chap 4 nutrition in humans

• the oesophagus has 2 layers of muscles: longitudinal muscles on the outer side and circular muscles on the inner side
• peristalsis is the rhythmic, wave-like muscular contractions in the wall of the alimentary canal
• circular and longitudinal muscles are antagonistic
• when the circular muscles contract, the longitudinal muscles relax, causing the wall of the gut constricts. hence, the food is pushed forward
• when the longitudinal muscles contract, the circular muscles relax. the gut dilates, becoming wider and shorter, widening the lumen for the food to enter
• the first part of the small intestines is called duodenum
• the second part of the small intestine is called ileum
• large intestines consist of the colon, rectum, and anus
• liver cells produce and secrete bile, an alkaline greenish- yellow liquid containing bile salts
• the bile from the liver flows into the duodenum via the bile duct
• the pancreas is connected to the duodenum by the pancreatic duct
• what does the pancreas secrete?

  pancreas produces pancreatic juice which contains amylase, lipase and protease

  it also secretes hormones insulin and glucagon
• digestion involves physical and chemical digestion
• physical digestion in mouth is chewing food by the teeth
• physical digestion in the stomach, where the wall of the stomach churns, contracts and relaxes, breaking up the food and mixing them with digestive enzymes
• physical digestion in the small intestine, where fats are broken up into small fat molecules by bile salts
• describe how food is digested in the mouth

  1. food stimulates the salivary glands in the mouth to secrete saliva
  2. saliva mixes with food to soften it
  3. saliva contains salivary amylase (optimum pH is 7), digesting starch to maltose
  4. chewing breaks the food up into smaller pieces, increasing surface area to volume ratio
  5. the tongue rolls the food into small, slippery boli which is swallowed and passed down the oesophagus via the pharynx
  6. peristalsis in the walls of the oesophagus pushes the bolus of food into the stomach
• describe how food is digested in the mouth

  1. food stimulates the gastric glands to secrete gastric juice into the stomach cavity
  2. peristalsis in the stomach wall churns and breaks up the food and mixes the food with the gastric juice
  3. gastric juice stops the salivary amylase by denaturing it and kills harmful microorganisms in food
  4. gastric juice contains pepsin (protease) and digests proteins into polypeptides
  5. food remains in the stomach for about 3 to 4 hours before forming chyme and being passed in small amounts in the duodenum when the pyloric sphincter relaxes and opens
• what is the function of the mucus layer in the stomach ?

  • it protects the stomach wall against being digested by the enzymes
  • the mucus moistens food to allow easy movement within the stomach
• what is the optimum pH of pepsin ?

  pH 2
• how does gastric juice stops the action of salivary amylase ?

  gastric juice contains hydrochloric acids which causes the environment to be about pH 2, but salivary amylase optimum pH is about pH 7 so pH 2 is too extreme, hence denaturing the salivary amylase
• what kills the harmful microorganisms in food in the stomach >?

  the hydrochloric acid present in the gastric juice
• describe how food is digested in the small intestine

  the chyme stimulates

  1. the pancreas to secrete pancreatic juice, which enters the duodenum via the pancreatic duct
  2. the gall bladder to release bile which enters the duodenum via the bile duct
  3. epithelial cells in the small intestine to produce maltase, protease and lipase

  the 3 fluids neutralises the acidic chyme and provides a suitable alkaline medium (pH 8) for the pancreatic and intestinal enzymes
• what are some examples of carbohydrases ?

  amylase and maltase
• carbohydrates are digested by carbohydrases
• where is carbohydrates fully digested ?

  they are fully digested into simple sugars in the small intestine
• where is carbohydrates digested ?

  in the mouth and small intestine
• why is little starch digested in the mouth ?

  food does not stay long in the mouth for more starch to be digested into maltose and glucose
• why are carbohydrates not digested in the stomach ?

  there are no carbohydrases present in the stomach
• where does proteins digestion take place ?

  the stomach and small intestine
• where is proteins fully digested ?

  the small intestine
• describe the process of emulsification

  bile salts in bile emulsify fats, lowering surface tension of the fats by reducing attractive forces between the fat molecules

  this causes the fats to break into tiny fat droplets suspended in water, forming an emulsion
• how does emulsification help with digestion of fats ?

  it increases the surface area to volume ratio of fats, speeding up their digestion by lipase
• emulsification is the breaking up of fats into tiny fat droplets
• what digestive substances are present in the mouth and where is it secreted from ?

  saliva containing salivary amylase secreted from salivary glands
• what digestive substances are present in the stomach and where is it secreted from ?

  gastric juice, containing protease, secreted from gastric glands
• what digestive substances are present in the small intestine and where is it secreted from ?

  bile secreted form the liver

  pancreatic juice, containing proteases, pancreatic amylase, lipase (LAP), secreted from the pancreas

  intestinal juice, containing lipase, maltase, protease (LMP), secreted from epithelial cells of the small intestine
• what is absorption ?

  is it the process where digested food substances are absorbed into the body cells
• how is the small intestine adapted for absorption (increased surface area to volume ratio)?

  • the wall of the ileum is folded extensively and has many villi to increase surface area to volume ratio
  • each villus has many microvilli to further increase the surface area to volume ratio
• how else is the small intestine adapted for absorption ?

  • each villus has many blood capillaries that allow the blood to transport absorbed nutrients like amino acids and glucose to maintain a steep concentration gradient
  • each villus contains a lacteal to transport absorbed fats away and maintain a steep concentration gradient
  • the wall of villus is 1 cell thick to provide a short diffusion distance
  • epithelial cells contain many mitochondria to provide energy for active transport of nutrients