• Autotrophic – where an organism makes their own food via photosynthesis
  • Heterotrophic – where organisms feed off of autotrophs for energy
    • Holozoic feeders – where organisms eat and digest food internally
    • Saprophytes/saprobiontssecrete enzymes to digest extracellularly and then absorb the nutrients via diffusion (all fungi and some bacteria decomposers)
    • Parasites – highly specialised and feed on another living organism usually causing harm/death as a result

The digestive system

The gut is essentially a long hollow tube where food passes, to be broken down and its nutrients absorbed by the body. Some animals have a simple tube to digest one type of food. Other, more complicated animals, have specialised areas of the gut to complete different tasks.

  • Ingestion – taking food into the body through the mouth (eating)
  • Digestion – the chemical breakdown of large insoluble food molecules into simple soluble molecules via enzymes. In humans, we also have physical breakdown of food with our teeth mashing up food and our gut walls pushing and pulling on its contents.
  • Absorption – digested food passing through the gut wall to the blood
  • Egestion – the elmination of food that cannot be digested eg. cellulose

Structure of gut wall


Different enzymes are required to break down different types of food:

  • Carbohydrates -> glucose
    • Amylase: Polysaccharides -> disaccharides
    • Maltase: disaccharide -> monosaccharide
  • Proteins -> amino acids
    •  Peptidases
      • Endopeptidase: breaks peptide bonds within the protein
      • Exopeptidase: breaks peptide bonds at ends of protein
  • Fat -> fatty acids + glycerol
    • Lipase

The Mouth

  • Food is broken up by teeth and saliva (containing amylase)
    • The watery, alkaline nature of saliva helps to lubricate and create optimum conditions for amylase

The stomach

  • Holds food in gastric juices (secreted by glands in wall) via 2 muscle rings at top and bottom
  • Mixes food up with contractions
  • Low pH of 2 -> optimum enzyme conditions and kills many bacteria
  • Heavy mucus lining protects the body from acid and enzymes

Stomach and gland structure

The small intestine

  • Duodenum
    • 1st 20cm of small intestine
    • receives secretions from liver (bile) and pancreas (pancreatic juice)
    • also secretes alkaline juice and mucus itself
  • Bile
    • stored in the gall bladder
    • contains emulsifiers that break up fat globules into smaller droplets for easy access of lipase
    • also increases pH
  • Pancreatic juice
    • Secreted from exocrine glands
    • passes through pancreatic duct
    • Contains nearly all the enzymes (endopeptidase, amylase, lipase)

Small intestine pic (liver and pancreas)

Small intestine also have villi that increase surface area. At tips, all other enzymes are secreted (maltase, endopeptidase and exopeptidase)

  • Ileum
    • the rest of the small intestine, very long and tightly packed in
    • Villi and microvilli
    • Most absorption takes place (thus lots of mitochondria in cells!)
    • Glucose and amino acids are absorbed by diffusion through the villi and into the blood stream
      • They also has active transport to make up where diffusion cant keep up
    • Fatty acids pass into the lacteal (part of the lymph system) that eventually passes into the blood at the thoratic duct

The large intestine

  • 1.5M long
  • Caecum, appendix, colon and rectum areas
  • Mostly water, minerals and vitamins are absorbed
  • Bacteria make some of the vitamins we need (vitamin K, folic acid)
  • At the end, any undigested food (cellulose, bacteria, cells) then goes to the rectum as faeces to be defacated