Ch 1 · Enzymes · IGCSE Biology 0610
Topic 1 of 1 in Enzymes — IGCSE Biology 0610 — 48 lessons.
Introduction
Almost every chemical reaction in a living cell, from digesting a meal to copying DNA, is controlled by an enzyme. Without enzymes these reactions would be far too slow at body temperature to keep an organism alive. Learn the definitions and the lock-and-key model word for word; temperature and pH investigations are also a favourite Paper 6 context.
Key ideas — Catalysts and enzymes
A catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction.
🧑🏫 Teacher's explanation
Imagine you are trying to move a massive pile of bricks from one side of a garden to another. If you carry them one by one, it will take all day. Now, imagine you have a wheelbarrow. The wheelbarrow doesn't get used up or disappear after one trip; it just makes the process of moving bricks much faster. In this analogy, the bricks are your substrates, the garden is your cell, and the wheelbarrow is the enzyme.
📖 Definitions
🔍 Worked example — Easy
A student measures the rate of an enzyme-catalyzed reaction at two different temperatures. At 20^° C, the rate is 5 units/min. At 40^° C, the rate is 15 units/min. Calculate the percentage increase in the rate of reaction.
🔍 Worked example — Standard
Describe the effect of increasing temperature from 10^° C to 35^° C on the rate of an enzyme-controlled reaction.
🔍 Worked example — Exam-style
Explain why an enzyme stops functioning when the temperature rises above its optimum temperature (e.g., 50^° C for a human enzyme).
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Temperature (^° C): 10, 20, 30, 40, 50 Rate of reaction (units/min): 2, 8, 15, 12, 0
Key ideas — How enzymes work: the lock-and-key model
In the lock-and-key model the substrate is the key and the active site is the lock. Just as a lock accepts only the correctly shaped key, an enzyme catalyses only the reaction of the substrate that fits its active site. The enzyme is unchanged and can be reused.
🧑🏫 Teacher's explanation
Imagine you are trying to open a door. You have a specific key (the substrate) and a specific lock (the enzyme). Only the key with the exact right teeth can fit into the lock's keyhole (the active site). Once you turn the key, the door opens (the reaction occurs), but the key itself remains unchanged and ready to be used on the next door.
📖 Definitions
🔍 Worked example — Easy
A student observes that an enzyme-catalyzed reaction produces 50 mumol of product in 10 minutes. Calculate the rate of reaction in mumol per minute.
🔍 Worked example — Standard
Explain why an enzyme that digests starch (amylase) cannot digest proteins.
🔍 Worked example — Exam-style
An enzyme's activity is measured at 20 C and 40 C. At 40 C, the rate is 10 units/min. At 60 C, the rate is 0 units/min. Explain the result at 60 C in terms of enzyme structure.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Identify the optimum pH for this enzyme. [1] (b) Explain why the rate of reaction decreases when the pH moves from 7 to 9. [3] [4 marks]
Key ideas — Effect of temperature
A typical investigation uses amylase and starch. Equal volumes of the two solutions are placed in separate tubes in water baths at a range of temperatures, then mixed. Every 30 seconds a drop is tested with iodine solution on a spotting tile. The time taken for the iodine to stay orange-brown (no starch left) is recorded; a shorter time means a higher rate. Volumes, concentrations and pH are kept constant and each temperature is repeated.
🧑🏫 Teacher's explanation
Think of an enzyme like a specialized key and the substrate like a lock. For a reaction to happen, the key must fit perfectly into the lock. As you turn up the heat, it is like the molecules are dancing faster and faster around the room; because they are moving so quickly, they are much more likely to bump into each other, making the 'lock and key' connection happen much more often.
📖 Definitions
🔍 Worked example — Easy
A student measures the time taken for starch to be completely broken down by amylase at different temperatures. At 20 ^circC, it takes 300 s. At 40 ^circC, it takes 60 s. Calculate the factor by which the rate of reaction has increased.
🔍 Worked example — Standard
Describe the effect of increasing temperature from 10 ^circC to 35 ^circC on the rate of an enzyme-catalyzed reaction.
🔍 Worked example — Exam-style
An experiment shows that at 50 ^circC, the rate of reaction for the enzyme sucrase is zero. Explain this observation in terms of enzyme structure.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Effect of pH
To investigate pH, the same amylase and starch method is used at constant temperature, with a few drops of a different buffer solution (for example pH 3, 5, 7, 9 and 11) added to each tube. Rate is again judged from the time taken for starch to disappear.
🧑🏫 Teacher's explanation
Think of an enzyme like a specialized key and the substrate like a lock. For the key to work, the lock must have a very specific shape. Now, imagine that the lock is made of a soft, flexible material like wax. If you change the environment—for example, by dipping the lock in acid or a strong alkali—the wax might melt or warp. Even if you return it to normal, the shape of the keyhole has changed, and the key no longer fits.
📖 Definitions
🔍 Worked example — Easy
An experiment measures the time taken for starch to be digested by amylase at different pH levels. At pH 7, starch disappears in 30 seconds. At pH 4, starch disappears in 150 seconds. Calculate the factor by which the rate of reaction decreased when moving from pH 7 to pH 4.
🔍 Worked example — Standard
A student investigates the effect of pH on pepsin activity. The enzyme works best at pH 2. Describe the effect on the enzyme's structure if the solution is changed to pH 10.
🔍 Worked example — Exam-style
In an experiment, the rate of reaction for an enzyme was measured at pH 5, 7, and 9. The rates were 0.2 units/min, 0.8 units/min, and 0.3 units/min respectively. Interpret this data to identify the optimum pH and explain the trend.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Question. A student investigated the effect of temperature on catalase, which breaks down hydrogen peroxide, by measuring the volume of oxygen released in one minute. The volume rose from 4 cm³ at 10 °C to 22 cm³ at 40 °C, then fell to 1 cm³ at 60 °C. Explain these results. [5]
🔍 Worked example — Easy
Calculate the percentage increase in the volume of oxygen released when the temperature increases from 10 ^circC to 40 ^circC (from 4 cm^3 to 22 cm^3).
🔍 Worked example — Standard
A student observes that an enzyme's rate of reaction is highest at 37 ^circC. Describe what happens to the rate of reaction if the temperature is increased to 60 ^circC.
🔍 Worked example — Exam-style
An experiment measures the volume of gas produced by an enzyme over time. At 20 ^circC, 10 cm^3 of gas is produced in 5 minutes. At 40 ^circC, 30 cm^3 of gas is produced in 5 minutes. Explain why the rate is higher at 40 ^circC.
⚠️ Common errors
Common mistakes
Practice
Answers A substance that increases the rate of a chemical reaction [B1] and is not changed by the reaction [B1]. Enzymes speed up the chemical reactions of metabolism [B1]; without them, reactions would be too slow at body temperature to sustain life [B1]. The substrate fits into the complementary active site [B1]; an enzyme-substrate complex forms [B1]; the substrate is broken down and the products leave the active site, which is unchanged [B1]. Add amylase and starch solutions, each with a buffer of known pH, to tubes [M1]; use at least five pH values, keeping volumes, concentrations and temperature constant [M1]; test drops with iodine solution at regular intervals [M1]; record the time for the blue-black colour to stop appearing, and repeat each pH [M1]. The active site of lipase is complementary in shape to fat only [B1]; starch has a different shape so cannot bind and no enzyme-substrate complex forms [B1].