Organisation

• A tissue is a group of cells with a similar structure or function.
• An organ is a group of tissues working together for a specific function.
• Organs are grouped into organ systems which work together to from organisms.
• Food contains three main nutrients: carbohydrates, protein and lipids. These are large molecules meaning they are too large to be absorbed into the blood stream, so they have to be digested.
• During digestion, large food molecules are broken down into small molecules by enzymes. The small molecules can then be absorbed into the bloodstream.
• Digestive System


  1. Food is chewed in the mouth

  2. Enzymes in the saliva begin to digest the starch into smaller sugar molecules

  3. Food passes down the oesophagus into the stomach

  4. In the stomach, enzymes begin the digestion of proteins

  5. The stomach also contains hydrochloric acid which helps the enzymes to digest proteins

  6. The churning action of the stomach muscles turns the food into a fluid, increasing the surface area for enzymes to digest

  7. The fluid now passes into the small intestine

  8. Chemicals are released into the small intestine from the liver and the pancreas

  9. The pancreas releases enzymes which continue the digestion of starch and protein. They also start the digestion of lipids

  10. The liver releases bile which helps to speed up the digestion of lipids. Bile also neutralises the acid released from the stomach

  11. The walls of the small intestine release enzymes to continue the digestion of protein and lipids

  12. In the small intestine, the small food molecules produced by digestion are absorbed into the bloodstream either by diffusion or active transport

  13. The fluid makes its way through the large intestine where water is absorbed into the bloodstream and the faeces is released from the body
• In the digestive system, large food molecules are digested into smaller molecules and then the products of digestion are absorbed into the bloodstream.
• The products of digestion are used by the body to build new carbohydrates, lipids and proteins.
• Enzymes catalyse chemical reactions.
• Enzymes are large protein molecules and they have a groove on their surface called the active site.
• The active site is where the substrate attaches.
• Enzymes are specific. The substrate must fit perfectly into the active site.
• Proteins are broken down by enzymes called proteases. We find these in the stomach, the pancreatic fluid and the small intestine.
• Proteins are long chains of chemicals called amino acids.
• When we digest proteins, the protease enzymes convert the protein back to the individual amino acids, which are then absorbed into the bloodstream.
• When amino acids are absorbed by body cells, they are joined together in a different order to make human proteins.
• Starch consists of a chain of glucose molecules.
• Carbohydrates are broken down by enzymes called carbohydrases. In the case of starch this is called amylase.
• When carbohydrates like starch are digested, we produce simple sugars.
• Amylase is found in the saliva and in pancreatic fluid.
• A lipid molecule consists of a molecule of glycerol attached to three molecules of fatty acids.
• Lipid molecules are digested by the enzyme lipase. This produces glycerol and fatty acids.
• Lipase is found in pancreatic fluid and in the small intestine.
• Bile is made in the liver and it is stored in the gall bladder.
• Bile helps to speed up the digestion of lipids but bile is not an enzyme.
• Bile converts large lipid droplets into smaller droplets. It emulsifies the lipid which increases the surface area of lipid droplets and increases the rate of lipid breakdown by lipase.
• Bile is alkaline, which allows it to neutralise stomach acid, creating alkaline conditions in the small intestine. This increases the rate of lipid digestion by lipase.
• As we increase temperature, the activity of enzymes increases because the enzyme and the substrate begin moving faster so there are more collisions per second between the substrate and the active site
• At a certain temperature, the enzyme is working at the fastest possible rate. This is called it's optimum temperature. At this point, there is the maximum frequency of successful collisions between the substrate and the active site.
• As we increase temperature past the optimum, the activity of the enzyme rapidly decreases to zero. At high temperatures, the enzyme molecule vibrates and the shape of the active site changes, meaning the substrate no longer fits perfectly into the active site as it is denatured.
• Enzymes have an optimum pH where the activity is at its maximum. If we make the pH more acidic or more alkaline then the activity drops to zero because the active site denatures if the conditions are too acidic or too alkaline.
• The human small intestine has a length of approx. 5m. This provides a very large surface area for absorption of the products of digestion.
• The interior of the small intestine is covered with millions of villi. These increase the surface area for the absorption of molecules. Microvilli on the surface increase surface area even further. Villi have a good blood supply so the bloodstream rapidly removes the products of digestion, this increases the concentration gradient. The villi also have a thin membrane, ensuring a short diffusion path. Any molecules that cannot be absorbed by diffusion, are absorbed by active transport.
• The problem with a single circulatory system is that the blood loses a lot of pressure before it passes to the organs, so the blood travels to the organs slowly and it cannot deliver a lot of oxygen.
• Humans have a double circulatory system, where blood is pumped from the heart to the lungs and is oxygenated before returning to the heart. The heart now pumps the oxygenated blood to the organs where the oxygen is transferred to the body cells and is then taken back to the heart.
• The benefit of a double circulatory system is that because blood travels through the heart twice, it can travel rapidly to the body cells.
• The heart is an organ consisting mainly of muscle tissue.
• The heart has four chambers: the right atrium, left atrium, right ventricle and left ventricle. The atria are separated from the ventricles by valves.
• 1. The deoxygenated blood passes into the heart through the vena cava.
  2. The blood now passes from the heart to the lungs in the pulmonary artery, the blood collects oxygen in the lungs.
  3. Oxygenated blood passes from the lungs to the heart in the pulmonary vein.
  4. Oxygenated blood is pumped from the heart to the body in the aorta.
• Pattern of blood flow in the heart:

  1. First, blood enters the left atrium and the right atrium.
  2. The atria contract and the blood is forced into the ventricles.
  3. The ventricles now contract and force blood out of the heart.
  4. The valves prevent backflow into the atria when the ventricles contract.