AQA GCSE Biology: Metabolism

Metabolism is the sum of all the chemical reactions going on in a cell or a body. This short guide explains what metabolism means and gives the examples you need of reactions that build molecules up and break them down. The pattern to spot is that energy from respiration drives the building reactions.

What metabolism is

Metabolism is the sum of all the chemical reactions taking place in a cell or in the body. The metabolic rate is the overall rate at which these reactions happen.

Every one of those reactions is controlled by an enzyme. That is not a detail: it is why temperature and pH change how fast an organism runs, and why anything that denatures enzymes stops metabolism rather than just slowing it.

Metabolic reactions fall into two groups: those that build larger molecules from smaller ones, and those that break larger molecules into smaller ones.

The building reactions are the ones that need paying for. The energy transferred by respiration is what pays for them, which is why respiration runs continuously in every living cell rather than only when an organism is active. Nothing in a cell assembles a larger molecule for free.

Building molecules up

Examples of reactions that build larger molecules include:

• joining many glucose molecules to make starch (stored in plants), glycogen (stored in animals), or cellulose (in plant cell walls);
• combining one glycerol molecule with three fatty acids to make a lipid;
• combining glucose with nitrate ions to make amino acids, which are then built into proteins.

Three of those five products start from glucose, which is the thread running through the whole topic: photosynthesis makes it, and metabolism spends it. The one that does not is the lipid, and the numbers there are worth holding, because questions ask for them: one glycerol and three fatty acids.

Starch and glycogen are useful stores because they are insoluble, so they can hold a lot of glucose without lowering the water potential of the cell and drawing in water by osmosis. When energy is needed, they are broken back down into glucose.

Breaking molecules down

Examples of reactions that break molecules down include:

• respiration, in which glucose is broken down to release energy;
• breaking down excess proteins (in animals) to form urea, which is excreted by the kidneys.

Protein is the one thing on that list the body cannot store. Carbohydrate is kept as glycogen and fat is kept as fat, but surplus amino acids have nowhere to go, so they are broken down in the liver, the nitrogen-containing part becomes urea, and the blood carries it to the kidneys to be removed in urine. That journey is also covered in the blood guide, from the plasma's side.

Telling the two apart

Questions on this topic often give a reaction you have never seen and ask which kind it is. Look at the molecules rather than the names. If the product is larger than what went in, it is a building reaction and it needed energy from respiration. If the products are smaller, it is a breakdown and energy was released.

That test also catches the two reactions students most often place in the wrong group. Making starch from glucose builds, even though starch is a store and stores sound passive. Respiration breaks down, even though it is the reaction that makes everything else possible.

How this fits together

Metabolism ties Topic 4 together: photosynthesis and respiration are the central metabolic reactions, one building glucose and storing energy, the other breaking it down to release energy. These reactions supply the energy and raw materials for everything else the organism does.