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Overview of Carbohydrate
Metabolism
Introduction
Carbohydrates
are carbon compounds
that contain large quantities of hydroxyl
groups.
The presence of the hydroxyl groups
allows carbohydrates to interact with the
aqueous solution and to participate in
hydrogen bonding
All carbohydrates can be classified as
either
 monosaccharides, oligosaccharides
(from 2 – 10 monosaccharide units),or
polysaccarides.


The monosaccharides commonly
found in humans are classified
according to the number of
carbons they contain in their
backbone structures.

The major monosaccharides
contain four to six carbon atoms.
#
Category Name Relevant examples
Carbons
3
Triose
Glyceraldehyde,
Dihydroxyacetone
4
Tetrose
Erythrose
,Ribose, Ribulose
Xylulose
,Glucose, Galactose
Mannose, Fructose
5
Pentose
6
Hexose
7
Heptose
Sedoheptulose
Nonose
Neuraminic (Sialic)
acid
9

The glycosidic Covalent linkage of
two monosaccharides result in
disaccharides .

Several physiogically important
disaccharides are sucrose, lactose
and maltose.
•Sucrose : prevalent in sugar cane and
sugar beets, is composed of glucose and
fructose through an a-(1,2)b-glycosidic
bond

Lactose : is found exclusively in the
milk of mammals and consists of
galactose and glucose in a b-(1,4)
glycosidic bond.

Maltose : the major degradation
product of starch, is composed of
glucose monomers in an a-(1,4)
glycosidic bond.

Most of the carbohydrates found in
nature occur in the form of high
molecular weight polymers called
polysaccharides.

The monomeric building blocks found
in polysaccharides is D-glucose.

When polysaccharides are composed
of a single monosaccharide building
block, they are termed
homopolysaccharides.

Polysaccharides with more than one
type of monosaccharide are termed
heteropolysaccharides.

Glycogen is the major form of stored
carbohydrate in animals.

This molecule is a homopolymer of
glucose in a-(1,4) linkage; it is also
highly branched, with a-(1,6) branch
linkages occurring every 8-10
residues.

Glycogen is a very compact structure
that results from the coiling of the
polymer chains.

This compactness allows large
amounts of carbon energy to be
stored in a small volume, with little
effect on cellular osmolarity.

Starch is the major form of stored
carbohydrate in plant cells.

Its structure is identical to glycogen,
except for a much lower degree of
branching (about every 20-30
residues).

Unbranched starch is called amylose;
branched starch is called amylopectin.
Dietary carbohydrate from which humans
gain energy enter the body in complex forms,
such as disaccharides and the polymers starch
(amylose and amylopectin) and glycogen.
The polymer cellulose is also consumed but
not digested . (WHY)
 The breakdown of polymeric sugars begins
in the mouth.
 Saliva has a slightly acidic pH of 6.8 and
contains lingual amylase that begins the
digestion of carbohydrates.
 The mixture of gastric secretions, saliva,
and food, known collectively as chyme,
moves to the small intestine.
 a-amylase enzyme is secreted by the
pancreas and has the same activity as
salivary amylase, producing disaccharides.
 Which converted to monosaccharides by
the specific intestinal disaccharidases,
sucrase, lactase, and maltase.
 The resultant glucose and other simple
carbohydrates are transported across the
intestinal wall to the hepatic portal vein and
then to liver parenchymal cells and other tissues.
 There they are converted to fatty acids, amino
acids, and glycogen, or
 oxidized by the various catabolic pathways of
cells.
Blood sugar levels are controlled by
three hormones: insulin, glucagon,
and epinephrine.
 If the concentration of glucose in the
blood is too high, insulin is secreted by
the pancreas.
 Insulin stimulates the transfer of glucose
into the cells, especially in the liver and
muscles, other organs are also metabolize
glucose.

In the liver and muscles, most of the
glucose is changed into glycogen by the
process of glycogenesis (anabolism).
 Glycogen is stored in the liver and
muscles until needed at some later time
when glucose levels are low.
 If blood glucose levels are low, then
epinephrine and glucogon hormones
are secreted to stimulate the conversion
of glycogen to glucose.
 This process is called glycogenolysis
(catabolism).

If glucose is needed immediately when
entering the cells to supply energy, it
begins the metabolic process called
glycoysis (catabolism).
 The end products of glycolysis are
pyruvic acid and ATP.

Since glycolysis releases little ATP, further
reactions continue to convert pyruvic acid
to acetyl CoA and then citric acid in the
citric acid cycle.
 The majority of the ATP is made from
oxidations in the citric acid cycle in
connection with the electron transport
chain.

During exhausting muscular activity,
pyruvic acid is converted into lactic acid
rather than acetyl CoA.
 During the resting period, the lactic acid
is converted back to pyruvic acid.
 The pyruvic acid in turn is converted back
to glucose by the process called
gluconeogenesis (anabolism).
 If the glucose is not needed at that
moment, it is converted into glycogen by
glycogenesis.

These processes are summarized in the
Metaboism Summary in the graphic
below.
 Each of these processes will be developed
in greater detail in subsequent lectures
