Topic 6: Human Physiology
6.1 Digestion and Absorption
∑ The pancreas secretes enzymes into the lumen of the small intestine
∑ Enzymes digest most macromolecules in food into monomers in the small intestine
Digestion process:
Mouth
Chemical digestion
o Food enters the mouth, stimulating secretion of saliva by salivary glands.
o Mucin in saliva softens food; salivary amylase digests starch into maltose
o Saliva’s pH is neutral, the optimum pH for amylase
Mechanical digestion
o Chewing breaks food into smaller pieces and increases SA:V ratio for salivary
amylase to work on
Swallowing
o Voluntary phase: tongue rolls food into bolus that is swallowed
o Involuntary phase:
Pharynx receptors stimulated by food
Soft palate rises, larynx rises slightly, epiglottis closes opening to
trachea, tongue pushes food further, food enters oesophagus.
Esophagus
Bolus passes down the oesophagus via the pharynx (epiglottis, the flap, is closed)
Peristalsis mainly helps to bring it down into the stomach, although gravity also helps.
Stomach
Lower esophageal sphincter (LES) regulates food movement into stomach
Chemical digestion
o Food stimulates secretion of gastric juice into stomach cavity by gastric
glands made of parietal cells
o Dilute HCl in gastric juice
Stops salivary amylase action
Changes inactive enzymes to active (pepsinogen pepsin; Prorennin
rennin). Enzymes are produced in inactive form to prevent
digestion of source cell’s proteins.
Acidic medium suitable for gastric enzymes
Kills harmful microorganisms in food
o Pepsin
Digests protein into polypeptides
o Renin
Clots and curdles milk proteins by converting soluble caseinogen into
soluble casein which remains long enough to be digested by pepsin
Mechanical digestion
o Peristalsis of stomach walls churn and break up food to increase SA:V ratio
and mixes it with gastric juice
After 3-4h, partially digested food becomes liquefied, forming chyme
Chyme passes into duodenum in small amounts when pyloric sphincter relaxes
(based on feedback system)
, Topic 6: Human Physiology
*Some small molecules (eg. alcohol) diffuse through stomach lining before reaching small
intestine
Small Intestine [duodenum (mostly digestion), jejunum, ileum (mostly absorption)]
Chyme simulates the secretion of 3 alkaline fluids that neutralises acidic chyme and
provides suitable alkaline medium for pancreatic and intestinal enzymes
Large molecules are often insoluble and too large to diffuse across villi membranes
for absorption. Hence, they need to undergo hydrolysis.
Wall of intestine produces many enzymes (those by gland cells secreted in intestinal
juice; but most remain immobilised in plasma membrane of epithelium cells lining
intestine)
Chyme takes hours to pass through small intestine
Pancreatic juice secreted by pancreas through pancreatic duct into duodenum.
o Secretion controlled by hormones synthesized and secreted by stomach and
enteric nervous system.
o Digestive enzymes synthesized by RER ribosomes, processed in Golgi
apparatus, secreted by exocytosis. Ducts within pancreas merge into larger
ducts and finally forming one pancreatic duct.
o Alkaline solution rich in bicarbonates
o Amylase (starch maltose)
o Trypsinogen (proteins polypeptides)
o Lipase (fats glycerol + fatty acid/ monoglycerides)
o Phospholipase (phospholipids fatty acids, glycerol, phosphates)
*Trypsin is an endopeptidase: breaks peptide bonds from non-terminal amino acids. Hence,
cannot break polypeptide into monomers.
Intestinal juice secreted by intestinal glands
o Enterokinase (activates trypsinogen into trypsin)
o Disaccharidases (maltase, sucrase, lactase)
o Erepsin (polypeptides amino acids)
o Exopeptidases (digest peptides by removing single amino acids either from
carboxyl or amino terminal till a dipeptide is left)
o Dipeptidases (dipeptide amino acids)
o Lipase and Nucleases
Bile released by gall bladder through bile duct
o Bile salts (emulsify fats in duodenum by lowering surface tension of fats,
causing it to break into tiny flat globules suspended in water, or an emulsion.
Increased SA:V ratio increases digestion by lipase)
, Topic 6: Human Physiology
∑ The contraction of circular and longitudinal muscle layers of the small intestine
mixes the food with enzymes and moves it along the gut
2 smooth muscle layers in wall of gut: circular and longitudinal muscle. They are
short cells that exert continuous moderate force rather than interspersed vigorous
contractions.
Circular layer is inner, longitudinal is outer.
Peristalsis caused by the 2 muscle layers is a unidirectional, wave-lie, rhythmic
contractions that enables food to be mixed with digestive juices along the gut, and
prevents food from being pushed backwards.
Contractions are controlled unconsciously by the enteric nervous system rather than
the brain.
o Circular contract, longitudinal relax gut wall constricts (becomes long and
narrow) food is squeezed forward
o Longitudinal contract, circular relax gut wall dilates (becomes wider and
shorter) widens lumen, food enters.
