Carbohydrate metabolism is a fundamental biochemical process that ensures a constant supply of energy to living cells. The most important carbohydrate is glucose, which can be broken down via glycolysis, enter into the Krebs's cycle and oxidative phosphorylation to generate ATP

1. Digestion &
metablism of
carbohydrates

2. Digestion of Carbohydrate
 In the mouth digestion of starch and
glycogen begins during mastication under
the effect of salivary amylase. They are
converted to dextrins and maltose
Salivary amylase
Starch & Glycogen Dextrins + Maltose
pH 6-8

3.  In the stomach the action of salivary
amylase stops due to the acidic medium
because the enzyme needs alkaline
medium.

4. In the duodenum starch and glycogen digestion is
completed by the action of :
 pancreatic amylase
 maltase
 disaccharidases ( found in the brush border of
intestinal cells ).
Finally monosaccharides are obtained.
Pancreatic amylase
Starch + Dextrins Maltose
 pH 7

5. Maltase
Maltose 2 Glucose
Sucrase
Sucrose Glucose + Fructose
Lactase
Lactose Glucose + Galactose

6. Lactose intolerance
Is due to deficiency of lactase enzyme
It may be:
 Congenital (Primary) :
due to genetic defect of the enzyme, they can not tolerate
drinking milk
 Acquired ( secondary) :
due to gastroenteritis especially in children and loss of
the brush border of intestinal cells by infectious agents.
 Clinical picture ,
Lactose accumulates in the lumen, by the osmotic effect
influx of water in the small intestine occurs :
intestinal distention , nausea , colic, and watery
diarrhea.

7. Absorption of Carbohydrates
 The end products of carbohydrate digestion are :
Glucose , Fructose and Galactose
 They are absorbed as monosaccharides
1- Facilitated transport
Requires specific protein carriers called glucose transporter
proteins GLUT. They combine with fructose and facilitate its
entry into the cell.
2- Active transport
Glucose and galactose are transported by this method .
They are absorbed against a concentration gradient, so need
energy and carrier protein.

8. Fate of absorbed sugars
 The absorbed sugars pass from intestine to
the liver through portal circulation .
 Fructose & galactose are converted into
glucose.
 The liver acts as a blood glucostat i.e It
converts excess absorbed glucose into
stored glycogen
 Reconverts the stores to glucose during
starvation to maintain adequate level of
glucose in blood.

9. Metabolic fate of
Glucose

10. Glucose
Glycogenesis
to form glycogen
Glycolysis &
TCA cycle
to produce energy
HMP pathway
for synthesis of
pentoses
Provides acetyl CoA
for synthesis of:
Fatty acids
Cholesterol
Ketone bodies and
steroids
Non essential amino
acids
Synthesis of other
carbohydrates:
* Galactose for
lactose synthesis

11.  Glycolysis means breakdown of
glucose . It occurs in cytoplasm of all
body cells either:
 in presence of oxygen ( aerobic)
 or in absence of oxygen ( anaerobic)
Glycolysis

13. 2 ATP
2 pyruvate 2 NADH+2H
End products of glycolysis
Kreb,s Cycle Respiratory Chain
In mitochondria
To give 6 ATP
(3 ATP for each one)
So the overall energy is 8 ATP.

14. Pyruvate will enter the mitochondria
to be converted to Acetyl-CoA ,which
enter Kreb’s cycle .
So the overall energy is 8 ATP.
Glycolysis under Aerobic conditions
2

15. Glycolysis under Anaerobic conditions
the mitochondrial fate of pyruvate and NADH is
absent so, the net energy produced is only 2
ATP only .
1- in RBC
2- cornea, lens, retina, renal medulla
non functioning mitochondria due to decreased
blood supply and oxygen
3- during stressful muscular exercise. Glycolysis
is considered the main source of ATP with
production of lactic acid
2

16. The tricarboxylic acid cycle is also called
Citric acid cycle or Kreb’s cycle.
It is the final common pathway for
oxidation of carbohydrates, lipid and
proteins because their oxidation give
Acetyl-CoA.
All enzymes of the cycle are located in
the mitochondria.
Tricarboxylic Acid Cycle
(TCA cycle)

18. Functions of Citric acid cycle
 TCA cycle is amphibolic :
serves both
catabolic ( provides energy, ATP)
anabolic ( synthesis of important
substances)

19. So its functions include:
1-It provides ATP
.
2-It provides citrate that is the precursor for
synthesis of fatty acids
.
3-Synthesis of non essential amino acids
4- TCA cycle shares in gluconeogenesis.
Gluconeogenesis is synthesis of glucose
from non carbohydrate sources
5- It gives the precursors for Heme synthesis ,
that is an important part of hemoglobin

20. TCA cycle

21. Energy production of TCA cycle
One NADH+H 3ATP
One FADH2 2ATP
3 NAD (3 ×3ATP) + 1 FAD( 2ATP) + 1GTP= 12 ATP
So one molecule of Acetyl-Co oxidation
in TCA cycle will give 12 ATP

22. So, one glucose molecule
gives: 8 ATP from glycolysis, 6
ATP from intermediate reaction
(3x2 pyruvate), 24 ATP from
TCA cycle (12x2 acetylCoA).
Total: 38 ATP.