Aerobic and anaerobic respiration: chemical reactions to build larger molecules

The idea

Respiration transfers energy by breaking down glucose, and this energy is used by cells for chemical reactions that build larger molecules from smaller ones. In animals, glucose molecules are joined to form glycogen for storage, and amino acids are joined to form proteins. In plants, glucose is combined to form cellulose for cell walls, starch for storage, and lipids for seeds. These synthesis reactions require energy, supplied by continuous cellular respiration, and are controlled by enzymes. For example, many glucose units link together to form a long starch or glycogen chain. This links respiration to growth, repair, and storage, and explains why cells that respire actively can also synthesise large molecules.

What earns the marks

• Respiration supplies the energy needed for reactions that build larger molecules from smaller ones.
• Glucose is joined together to form glycogen in animals, and to form starch or cellulose in plants.
• Amino acids are joined together to form proteins, including enzymes and structural proteins.
• Fatty acids and glycerol are joined together to form lipids, such as fats for storage and oils in plants.
• The products are used for growth, repair, storage, and the building of new cells and tissues.

Common mistakes and how to put them right

• Thinking respiration only breaks down molecules.

  Put it right: , respiration also provides the energy that powers building reactions such as protein synthesis.

• Confusing glycogen with glucagon.

  Put it right: , glycogen is the storage carbohydrate made from glucose, while glucagon is a hormone.

• Writing that plants make cellulose from amino acids.

  Put it right: , cellulose is built from glucose units, whereas amino acids build proteins.

Exam tips

• Name the small molecule and the large product in each example, for example glucose to glycogen or amino acids to protein.
• Link every building reaction back to energy transferred from respiration rather than describing it as spontaneous.
• Remember that enzymes control these synthesis reactions in both plants and animals.