The Digestive system

Remember it!

Remember it!

One interface with the environment is the digestive system. The digestive system is composed of the:

  • oesophagus
  • stomach
  • small intestine
  • large intestine
  • rectum

It’s also composed of two glands:

  • the salivary glands
  • the pancreas

The process of digestion involves large molecules being broken down into smaller ones in a process known as hydrolysis. These smaller molecules can than then be absorbed by the body and assimilated.

See below for a summary of the key notes on the Digestive System for your As and A level biology revision :



  • Digestion involves hydrolysis, absorption and assimilation.
  • Proteins are composed of amino acids
  • They have four structures: primary, secondary, tertiary and quaternary.
  • A protein can be either globular or fibrous.
  • The biuret test can be used to test for proteins.
  • Enzymes are biological catalysts which lower the activation energy required for a reaction to take place.
  • Enzymes are composed of an active site which is specific to a substrate. They bond to form an enzyme-substrate (ES) complex which becomes an enzyme-product (EP) once the reaction is complete.
  • The lock-and-key hypothesis stated that the enzyme and substrate fit exactly together.
  • The induced-fit hypothesis is more widely accepted and states that the enzyme changes shape slightly to allow the substrate to fit.
  • An enzyme rate of reaction can be affected by temperature, pH level and substrate concentration.
  • Inhibitors slow down the rate of reaction.
  • Carbohydrates are composed of monsaccharides which contain carbon, hydrogen and oxygen.
  • ?-glucose molecules bond together via glycosidic bonds produced through condensation.
  • Someone who’s lactase intolerant cannot produce enough lactase.
  • Benedict’s reagent can be used to test for reducing sugars.
  • Starch can be tested for with iodine.
  • Eukaryotic cells are made up of a number of different organelles.
  • Scanning electron microscopes provide high magnification and resolution.
  • Cell components can be separated using ultracentrifugation.
  • The Fluid Mosaic Model describes the structure of plasma membranes which are composed of phospholipids, proteins and carbohydrates.
  • Phospholipids are composed of a hydrophilic, phosphate head and two hydrophobic, fatty acid tails.
  • Lipids can be test for using the ethanol emulsion test.
  • Proteins have a number of roles: transport, structural, recognition and as enzymes.
  • Substances can travel across the membrane by passive transport (simple diffusion, facilitated diffusion or osmosis) or active transport.
  • Starch is broken down by enzymes into glucose before being absorbed into the blood stream.
  • Glucose is absorbed by coupled active transport.
  • Cholera affects the small intestine causes the sufferer to dehydrate by losing too much water through diarrhoea.
  • ORS are sugar and salt solutions which can be used to rehydrate cholera sufferers.