metabolism

• metabolism is all the chemical reactions in an organism
• in a cell there are lots of chemical reactions happening all the time, which are controlled by enzymes
• many of these reactions are linked together to form bigger reactions:

  $reactant -> product -> product -> product$
• in some of these reactions, larger molecules are made from smaller ones
• examples of large molecules made from smaller ones:

  • lots of small glucose molecules are joined together in reactions to form starch (a storage molecule in plant cells), glycogen (a storage molecule in animal cells) and cellulose (a component of plant cell walls)
  • lipid molecules are each made from one molecule of glycerol and three fatty acids
  • glucose is combined with nitrate ions to make amino acids, which are then made into proteins
• in other reactions, larger molecules are broken down into smaller ones
• glucose is broken down in respiration, respiration transfers energy to power all the reactions in the body that make molecules
• excess protein is broken down in a reaction to produce urea - urea is then excreted in urine
• the sum (total) of all the reactions that happen in a cell or the body is called the metabolism