Chapter 5: Nutrition in humans

• Nutrition: process by which organisms obtain food and energy for growth, repair and maintenance of the body
• Digestion: large food molecules are broken down into smaller soluble molecules that can be absorbed into the body cells
• Assimilation: nutrients are used by cells to provide energy or to make new cytoplasm for growth
• Egestion: undigested materials are removed from the body
• 1: mouth and buccal cavity

  2: oesophagus

  3: stomach

  4: liver

  5: gall bladder

  6: bile duct

  7: duodenum

  8: small intestine

  9: ileum

  10: pancreas

  11: large intestine

  12: rectum

  13: anus

  
• Mouth and buccal cavity contains:

  • teeth that breaks large pieces of food into smaller pieces, helping to increase the surface area of the food so enzymes can act on it more efficiently
  • salivary gland that secret saliva into mouth
  • tongue that mixes food with saliva and moves food to the back of mouth during swallowing
• Pharynx:

  • connects buccal cavity to oesophagus and larynx (voice box)
• Oesophagus:

  • narrow muscular tube that joints the mouth to stomach
  • contains longitudinal muscles on the outside and circular muscles on the inside
  • both muscles produce long, slow contractions that move food along the gut by peristalsis
• Peristalsis:

  • rhythmic wave-like muscular contractions in the walls of the alimentary canal
  • it allows food to be mixed with digestive juices and pushes food along the gut
  • When circular muscles contract, longitudinal muscles relax, the walls of the gut constricts and becomes narrower, pushing the food forward
  • When the longitudinal muscles contract, the circular muscles relax, the walls of the gut dilates, becoming wider and shorter, widening the lumen for food to enter
• Stomach:

  • walls has numerous pits lined with gastric glands
  • gastric glands secret gastric juice which helps in digestion
• Small intestine:

  • consists of the duodenum connecting the stomach to the ileum and the coiled portion known as the ileum
  • walls contains glands that secret enzymes that carry out digestion
• Large intestine:

  • Consists of colon, rectum and anus
  • The colon absorbs 90% of the remaining water and mineral salts from undigested matter
  • Faeces (undigested matter) is temporarily stored in the rectum before being expelled through the anus
• Liver and gall bladder:

  • liver cells produce and secret bile that aids in physical digestion of fats by emulsification
  • Bile is temporarily stored in the gall bladder and flows into the duodenum via the bile duct when the gall bladder contracts
• Pancreas:

  • Connected to the duodenum via the pancreatic duct
  • Produces pancreatic juice containing digestive enzymes and hormones such as insulin and glucagon
• Digestion in mouth:

  1. Presence of food stimulates salivary glands to secret saliva
  2. Saliva is mixed with food to soften it
  3. Salivary amylase digests starch into maltase
  4. Chewing breaks down food into smaller pieces increasing its surface area for enzymes to act on
  5. Tongue rolls up food into small round masses (boli)
  6. Boli are swallowed and passed down into the oesophagus via the pharynx
  7. Peristalsis in walls of oesophagus pushes each bolus into the stomach
• Digestion in stomach:

  1. Presence of food stimulates gastric glands to secret gastric juices
  2. Peristalsis churns and breaks up food, it also mixes food with gastric juices
  3. Gastric juices is made up of dilute hydrochloric acid and pepsin (protease)
  4. Pepsin digests protein to polypeptides
  5. Mucus layer of stomach walls protects it from being digested
  6. Food remains in stomach for 3-4 hours, becoming liquefied, forming chyme
  7. Chyme passes in small amounts into duodenum when pyloric sphincter relaxes and opens
• Digestion in small intestine:

  1. Presence of chyme in duodenum stimulates the pancreas to produce pancreatic juices that passes through the pancreatic duct into the duodenum. The gall bladder is also stimulated to release bile that passes through the bile duct into the duodenum. The epithelial cells in the small intestines are also stimulated to produce maltase, protease and lipase
  2. Chyme (acidic) comes in contact with pancreatic juice, bile and intestinal juices (alkalis) and gets neutralized, providing an alkaline medium for pancreatic and intestinal enzymes to act
• Digestion of carbohydrates:

