Chapter 5

• Nutrition

  The process by which organisms obtain food and energy for growth, repair and maintenance of the body
• Feeding or ingestion

  Food is taken into the body
• Digestion

  Large food molecules are broken down into smaller, soluble molecules that can be absorbed through the pores of partially permeable membranes into the body cells
• Absorption

  Digested nutrients move from the small intestine into the bloodstream
• Assimilation

  Nutrients are used by cells for maintenance of the body e.g. to provide energy or to make new cytoplasm for growth
• Egestion

  Undigested matter is removed from the body
• Mouth

  • Breaks up large pieces of food into smaller pieces
  • Chewing increases the surface area to volume ratio of the food so enzymes can act on it more efficiently
  • Secrete saliva into the mouth, which contains salivary amylase to chemically digest starch into maltose
• Oesophagus

  • A narrow, muscular tube that passes through the thorax (chest) and diaphragm
  • Connects the mouth and stomach
  • Consists of a layer of longitudinal muscles and a layer of circular muscles
  • These muscles contract and relax to move food along via peristalsis
• Peristalsis

  Rhythmic, wave-like muscular contractions in the wall of the alimentary canal that enable food to be mixed with digestive juices and help to push or propel the food along the gut
• Stomach

  • Lies beneath the diaphragm, to the left side of the abdomen, and is partly covered by the liver
  • Has numerous pits and gastric glands found in the stomach lining
  • Gastric glands secrete gastric juice, which contains hydrochloric acid, mucus and pepsin (a protease)
  • Hydrochloric acid provides an optimum pH of 2 for pepsin to function and helps to kill any microorganisms in the food
  • After about 3-4 hours, partially digested food becomes liquefied into chyme, which passes into the duodenum
• Small Intestine

  • Consists of a U-shaped duodenum, the jejunum, and the much-coiled ileum
  • Carries out most of the digestive processes – digestive enzymes are secreted from the pancreas and intestinal wall into the duodenum
  • Absorption of water and digested nutrients usually takes place in the ileum – possess adaptations to facilitate this

  
• Large Intestine

  • Is shorter and broader than the small intestine
  • Consists of the colon, rectum and anus
  • Absorbs remaining water and mineral salts from undigested food
  • Temporarily stores faeces in the rectum

  
• Liver

  • Largest gland in the body, located below the diaphragm
  • Three blood vessels are attached to the lower surface of the liver: hepatic artery, hepatic vein, and hepatic portal vein
  • One important function is the production and secretion of bile, which contain bile salts that aid in the emulsification of fats
  • After secretion by the liver, bile is stored temporarily in the gall bladder before being released into the duodenum via the bile duct
• Gall bladder

  • Attached to the liver
  • Temporarily stores bile that is produced and secreted by the liver
  • When the gall bladder contracts, bile flows via the bile duct into the duodenum, which is part of the small intestine
• Pancreas

  • Connected to the duodenum by the pancreatic duct
  • Produces pancreatic juice, which contains digestive enzymes such as pancreatic amylase, lipase and protease
  • Secretes the hormones insulin and glucagon, which help regulate the blood sugar concentration in the body
• Hydrochloric acid

  Digestive fluid secreted by the stomach
• Pepsin


  Enzyme that digests proteins into polypeptides
• Optimum pH for pepsin is 2
• Gall bladder releases bile

  Into the duodenum via the bile duct
• Bile salts


  Emulsify large fat droplets into tiny fat droplets
• Pancreas secretes pancreatic juice

  1. Into the duodenum via the pancreatic duct

  2. Contains pancreatic amylase, protease, and lipase
• Intestinal wall secretes intestinal enzymes

  1. Maltase digests maltose into glucose

  2. Protease digests polypeptides into amino acids

  3. Lipase digests fats into fatty acids and glycerol
• Examples of carbohydrates

  • Starch
  • Sucrose
  • Cellulose
• Starch digestion


  Broken down into maltose by salivary amylase in the mouth and pancreatic amylase in the small intestine
• Maltose digestion

  Broken down into glucose by maltase in the small intestine
• Proteases

  Enzymes that digest proteins
• Protein digestion

  1. Starts in the stomach with pepsin digesting proteins to polypeptides

  2. Undigested proteins entering the small intestine are digested by protease to polypeptides

  3. Polypeptides are further digested to amino acids by intestinal protease
• Fat digestion

  1. Bile salts emulsify fats by lowering the surface tension, increasing the surface area-to-volume ratio for enzyme action

  2. Emulsified fats are digested by lipases into fatty acids and glycerol in the small intestine
• Summary of digestion

  • Starch -> enzyme: Amylase -> Maltose
  • Maltose -> enzyme: Maltase -> Glucose
  • Protein -> enzymes: Pepsin (stomach) + Protease (small intestine) -> Polypeptides -> enzyme: Intestinal protease -> Amino acids
  • Fats -> enzyme: Lipase -> Fatty acids and glycerol
• Function of Glucose

  When there is enough glucose, fats are not broken down and are used to build protoplasm like cell membranes or stored in adipose tissues
• Adipose tissues

  Protect organs by acting as shock absorbers
• Fats

  When glucose is in short supply, fats are broken down to release the energy needed for vital activities in the body
• Function of Liver

  • Production and secretion of bile
  • Deamination of amino acids into urea
  • Detoxification of harmful substances
  • Breakdown of hormones
  • Regulation of blood glucose concentration
• When there is High blood glucose concentration…

  • Detected by islets of Langerhans in the pancreas, which secretes insulin
  • Insulin transported to liver and muscle cells
  • Stimulate liver and muscle cells to convert excess glucose into glycogen
  • Make cell membranes more permeable to glucose, increasing uptake of glucose into cells
  • Increases use of glucose for respiration
• When there is low blood glucose concentration…

  • Detected by islets of Langerhans in the pancreas, which secretes glucagon
  • Glucagon transported to liver
  • Stimulate liver to convert stored glycogen into glucose
  • Stimulate liver to convert fats and amino acids into glucose
• Harmful effects of Alcohol consumption on the digestive system

  • Stimulates acid secretion in the stomach
  • Increases risk of gastric ulcers
  • May lead to cirrhosis of the liver
  • Cirrhosis is a disease in which the liver cells are destroyed and replaced with fibrous tissue, making the liver less able to function
  • Alcoholic cirrhosis may lead to haemorrhage in the liver, liver failure and subsequently, death
• Impacts on the nervous system

  • Depressing brain function
  • Reduced self-control
  • Slower reaction time+
  • ’Wet brain’ - a form of dementia
  • Shrinkage of brain volume
• Long-term impacts on the brain

  • Wet brain (a type of dementia caused by brain damage)
  • Shrinkage of brain volume
• Social implications of alcohol

  • Some people may become addicted to alcohol
  • Alcoholics may neglect work and families, exhibit violent behaviour, and be at higher risk of committing crimes