Metabolism (AQA GCSE Biology: Combined Science)

What is metabolism?

• Metabolism can be defined as

the sum of all the reactions in a cell or the body

• Metabolic reactions happen all of the time powered by energy transferred by respiration in cells
• Enzyme-controlled processes use this energy in metabolism to synthesise new molecules
• Metabolic reactions are often linked, where the products of one reaction are the reactants for another

• Metabolic reactions include the synthesis and breakdown of carbohydrates, lipids and proteins 
  • Glucose (a sugar) is used in the synthesis of starch, glycogen or cellulose
  • Fatty acids and glycerol are used in the synthesis of lipids
  • Amino acids are used in the synthesis of proteins
• These substances act as substrates in many key reactions

Metabolic reactions

The products of digestion are used to build substances, and some of these are then broken down – this is metabolism

• Metabolism includes:
  • Conversion of glucose to cellulose in plants to build and strengthen cell walls
  • Conversion of glucose into glycogen in animals and starch in plants for storage
  • The formation of lipid molecules from a molecule of glycerol and three molecules of fatty acids to form triglycerides which are used for energy storage and as insulation in animals
  • In plants: the use of glucose and nitrate ions to form amino acids which in turn are used to synthesise proteins required by cells (such as enzymes)
  • Glucose is broken down in the process of respiration to release energy in all cells
  • In animals, the breakdown of excess proteins to form urea for excretion