Chapter 3 Biochemical reactions in cells

Question 1

3.1 What are enzymes?

Answer 1

Enzymes are globular proteins that act as biological catalysts (they speed up chemical reactions without being used up in the process)

Question 2

3.1 What is the activation energy?

Answer 2

The activation energy refers to the minimum amount of energy needed to start a reaction.

Question 3

3.1 How do enzymes speed up chemical reactions?

Answer 3

The acitivation energy level acts as an energy barrier which must be inititally overcome before the reaction can proceed. Enzymes speed up chemical reactions by lowering this activation energy level (Ea) , which means enzymes allow reactions to take place at a lower temp. than usual.

Question 4

3.1 What is the structure of an enzyme?

Answer 4

• It is a globular protein, that has a specific 3D shape as a result of its primary structure. Also, although enzymes are a large molecule only a small region is functional which is known as the active site. The active site is made up of a small number of amino acids and forms a small hollow depression within the much larger enzyme molecule

Question 5

3.1 What is the substrate?

Answer 5

• The substrate refers to the molecule on which the enzyme acts on

Question 6

3.1 How do the substrate and enzyme interact with one another?

Answer 6

• The enzyme and substrate bind together at the active site to form an enzyme-substrate complex. The substrate molecule is held within the active site by bonds that temporarily form between certain amino acids of the active site and groups on the substrate molecule

Question 7

3.1 What is the lock and key model? (include how the model is supported)

Answer 7

• The lock and key model proposes that enzymes work in a similar way to how a key operates a lock.
• The key has a specific shape that compliments only one lock
• In a similar way, the substrate molecule has a specific shape that only compliments the active site of one enzyme
• This model is supported by the observation that ensymes are specific in the reactions they catalyse

Question 8

3.1 What are the limitations of the lock and key model?

Answer 8

• One limitation of the lock and key model is that the enzyme, like a lock, is considered to be a rigid structure. However, scientists have observed that other molecules could bind to enzymes at sites other than the active site, altering the enzyme’s activity. This suggests that the binding molecule alters the enzyme’s shape, leading to the conclusion that an enzyme’s structure is flexible rather than rigid. And has lead to another model being proposed to better explain enzyme action.

Question 9

3.1 What is the induced fit model?

Answer 9

The induced fit model proposes that although the enzyme has a general shape, it is flexible and its shape can be altered in the presence of a substrate molecule in order to fit its profile. This is simlar to how a glove which has a general shape can mould itself to fit a hand. Due to the enzyme’s shape altering, it puts a strain on the substrate molecule, distorting a particular bond, which lowers the activation energy needed to break it and start the reaction

Question 10

3.1 Why is the induced fit model a better explanation of enzyme action?

Answer 10

• Exaplains how the activation energy is lowered
• Explains how other molecules affect enzyme activity

Question 11

3.2 What does an enzyme need to function properly?

Answer 11

• It must come into physical conatct with its substrate
• It must have an active site which fits the substrate

Question 12

3.2 How do we measure the rate of an enzyme-controlled reaction?

Answer 12

We either measure the time course of :
- formation of the products of the reaction
- the disappearance of the substrate

Question 13

3.2 What is the explanantion of both of their graph shapes

Answer 13

Both have the same explanation:
1. At first there is a lot of substrate and no product
2. Therefore it is very easy for the substrate molecules to bind to the free active sites of enzyme molecules
3. The substrate molecules are then broken down, resulting in the production of product
4. Now there is a decrease in substrate, resulting in an increase in product
5. As the reaction proceeds, there is less and less substrate and more and more product
6. It becomes more difficult for the substrate molecules to come into contact with the enzymes due to there being fewer substrate molecules/ the product molecules “getting in the way”
7. Therfore takes longer for the substrate molecules to be broken down by the enzyme and the rate of dissappearance slows / rate of formation of product slows . Both graphs “tail off”
8. Rate of reaction continues to slow untill there is so little substrate that a decrease in concentration can’t be measured
9. Graphs flatten out because all the substrate is used up and so no new product is formed

http://biology4alevel.blogspot.com/2014/08/17-following-course-of-enzyme-catalysed.html

Question 14

3.2 What is the affect of temperature on enzyme action?

Answer 14

• As the temperature gets higher, the molecules gain kinetic energy and in this case, the enzyme and substrate molecule come together more often increasing the rate of enzyme action untill it reaches the optimum temp. ( Shown on a graph as a rising curve)
• After the optimum temp. an increase in temp. will result in the hydrogen and other bonds in the enzyme to begin to break. This causes the enzyme including its active site to change shape/beign to deanture. At first the substrate may fit less easily into the changed active site, slowing the rate of enzyme action
• However, as the temperature keeps increasing, the enzyme becomes denatured. This means it stops working altogether and the enzyme can’t function anymore because denaturation is a permeanant change (Shown in a graph as a falling curve)

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Question 15

3.2 Why is our body temp. at 37 when many enzymes in our body have a optimum temp of 40?

Answer 15

• Other proteins apart from enzymes may deanture at higher temperatures
• At higher body temperatures, any further increase in body temperature (e.g. due to illness) could lead to denaturation of enzymes
• Although higher body temp will increase the metabollic rate slightly, additional energy (food) would be required to maintain the higher temperature

Question 16

3.2 What is the affect of pH on enzyme action?

Answer 16

• Change in pH alters the charges on the amino acids that make up the active site. As a result, the substrate can no longer become attached to the active = the enzyme-substrate complex can’t be formed
• Change in pH can cause bonds that maintain the enzyme’s teritiary structure to break. The enzyme’s shape is altered which can alter the shape of the active site and the substrate molecule can no longer fit = ezyme has been denatured
• As pH fluctuations inside organisms are usually small, pH is more likely to reduce an enzyme’s activity than denature it.

Question 17

3.2 What is the effect of substrate concentration on enzyme action?

Answer 17

• If there is a fixed amount of enzymes and substrate molecules are slowly added, the rate of enzyme action increases in proportion to the amount of substrate molecules added
• When there is a low substrate conc.= too few molecules and so the enzymes are not working at full potential (there will be a decrease in possible max rate of enzyme action)
• When there is an intermediate substrate conc.= all the active sites are occupied and the rate of reaction reaches its maximum
• When there is high substrate conc.=The additition of further substrate will have no effect on enzyme action

https://zhtutorials.com/2021/05/15/factors-affecting-enzyme-activity/