Objectives
Carbohydrates: Brief introduction to the
digestion and absorption of carbohydrates,
Aerobic and anaerobic breakdown of
Glucose, Glycolysis, Pentose Phosphate
Pathway, Glycogenolysis, Glycogenesis,
Gluconeogenesis, Citric acid cycle,
Energetics of various metabolic processes.
The
major
source
of
carbohydrate is found in plants.
Dietary carbohydrate principally
consist of
Polysaccharides
:- Starch,
glycogen & cellulose.
Disaccharides :- Sucrose and
Maltose.
Monosaccharides :- glucose
and fructose.
Monosaccharides does not need digestion.
Action of ptylin (salivary amylase)
• Location: mouth
• It is α-amylase and requires Cl− ion for
activation with an optimum pH of 6.7 (Range
6.6 to 6.8).
• The enzyme hydrolyses α-1→ 4 glycosidic
linkages
deep
inside
polysaccharide
molecules.
• However, ptylin action stops in the stomach
when the pH falls to 3.0.
There are two phase of intestinal digestion….
Digestion due to pancreatic amylase
Digestion due to intestinal brush border enzyme.
Summary
Active Transport. The transport of glucose
and Galactose across the brush boarder of
mucosal cells occur by active transport. It is
an energy requiring process that requires
transport protein and presence of sodium
ions. A sodium dependent glucose
transporter (SGLT- 1) binds both glucose
and sodium at separate sites and
transports them into the cell. The sodium
transport down conc. gradient & glucose
transported against conc. gradient. This
process is called cotransport or symport.
The energy for this reaction is provided by
ATP linked to sodium pump.
Secondary active transport, is transport of
molecules across the cell membrane utilizing
energy in other forms than ATP. This energy
comes from the electrochemical gradient
created by pumping ions out of the cell.
This Co-Transport can be either via antiport or
symport
Factors affecting rate of absorption of Monosaccharides
• The absorption is faster through intact mucosa. The absorption
is decreased if there is some inflammation or injury to the
mucosa.
• Thyroid hormones ↑ the rate of absorption of glucose.
• Mineralocorticoid,i.e Aldosterone ↑ the rate of absorption.
• Vitamin B6,B12, pantothenic acid, folic acid are
required for absorption of glucose.
• With advancing age, rate of absorption declines.
• Note: Insulin has no role in the absorption of
monosaccharide like glucose.
Why cellulose is not digested by humans?
Cellulose is polysaccharide found in plants. It
contain β- (1-4) glycosidic bond in its structure.
Humans cannot synthesize the enzyme which
can break β glycosidic bond. So, cellulose is
not digested by humans. But, although is not
digested it is one of the imp component in the
diet. Because, undigested cellulose provide
bulk or fiber in the diet. Fiber helps in intestinal
motility & as a stool softer.
Recap
The major source of carbohydrate is found in plants. Dietary carbohydrate principally consist of
Polysaccharides
Two types of enzymes are important for digestion
Alpha amylase (Salivary, Panceriatic) and Disaccharidase both amylase require Cl- for activation
No enzymatic breakdown occur in Stomach for carbohydrates
There are two phase of intestinal digestion….
Digestion due to pancreatic amylase
Digestion due to intestinal brush border enzyme.
Mechanisms of absorption
1- passive diffusion 2- facilitated diffusion 3- active transport
Monosaccharides absorbed by facilitated diffusion i.g Glucose and Na+ co transport
Humans cannot synthesize the enzyme which can break β glycosidic bond.
Objectives
Glycolysis, Pentose Phosphate
Pathway,
What is glycolysis?
Glycolysis is a series of reactions that extract energy
from glucose by splitting it into two three-carbon
molecules called pyruvates. Glycolysis is an ancient
metabolic pathway, meaning that it evolved long ago,
and it is found in the great majority of organisms alive
today
In organisms that perform cellular respiration, glycolysis
is the first stage of this process. However, glycolysis
doesn’t require oxygen, and many anaerobic
organisms—organisms that do not use oxygen—also
have this pathway.
Glucose + 2NAD+2ADP + 2Pi = 2Pyruvate + 2NADH + 2H+ + 2ATP + 2H2O
It is also called as Embden-Meyerhof Pathway (EMP)
→ it is defined as the sequence of reactions converting
glucose or glycogen to pyruvate or lactate with
production of ATP.
→ Enzymes takes place in cytosomal fraction of the
cell.
→ major pathway in tissues lacking mitochondria like
erythrocytes, cornea, lens etc.
→ it is essential for brain which is dependent in
glucose for energy.
Historical Perspective
Glycolysis was the very first biochemistry or oldest biochemistry
studied.
It is the first metabolic pathway discovered.
Louis Pasture 1854-1864: Fermentation is caused by microorganism.
Pastuer’s effect: Aerobic growth requires less glucose than anaerobic
condition.
Buchner; 1897: Reactions of glycolysis can be carried out in cell-free
yeast extract.
Harden and Young 1905: 1: inorganic phosphate is required for
fermentation. 2: yeast extract could be separated in small molecular
weight essential coenzymes or what they called Co-zymase and bigger
molecules called enzymes or zymase.
1940: with the efforts of many workers, complete pathways for glycolysis
was established.
Inorganic
Phosphate
Stage 1: A preparatory stage in which glucose is phosphorylated,
converted to fructose which is again phophorylated and cleaved into two
molecules of glyceraldehyde-3-phosphate. In this phase there is an
investment of two molecules of ATP.
Stage 2: The two molecules of glyceraldehyde-3- phosphate are converted
to pyruvate with concomitant generation of four ATP molecules and two
molecules of NADH. Thus there is a net gain of two ATP molecules per
molecule of Glucose in glycolysis.
Glucose + 2NAD+2ADP + 2Pi = 2Pyruvate + 2NADH + 2H+ + 2ATP + 2H2O
The pentose phosphate pathway (also called
the phosphogluconate pathway and the hexose
monophosphate shunt) is a metabolic
pathway parallel to glycolysis.
It generates NADPH and pentoses (5carbon sugars) as well as ribose 5-phosphate, the
last one a precursor for the synthesis of
nucleotides. While it does involve oxidation
of glucose, its primary role is anabolic rather
than catabolic.
Pentose phosphate pathway takes place in the cytosol;
Ribose-5-phosphate


Demand for NADPH
Biosynthetic pathways
• FA synthesis (liver, adipose, mammary)
• Cholesterol synthesis (liver)
• Steroid hormone synthesis (adrenal, ovaries, testes)
Detoxification (Cytochrome P-450 System) –
liver
 Reduced glutathione as an antioxidant (RBC)
 Generation of superoxide (neutrophils)


Oxidative phases
Reactions producing
NADPH
 Irreversible

Non-oxidative
phases

Produces ribose-5-P
 Reversible reactions
feed to glycolysis

Oxidative phase
Oxidative phase of pentose phosphate pathway.
Glucose-6-phosphate (1), 6-phosphoglucono-δ-lactone (2), 6phosphogluconate (3), ribulose 5-phosphate (4)
The overall reaction for this process is:
Glucose 6-phosphate + 2 NADP+ + H2O → ribulose 5-phosphate + 2 NADPH + 2 H+ + CO2
Net reaction: 3 ribulose-5-phosphate → 1 ribose-5-phosphate + 2 xylulose-5-phosphate → 2
fructose-6-phosphate + glyceraldehyde-3-phosphate