BIOLOGY - ORGANISATION

The human digestive system

The digestive system is an organ system - made up of organs working together to perform a function. The food we eat is insoluble and needs to be broken down into a form that can be absorbed by cells

Glands: ie the pancreas/salivary glands. They produce digestive juices containing enzymes that break down food

Stomach: produces hydrochloric acid to kill bacteria and provide optimum conditions for the protease enzyme to work

Small intestine: where soluble food molecules are absorbed into the blood

Liver: produces bile which is stored in the gallbladder, and helps the digestion of lipids

Large intestine: absorbs water from undigested food to produce faeces, which passes out of your body through rectum and anus

Enzymes

enzymes are biological catalysts that speed up the rate of a reaction without being used up

enzymes can break up large molecules and join up smaller ones

each enzyme has its own unique shaped active site where the substrate binds

enzmyes are present in many reactions so that they can be controlled

The Lock and Key Hypothesis:

  1. The shape of the substrate is complementary to the shape of the active site, so when they bond, it forms an enzyme-substrate complex
  2. Once bound, the reaction takes place and the products are released from the surface of the enzyme

Enzymes require an optimum pH (usually 7 but some in the stomach are lower) and temperature (around 37 degrees C)

The rate of reaction will increase as the temperature increases up to the optimum, but above this the temperature will rapidly decrease and the reaction will stop

When the temperature becomes too hot, the bonds in the enzyme structure will break. This changes the shape of the active site so the substrate will no longer fit. We say the enzyme is denatured

If the pH is too high or low, the forces that hold the amino acid chain will be affected, the active site shape will change (...) enzyme is denatured

Specific enzymes

Carbohydrases: break down carbohydrates into simple sugars

  • ie amylase, breaks down starch into maltose
  • produced in salivary glands, pancreas and small intestine

Proteases: break down proteins into amino acids

  • ie pepsin, produced in stomach
  • other proteases produced in pancreas and small intestine

Lipases: convert lipids into glycerol and fatty acids

  • produced in the pancreas and small intestine

Soluble glucose, amino acids, glycerol and fatty acids pass into the bloodstream to be carried to all cells in the body, to build new carbohydrates, lipids and proteins, and some glucose is used in respiration

Food tests

Iodine test: iodine solution tests for the presence of starch. It turns blue-black if starch is present

Benedicts test: Benedicts solution tests for the presence of sugar. It turns brick-red if sugar is present

Biuret test: Biuret solution tests for the presence of protein. It turns purple if protein is present

Emulsion test: To test for lipids, add ethanol. If lipids are present, a cloudy layer will form

Sudan III test: To test for lipids, add Sudan III solution. If lipids are present, a red layer forms on top

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Bile

Bile is produced in the liver and stored in the gallbladder

Bile is an alkaline to neutralise the hydrochloric acid that comes from the stomach, to provide a more optimum pH for enzymes in the small intestine

Bile emulsifies fats (breaks down large drops into smaller ones). This larger surface area allows lipase to break down lipids into glycerol and fatty acids faster