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Digestions of Food: The Alimentary Canal (Gut)

1) MOUTH where food is chewed and swallowed

• Hydrolysis of starch to maltose by salivary amylase
• Alkaline conditions assist to break the glycosidic bonds in starch

2) Food travels down the OESOPHAGUS by peristalsis

3) To the STOMACH

• Acidity kills bacteria / Inhibits salivary amylase
  • Gastric glands are stimulated by gastrin to secrete gastric juice
  • Contains HCl and pepsinogen (inactive pepsin)
• Active pepsin digests proteins → would damage glandular tissue
• Damage to stomach wall by acidic gastric juice is prevented by mucus
• Stomach digests proteins by hydrolytic endopeptidase
  • pepsinogen + HCl + pepsin → pepsin
  • Endopeptidases (pepsin/trypsin) break peptide bonds in the middle of polypeptides

4) To the SMALL INTESTINE (duodenum → ileum)

• Liver in the upper abdomen secretes bile
  • Liver detoxifies blood by removing poisonous substances / destroys old red blood cells / converts Hb to bilirubin (present in bile) / produces bile / produces urea from amino groups and ammonia
  • Gall bladder stores bile
  • Secreted into small intestine by bile duct
• Muscles in wall of small intestine mix H2O and oil forming small droplets/emulsion
  • Larger surface area / higher lipase activity
  • Bile prevents droplets from running together
• Exocrine gland of the pancreas secretes pancreatic juice into the duodenum
  • Contains amylase, lipase, exopeptidase, trypsinogen (inactive trypsin)
  • Exopeptidases break peptide bonds at the end of smaller polypeptide chains
• Intestinal brush border contains enterokinase
  • Trypsinogen + enterokinase → trypsin
• Intestinal brush border contains peptidase
• Intestinal brush border contains maltase
  • Maltose in lumen of small intestine binds to maltase
  • Resulting glucose diffuses into the cytoplasm of epithelial cells
  • Glucoseis also released back into the intestinal lumen and absorbed further down
  • Thus, duodenum digests food by HYDROLYTIC enzymes (→H2O)
• Ileum absorbs food
  • Last and longest part of small intestine
  • Microvilli (brush border) increase surface area

5) To the LARGE INTESTINE (cecum → colon → rectum) to anus

• Larger in diameter than small intestine but shorter in length
• Stores undigested food before it is egested as faeces

Lactose and Lactose Intolerance

• Lactase splits lactose into β-glucose and galactose
• Lactose intolerant person lacks lactase → lactose is neither digested nor absorbed
• High levels of soluble lactose remain in small intestine
  • Supports large populations of bacteria / ferment lactose producing gas / causing discomfort
  • Water potential becomes more negative / H2O moves into small intestine / not reabsorbed / diarrhoea
• Adults rarely produce lactase / gene is switched off in adulthood

Absorption of Products of Digestion

Histology of the Ileum in Relation to its Secretory and Absorptive Functions

• Na+, Cl-, digestive juice secreted into duodenum → LOWERS water potential
• Thus, H2O moves from epithelial cells into lumen by osmosis
• Increases efficiency of digestion (hydrolytic reactions) and absorption
• Ileum absorbs ions by active transport → INCREASES water potential
• Thus, H2O moves back into epithelial cells

The Layers of the Gut Wall and the Ultrastructure of the Epithelium

• Hollow organs with a layer of epithelial cells surrounding the lumen
• Walls of the lumen contain muscles and blood vessels
• Small change in structure has a specific function
  • Small intestine
    • Tube with a thick wall surrounding a hollow lumen
    • Epithelial cells have microvilli on their surface
    • Epithelial cells secretes mucus

Absorption And Active Uptake Of The Products Of Digestion In Small Intestine

Glucose

• Absorbed by epithelial cells using a protein carrier
• This protein carrier works by secondary active transport (co-transport system)
• Glucose and Na+ are transported across the membrane into the intestinal cell
• Further transport of glucose into blood capillaries by facilitated diffusion
• NOTE: Fructose moves entirely by facilitated diffusion!

