digestion

• Digestion begins in the mouth, where food is chewed and mixed with saliva.
• Lactase is an enzyme produced by the small intestine that breaks down lactose into glucose and galactose.
• The stomach secretes hydrochloric acid to kill bacteria and denature proteins.
• Salivary amylase breaks down carbohydrates into simple sugars.
• The stomach is the organ that receives food from the esophagus, mixes it with gastric juices to form chyme, and stores it until it can be further processed by the small intestine.
• digestion is the breaking down of food we eat into other substances that our bodies absorb and use
• Stages of digestion

  Different things happen to food as it passes through the digestive system:

  • food is digested in the mouth, stomach and small intestine
  • digested food is absorbed into the blood stream in the small intestine
  • excess water is absorbed back into the body in the large intestine
  • any undigested food passes out of the anus as faeces (or poo) when we go to the toilet
• the digestive system goes: mouth, esophagus, stomach, small intestine, large intestine, rectum, anus
• The liver produces bile, which helps the digestion of lipids (fats and oil).
• The pancreas produces biological catalysts called digestive enzymes which speed up the digestive reactions.
• Food is moved through the digestive system by a process called peristalsis. Two sets of muscles in the gut wall are involved, they work together to produce wave-like contractions. These have a ‘squeezing action’ that pushes the bolus through the digestive syste
• Food has to be broken down chemically into really small particles before it can be absorbed. This is called chemical digestion
• Different types of enzymes can break down different nutrients:

  • amylase and other carbohydrase enzymes break down carbohydrates into sugar e.g. starch into glucose.
  • protease enzymes break down proteins into amino acids.
  • lipase enzymes break down lipids (fats and oils) into fatty acids and glycerol.
• different types on enzymes that break down certain type of food into smaller particles

  
• Digestion of fat in the small intestine is helped by bile, made in the liver. Bile breaks the fat into small droplets that are easier for the lipase enzymes to work on. Bile is not an enzyme.
• Minerals, vitamins and water are already small enough to be absorbed by the body without being broken down, so they are not digested.
• Digested food molecules are absorbed in the small intestine. This means that they pass through the wall of the small intestine and into our bloodstream.
• The inner wall of the small intestine has adapted so that substances pass across it quickly and efficiently. It:

  • has a thin wall, just one cell thick, so there is a short diffusion distance
  • is long (over 6 m), to increase surface area
  • has many tiny villi (small, slender, projections), to increase surface area
  • has good blood supply, to create a high diffusion gradient
  • is permeable, to allow molecules to pass through easily
• Excess water is absorbed back into the body in the large intestine.
• undigested food is stored in the rectum
• Take care not to confuse egestion (removal of undigested food from the body) with excretion (removal of waste products from chemical reactions from the body).