How are the chemical reactions of metabolism organised and controlled?
Metabolic pathways as integrated networks of enzyme-controlled reactions, anabolic and catabolic reactions, the control of pathways by enzymes, induced fit, competitive and non-competitive inhibition, feedback inhibition, and the role of membranes in metabolism.
An SQA Higher Biology answer on metabolic pathways and their control, covering anabolic and catabolic reactions, enzyme action and induced fit, competitive and non-competitive inhibition, feedback inhibition, and the role of membranes in metabolism.
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What this key area is asking
The SQA wants you to describe metabolism as an integrated network of enzyme-controlled pathways, distinguish anabolic from catabolic reactions, explain how enzymes work by induced fit, explain competitive, non-competitive and feedback inhibition, and describe the role of membranes in metabolism.
Anabolic and catabolic reactions
Pathways are integrated and controlled, with reversible steps, irreversible steps and alternative routes that let a cell respond to its needs. Reversible steps can run in either direction depending on the concentrations of reactants and products, while irreversible steps act as control points and one-way valves. Alternative routes let a cell switch a substrate towards whichever product is needed at the time.
Enzymes and induced fit
Because the active site moulds around the substrate, the fit becomes very precise, which makes enzymes highly specific to their substrates. Lowering the activation energy means the reaction can happen at body temperature and at a useful rate, which would otherwise be far too slow.
Competitive and non-competitive inhibition
A non-competitive inhibitor binds to a site away from the active site, changing the shape of the active site so the substrate no longer fits. Increasing substrate concentration does not reverse this effect, because the active site has been altered rather than just blocked. Many poisons and some drugs act as non-competitive inhibitors of specific enzymes.
Feedback inhibition
This prevents the cell from wastefully producing too much of a substance and is a non-competitive effect, because the end product binds away from the active site of the early enzyme. As the end product is used up, the inhibition is lifted and the pathway runs again, so the cell automatically matches production to demand.
The role of membranes
Membranes provide a large surface area for reactions and the proteins (such as enzymes and carriers) involved, and they create compartments so that different reactions can be kept separate and organised efficiently. For example, the inner membrane of the mitochondrion holds the electron transport chain and is highly folded to increase its surface area.
Examples in context
Example 1. Statins as competitive inhibitors. Statins are drugs used to lower blood cholesterol. They work by competitively inhibiting an enzyme (HMG-CoA reductase) that controls an early step in the pathway that makes cholesterol. By competing with the substrate for the active site, statins slow cholesterol production, showing how understanding enzyme inhibition leads directly to important medicines.
Example 2. Feedback inhibition of amino acid synthesis. Bacteria make the amino acid isoleucine through a short pathway. When enough isoleucine has been produced, it binds to and inhibits the first enzyme in its own pathway, switching production off. When isoleucine is used up in making proteins, the inhibition is released and synthesis restarts. This classic example shows feedback inhibition preventing the waste of resources.
Try this
Q1. State the difference between an anabolic and a catabolic reaction. [2 marks]
- Cue. Anabolic builds larger molecules and uses energy; catabolic breaks molecules down and releases energy.
Q2. Explain how feedback inhibition controls a metabolic pathway. [2 marks]
- Cue. The end product inhibits an early enzyme, stopping further production once enough has been made.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20184 marksExplain the difference between competitive and non-competitive inhibition, and explain how the effect of each is influenced by substrate concentration.Show worked answer →
A 4-mark answer needs where each inhibitor binds and the effect of substrate.
A competitive inhibitor binds to the active site of the enzyme, so it competes directly with the substrate. Increasing the substrate concentration makes it more likely that substrate, rather than inhibitor, binds, so the inhibition can be reversed.
A non-competitive inhibitor binds to a site away from the active site, changing the shape of the active site so the substrate no longer fits. Increasing the substrate concentration does not reverse this, because the active site itself has been altered.
Markers reward (1) competitive binds the active site, (2) it is overcome by more substrate, (3) non-competitive binds elsewhere and changes the active site and (4) more substrate does not reverse it.
SQA Higher 20213 marksDescribe how feedback inhibition controls a metabolic pathway, and explain why this benefits the cell.Show worked answer →
A 3-mark answer needs the mechanism plus the benefit.
In feedback inhibition, the final product of a metabolic pathway binds to and inhibits an enzyme early in the pathway. This is a non-competitive effect, because the end product binds away from the active site.
As a result, when enough end product has been made, the pathway is switched off. When the product is used up, the inhibition is lifted and the pathway runs again.
This benefits the cell because it prevents the wasteful overproduction of substances and saves resources and energy.
Markers reward the end product inhibiting an early enzyme, the switching off of the pathway, and the benefit of preventing waste.
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Sources & how we know this
- SQA Higher Biology Course Specification — SQA (2018)