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How enzymes work, why temperature and pH change their rate, and how to measure it in the amylase practical.
Specification references this topic covers: Edexcel B1.7, B1.8, B1.9, B1.10, B1.11, B8.11.
Enzymes are tiny workers inside living things. They help the chemical reactions in your body run fast enough to keep you alive. Without them, digesting a meal would be far too slow.
This matters because your food, your energy and your growth all depend on enzymes. It also explains why a very high body temperature is dangerous.
By the end you will be able to explain how an enzyme works using the lock and key idea. You will say what happens when it gets too hot or the pH changes. You will also work out a rate from the amylase practical.
A reaction is when substances change into new substances. Many reactions in your body would be far too slow on their own. A catalyst is a substance that speeds up a reaction and is not used up.
Enzymes are biological catalysts, which means catalysts made by living things. Enzymes are proteins. Proteins are long chains of smaller units, folded into a particular shape.
Because an enzyme is not used up, the same one works again and again. Think of a whisk. It helps with the mixing, and it is still there when you finish.
Each enzyme has a special dent on its surface called the active site. The molecule that the enzyme works on is called the substrate.
The substrate fits into the active site, like a key in a lock. Then the reaction happens. The new substances made are called the products, and they leave the active site.
The enzyme is unchanged afterwards, so it is ready for the next substrate. Then it can start the job again.
The active site has one particular shape. So it fits only one kind of substrate. We say the enzyme is specific, which means it works on one thing only.
Amylase works on starch, but it cannot break down protein. Protein needs a different enzyme with a different shaped active site.
Think of your keys. The key for your front door will not open a bike lock. Only the matching shape works, and the same is true for an enzyme and its substrate.
Particles in a liquid are always moving. When it is warmer, they move faster. The enzyme and the substrate bump into each other more often. More bumps mean a faster reaction.
Each enzyme has an optimum temperature, which is the temperature where it works fastest. For human enzymes this is about 37 °C, close to normal body temperature.
Think of a busy school corridor. If everyone walks faster, more people meet each other.
The rate also depends on the concentration of the substrate. With little substrate the rate is low. As the substrate concentration rises the rate rises, until every active site is busy, and then the rate levels off.
Enzymes are used to build molecules as well as to break them down. They join sugars into carbohydrates such as starch, amino acids into proteins, and fatty acids and glycerol into lipids.
Go past the optimum temperature and the enzyme is in trouble. The bonds that hold its shape start to break. The active site changes shape, so the substrate no longer fits. We say the enzyme is denatured.
Denaturing is permanent. Think of a fried egg. You cannot turn it back into a runny egg. Enzymes are not alive, so they are not killed.
The pH can do the same. pH is a scale that shows how acid or alkaline a liquid is. Most enzymes work best near pH 7. Stomach protease works best at about pH 2.
Amylase breaks starch down into sugar. Iodine turns blue-black when starch is present. Mix amylase with starch and a buffer, which is a liquid that keeps the pH steady. Every 30 seconds, put a drop on iodine. Note when it stops turning blue-black.
Repeat with buffers of different pH. Keep the temperature, enzyme volume and starch the same, so the test is fair.
A shorter time means a faster rate. Use rate = 1000 ÷ time in seconds. Starch gone in 50 s gives 1000 ÷ 50 = 20.
Enzymes are protein catalysts. They speed up reactions and are not used up.
The substrate fits the active site like a key in a lock, so each enzyme is specific.
Too hot, or the wrong pH, changes the active site. The enzyme is denatured.
Rate = 1000 ÷ time. A shorter time means a faster rate.
Name the part of an enzyme that the substrate fits into.
Answer: the active site
The time for amylase to digest all the starch at pH 7 is 40 s. Calculate the rate using rate = 1000 ÷ time.
Answer: 25
A student boils amylase, then adds it to starch. The starch is not broken down. Explain why.
Answer: The enzyme is denatured because the active site changes shape
Enzymes practice questions for GCSE Combined Science Biology. Try each one, then open the answer. Easy questions come first.
What type of molecule is an enzyme?
Answer: protein
Name the region of an enzyme where the substrate binds.
Answer: active site
What word describes an enzyme that has permanently changed shape and no longer works?
Answer: denatured
What colour does iodine solution go when starch is present?
Answer: blue-black
What is the name of the molecule that an enzyme works on?
Answer: substrate
What word describes the temperature at which an enzyme works fastest?
Answer: optimum
Starch takes 50 s to break down. Calculate the rate using rate = 1000 ÷ time.
Answer: 20
Starch takes 25 s to break down. Calculate the rate using rate = 1000 ÷ time.
Answer: 40
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
(a) Explain why an enzyme is specific to one substrate. [2 marks] (b) Explain why the rate of an enzyme reaction falls above the optimum temperature. [2 marks]
Answer: (a) the substrate has a shape that fits the active site of the enzyme, and no other shape fits (b) the enzyme is denatured because the bonds break and the active site changes shape, so the substrate no longer fits
A student investigates the effect of pH on the rate at which amylase breaks down starch. Describe how the student could do this practical. [4 marks]
Answer: Mix amylase and starch solution with a buffer of a set pH, in a water bath. Every 30 seconds put a drop of the mixture onto iodine on a spotting tile. Time how long until the iodine no longer turns blue-black. Repeat at other pH values and keep the temperature and volumes the same.
Saying a high temperature kills the enzyme
Enzymes are not alive. They are denatured, which means the shape of the active site changes.
Saying the enzyme is used up in the reaction
Enzymes are catalysts. They are not changed and can be used again.
Saying a higher temperature always speeds up the enzyme
It speeds up only to the optimum. Beyond that the enzyme is denatured.
Saying the substrate is the enzyme
The substrate is the molecule that the enzyme works on.
Enzymes is on both Foundation and Higher papers for GCSE Combined Science Biology, aimed at around grade 5.
Saying a high temperature kills the enzyme. Enzymes are not alive. They are denatured, which means the shape of the active site changes.
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.
See how the papers are set: AQA Combined Science papers and Edexcel Combined Science papers.
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