∑ Villi increase the surface area of epithelium over which absorption is carried out
∑ Villi absorb monomers formed by digestion as well as mineral ions and vitamins
Villi increase surface area by factor of ~10
Macromolecules have to be broken down to monomers/ smaller molecules for
absorption to occur through villi membranes.
o Monosaccharides, amino acids, fatty acids, monoglycerides, glycerol, bases
Villi absorb some substances that do not require digestion
o Mineral ions (K, Ca, Na), vitamins
Once in blood, substances absorbed are carried to tissues where they are
assimilated
Unwanted substances may pass through
o Unwanted harmful substances are subsequently removed from blood and
detoxified by liver.
o Harmless unwanted substances are absorbed (eg. colouring and flavouring),
which pass out in urine.
o Small numbers of bacteria pass through epithelium but are quickly removed
by phagocytic cells in liver.
, Topic 6: Human Physiology
∑ Different methods of membrane transport are required to absorb different nutrients
Many different mechanisms to move nutrients into and out of villus epithelium cells:
simple diffusion facilitated diffusion, active transport and exocytosis
Triglycerides:
o Digested into fatty acids and monoglycerides that can enter villus cells via
simple diffusion
o Fatty acids also absorbed by facilitated diffusion (fatty acid transporters)
o Fatty acid and monoglyceride in epithelium cells then combine to form
triglycerides that cannot diffuse out into lumen
o Triglyceride coalesce with cholesterol to form droplets coated in
phospholipids and protein
o Lipoprotein particles are released by exocytosis through plasma membrane
on inner side of villus epithelium cells as chylomicrons (specialised vesicles
for fat transport)
o They enter lacteal (lymphatic capillary that absorbs digested fats)
Glucose
o Polar and hydrophilic. Hence, it cannot pass through plasma membrane by
simple diffusion.
o Na-K pumps in inward-facing part of plasma membrane pumps sodium ions
in, potassium ions out of cytoplasm. This creates low concentration Na ions
inside villus epithelium cells by active transport.
o Sodium-glucose co-transporter proteins in microvilli transfer sodium ion and
glucose molecule together from intestinal lumen to cytoplasm of epithelium
cells. This facilitated diffusion depends on the concentration gradient
generated by active transport.
o Glucose channels allow glucose to move by facilitated diffusion from
cytoplasm to interstitial spaces inside villus into blood capillaries.
All monosaccharides across the basolateral membrane via facilitated diffusion
Amino acids are absorbed by active transport into villus cells, and leave it by
facilitated diffusion and enter capillary via intercellular clefts.
6.1 Digestion and Absorption
∑ The pancreas secretes enzymes into the lumen of the small intestine
∑ Enzymes digest most macromolecules in food into monomers in the small intestine
Digestion process:
Mouth
Chemical digestion
o Food enters the mouth, stimulating secretion of saliva by salivary glands.
o Mucin in saliva softens food; salivary amylase digests starch into maltose
o Saliva’s pH is neutral, the optimum pH for amylase
Mechanical digestion
o Chewing breaks food into smaller pieces and increases SA:V ratio for salivary
amylase to work on
Swallowing
o Voluntary phase: tongue rolls food into bolus that is swallowed
o Involuntary phase:
Pharynx receptors stimulated by food
Soft palate rises, larynx rises slightly, epiglottis closes opening to
trachea, tongue pushes food further, food enters oesophagus.
Esophagus
Bolus passes down the oesophagus via the pharynx (epiglottis, the flap, is closed)
Peristalsis mainly helps to bring it down into the stomach, although gravity also helps.
Stomach
Lower esophageal sphincter (LES) regulates food movement into stomach
Chemical digestion
o Food stimulates secretion of gastric juice into stomach cavity by gastric
glands made of parietal cells
o Dilute HCl in gastric juice
Stops salivary amylase action
Changes inactive enzymes to active (pepsinogen pepsin; Prorennin
rennin). Enzymes are produced in inactive form to prevent
digestion of source cell’s proteins.
Acidic medium suitable for gastric enzymes
Kills harmful microorganisms in food
o Pepsin
Digests protein into polypeptides
o Renin
Clots and curdles milk proteins by converting soluble caseinogen into
soluble casein which remains long enough to be digested by pepsin
Mechanical digestion
o Peristalsis of stomach walls churn and break up food to increase SA:V ratio
and mixes it with gastric juice
After 3-4h, partially digested food becomes liquefied, forming chyme
Chyme passes into duodenum in small amounts when pyloric sphincter relaxes
(based on feedback system)
, Topic 6: Human Physiology
*Some small molecules (eg. alcohol) diffuse through stomach lining before reaching small
intestine
Small Intestine [duodenum (mostly digestion), jejunum, ileum (mostly absorption)]
Chyme simulates the secretion of 3 alkaline fluids that neutralises acidic chyme and
provides suitable alkaline medium for pancreatic and intestinal enzymes
Large molecules are often insoluble and too large to diffuse across villi membranes
for absorption. Hence, they need to undergo hydrolysis.
