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

Metabolic pathways can have reversible steps, irreversible steps and alternative routes.

Reactions within metabolic pathways can be anabolic or catabolic. 

• Anabolic reactions build up large molecules from small molecules and require energy. 

• Catabolic reactions break down large molecules into smaller molecules and release energy.

Protein pores, pumps and enzymes are embedded in membranes.

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

Induced fit and the role of the active site of an enzyme in affecting activation energy and the affinity of the substrate and products for the active site.

• Induced fit occurs when the active site changes shape to better fit the substrate after the substrate binds.

The effects of substrate and product concentration on the direction and rate of enzyme reactions.

• The substrate molecule(s) have a high affinity for the active site and the subsequent products have a low affinity, allowing them to leave the active site.

• 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.

Control of metabolic pathways through competitive, non-competitive and feedback inhibition of enzymes.

• Competitive inhibitors bind at the active site preventing the substrate from binding. 

• Competitive inhibition can be reversed by increasing substrate concentration.

• Non-competitive inhibitors bind away from the active site but change the shape of the active site preventing the substrate from binding. 

• Non-competitive inhibition cannot be reversed by increasing substrate concentration.

• 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.