4.4.2.3 - Metabolism

What metabolism means

Cells are constantly carrying out chemical reactions. Some reactions build larger molecules from smaller molecules; these are synthesis reactions. Other reactions break molecules down, for example to transfer energy or to form a waste product that can be excreted.

Most metabolic reactions are controlled by enzymes. Enzymes make the reactions happen fast enough at body or cell temperature, and each enzyme controls a specific reaction or set of reactions.

Respiration is part of metabolism because it is a reaction in cells. The energy transferred by respiration is used for continual enzyme-controlled processes, including the synthesis of new molecules. AQA often rewards the phrase energy transferred, so avoid saying that respiration "makes energy".

Sugars and carbohydrates

Sugars are small carbohydrate molecules. The sugar named most often in this part of GCSE Biology is glucose.

Glucose is important because it can be built into larger carbohydrate molecules:

These are synthesis reactions because smaller glucose molecules are used to make larger carbohydrate molecules. Carbohydrates can also be broken down. The key breakdown reaction to link here is respiration, where glucose is broken down in cells and energy is transferred for living processes.

Do not treat "sugar" and "energy" as the same thing. Glucose is a molecule. Respiration transfers energy from glucose in a reaction; that transferred energy can then be used for other metabolic reactions.

Lipids, proteins and urea

Lipids and proteins also fit the same build-and-breakdown idea. The important GCSE detail is the smaller molecules used to build them.

Lipids are formed from glycerol and fatty acids.

glycerol + three fatty acids -> lipid molecule

That "three" matters. AQA expects the formation of a lipid molecule from one molecule of glycerol and three molecules of fatty acids. If lipids are broken down, fatty acids and glycerol are the important smaller molecules to name at this level.

Proteins are built from amino acids. In plants, glucose and nitrate ions are used to form amino acids; those amino acids are then used to synthesise proteins.

glucose + nitrate ions -> amino acids -> proteins

Proteins can also be broken down in metabolism. One AQA named link is that excess proteins are broken down to form urea for excretion. You do not need detailed urea-cycle chemistry here; the AQA-level link is excess protein breakdown to urea.

Named reactions linked together

The metabolism section acts like a summary map. The individual reactions are taught in more detail elsewhere, but here you must be able to link the named molecules and processes.