Understanding Biological molecules: Edexcel IGCSE Human Biology Explained

Biological molecules are the raw materials of life, and this section of the edexcel igcse human biology course is where you learn exactly what those materials are, how to test for them, and how enzymes make the body's chemistry fast enough to keep you alive.

The specification groups everything here under one topic: biological molecules and enzymes. Do not let that compact heading fool you. This is one of the most heavily examined areas of edexcel igcse human biology biological molecules being a topic that connects to digestion, respiration and enzyme action throughout the 4HB1 course. You need to know the chemical composition of the three main macromolecule groups, how to test for them in a laboratory, how enzymes work, and how their activity is affected by temperature, pH, substrate concentration, and inhibitors. These edexcel igcse human biology revision notes give you all of that, stripped down to what the exam actually tests.

The three macromolecule groups

Every large biological molecule is built from smaller units. The edexcel specification expects you to know the chemical elements present in each group and how the building blocks assemble into the larger molecules.

Food tests

The specification requires you to know the standard laboratory tests for glucose, starch, lipid and protein. Here is what you need to recall, fast and clean:

Worked example: interpreting a food test

A student adds Benedict's solution to a food sample and heats it. The solution remains blue. What does this tell you?

Answer: The food sample does not contain a reducing sugar (such as glucose). Benedict's solution stays blue when no reducing sugar is present. The sample might contain a non-reducing sugar (such as sucrose), starch, or no carbohydrate at all, but the Benedict's test alone cannot distinguish those.

Vitamin C investigation

The specification also requires you to investigate the vitamin C content of food. The standard method uses DCPIP (dichlorophenolindophenol), a blue dye that is decolourised by vitamin C. You add the food extract drop by drop to a fixed volume of DCPIP until the blue colour disappears. The fewer drops needed, the higher the vitamin C concentration.

Energy content of food

To investigate the energy content of food, you burn a known mass of food beneath a boiling tube containing a known volume of water. Measure the temperature rise of the water. Calculate energy released using:

Energy (J) = mass of water (g) x 4.2 x temperature rise (degrees C)

Worked example: energy calculation

A student burns 0.5 g of peanut beneath a boiling tube containing 20 cm³ of water. The water temperature rises from 22 degrees C to 47 degrees C. Calculate the energy released per gram of peanut.

Step 1: Energy released = 20 x 4.2 x (47 - 22) = 20 x 4.2 x 25 = 2100 J

Step 2: Energy per gram = 2100 / 0.5 = 4200 J/g (or 4.2 kJ/g)

Enzymes: the biological catalysts

Enzymes are biological catalysts: they speed up metabolic reactions without being used up in the process. Each enzyme has a specific shape at its active site that fits a particular substrate, like a key fitting a lock. The substrate binds to the active site, the reaction occurs, and the products are released.

Factors affecting enzyme activity

Temperature: As temperature increases, the rate of enzyme-catalysed reactions increases because molecules have more kinetic energy and collide more frequently. Above the optimum temperature (around 37 degrees C for human enzymes), the enzyme begins to denature: the active site changes shape, the substrate can no longer fit, and the reaction rate drops sharply. At very high temperatures, the enzyme is fully denatured and the reaction stops.

pH: Each enzyme has an optimum pH. For most human enzymes this is around pH 7, but pepsin in the stomach works best at around pH 2. Moving away from the optimum pH changes the shape of the active site, reducing the rate of reaction. Extreme pH values denature the enzyme.

Substrate concentration: Increasing substrate concentration increases the rate of reaction because there are more substrate molecules to collide with enzyme active sites. Eventually, all active sites are occupied (the enzyme is saturated) and the rate plateaus. Adding more substrate beyond this point has no effect unless more enzyme is added.

Competitive and non-competitive inhibitors

Competitive inhibitors have a shape similar to the substrate. They bind to the active site, blocking the substrate from entering. Increasing substrate concentration can overcome competitive inhibition because substrate molecules outcompete the inhibitor for the active site.

Non-competitive inhibitors bind to a different part of the enzyme (not the active site). This changes the shape of the active site so the substrate can no longer fit. Increasing substrate concentration does not overcome non-competitive inhibition because the inhibitor is not competing for the same binding site.

Immobilised enzymes

Immobilised enzymes are enzymes that are attached to an insoluble material such as alginate beads. They can be reused, are easier to separate from the product, and are more stable at higher temperatures. The specification requires you to know three applications:

• Production of lactose-free milk: The enzyme lactase is immobilised in alginate beads. Milk is passed over the beads, and lactase breaks down lactose into glucose and galactose. People who are lactose intolerant can drink the resulting milk without digestive discomfort.
• Conversion of sucrose into glucose and fructose: Immobilised sucrase (invertase) converts sucrose into its sweeter component sugars. This is used in the food industry to produce sweeter syrups from cheaper sucrose.
• Glucose testing strips for diabetics: The enzyme glucose oxidase is immobilised on a test strip. When a drop of blood is applied, the enzyme reacts with glucose and produces a colour change proportional to the glucose concentration.

Practical: preparing alginate beads

Mix the enzyme solution with sodium alginate. Using a syringe or pipette, drop the mixture into a calcium chloride solution. The drops solidify into small beads that trap the enzyme. The beads are then washed and can be packed into a column or used in a container. Substrate solution is passed over or through the beads, and the product is collected at the other end.

Self-check questions

Test yourself before moving to the next topic. Write your answer, then check it against the guidance below.

1. Name the elements found in proteins but not in carbohydrates.
2. Describe the test for lipid and state the positive result.
3. A student heats an enzyme to 80 degrees C. The reaction rate drops to zero. Explain why.
4. Distinguish between competitive and non-competitive inhibition.
5. Give two advantages of using immobilised enzymes in industry.
6. In a Benedict's test, a sample turns orange. What does this indicate?
7. Explain why increasing substrate concentration eventually has no further effect on reaction rate.
8. Describe how you would investigate the effect of pH on enzyme activity.

These edexcel igcse human biology notes on biological molecules cover the full scope of what the exam tests. You can find biological molecules edexcel igcse practice questions on the Green Bridge CBT platform, where igcse 4hb1 biological molecules questions are organised by difficulty and mapped to the specification. For a broader view of how this topic connects to digestion and respiration, see the edexcel igcse human biology explained series, and use edexcel igcse human biology practice questions across all topics to build exam confidence.