Biology (0610) | Enzymes

Hello IGCSE Biologists! Welcome to Enzymes – The Busy Workers of Your Cells

Welcome to one of the most exciting topics in Biology! Enzymes might sound technical, but they are the tiny, essential workers inside every living thing, running all the chemical reactions that keep you alive, from breathing to digesting your lunch.

In this chapter, we will learn exactly what enzymes are, how they perform their incredible jobs with speed and precision, and what happens when their environment gets too hot or too acidic. Getting this topic right is crucial for success, as it links directly to digestion, respiration, and many practical experiments! Let's get started.

5.1 Defining Enzymes and Catalysts (The Basics)

What is a Catalyst? (Core 5.1.1)

A catalyst is a substance that increases the rate of a chemical reaction without being used up itself during the reaction. Think of a catalyst like a coach for a race: the coach speeds up the training (reaction rate) but isn't part of the final team running the race (product).

Enzymes: Biological Catalysts (Core 5.1.2 & 5.1.3)

Enzymes are special catalysts found in living organisms.

• Enzymes are made of proteins.
• They function as biological catalysts.
• They are essential for all metabolic reactions (the chemical reactions in cells) because, without them, these reactions would happen far too slowly to sustain life.

Did You Know? Metabolism is just a fancy word for all the building (anabolism) and breaking down (catabolism) reactions happening constantly in your body!

Quick Review: Enzymes are protein catalysts that speed up the chemical reactions necessary for life.

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How Enzymes Work: The Lock and Key Model

The way an enzyme works is highly specific—it only reacts with one particular molecule. We explain this using the famous Lock and Key Model (Core 5.1.4, Supplement 5.1.6 & 5.1.7).

Key Terms to Understand Enzyme Action

• Substrate: This is the molecule that the enzyme acts upon. (The key)
• Active Site: This is the specific region on the enzyme where the substrate binds. It has a unique, precise shape. (The lock)
• Product: The new molecule(s) formed after the reaction is complete.

Step-by-Step Enzyme Action (The Key Turning the Lock)

The enzyme action is a temporary process that changes the substrate into product, releasing the product and leaving the enzyme ready for another reaction.

1. The substrate approaches the enzyme.
2. The substrate fits perfectly into the enzyme's active site because their shapes are complementary (like a specific key fitting a specific lock).
3. They join together temporarily to form an enzyme-substrate complex.
4. The chemical reaction takes place (e.g., breaking down a large molecule or building a new one).
5. The new substance(s), the products, are released from the active site.
6. The enzyme remains unchanged and is immediately ready to bind to a new substrate molecule.

We can summarise the reaction like this (Supplement 5.1.6):
\( \text{Enzyme} + \text{Substrate} \rightarrow \text{Enzyme-Substrate Complex} \rightarrow \text{Enzyme} + \text{Product} \)

Enzyme Specificity (Supplement 5.1.7)

Enzymes are highly specific. This means each type of enzyme can usually only catalyse one type of reaction or act on one specific substrate.

Why? Because the shape of the active site is complementary to only one substrate molecule. If the shape doesn't match, the reaction cannot happen.

Key Takeaway: Enzyme action relies on the complementary shape between the substrate and the active site, often described by the Lock and Key model.

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Factors Affecting Enzyme Activity (Core 5.1.5, Supplement 5.1.8 & 5.1.9)

Enzymes are delicate protein structures, and their activity is highly sensitive to changes in their environment, especially temperature and pH.

1. The Effect of Temperature

When you measure the rate of reaction, you plot a graph showing how enzyme activity changes with temperature.

A. Lower Temperatures (0°C to Optimum)

As the temperature increases, the rate of reaction increases (Supplement 5.1.8).

• The substrate and enzyme molecules have low kinetic energy (they move slowly).
• The frequency of effective collisions (collisions resulting in the substrate binding to the active site) is low.
• Increasing temperature increases kinetic energy, making collisions more frequent and successful, thus increasing the reaction rate.

B. Optimum Temperature

The optimum temperature is the temperature at which the enzyme is most active and the reaction rate is fastest. For most human enzymes, this is around 37°C.

C. High Temperatures (Above Optimum)

Once the temperature goes too high (usually above 40°C–60°C), the rate plummets dramatically. This is due to denaturation.

Denaturation Explained (Supplement 5.1.8):

1. High temperatures cause intense vibrations within the enzyme molecule.
2. These vibrations break the weak bonds holding the 3D structure of the protein together.
3. The specific shape of the active site changes (it becomes permanently damaged).
4. Because the active site is no longer complementary to the substrate, the substrate cannot bind.
5. The enzyme is now denatured and cannot function. This damage is usually irreversible.

Analogy: Imagine the enzyme is a plastic key. If you heat it slightly, it moves faster. If you melt it (denature it), it completely changes shape and can no longer open the lock.

2. The Effect of pH

Enzymes work best within a very narrow pH range.

A. Optimum pH

The optimum pH is the pH value at which the enzyme achieves its maximum reaction rate.

• The optimum pH varies widely for different enzymes. For example, the enzyme amylase in your mouth works best near pH 7 (neutral), while pepsin in your stomach works best around pH 2 (highly acidic).

B. Extreme pH (Supplement 5.1.9)

If the pH moves significantly above or below the optimum level, the enzyme will denature.

• Extreme pH values disrupt the weak forces (like ionic bonds) that maintain the enzyme's specific 3D structure.
• The shape of the active site is altered.
• The substrate can no longer fit into the active site, and the reaction stops.

Don't worry if this seems tricky at first—just remember that both too much heat and the wrong pH ruin the active site's unique shape!

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Enzymes in Action: Relating to Other Chapters

Enzymes are not just theoretical! They are the tools used constantly throughout your body (and in industry).

Enzymes in Digestion (Core 7.4.3 & 7.4.4)

The digestive system relies completely on enzymes (chemical digestion) to break down large, insoluble food molecules into small, soluble molecules that can be absorbed into the blood.

• Amylase: Breaks down starch into simple reducing sugars (like maltose). Secreted in the mouth (salivary glands) and pancreas, acting in the mouth and small intestine.
• Proteases (e.g., Pepsin, Trypsin): Break down protein into amino acids. Secreted in the stomach (pepsin, acidic pH) and pancreas/small intestine (trypsin, alkaline pH).
• Lipase: Breaks down fats and oils (lipids) into fatty acids and glycerol. Secreted by the pancreas, acting in the small intestine.

Common Mistake to Avoid: The stomach uses hydrochloric acid (low pH) to kill microbes and provide the acidic optimum conditions for the protease pepsin, but the acid itself is not an enzyme.

Key Takeaway: Temperature and pH must be controlled (e.g., in digestion or experiments) to ensure enzymes function correctly and do not denature.