Term 3
Carbohydrate absorption
1. Explain the breakdown of complex carbohydrates in different locations in the
gastrointestinal tract (GIT).
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Major dietary carbohydrates are polysaccharides of plant origin [starch and cellulose] and
of animal origin [ glycogen].
After consuming a meal contains carbohydrates digestion begins in the oral cavity /
mouth and terminates at large intestine.
In the mouth, during mastication, salivary alpha amylase, acts briefly on the dietary starch and
glycogen.
It hydrolyzes random alpha 1-4 glycosidic bonds and form a mixture of short branched and
unbranched oligosaccharides called dextrins.
disaccharides are resistant to amylase.
Also, alpha amylase cannot hydrolyze the alpha 1-6 glycosidic bonds found in branched
amylopectin.
Salivary amylase is inactivated in the stomach due to the highly acidic microenvironment (pH
1.8 - 3.5) and no further digestion occurs.
Then the acidic chyme enters the duodenum through the pylorus, chyme is neutralized
by HCO3-ions secreted by the pancreatic juice.
Secretion of the pancreatic juice is stimulated by secretin and cholecystokinin
hormones.
Pancreatic amylase enzyme in the pancreatic juice enters the duodenum and continues
the process of starch and glycogen digestion.
Final digestive process occurs primarily in the mucosal lining of the duodenum and
upper jejunum via
membrane bound disaccharidases on the luminal surface of the villi of the
enterocyte.
These enzymes are transmembrane proteins of the brush border called brush
border enzymes - isomaltase, maltase, sucrase and lactase.
Isomaltase cleaves the alpha 1-6 glycosidic bond in isomaltose and form glucose.
Maltase cleaves alpha 1-4 glycosidic bond in maltose and maltotriose to form
glucose
Sucrase cleaves alpha 1-2 bond of sucrose and forms glucose and fructose.
Lactase cleaves beta 1-4 glucosidic-galactosidic bond in lactose and form glucose
and galactose, digestion completed forming monosaccharides.
Absorption of monosaccharides occur in the small intestine
In the large intestine, dietary fibres, resistant starches and other indigestible
carbohydrates are partially broken down by intestinal bacteria
and form short chain fatty acids and some gases.
Products of bacterial digestion are absorbed into cells of the colon
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small amount transported to the liver and some amount used by bacteria to make
energy and growth.
Water is absorbed from undigested fibres and eliminated in feces with bacterial
degradation products.
2. Outline the process of glucose transport across the intestinal mucosal cell.
• Upper jejunum, predominantly, and duodenum absorb the monosaccharides, which are
the ultimate products of carbohydrate digestion – glucose, fructose and galactose into
the enterocyte via different transport mechanisms.
• There are two ways of monosaccharide absorption.
1. Facilitated diffusion – a passive process via GLUTs
2. Secondary active co-transport – an active process occurring down the electrochemical
gradient established through primary active transport by hydrolysis of ATP.
• Galactose and glucose are transported into the mucosal cells by a secondary active
mechanism via sodium dependent glucose cotransporter 1 (SGLT- 1) on the luminal
membrane.
• This process requires a concurrent uptake of sodium ions and the electrochemical gradient
of Na+ is established by the primary active transporter Na+/K+-ATPase pump in the
basolateral membrane of enterocytes that moves 3Na+ ions out of the enterocyte in
exchanges to 2K+ ions into the cell.
• Thus, forming a relative deficiency of Na+ ions in the intracellular compartment, along which
Na+ ions enter the intestinal mucosal cell.
• Fructose absorption utilizes an energy and sodium independent monosaccharide transporter
(GLUT-5) on the luminal membrane.
• Fructose is taken up by facilitated diffusion.
• All three monosaccharide types are transported from the enterocyte into the portal
circulation via GLUT - 2 transporter on the basolateral membrane by facilitated
diffusion.
• In healthy individuals, ordinarily all digestible dietary carbohydrates are absorbed by the
time ingested material reach the lower jejunum.
3. Explain why glucose is a component of oral rehydration fluid.
 In the healthy state there are glucose-dependent and glucose independent methods of
absorbing Na+ in the gut.
