Unit 2 KA 1

Question 1

Metabolism

Answer 1

Metabolism is the sum of all the chemical reactions that take place within a cell.

Question 2

Metabolic Pathways

Answer 2

Metabolic pathways are integrated and controlled pathways of enzyme- catalysed reactions within a cell

There are two types of reactions within a metabolic pathway:-anabolic &
catabolic.

Metabolic pathways are controlled by enzymes which catalyse specific reactions. Some steps will be irreversible; some reversible. Some metabolic pathways may have alternative routes which allow steps in the pathway to
be bypassed.

Question 3

Anabolic Pathways

Answer 3

Involve the biosynthesis (build-up) of large molecules from small molecules

e.g. proteinsynthesis

This requires energy.

Question 4

Catabolic pathways

Answer 4

Involve the breakdown of large molecules into smaller molecules.

e.g. aerobic respiration

This involves the release of energy.

Question 5

cell membrane structure

Answer 5

Membrane is composed of protein and phospholipid molecules.
The phospholipids form a double layer (phospholipid bilayer). The phospholipids are constantly moving giving the membrane a fluid nature and allowing it to be flexible.
The proteins are scattered in a patchy mosaic pattern within this bilayer.
The membrane structure is sometimes referred to as the fluid mosaic model because of its fluid nature and arrangement of proteins.

Question 6

Channel-forming proteins

Answer 6

Contain pores which control the diffusion of small molecules across the membrane (down a concentration gradient). These pores make the membrane selectively permeable.

Question 7

Specialised protein pumps(carrier protein)

Answer 7

Recognise specific ions (e.g. sodium, potassium) and transfer them across the membrane by active transport. Since this movement is against the concentration gradient, energy from ATP is required.

Question 8

Control of Metabolic Pathways

Answer 8

Metabolic pathways are controlled by the presence or absence of particular enzymes and the regulation of the rate of reaction of key enzymes.

If any of the enzymes involved in a metabolic pathway is absent or non- functional (due to a gene mutation), then a metabolic block occurs.

Question 9

Activation Energy and Enzyme Action

Answer 9

The energy needed to start a chemical reaction by breaking chemical bonds is known as the activation energy. Enzymes speed up the rate of a reaction by lowering its activation energy.

Question 10

Induced Fit

Answer 10

Each enzyme has an area on its surface called an active site.
Each enzyme’s active site has a particular shape and is complementary to one substrate only (like a lock and a key). For this reason, enzymes are said to be specific.
A substrate molecule(s) have a high affinity for an active site – it is chemically attracted to it.
The active site is flexible and when the substrate binds to it, the active site changes shape to better fit the substrate and increases the chance of a reaction occurring. This is called induced fit.

The shape of the active site determines the orientation of the substrate. This ensures that it held together in such a way that the reaction can take place by reducing the activation energy.
After the reaction, the product(s) has a low affinity for the active site,
allowing them to leave the active site. The active site returns to its original shape and the enzyme is now free to repeat the process with other substrate molecules

Question 11

Factors Affecting Enzyme Action

Answer 11

 a suitable temperature
 a suitable pH
 a supply of the appropriate substrate

Question 12

Substrate Concentration

Answer 12

At low concentrations of substrate, reaction rate is low as there are too few substrate molecules present to bind with all the enzymes’ active sites.
An increase in substrate concentration results in an increase in rate as more and more active sites become bound with the substrate.
When all the active sites are bound to the substrate, the rate remains steady and the graph levels off – the enzyme concentration is a limiting factor.

Question 13

Reversibility

Answer 13

Some metabolic reactions are reversible and the presence of a substrate or the removal of a product will drive a sequence of reactions in a particular direction

Question 14

Control of Metabolic Pathways by Regulating Enzyme Action

Answer 14

Some metabolic pathways have to operate continuously e.g. those for respiration.
The genes which produce these enzymes therefore are always switched on and these enzymes are always present in the cell.
These enzymes are controlled through the regulation of their rates of reaction.

Question 15

Effect of Inhibitors

Answer 15

Metabolic pathways can also be controlled by inhibitors. Inhibitors decrease the rate of an enzyme-controlled reaction.
There are 3 types of inhibitors:-
 Competitive
 Non-competitive
 Feedback inhibition

Question 16

Competitive inhibitor

Answer 16

Competitive inhibitors are so similar in shape to the substrate that they can fit into the enzyme’s active site.
Competitive inhibitors bind at the active site preventing the substrate from binding to the enzyme. Thus the rate of reaction is reduced.
Competitive inhibition can be reversed by increasing the substrate concentration.

As the number of substrate molecules begin to outnumber those of the competitive inhibitor, the chances of the substrate binding to the active sites instead of the inhibitor is increased. The reaction rate increases until all the active sites are occupied, mostly by substrate.

Question 17

Non competitive inhibitor

Answer 17

Non-competitive inhibitors have a different shape to that of the substrate.
Active site has changed shape
Non-competitive inhibitor
They bind away from the active site but change the shape of the active site so that it is unable to combine with the substrate.
The more enzyme molecules affected, the slower the reaction. The effect is permanent and cannot be reversed. Increasing the substrate concentration has no effect.

Question 18

Feedback inhibition

Answer 18

Feedback inhibition occurs when the end product in the metabolic pathway reaches a critical concentration.
This prevents the pathway from proceeding. The end-product then inhibits an earlier enzyme, blocking the pathway, and so prevents further synthesis of the end-product.
However, when the concentration of the end-product decreases, fewer of the enzymes at the start are affected and the pathway can then begin to proceed as normal.
This control avoids the build-up of end product and wasteful conversions