Chapter 5: Nutrition in Humans

• 5 steps of nutrition:

  1. Ingestion : Food being taken into the body
  2. Digestion : Large food molecules are broken down into smaller, soluble molecules that can be absorbed into the body cells
  3. Absorption : Nutrients move from the smaller intestine into the blood stream
  4. Assimilation : Nutrients are used by cells to provide energy or to make new cytoplasm for growth
  5. Egestion : Undigested matter is removed from the body
• Digestion :

  -> Chemical :

  involves enzymes

  -> Physical :

  e.g. chewing
• Physical digestion of Mouth / buccal cavity :

  • Teeth break down large pieces of food into smaller pieces by chewing.
  • This increases the surface area-to-volume ratio for enzyme salivary amylase to act on
• Chemical Digestion of Mouth / buccal cavity :

  • Saliva containing salivary amylase are secreted to the buccal cavity from the salivary glands.
  • salivary amylase catalyses the breakdown of starch into maltose
  • Starch ----> Maltose
• Other processes of Mouth / buccal cavity :

  • Tongue will mix the food with saliva and roll into a bolus, which then move to the back of the mouth to be swallowed
• Pharynx:

  part of the alimentary canal that connects the buccal cavity to the oesophagus and larynx (voice - box) and trachea (windpipe)
• Physical digestion of oesophagus:

  no physical digestion occurs here

  Chemical digestion of oesophagus:

  No chemical digestion occurs here
• Other processes of Oesophagus:

  Wall of the oesophagus contains 2 layers of antagonistic (one contract the other relaxes) muscles:

  • >longitudinal muscles on the outer side of the wall
  • >circular muscles on the inner side of the wall

  The contraction and relaxation of either muscles move food along the oesophagus via peristalsis.

  
  

  Peristalsis is the rhythmic, wave-like muscular contractions in the wall of the alimentary canal. This enables food to be mixed with digestive juices and pushes the food along the alimentary canal
• Other processes of Oesophagus [PT 2]:

  Both the longitudinal and circular muscles are antagonistic. When one muscle contracts, the other one will relax, vice versa.

  -> when the circular muscles contracts, the longitudinal muscles will relax. This results in the wall of the gut to constrict (becomes narrower and longer) to push the food forward.

  -> When the logitudinal muscles contracts, the circular muscles will relax. This results in the wall of the gut to dilate (becomes wider and shorter) for the food to enter.
• Peristalsis occurs through the gastrointestinal tract (oesophagus, stomach, small intestine, large intestine)
• Physical digestion of stomach:

  Peristalsis in the stomach wall will churn and physically break down the food into smaller pieces. This action also helps to mix the food with the gastric juice.
• Chemical digestion of stomach :

  Gastric juice containing hydrochloric acid and enzyme pepsin are secreted into the stomach cavity from the gastric glands.

  Pepsin catalyses the breakdown of proteins into polypeptides.

  Protein ---> polypeptide

  Hydrochloric acid provides an acidic environment within the stomach (approx. pH2) which is the optimum pH for enzyme pepsin. In addition, the acidic condition is able to kill certain potentially harmful microorganisms in food.

  Acidic environment will denature enzyme salivary amylase
• Other processes of stomach :

  • mucus layer (prevents stomach from corroding [gastric pain] from hydrochloric acid) is present on the walls of the stomach to prevent stomach from being digested by the enzymes it secreted. In addition, it helps to moisten the food for easier movement within the stomach
  • Food will be partially digested into chyme and passes into the duodenum via the contraction of the pyloric sphincter (prevents food from going into small intestine) [found at the bottom of the stomach]
• small intestine is divided into 3 sections: duodenum, dejunum and ileum
• Physical digestion of small intestine:

  Bile, produced by the liver and stored in the gall bladder, is released into the duodenum. The bile salt will emulsify fats into smaller fat dropslets. This increase surface area-to-volume ratio for enzyme lipase to act on.

  * Emulsification is a physical process

  * Bile is not an enzyme

  
• Chemical digestion of small intestine:

  Pancreatic juice (from pancreas) containing enzyme pancreatic amylase, protease and lipase are secreted into the duodenum from the pancreas through the pancreatic duct.