 However, during conditions like diarrhoea, the glucose independent transporters of
sodium are inactivated
 The Na+ in the intestinal lumen is transported into the enterocytes via the Sodium
Glucose Cotransporter (SGLT-1) which is a symport (secondary transport of glucose)
 SGLT-1 requires the presence of both sodium and glucose in the lumen
 Therefore, glucose must be present (along with Na+) in the ORS fluid
 Na+ that enters the cell is then pumped into the blood using ATP by the Na+ /K+ ATPase
pump in the basolateral surface of the enterocytes
 Glucose enters the blood from the enterocytes via the GLUT-2 transporter
 This maintains a low concentration of Na+ inside the cell, thus maintaining a
concentration gradient of Na+ from the lumen to the cell for continued entry of Na+ (and
glucose)
 Since both Na+ and glucose are osmotically active substances, water will also move in
from the lumen to the enterocytes and then to the blood.
 Therefore, both lost water and electrolytes (Na+) are replaced due to ORS.
 For this to happen, glucose is a required component in ORS fluid.
4. Explain the differences in transport mechanisms between sodium channels and the
sodium potassium pump in the gut epithelial cells.
Epithelial Na+ channels
Na+/K+ pump
Occurs via simple diffusion.
Occurs via primary active transport.
Energy from ATP not needed. (Passive)
Energy in the form of ATP is needed. (Active)
Molecules travel along electrochemical
gradient.
Molecules pumped against electrochemical
gradient.
The channels protein does not change
shape during action.
Conformational change in the protein is
seen during action.
Does not show saturation kinetics.
Does show saturation kinetics.
MCQ
1. T/F
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B.
C.
D.
E.
Humans can breakdown β [1-4] glycosidic bonds in cellulose.
α amylase cannot hydrolyze α [1-6] glycosidic bonds.
Osmotic diarrhoea occurs in lactase deficiency.
Rice kanji is not suitable for an already dehydrated patient.
Sports drinks are relatively high in sodium and can be used as ORS.
FTTTF
2. Regarding carbohydrate absorption,
A. GLUTs display a tissue specific pattern of expression.
B. GLUT 1 and 3 are responsible for maintaining the basal rate of glucose uptake.
C. GLUT 2 has low Km compared to GLUT 1.
D. Metformin drug increases sensitivity to insulin by phosphorylating GLUT 4.
E. GLUT 5 transports glucose.
TTFTF
Protein digestion
1. Discuss the fate of amino acids after absorption
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Protein digestion occurs in stomach and small intestine, and the absorption of proteins occur in
the small intestine.
Free amino acids, dipeptides and tripeptides are made by protein digestion as the final
products.
Free amino acids are absorbed into the enterocytes, by amino acid-Na+ cotransporters, using
secondary active transport.
Dipeptides and tripeptides are absorbed into enterocytes, by proton linked transport systems
(PepT 1), using secondary active transport.
These dipeptides and tripeptides are hydrolyzed into amino acids by intracellular
dipeptidases and tripeptidases in the cytosol, respectively.
All amino acids are released into the portal vein by facilitated diffusion.
∴ Only free amino acids are found in the portal vein after a meal containing
protein.
They are uptaken by liver.
They are either metabolized by the liver or released into the general circulation.
Branched-chain amino acids are not metabolized by the liver and instead, are taken up by
muscles, via blood.
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Unlike fats and carbohydrates, amino acids are not stored by the body.
∴ In liver excess amino acids are converted into fat or glucose and are used for energy
production.
Amino acids are also used for synthesis of non-protein molecules that contain Nitrogen.
Any amino acids in excess of body needs of the cell are rapidly degraded.
Amino acid catabolism occurs through Citric acid cycle and Urea cycle (NH4+).
MCQ
1.Regarding protein digestion
A.
B.
C.
D.
E.
F.
G.
H.
Trypsin is an endopeptidase.
Carboxypeptidase cleaves terminal bonds from the peptide chain.
HCl denatures proteins to make them more susceptible for digestion.
Pepsin does not have an autocatalytic activity.
Pancreatic juice contains exopeptidases only.
Trypsinogen is activated by enterokinase.
In cystic fibrosis proteins may be present in stools.
Exopeptidase cleaves C terminal amino acids.
Lipid digestion
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1. Outine why triglycerols are considered good for storage of energy.
Fatty acid portion of TAG is highly reduced.
Nonpolar and therefore stored in anhydrous form.
High energy per mole/9 kcal/g.
