The Digestive System

The Alimentary canal

  • A long tube going from the mouth to the anus

  • Along the tube there are glands where substances called enzymes are produced

  • The intake of food has 5 steps:

Ingestion - the intake of food

Digestion - the physical or chemical breakdown of food, so it can be absorbed into the bloodstream

Absorption - the release of digested food into the blood stream

Assimilation - the use of digested food in the organisms cells

Egestion - the removal of indigestible waste materials from the body


Enzymes

  • Enzymes are biological catalysts

  • Enzymes speed up the breakdown of food substances and other materials

  • They work by combining with the food (substrate) forming the enzyme-substrate complex

  • The enzyme-substrate complex breaks down and forms the product and releases the enzyme

  • The enzyme can the be used again

  • Enzymes are very specific and will only work on one substrate

  • Enzymes work at specific pH’s

  • Most at pH 7 but pepsin works only at pH 2 & 3

  • Enzymes are made from proteins, so if heated will become denatured, and will not work


The mouth

  • Food is mechanically broken down by the teeth, increasing the surface area for the enzyme to work

  • Saliva (salivary amylase) breaks down starch and maltose

  • Food is brought down to the stomach, in a structure called a bolus through the oesophagus by peristalsis

  • Physical and chemical digestion 


The stomach

  • The stomach contains Hydrochloric Acid (pH - 1-2)

  • Hydrochloric acid loosens food, kills bacteria, triggers stomach enzymes and turns off amylase

  • Inactive pepsinogen is activated by the low pH turning it into pepsin (proteinase)

  • Proteinase breaks down proteins into amino acids

  • The stomach churns the food for a number of hours until the food is a thick sloppy liquid called Chyme

  • Sphincter muscles relax and the chyme enters the duodenum

  • Physical digestion by the churning process, chemical digestion by the action of enzymes




The duodenum

  • Lots of enzymes are secreted from the wall of the duodenum, the pancreas and the gall bladder

  • Enzymes including lipase, proteinase (trypsin), amylase, maltase, sucrase, and lactase are found there

  • Bile is also secreted from the gall bladder

  • Bile emulsifies fat, so it can be digested

  • Bile also neutralises the stomach acids


The ileum (small intestine)

  • The ileum contains the same enzymes as the duodenum

  • The wall of the intestine contains millions of finger like projections called villi (villus) so the surface area is increased

  • Sugars, amino acids, vitamins, minerals and water are absorbed into the blood stream

  • Fatty acids and glycerol are transported by the lymph vessels within the villus itself (via lacteal)


The large intestine

  • Here, any remaining broken down food is absorbed

  • Water is removed from the waste

  • Excess substances are moved to other parts of the body for storage

  • Excess glucose is stored when too much is ingested than needed

  • Glucose can be stored in the short term as glycogen. It is stored in the liver

  • The long term storage of glucose fat

  • Faeces are formed and can move to the rectum before being egested from the anus