Digestion

• Principles of Organisation

  Cells make up all living things.

  A tissue is a group of specialised cells with a similar structure and function e.g. muscular and epithelial tissue.

  Organs are formed from a number of different tissues, working together to produce a specific function e.g. the stomach is made of muscular and epithelial tissue.

  Organs are organised into organ systems, which work together to perform a specific function e.g. the digestive system is made up of organs like the stomach, liver and small intestine.
• Digestion

  Process by which large, insoluble food molecules e.g. starch and protein are broken down into smaller, soluble molecules e.g. glucose and amino acids that can be absorbed by the bloodstream and delivered to the cells.
• Components of digestive system

  1: Mouth

  2: Oesophagus

  3: Stomach

  4: Small intestine

  5: Large intestine

  6: Pancreas

  7: Liver
• Mouth

  Where mechanical digestion (chewing) takes place.

  Teeth break food down into smaller pieces.

  Salivary glands secrete saliva which contains amylase to break down starch.
• Oesophagus

  Tube connecting mouth to stomach; delivers chewed food.
• Stomach

  Food is mechanically digested by the contractions of muscular tissue causing churning.

  Protease enzymes present to break down proteins into amino acids.

  Hydrochloric acid present to kill bacteria in food and provide optimum pH for protease enzymes.
• Small intestine

  Digested food molecules are delivered here for absorption into the bloodstream.

  Lined with villi to increase surface area over which absorption takes place.
• Large intestine

  Water is absorbed from remaining material to form faeces.

  Faeces are stored in rectum and expelled by the anus.
• Pancreas

  Produces digestive enzymes: Amylase, Protease and Lipase.
• Liver

  Produce bile to emulsify (break down) lipids.

  Amino acids that aren't used are broken down (deamination) which produces urea as a waste product.
• Enzymes

  Biological catalysts that are made up of a single protein molecule.

  Increase the rate of reaction without being used up in the process.
• Lock and Key Theory

  Shape of an enzyme is vital to its function.

  This is because each enzyme has a uniquely shaped active site where the substrate binds.

  The shape of the substrate is complementary to the shape of the active site so when they bind it forms an enzyme-substrate complex.

  Reaction then takes place and products are released.
• Optimum temperature for enzymes

  Around 37 degrees.

  Rate of reaction increases with an increase in temperature up to this optimum.

  Above this temperature the rate of reaction rapidly decreases and stops.

  When temperature is too hot, bonds in the structure will break.

  Enzyme is denatured and can no longer function.
• Optimum pH for enzymes

  7 for most enzymes, with exception of enzymes in stomach, which require acidic conditions.

  If pH is too high or too low, forces that hold the amino acid chain together are affected.

  This changes shape of the active site so substrate can no longer fit in.

  Enzyme is denatured and no longer functions.
• Types of digestive enzyme

  Carbohydrases convert carbohydrates into simple sugars.

  Proteases convert proteins into amino acids.

  Lipases convert lipids into fatty acids and glycerol.
• Food tests PRACTICAL

  Benedict's solution to test for sugars (turns brick-red if present).

  Iodine solution to test for starch (turns blue-black if present).

  Biuret solution to test for protein (turns purple if present).

  Ethanol to test for lipids (results in a cloudy emulsion if present).
• Role of bile

  Produced in the liver and stored in the gallbladder.

  Is alkaline to neutralise hydrochloric acid from the stomach.

  Breaks down large drops of fat into smaller ones (emulsification) increasing surface area; allows for lipases to break lipid down into fatty acids + glycerol faster.
• Enzyme PRACTICAL

  Place single drops of iodine solution onto a tile.

  Place 2cm^3 of amylase in the test tube of the pH you're testing.

  Place 1cm^3 of buffer solution into the test tube.

  Add 2cm^3 of starch solution and start the stopwatch whilst mixing.

  After 10 seconds, place one drop of the mixture on the first drop of iodine; should turn blue back.

  Wait another 10 seconds and place another drop of mixture on the next drop of iodine.

  Repeat every 10 seconds until iodine solution remains orange-brown.

  Repeat experiment at different pH values.