  Pancreatic amylase catalyses the breakdown of starch into maltose

  starch --> Maltose

  Pacreatic protease catalyses the breakdown of protein into polypeptide

  Protein --> polypeptide

  Pacreatic lipase catalyses the breakdown of fat into glycerol and fatty acids

  Fat --> fatty acids + glycerol
• Chemical digestion of small intestine [pt2]:

  intestinal juice (from small intestine) containing enzyme intestinal maltase, protease and lipase are secreted into the duodenum from the epithelial cells in the small intestine

  Maltase catalyses the breakdown of maltose into glucose

  Maltose --> Glucose

  Intestinal protease catalyses the breakdown of polypeptide into amino acids

  Polypeptide --> amino acids [smaller and can be used]

  Lipase catalyses the breakdown of fat into glycerol and fatty acids

  Fats --> fatty acids + glycerol
• other processes of small intestine:

  intestinal juice, bile and pancreatic juice are alkaline which provides an alkaline environment within the small intestine (approx pH 8) which is the optimum pH for the various enzymes
• Absorption of digested food into the bloodstream. Structural adaptation of the small intestine for efficient absorption:

  • inner suface of ileum has extensive folding and numerous minutee finger-like projection called villi to increase surface area-to-volume ratio for absorption of digested food
  • Epithelium of the villus is one cell thick to provide a short diffusion distance for nutrients to pass through for more efficient diffusion
• • the cell of the epithelium has microvilli to further increase surface area-to-volume ratio for absorption of digested food for more efficient diffusion
  • each villius has extensive blood capillaries to allow blood to transport the absorbed glucose and amino acids away to maintain steep concentration gradient
• • each villius has lacteal to allow blood to transport the absorbed fats away to maintain steep concentration gradient

  *Fats molecules are too large to be absorbed into the blood capillaries, thus they are absorbed into the lymphatic system

  • epithelium cells contain numerous mitochondria to provide energy, via aerobic respiration, for active transport of nutrients into the villi
• Pancreas :

  A gland located in the loop of the duodenum. Connected to the duodenum by the pancreatic duct. The pancreas produces pancreatic juice which contains enzymes pancreatic amylase, protease and lipase. These will be secreted into the duodenum

  
• pancreas contain a special group of cells known as islets of Langerhans which secretes hormones:

  1. Insulin : stimulates the conversion of glucose into glycogen in the liver
  2. Glucagon : stimulates the conversion of glycogen into glucose in the liver

  Both insulin and glucagon are involved in controlling the blood sugar level in the body
• Hormones:

  • not enzymes
  • chemical substances produced in minute quantities. It is transported in the bloodstream to target organ (s) where it exerts its effect (s).
  • Target Organ: an organ which hormone exerts its effects
• Liver:

  • Largest gland in the body located in the upper right-hand portion of the abdominal cavity, beneath the diaphragm and top of the stomach, right kidney, and intestines
  • After absorption of nutrients (containing mainly sugar and amino acids) into the blood capillaries, the blood rich in nutrients will flow through the Hepatic portal vein from the small intestine to the liver. Assimilation occurs

  
• Liver:

  • all cells use glucose as a source of energy. Glucose is assimilated and broken-down during respiration to release energy for the cells.
  • Amino acids that enter the cells are converted into new cytoplasm that Is used for growth and repair of worn-out parts of the body
  • amino acids are also used to form enzymes and hormones
• bile is not an enzyme
• Liver produces bile

  liver cells produces bile, which emulsify fats into smaller fat droplets that has larger surface area-to-volume ratio for enzyme lipase to act on. Bile is temporally stored in the gall bladder. When the gall bladder contracts, bile flows into the small intestine via the bile duct
• Deamination of amino acids in the liver

  • deamination is the process by which amino groups are removed from the amino acids and converted to urea.
  • urea is removed from the body in the urine. The remains of the deaminated amino acids are converted into glucose in the liver. Any excess glucose will be converted into glycogen
  • Hydrogen peroxide (H2O2) ---> water + oxygen
• regulation of blood sugar concentration in the liver

  plays an important role in carbohydrate metabolism by maintaining the amount of glucose in the blood constant .

  Hormone insulin stimulates liver cells to convert excess glucose into glycogen. Glycogen acts a temporary storage of glucose and will be primarily stored in the liver

  When the body requires energy, hormone glucagon stimulates liver cells to convert glycogen into glucose. Glucose is then transported by the blood to the cells.
• hormones Insulin and glucagon are not enzymes.

  pancreas produces insulin, glucagon

  glucose - sugar

  glycogen - storage form of glucagon

  glucagon - hormone
• breakdown of hormones in the liver

  hormones are broken down in the liver
• detoxification in the liver

  harmful substances may be absorbed into the blood from the gut. The liver is involved in converting harmful substances into harmless ones and the process is known as detoxification

  alcohol is a harmful substance (can cause damage to digestive system and slow down brain function) that can be absorbed into the bloodstream. Liver contains an enzyme alcohol dehydrogenase which can break down alcohol into compounds that can be used in respiration providing energy of cells
• Large intestine consists of colon, rectum and anus:

  No physical digestion occurs here

  No chemical digestion

  other processes:

  • absorb water and mineral salts from the undigested food
  • faeces will be temporarily stored in the rectum. When the rectum contracts, the faeces will be expelled through the anus