  • Starts in mouth where salivary amylase digests starch into maltose. Only small amounts of starch are digested as food does not remain in mouth for long
  • Digestion of carbohydrates do not occur in stomach
  • In the small intestine, carbohydrates are fully digested into simple sugars. Starch is digested by pancreatic amylase into maltose before being digested by maltase into glucose
• Digestion of proteins:

  • Starts in the stomach where protease (pepsin) digests proteins into polypeptides
  • In the intestines, undigested proteins are digested by protease into polypeptides. The polypeptides are further digested to amino acids by protease
• Fat digestion:

  • Bile produced by the liver and stored in the gall bladder is released into the duodenum. In the small intestine, bile salts in bile lowers the surface tension of fats, reducing the attractive forces between fat molecules, breaking them into tiny fat droplets, increasing the surface area to volume ratio of the fats, speeding up their digestion by lipase
  • Emulsified fats are digested by intestinal and pancreatic lipases into glycerol and 3 fatty acid molecules
• Emulsification: breaking up of fats into tiny fat droplets
• Absorption: process where digested food substances are absorbed into the body cells
• The small intestine is long to allow for sufficient time for digestion and absorption of food.
• The small intestine has extensive folds of the inner surface of the ileum and numerous minute finger-like projections called villi to increase the surface area for more adsorption of food to take place.
• The epithelium of the small intestine is one cell thick, reducing the diffusion distance, allowing for food to diffuse faster into the blood capillaries.
• Cells of the epithelium have multiple microvilli to further increase the surface area for more diffusion to take place.
• Each villus of the small intestine has many blood capillaries that transport the absorbed glucose and amino acids away from the small intestine to allow for a steep concentration gradient to allow the substances to constantly diffuse at a high rate.
• Epithelial cells of the small intestine have many mitochondria to provide energy for active transport of nutrients.
• Glucose and amino acids are absorbed by diffusion into the blood capillaries of the villi
• Glucose and amino acid are also absorbed by active transport in the villi when there is low concentration of these substances in the small intestine
• Glycerol and fatty acids diffuse into the epithelium, combining to form fat globules that enters the lacteals
• undigested and unabsorbed materials are discharged as faeces through the anus. This is known as egestion / defecation
• How are glucose and amino acids used:

  • After glucose and amino acids are absorbed, they would be transported to the liver through the hepatic portal vein
  • Glucose and amino acids is transported by the hepatic vein to the rest of the body to be used
  • Excess glucose is converted to glycogen for storage
  • Excess amino acid is converted to urea
• Uses of glucose:

  • Is an energy source for cells
  • Excess would be returned to the liver to be converted to glycogen due to the hormone insulin
  • Glycogen can be broken down by glucagon to be used for energy when needed
• Uses of amino acids:

  • Use to create new cytoplasm for growth, repair
  • Used to form enzymes and hormones
  • Excess amino acids are deaminated in the liver
• How are fats transported:

  • Absorbed in lymphatic capillaries that join to lymphatic vessels which discharge fats into the blood stream
  • Blood carries fats into all parts of the body, especially the liver
  • In the liver, fat is converted to forms that is either broken down or stored
• Utilisation of fats:

  • When there is sufficient supply of glucose, fats are not broken down and is instead used to make protoplasm (cytoplasm, nucleus)
  • When glucose is short in supply, fats are broken down to provide energy for cellular activity
  • Excess fats are stored in adipose tissues located near the heart and kidneys. They act as shock absorbers to protect these organs
• Functions of liver:

  1. Producing bile
  2. Deamination of excess amino acids to form urea
  3. Regulating blood glucose concentration
  4. Breaking down of hormones
  5. Detoxification
• Deamination of amino acids:

  • Amino group (NH2) is removed from amino acids to form urea
  • Urea is removed from body via urine
  • Remains of deaminated amino acids are covered to glucose in the liver, if glucose is in excess, it would be converted to glycogen
• Regulation of blood glucose concentration:

  • When blood glucose level rises above normal, insulin produced by the islets of Langerhans of the pancreas is released, liver cells will convert glucose into glycogen, allowing the blood glucose level to drop back to normal
  • When blood glucose level drops below normal, glucagon is released by the islet of Langerhans, liver cells convert glycogen to glucose to be released into the blood, blood glucose level rises back to normal