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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