Metabolism (AQA GCSE Biology): Revision Note

Metabolism: Basics

• Metabolism can be defined as:

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

• Energy released during respiration allows enzyme-controlled reactions to occur inside cells, e.g. to produce new molecules

Synthesis & breakdown of molecules

• Organisms gain organic molecules either by consuming food, e.g. in animals, or carrying out photosynthesis, e.g. in plants

• The molecules gained can then be broken down and used to synthesise new molecules

• Metabolic reactions include the synthesis and breakdown of:

  • carbohydrates

    • Dietary carbohydrates are digested into sugars, e.g. glucose

  • lipids

    • Dietary lipids are broken down into fatty acids and glycerol

  • proteins

    • Dietary proteins are broken down into amino acids

Examples of metabolic reactions

Carbohydrates

• Glucose is produced during digestion (animals) or photosynthesis (plants)

• Glucose is used in the synthesis of:

  • starch; energy storage in plants

  • glycogen; energy storage in animals

  • cellulose; strengthens plant cell walls

• Glucose is broken down during respiration to release energy

Lipids

• Fatty acids and glycerol are produced during digestion (animals) or from the products of photosynthesis (plants)

• Glycerol can be combined with three fatty acids in the synthesis of new lipids, which can be used in energy storage

Proteins

• Amino acids come from the breakdown of dietary proteins (animals) or are produced from glucose and nitrates (plants)

• Amino acids are used in the synthesis of new proteins

• Excess proteins are broken down to produce urea, which is excreted from the body

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