Key Area 1 - Metabolic Pathways

Question 1

What is ‘cell metabolism’?

Answer 1

Cell Metabolism refers to all the chemical reactions that take place inside living cells. Organisms must control their own metabolism to stay alive.

Question 2

What are metabolic pathways?

Answer 2

Metabolic pathways are integrated (linked) and controlled pathways of enzyme-catalysed reactions within a cell. They start with a substrate and end with a product. These metabolic pathways can have reversible steps, irreversible steps and alternative routes.

Question 3

What 3 things can metabolic pathways have?

Answer 3

Reversible steps

Irreversible steps

Alternative routes

Question 4

What are anabolic reactions?

Answer 4

Anabolic reactions build up large molecules from small molecules.

Question 5

Do anabolic reactions require energy or release energy?

Answer 5

Anabolic reactions require energy.

Question 6

Give an example of an anabolic reaction.

Answer 6

Photosynthesis

Protein synthesis

DNA replication

Question 7

What are catabolic reactions?

Answer 7

Catabolic reactions break down large molecules into smaller molecules.

Question 8

Do catabolic reactions require energy or release energy?

Answer 8

Catabolic reactions release energy.

Question 9

Give an example of a catabolic reaction.

Answer 9

Digestion

Respiration

Degradation of hydrogen peroxide

Question 10

What is the function of protein pores embedded in the cell membrane?

Answer 10

Protein pores allow larger molecules and ions to be transported into or out of the cell. This is a passive process. Molecules and ions move from an area of high contraction to an area of low concentration.

Question 11

What is the function of protein pumps embedded in the cell membrane?

Answer 11

Pumps recognise specific ions and transfer them across the cell membrane against the concentration gradient. This requires energy in the form of ATP.

Question 12

Give 2 ways metabolic pathways are controlled.

Answer 12

Metabolic pathways are controlled by:

1. The presence or absence of particular enzymes.
2. ​​The regulation of the rate of reaction of key enzymes.

Question 13

Explain what is meant by the ‘induced fit’ model of enzyme activity.

Answer 13

When the substrate joins with an enzyme, the enzyme’s active site changes shape (slightly) to better fit the substrate after the substrate binds. This is called an induced fit. The induced fit ensures the active site is in contact with the substrates, increasing the chances of the reaction taking place.

Question 14

Why do substrates bind to the active site?

Answer 14

The substrate molecule(s) have a high affinity (chemical attraction) for the active site.

Question 15

What happens to the enzyme once the substrate binds?

Answer 15

Once the substrate binds, the enzyme’s active site changes shape slightly. The enzyme fits the substrates better and the activation energy is reduced.

Question 16

Sketch a graph to show the effect of enzymes on activation energy.

Question 17

Why do the products leave the enzyme?

Answer 17

The products have a low affinity (chemical attraction) for active site so they are released from the enzyme.

Question 18

Draw a diagram to illustrate the induced fit model of enzyme activity.

Question 19

Describe how the rate of enzyme controlled reactions is affected by substrate concentration.

Answer 19

The rate of enzyme reactions can be affected by the substrate concentration. As the substrate concentration increases the rate of reaction increases until all the active sites are occupied. At this point the enzyme is described as saturated.

Question 20

Sketch a graph to illustrate the effect of increasing substrate concentration on the rate of reaction.

Question 21

What is the ‘point of saturation’?

Answer 21

The point of saturation is the substrate concentration which results in all active sites being occupied.

Question 22

What effect does the presence of a substrate have on the direction that a reversible reaction is driven in?

Answer 22

The presence of a substrate will drive the reaction towards the product in a reversible reaction.

Question 23

What effect does the removal of a product have on the direction that a reversible reaction is driven in?

Answer 23

The removal of the product will drive the reaction towards the product in a reversible reaction.

Question 24

What effect does an increase in the concentration of the product have on the direction of reversible reactions?

Answer 24

A build up of the product will drive a reversible reaction towards the substrates.

Question 25

Name 3 types of enzyme inhibition.

Answer 25

Competitive inhibition Non-competitive inhibition Feedback inhibition

Question 26

Describe how competitive inhibitors work.

Answer 26

Competitive inhibitors bind directly to the active site and prevent the substrate from binding. This can be reversed by increasing substrate concentration.

Question 27

Explain why increasing the substrate concentration reversed the effect of competitive inhibitors?

Answer 27

At high substrate concentrations the ratio of substrate to inhibitor becomes high enough that the effect of the inhibitor is negligible.

Question 28

Describe how non-competitive inhibitors work.

Answer 28

Non-competitive inhibitors bind away from the active site but change the shape of the active site preventing the substrate from binding. This cannot be reversed by increasing substrate concentration.

Question 29

Sketch a graph to show the effect of competitive and non-competitive inhibitors on enzyme activity as substrate concentration increases.

Question 30

Describe how feedback inhibition works.

Answer 30

Feedback inhibition occurs when the end product in the metabolic pathway reaches a critical concentration. The end-product then inhibits an earlier enzyme, blocking the pathway, and so prevents further synthesis of the end-product. Only when the end product concentration decreases will the pathway continue again.