Enzymes

Overview

Enzymes are the unsung heroes of the biological world. As highly specific protein catalysts, they drive the thousands of chemical reactions that sustain life, from digesting your food to synthesising new DNA. For your OCR GCSE Biology exam, a solid understanding of topic 1.6 is non-negotiable, as it forms a foundation for many other biological concepts. Examiners frequently test your knowledge of the 'Lock and Key' model, your ability to interpret data on reaction rates (especially from PAG 4), and your use of precise scientific language. This guide will equip you with the core knowledge, exam technique, and cognitive tools to master this topic and confidently tackle any question that comes your way.

Key Concepts

Concept 1: Enzymes as Biological Catalysts

At its core, an enzyme is a biological catalyst. This means it speeds up a chemical reaction without being chemically changed or used up in the process. Think of it like a very efficient factory worker who can assemble products (the reaction) over and over again without getting tired. Enzymes are large protein molecules, and their specific three-dimensional shape is crucial for their function. This shape creates a unique pocket or cleft called the active site.

Concept 2: The 'Lock and Key' Model

To explain the remarkable specificity of enzymes, we use the Lock and Key Model. This model is a central concept for OCR and is a frequent source of marks.

• The Lock: The enzyme itself, with its uniquely shaped active site.
• The Key: The substrate molecule, which has a shape that is complementary to the active site.

It is vital to use the word complementary, not 'the same shape'. Just as only one key can open a specific lock, only one type of substrate can fit into the active site of a specific enzyme. When the substrate binds to the active site, it forms a temporary structure called an enzyme-substrate complex. It is within this complex that the chemical reaction occurs, after which the products are released. The enzyme's active site is then free to bind with another substrate molecule.

Concept 3: Factors Affecting Enzyme Activity

Examiners love to ask questions about the factors that influence how fast enzymes work. You must be able to not only describe these effects but also explain the underlying mechanisms using collision theory.

Temperature

Temperature has a dramatic effect on enzyme activity. Your explanation must always be in two parts:

1. Increasing Temperature (up to the optimum): As temperature rises, both enzyme and substrate molecules gain more kinetic energy. They move around faster and more randomly, leading to more frequent and energetic collisions between them. This increases the chances of a successful collision where the substrate binds correctly to the active site, forming an enzyme-substrate complex. Consequently, the rate of reaction increases.
2. Increasing Temperature (beyond the optimum): If the temperature gets too high, the enzyme starts to denature. The high temperature provides enough energy to break the weak bonds holding the protein in its specific 3D shape. This causes the shape of the active site to change permanently. As a result, the substrate no longer has a complementary shape and cannot bind. The rate of reaction falls rapidly. A common mistake is to say the enzyme is 'killed'; this is incorrect as enzymes are not alive. The correct term is denatured.

pH

Like temperature, pH also has a significant impact. Every enzyme has an optimum pH at which it functions most effectively. For most enzymes in the human body, this is around neutral (pH 7). However, some are adapted to extreme environments, like pepsin in the stomach which has an optimum pH of around 2.

If the pH deviates too far from the optimum (either too acidic or too alkaline), it interferes with the bonds that maintain the enzyme's 3D shape. This causes the active site to change shape, and the enzyme becomes denatured. The substrate can no longer bind, and the rate of reaction decreases.

Mathematical/Scientific Relationships

In enzyme experiments, you often measure the time it takes for a reaction to complete. However, to get marks, you usually need to calculate the rate of reaction.

• Formula: Rate of reaction = 1 / time
• Units: The standard unit for time is seconds (s), so the unit for rate is per second (s⁻¹).
• Alternative: Sometimes, to avoid small decimal numbers, the rate is calculated as Rate = 1000 / time. Always check the question for instructions.

This formula is not given on the formula sheet and must be memorised.

Practical Applications

Enzymes are not just a textbook concept; they are essential in industry and in the required practicals you perform.

Industrial Uses

• Biological Washing Powders: Contain proteases and lipases to break down protein and fat stains (like blood and grease) at lower temperatures, saving energy.
• Food Production: Pectinase is used to break down pectin in fruit cell walls to extract more juice. Lactase is used to produce lactose-free milk for intolerant individuals.

Required Practical: PAG 4 - Investigating Enzyme Activity

This practical is a cornerstone of the topic. A common experiment is to investigate the effect of temperature on the activity of amylase, an enzyme that breaks down starch into maltose.

• Apparatus: Test tubes, water baths set at different temperatures, spotting tile, iodine solution, amylase solution, starch solution, syringe, timer.
• Method:
  1. Set up water baths at a range of temperatures (e.g., 20°C, 30°C, 40°C, 50°C, 60°C).
  2. Add starch solution to a test tube and place it in a water bath to acclimatise.
  3. Add amylase solution to the same test tube, start the timer immediately.
  4. At regular intervals (e.g., every 30 seconds), take a sample from the mixture and add it to a drop of iodine solution on a spotting tile.
  5. Record the time taken for the iodine solution to no longer turn blue-black, which indicates all the starch has been broken down.
  6. Repeat the experiment at each temperature.
• Control Variables: Enzyme concentration, substrate concentration, pH.
• Common Errors: Inaccurate temperature control, inconsistent timing for sampling, cross-contamination of solutions.