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Human Biology > Digestive System
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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

ATE

PRODUCTS

ENZYME

PRODUCED BY

Proteins

Smaller polypeptides

Endopeptidase: Pepsin

Gastric glands

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

SUBSTRATE

PRODUCTS

ENZYME

PRODUCED BY

Starch

Maltose

Pancreatic amylase

Pancreas

Maltose

Glucose

Maltase

Intestinal cells

Proteins

Smaller polypeptides

Endopeptidase: trypsin

Pancreas

Smaller polypeptides

Amino acids;
Dipeptides

Exopeptidase

Pancreas

Dipeptides

Amino acids

Peptidase

Intestinal cells

Triglycerides

Glycerol;
Monoglycerides;
Fatty Acids

Lipase

Pancreas

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