

Proteins are polymers built from amino acids joined in a chain. There are about 20 different amino acids, and the order in which they are arranged determines the shape and function of the protein. The chain folds into a specific three-dimensional shape, which is important for molecules like enzymes.

Proteins contain carbon, hydrogen, oxygen and nitrogen (C, H, O, N), and sometimes sulfur (S). The nitrogen is the key feature that separates proteins from carbohydrates and lipids.

Lipids

Lipids (fats and oils) are each made from one molecule of glycerol joined to three fatty acids. Like carbohydrates, lipids contain only carbon, hydrogen and oxygen (C, H, O), but they have far less oxygen in proportion to hydrogen, which is why they release more energy per gram.

Watch out

Don't muddle the building blocks. Carbohydrates are made of simple sugars, proteins of amino acids, and lipids of fatty acids + glycerol. A common exam slip is writing "glucose" as the unit for lipids.

Food tests

You can identify which biological molecule is present in a food sample using chemical tests. Learn the reagent, the procedure and the positive colour change for each one.

Exam tip

For Benedict's test, the amount of brick-red colour shows how much reducing sugar is present — more sugar gives a deeper red. Always state that the sample must be heated in a water bath, or you lose the mark.

Watch out

Iodine solution is orange-brown to start with. The mark is for the change to blue-black, not just saying "it goes blue-black". If starch is absent, the iodine simply stays orange-brown.

Enzymes — biological catalysts

A catalyst is a substance that speeds up the rate of a chemical reaction without being used up itself. Enzymes are biological catalysts: they are proteins made by living cells that speed up the reactions of metabolism, such as digestion and respiration.

Because an enzyme is not used up, a small amount can catalyse many reactions over and over again.

Key terms

Enzyme — a protein that acts as a biological catalyst, speeding up a reaction without being changed permanently.

Substrate — the molecule(s) an enzyme acts on.

Product — the new molecule(s) formed by the reaction.

Active site — the specific region of the enzyme where the substrate binds.

The lock-and-key model

Each enzyme has a region called the active site with a specific shape. Only a substrate with a complementary shape can fit into the active site — just as only one key fits a particular lock. This is the lock-and-key model.

When the substrate fits into the active site, an enzyme-substrate complex forms. The reaction takes place, the product(s) are released, and the enzyme is free to bind another substrate.

Watch out

The active site is complementary to the substrate, not "identical" or "the same shape". Say complementary in the exam.

Effect of temperature on enzyme activity

As temperature rises, enzyme molecules and substrate molecules move faster, so they collide more often. The rate of reaction increases up to the optimum temperature (around 37 °C for most human enzymes), where activity is fastest.

Above the optimum, the rate falls sharply. The heat makes the enzyme vibrate so much that the bonds holding its shape break. The active site changes shape, so the substrate no longer fits. The enzyme is said to be denatured and the reaction stops.

Watch out

Enzymes are denatured, not "killed". Enzymes are not alive, so they cannot die. After denaturation the change in shape is permanent — cooling down does not restore activity.

Effect of pH on enzyme activity

Each enzyme also has an optimum pH at which it works fastest. Many enzymes work best near neutral (pH 7), but some are adapted to extremes — the protein-digesting enzyme pepsin in the stomach has an optimum of about pH 2.

If the pH moves away from the optimum (too acidic or too alkaline), the bonds holding the enzyme's shape are disrupted, the active site changes shape, and the enzyme is denatured. Activity falls on either side of the optimum, giving a peaked curve.

Enzyme practicals

You should be able to describe simple experiments that show enzymes in action and how to make them a fair test.

Amylase and starch (using iodine)

Amylase breaks down starch into maltose (a sugar). You can follow the reaction using iodine solution:

1. Mix amylase solution with starch solution in a water bath at a set temperature.
2. At regular intervals, remove a drop and add it to iodine on a spotting tile.
3. At first the iodine turns blue-black (starch present).
4. When the starch has all been digested, the iodine stays orange-brown — record the time taken.
5. Repeat at different temperatures or pH values, keeping everything else the same.

Worked example

At 20 °C the iodine stopped going blue-black after 120 s; at 37 °C it took only 40 s. Explain.

At 37 °C the molecules have more kinetic energy and move faster, so there are more frequent successful collisions between amylase active sites and starch. The starch is therefore broken down more quickly, so the colour change happens sooner.

Catalase and hydrogen peroxide

The enzyme catalase (found in liver and potato) breaks down toxic hydrogen peroxide into harmless water and oxygen:

$$2H_2O_2 \rightarrow 2H_2O + O_2$$

Add a piece of liver to hydrogen peroxide and oxygen is released as bubbles of froth. You can measure the volume of oxygen collected in a fixed time, or the height of foam, to compare the rate of reaction at different temperatures or pH values.

Exam tip

When describing any enzyme practical, name the variable you change (e.g. temperature), the variable you measure (e.g. time for colour change, or volume of gas), and the variables you keep the same (e.g. enzyme concentration, volume of solution, pH). That is how you earn the "fair test" marks.

Real world

Enzymes are used widely in industry. Biological washing powders contain protease and lipase enzymes that digest protein and fat stains at low temperatures, saving energy. This is why such detergents work best on a cool wash and lose effectiveness if the water is too hot.