Can be easily hydrolyzed either
Chemically or
Enzymatically by lipases.
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2. Explain how lipids are digested within the gastrointestinal tract.
Digestion of lipids mainly starts at the stomach.
Lingual lipase gets activated by the acidic pH of the stomach.
Acts mainly on short or medium chain FA [<12]
About 30% of TAG is digested which is important in individuals with pancreatic insufficiency.
Lipids are emulsified in the small intestine.
Done by either mechanical mixing due to peristalsis which breaks lipids into small droplets or
Using detergent properties of bile salts.
This increases surface area of lipid droplets so digestive enzymes can act effectively.
Emulsifying agents stabilize the broken small lipid droplets and prevent them from coalescing.
Lipid digestion is done by enzymes found in the pancreatic juice.
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Secretion of enzymes in enhanced by CCK.
TAG is degraded by pancreatic lipase.
It removes fatty acids from carbons 1 and 3 forming DAGs MAGs and FFA.
FA on carbon 2 is retained.
Pancreatic cholesterol esterase degrades cholesterol esters.
Phospholipids are degraded by phospholipase A2.
Colipase binds to lipase and anchors it at lipid aqueous interface.
Primary products of digestion are (MAG, FFA, cholesterol, lysophospholipids, bile salts, fat
soluble vitamins) are packed within micelles and absorbed.
Short, medium chain fatty acids and glycerol does not need micelles to absorb as they are
water soluble.
MCQ
1. Regarding lipid digestion,
A. Bile salts are reabsorbed in the proximal ileum.
B. Inside enterocytes TAG and CE are resynthesized.
C. Bioavailability of vitamin A is dependent on the fat content of the diet.
D. TAG and CE are packed into VLDL and released via exocytosis.
E. Lipid malabsorption can lead to steatorrhoea.
FTTFT
Cholesterol metabolism
1.Explain how the biosynthesis of cholesterol is regulated.
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The main regulatory, rate limiting step of cholesterol synthesis is HMG CoA → Mevalonate
Enzyme HMG CoA reductase catalyzes this reaction.
This is regulated by short term and long term.
In short term HMG CoA reductase regulated by covalent modification via AMP activated
protein kinase.
When cellular AMP level are increased, AMPK is activated.
causing inactivation of HMG CoA reductase by phosphorylation.
This prevents the cholesterol synthesis.
Phosphorylation is enhanced by insulin and thyroxine.
When cellular AMP level low AMPK is inactivated
Causing activation of HMG CoA reductase by dephosphorylation.
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This increases cholesterol synthesis.
Dephosphorylation is enhanced by glucagon and epinephrine.
This is sterol independent phosphorylation
Sterol accelerated enzyme degradation also regulate the HMG CoA reductase
Increase in cellular concentration of cholesterol, steroids, oxidative forms of cholesterols,
stimulates proteolysis degradation of HMG CoA Reductase via Ubiquitination (Proteasomal
degradation of enzymes)
Statin drugs which are structural analogues to HMG CoA
Competitively inhibits HMG CoA reductase
In long term - enzyme expression is regulated at gene level
SREBP-2 is a transcriptional factor which is bound to ER.
When cellular Cholesterol(steroid) levels are high
proteolytic cleavage(detachment) of SREBP-2 from ER is halted
decreasing cholesterol synthesis.
2.List the mechanism that can be applied to control the body cholesterol level.
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Reduce cholesterol intake and absorption - Sterols and stanols
Reduce de novo synthesis of cholesterol. - statins
Increase HDL level and HDL function
Increase cholesterol catabolism and excretion.
Reduce recycling of bile acids - Resins
Increase cholesterol absorption by cells
MCQ
1. Regarding cholesterol
A. Cholesterol synthesis occurs in adrenal cortex.
B. Cholesterol is hydrophobic.
C. All carbon in cholesterol are from acetyl CoA.
D. Isoprenoids are 5 carbon intermediates.
E. Cholesterol can be found in plants.
TFTTF
2. Regarding bile acids
A. Excess cholesterol is excreted via bile in esterified form as bile acids.
B. Steroid ring is degraded in humans during formation of primary bile acids.
C. Primary bile acids are conjugated by lysine and taurine.
D. Secondary bile acids are formed in the gut.
E. During cholestyramine treatment resins prevent recycling of bile acids.
FFFTT