Amino Acids

• Absorbed by epithelial cells by secondary active transport
• Co-transport carrier proteins absorb amino acids and Na+ across the membrane
  • Different carrier molecules transport different types of amino acids
  • Carriers are associated with peptidase
• Passes from the epithelial cells into capillaries by facilitated diffusion
• Newborns don't produce trypsin, HCl → proteins are not digested before small intestine is reached
  • Whole proteins may be transported by endocytosis and exocytosis
  • Uptake by endocytosis, release into blood by exocytosis
  • Often occurs in newborns due to their immature mucosa
  • Allows passage of antibodies from mother's milk - provides passive immunity for the infant
  • Accounts for many early food allergies as the protein is recognized as "foreign"

Lipids

• Triglycerides digested into monogylcerate + glycerol + fatty acids by lipase
• Monoglycerides combine with bile to form micelles
  • 5mm in diameter / forms an emulsion / contains fatty acids and glycerol
• Micelles move to membrane of epithelial cells
• Monoglycerides + glycerol + fatty acids dissolve in bilayer
• Triglycerides re-synthesise in cytoplasm / move into lymph capillaries (→lacteals)
• Bile stays in small intestine

Oral Rehydration Therapy In The Control Of Gastro-Intestinal Infections

• High amounts of semi-liquid faeces result form toxins produced by microorganisms
  • Toxins block Na+ channels in cells lining small intestine
  • Stop reabsorption / conc. of Na+ ions in small intestine increases
  • Water potential gradient is in the opposite way (into small intestine)
  • Water is drawn out of epithelial cells and added to the contents of the gut
  • This results in diarrhoea
• Toxins have little effect on glucose co-transport carrier proteins
• TREATMENT: measured amounts of glucose and mineral salts are mixed with H2O
  • Drinking the solution stimulates Na+ and glucose uptake by co-transport proteins
  • H2O is absorbed from small intestine
  • ORT increases performance of co-transport proteins / adequate amounts of glucose and Na+ pass into intestinal cells / clears up attack of diarrhoea
• Thus, Na+ is absorbed by Na+ channels AND mainly by glucose-Na+-co-transport carrier proteins

Control Of Digestive Secretions

Nervous And Hormonal Control Of Salivary, Gastric And Pancreatic Secretions

• Mammal has 2 communication systems → nervous and endocrine system
• Nervous system is based on electrical impulses passing along nerve cells
  • Short-lasting effects, can be switched on or off rapidly
  • Secretes salvia when food enters mouth
• Endocrine system is based on hormones
  • Travel in blood to target organ
  • Produce long-lasting effects
  • Trigger secretion of bile and pancreatic juice
• Endocrine system is only activated with large amounts of food intake
  • Food takes a long time to reach small intestine
  • Food stays there for a long time
  • Digestive juice can be secreted as large amounts of food are present
  • Digestive juice contains trypsin and pepsin → both enzymes are peptidases which damage proteins → they would damage epithelial cells if only small amounts of food would be present

Importance Of Simple And Conditioned Reflexes And The Hormones

Nervous reflexes

• Nerve pathway involving small number of nerve cells (2/3) → rapid response
• Automatic response → particular stimulus has same effect

Condition reflex

• Salvia and gastric juice are secreted
• By various stimuli associated with food (smell/sight/sound)
• By contact of substances in food with taste buds on tongue

Hormones

• Secreted in response to presence of food in particular region of gut
• Hormones travel in blood to glands / in glands, stimulate secretion of digestive juices
• GASTRIN stimulates exocrine glands in stomach to release gastric juice
• Acids (chyme) from stomach, fatty acids in duodenum stimulate release of SECRETIN
  • Stimulates secretion of alkali (bicarbonate ions) from pancreas
    • Neutralises acidity from intestinal contents
    • When pH reaches neutrality, secretion of secretin is inhibited
  • Inhibits gastric gland secretion
• Acidic chyme from stomach, fat, amino acids in duodenum stimulate release of CHOLECYSTOKININ-PANCREOZYMIN CCK-PZ
  • Activates smooth muscle contraction/emptying of gall bladder (to release bile)
  • Triggers secretion of enzymes from pancreas
  • Stimulates Medulla oblongata which give a satiety signal
  • Once molecules stimulating CCK are digested → CCK inhibited again
• SOMATOSTATIN
  • Acts on stomach, duodenum, pancreas
  • Inhibits release of gastrin, secretin, and CCK-PZ