Wall of intestine produces many enzymes (those by gland cells secreted in intestinal
juice; but most remain immobilised in plasma membrane of epithelium cells lining
intestine)
Chyme takes hours to pass through small intestine
Pancreatic juice secreted by pancreas through pancreatic duct into duodenum.
o Secretion controlled by hormones synthesized and secreted by stomach and
enteric nervous system.
o Digestive enzymes synthesized by RER ribosomes, processed in Golgi
apparatus, secreted by exocytosis. Ducts within pancreas merge into larger
ducts and finally forming one pancreatic duct.
o Alkaline solution rich in bicarbonates
o Amylase (starch maltose)
o Trypsinogen (proteins polypeptides)
o Lipase (fats glycerol + fatty acid/ monoglycerides)
o Phospholipase (phospholipids fatty acids, glycerol, phosphates)
*Trypsin is an endopeptidase: breaks peptide bonds from non-terminal amino acids. Hence,
cannot break polypeptide into monomers.
Intestinal juice secreted by intestinal glands
o Enterokinase (activates trypsinogen into trypsin)
o Disaccharidases (maltase, sucrase, lactase)
o Erepsin (polypeptides amino acids)
o Exopeptidases (digest peptides by removing single amino acids either from
carboxyl or amino terminal till a dipeptide is left)
o Dipeptidases (dipeptide amino acids)
o Lipase and Nucleases
Bile released by gall bladder through bile duct
o Bile salts (emulsify fats in duodenum by lowering surface tension of fats,
causing it to break into tiny flat globules suspended in water, or an emulsion.
Increased SA:V ratio increases digestion by lipase)
, Topic 6: Human Physiology
∑ The contraction of circular and longitudinal muscle layers of the small intestine
mixes the food with enzymes and moves it along the gut
2 smooth muscle layers in wall of gut: circular and longitudinal muscle. They are
short cells that exert continuous moderate force rather than interspersed vigorous
contractions.
Circular layer is inner, longitudinal is outer.
Peristalsis caused by the 2 muscle layers is a unidirectional, wave-lie, rhythmic
contractions that enables food to be mixed with digestive juices along the gut, and
prevents food from being pushed backwards.
Contractions are controlled unconsciously by the enteric nervous system rather than
the brain.
o Circular contract, longitudinal relax gut wall constricts (becomes long and
narrow) food is squeezed forward
o Longitudinal contract, circular relax gut wall dilates (becomes wider and
shorter) widens lumen, food enters.
∑ Villi increase the surface area of epithelium over which absorption is carried out
∑ Villi absorb monomers formed by digestion as well as mineral ions and vitamins
Villi increase surface area by factor of ~10
Macromolecules have to be broken down to monomers/ smaller molecules for
absorption to occur through villi membranes.
o Monosaccharides, amino acids, fatty acids, monoglycerides, glycerol, bases
Villi absorb some substances that do not require digestion
o Mineral ions (K, Ca, Na), vitamins
Once in blood, substances absorbed are carried to tissues where they are
assimilated
Unwanted substances may pass through
o Unwanted harmful substances are subsequently removed from blood and
detoxified by liver.
o Harmless unwanted substances are absorbed (eg. colouring and flavouring),
which pass out in urine.
o Small numbers of bacteria pass through epithelium but are quickly removed
by phagocytic cells in liver.
, Topic 6: Human Physiology
∑ Different methods of membrane transport are required to absorb different nutrients
Many different mechanisms to move nutrients into and out of villus epithelium cells:
simple diffusion facilitated diffusion, active transport and exocytosis
Triglycerides:
o Digested into fatty acids and monoglycerides that can enter villus cells via
simple diffusion
o Fatty acids also absorbed by facilitated diffusion (fatty acid transporters)
o Fatty acid and monoglyceride in epithelium cells then combine to form
triglycerides that cannot diffuse out into lumen
o Triglyceride coalesce with cholesterol to form droplets coated in
phospholipids and protein
o Lipoprotein particles are released by exocytosis through plasma membrane
on inner side of villus epithelium cells as chylomicrons (specialised vesicles
for fat transport)
o They enter lacteal (lymphatic capillary that absorbs digested fats)
Glucose
o Polar and hydrophilic. Hence, it cannot pass through plasma membrane by
simple diffusion.
o Na-K pumps in inward-facing part of plasma membrane pumps sodium ions
in, potassium ions out of cytoplasm. This creates low concentration Na ions
inside villus epithelium cells by active transport.
o Sodium-glucose co-transporter proteins in microvilli transfer sodium ion and
glucose molecule together from intestinal lumen to cytoplasm of epithelium
cells. This facilitated diffusion depends on the concentration gradient
generated by active transport.
o Glucose channels allow glucose to move by facilitated diffusion from
cytoplasm to interstitial spaces inside villus into blood capillaries.
All monosaccharides across the basolateral membrane via facilitated diffusion
Amino acids are absorbed by active transport into villus cells, and leave it by
facilitated diffusion and enter capillary via intercellular clefts.
