Digestion in Humans

• Digestion is the process of breaking down large insoluble food molecules into smaller soluble food molecules that can be absorbed by the body.
• Ingestion: taking food into the body via the mouth
• Mechanical digestion: teeth chop up food - large food molecules are broken down into smaller food molecules
• Chemical digestion: enzymes break down large insoluble food molecules into smaller soluble food molecules
• Absorption: small soluble molecules taken from gut into the blood stream - transported into all body cells
• Egestion: undigested food forms faeces - removed from body via the anus
• Mouth: The opening of digestive system where food is chewed up by teeth (breaking up food molecule), salivary glands make saliva which contains carbohydrase and also mucus to lubricate food making it easier to swallow. pH 7
• Oesophagus: Part which connects the mouth to the stomach - peristalsis happens here, muscle in the gut wall contract and push food (bolus) along into stomach.
• Stomach: Churns up food with gastric juice (protease) and produces hydrochloride acid which kills bacteria. Stomach provides optimum pH for protease to work so protein digestion starts here. pH 2
• Liver: Produces bile (green fluid which helps break down fats and get rid of waste in body).
• Gall bladder: Stores bile produced by the liver and releases it into the small intestine via bile duct.
• Pancreas: Pancreatic juice contains all three enzymes, carbohydrase, protease and lipase. Added to the first part of the small intestine through pancreatic duct.
• Small intestine: Digestion is complete here - small soluble food molecules can pass through small intestine wall by diffusion into the blood stream and used for body cells - intestinal juice contains carbohydrase, protease and lipase. Bile neutralises HCl from stomach. pH 8.5
• Large intestine: Water is reabsorbed leaving a semi-solid waste called faeces, this is stored in the last part of the large intestine before egestion via anus.
• Digestive system: Mouth, oesophagus, stomach, small intestine, large intestine, rectum, anus.
• Mouth: Food enters mouth where teeth mechanically break down food into smaller pieces. Saliva from salivary glands lubricates food making swallowing easier. Contains amylase which begins starch breakdown.
• Bile contains bile salts which emulsify fats breaking down large fat globules into smaller fat globule. - Smaller fat globules have a greater total surface area compared to one fat globule, this gives lipase more surface area to act on so chemical digestion is faster.
• Biological catalysts: Protein molecules that speed up chemical reactions in the body and they are NOT used up or changed.
• Absorption in small intestine: 6m long, food can take a long time to pass through so more time for absorption - Millions of villi so larger surface area for absorption - Villi surface layer one cell thick so short distance for molecules to diffuse from gut into the blood - Villi contain blood capillaries which provide a goos transport system to all body cells maintaining a steep concentration gradient.
• Starch (carbohydrates) is broken down into glucose (energy source) by carbohydrase.

  Protein is broken down into amino acids (growth) by protease.

  Lipids (fats and oils) are broken down into fatty acids and glycerol by lipase.
• Testing for starch: Add iodine solution to sample, if present, it will change from orange/brown to blue / black.
• Testing for glucose: Add benedict’s solution, (must be boiled / heated with sample) if present, it will change from light blue to brick red
• Testing for protein: Add biuret solution to sample, if present, it will change from light blue to purple/lilac.
• Visking tube: A model for the gut

  • Has pores which allows small soluble molecules to pass through.
  • Starch is a larger insoluble molecule and cannot pass through.
  • Glucose is a smaller soluble molecule which can pass through.

  Limitations:

  • No blood supply so the diffusion gradient cannot be maintained.
  • No villi so the surface area is too small.
  • Too short, the tubing should be 6m long.
• A healthy balanced diet should contain: protein, carbohydrates, fat, vitamins, Minerals, fibre and water in correct amounts.
• Carbohydrates: for energy.

  Protein: used for growth, repair and replacement of tissues/bodies.

  Fat: provides/stores energy.

  Vitamins: small amounts to maintain healthy tissues, lack can cause deficiency disease e.g. lack of vitamin C can cause scurvy.

  Minerals: minimal amounts for good health, lack of these can cause deficiency disease e.g. lack of iron can cause anemia ( needed to make haemoglobin).

  Fibre: provides bulk and prevents constipation.

  Water: essential for body processes and functions e.g. sweating.
• Food has different energy contents which can be found by burning and using to heat water.
• Excess high energy foods can be stored as fat which leads to obesity.

  Excess sugar can lead to type 2 diabetes, tooth decay and obesity

  Excess fat, can lead to heart disease, circulatory disease and obesity.

  Excess salt can lead to high blood pressure.
• The production and release of useful substances is called secretion.
• Testing for lipids (fats & oils):

  • Add ethanol to sample, if present, forms a white cloudy emulsion.
• • Bile neutralises pH (in small intestine from stomach acid) - creates optimum pH.