Digestion and absorption of Carbohydrates

Digestion and absorption of Carbohydrates

1. Digestion and absorption of
Carbohydrates
Session 2

2. •Dietary carbohydrates principally consist of the polysaccharides: starch and
glycogen.
•It also contains disaccharides: sucrose, lactose, maltose and in small
amounts monosaccharideslike fructose and pentoses.
•Liquid food materials like milk, soup, fruit juice escape digestion in
mouth as they are swallowed, but solid foodstuffs are masticated
thoroughly before they are swallowed.
Digestion in Mouth
•Digestion of carbohydrates starts at the mouth, where they come in
contact with saliva during mastication. Saliva contains a carbohydrate
splitting enzyme called salivary amylase (ptyalin).

3. Digestion in Stomach
•No carbohydrate splitting enzymes are available in gastric juice. HClmay
hydrolyze some dietary sucrose to equal amounts of glucose and fructose.
Digestion in Duodenum
Pancreatic juice contains a carbohydrate-splitting enzyme pancreatic
amylase.It requires Cl-for activity. The enzyme hydrolyzes glycosidiclinkages
within the polysaccharide molecule.
Digestion in the small intestine
a.Pancreatic amylase –hydrolyze terminal glycocidiclinkage in
polysaccharide and oligosaccharide
b.Lactase –hydrolyses lactose to glucose and galactose
c.Maltase –hydrolysemaltose to two glucose units
d.Sucrase–sucrose to glucose and fructose

4. Absorption of Carbohydrates
•Products of digestion of dietary carbohydrates are practically completely
absorbed almost entirely from the small intestine.
Mechanism of absorption
1.Simple Diffusion
This is dependent on sugar concentration gradients between the intestinal
lumen. Mucosal cells and blood plasma. All the monosaccharidesare probably
absorbed to some extent by simple ‘passive’ diffusion.
2. Active transport
Glucose and galactose are absorbed very rapidly and hence it has been
suggested that they are absorbed actively and it requires energy.
fructose is not absorbed by simple diffusion alone and it is suggested that some
mechanism facilitates its transport, called as” facilitated transport”

5. Glycolysis
Oxidation of glucose or glycogen to pyruvate and lactate is called
glycolysis.
It occurs virtually in all tissues. Erythrocytes and nervous tissues derive
its energy mainly form glycolysis.
This pathway is unique in the sense that it can utilize O2 if available
(‘aerobic’) and it can function in absence of O2 also (‘anaerobic’)

6. •Aerobic Phase
Aerobic phase includes the conversion of glucose to pyruvate
Oxidation is carried out by dehydrogenation and reducing equivalent is transferred to
NAD. NADH + H+ in presence of O2 is oxidized in electron-transport chain producing
ATP.
•Anaerobic Phase
This phase includes the conversion of Glucose to lactate
NADH cannot be oxidized, so no ATP is produced in electron transport chain. But the
NADH is oxidized to NAD+ by conversion of pyruvate to Lactate, without producing ATP.
Anaerobic phase limits the amount of energy per molecule of glucose oxidized. Hence,
to provide a given amount of energy, more glucose must undergo glycolysis under
anaerobic as compared to aerobic.
Enzymes involved in glycolysis are present in cytoplasm.

8. Class Assignment 1 (10 marks)
Study the glycolysis pathway and;
1.Write the overall equation for glycolysis (indicating the net energy
yielded) (2.5 marks)
2.Explain the significance of the pathway (2.5 marks)
3.Discuss how fructose (2.5 marks) and galactose (2.5 marks) enter
glycolysis pathway

9. Oxidation of Pyruvate
•Pyruvate is common intermediate of many catabolic reactions.
•E.gcan be concerted to alanine, acetylcoA, Lactate, Oxaloacetate
•Oxidation of pyruvate into acetyl CoA-aerobic process (O2 terminal
electron-acceptor) which takes place in the mitochondrial matrix of
eukaryotic cells
•Pyruvate is transported into mitochondrial matrix by special
transporter.Insidematrix pyruvate is oxidized into acetylCoAby pyruvate
dehydrogenase complex which is complex of E1, E2 and E3 enzymes.
•This enzymerequiress five coenzymes-TPP, Lipoate, CoA, FAD and NAD+
•Where: E1 = pyruvate dehydrogenase,
E2 = dihydrolipoyltransacetylase,
E3 = dihydrolipoyldehydrogenase

10. •In your reading think about this….
How is TCA cycle regulated?

11. The TCA cycle/Krebs cycle
•Final common pathway for complete exudation of carbohydrates,
fatty acids and many amino acids. Common pathway for catabolism of
acetyl COA ,a common intermediate of different catabolic pathways
•

15. Regulation of the Krebs Cycle
•Primary function of the cycle is to provide energy, thus rate of the cycle is
adjusted to meet an animal cells ATP demand.
•Increased utilization of ATP increases the rate the cycle because of
availability of oxidized coenzymes necessary for the continuation of the
cycle (NAD+, FAD) and ADP which is needed for oxidative phosphorylation.
•High levels of ATP and NADH are inhibitoryindicating high energy status of
the cell. ATP inhibits both citrate synthase and isocitratedehydrogenase
where as both are activated by high levels of ADP. NADH inhibits isocitrate
dehydrogenase and α-Ketoglutaratedehydrogenase.
•Complementary mechanisms of controlling rate of acetyl CoA formation
and rate of acetyl CoA degradation is also involved