CARBOHYDRATES AND METABOLISM OF CARBOHYDRATES
PREPARED BY MR. ABHIJIT DAS
CARBOHYDRATES
DEFINITION
Carbohydrates are organic compounds made up of
carbon, hydrogen, and oxygen atoms.
They are one of the three macronutrients (along with
proteins and fats) that provide energy to the body.
CLASSIFICATION
Carbohydrates can be classified based on their
molecular size and complexity:
1.
Monosaccharides: These are the simplest
carbohydrates and cannot be broken down further by hydrolysis. They are
composed of a single sugar unit and are typically sweet-tasting. Examples
include glucose, fructose, and galactose.
2.
Oligosaccharides: These are
carbohydrates composed of 2 to 10 monosaccharide units joined by glycosidic bonds. Oligosaccharides are not as
sweet-tasting as monosaccharides. Examples include sucrose (glucose +
fructose), lactose (glucose + galactose), and maltose (glucose + glucose).
3.
Polysaccharides: These are complex
carbohydrates composed of many monosaccharide units joined by glycosidic bonds. Polysaccharides are typically not
sweet-tasting and are often used by organisms as a source of energy or as
structural components. Examples include starch (a storage polysaccharide in
plants), glycogen (a storage polysaccharide in animals), and cellulose (a
structural polysaccharide in plants).
CHEMICAL PROPERTIES
1.
Solubility: Most monosaccharides and
some disaccharides are highly soluble in water due to the presence of hydroxyl
(-OH) groups in their molecular structure.
2.
Reducing properties: Many carbohydrates,
such as glucose and fructose, have reducing properties because of their ability
to donate electrons.
3.
Acid-base properties: Carbohydrates can
act as both acids and bases, depending on the pH of the solution. For example,
in an acidic solution, the hydroxyl groups on a carbohydrate molecule can
donate a proton and act as an acid.
4.
Isomerism: Carbohydrates exhibit
stereoisomerism, where two molecules have the same chemical formula but differ
in their three-dimensional arrangement of atoms.
5.
Reactivity: Carbohydrates can undergo a
variety of chemical reactions, including hydrolysis, oxidation, reduction,
isomerization, and polymerization.
MONOSACCHARIDES
STRUCTURE OF GLUCOSE
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STRUCTURE OF FRUCTOSE
STRUCTURE OF GALACTOSE
DISACCHARIDES
STRUCTURE OF MALTOSE (GLUCOSE+GLUCOSE)
STRUCTURE OF LACTOSE (GALACTOSE+GLUCOSE)
STRUCTURE OF SUCROSE (GLUCOSE+FRUCTOSE)
POLYSACCHARIDES
CHEMICAL NATURE OF GLYCOGEN
Glycogen is a highly branched polymer of glucose molecules, which are linked together through alpha-1,4 glycosidic bonds. The branches are formed by alpha-1,6 glycosidic bonds, which create a highly branched structure.
CHEMICAL NATURE OF STARCH
Starch is a complex carbohydrate that is composed of two types of glucose polymers: amylose and amylopectin.
Amylose is a linear polymer of glucose units linked together by alpha-1,4 glycosidic bonds.
Amylopectin, on the other hand, is a branched polymer of glucose units that are linked together by alpha-1,4 glycosidic bonds, with branches occurring every 20 to 30 glucose units. The branches are formed by alpha-1,6 glycosidic bonds.
QUALITATIVE TESTS OF CARBOHYDRATES
1.
Benedict's Test: This test is used to
detect the presence of reducing sugars (e.g. glucose, fructose) in a sample.
Benedict's reagent is a mixture of copper sulfate, sodium citrate, and sodium
carbonate. When heated with a reducing sugar, the blue color of the reagent
changes to a brick-red precipitate.
2.
Barfoed's Test: This test is used to
distinguish between monosaccharides and disaccharides. A sample is heated with
Barfoed's reagent (copper acetate in acetic acid) and the presence of a
monosaccharide (e.g. glucose) will result in a brick-red precipitate within 2-3
minutes, whereas a disaccharide (e.g. sucrose) will not give a precipitate until
after 10-15 minutes.
3.
Molisch's Test: This test is used to
detect the presence of all carbohydrates. A sample is treated with
alpha-naphthol and sulfuric acid, and the presence of carbohydrates is
indicated by the formation of a purple ring at the interface of the two
liquids.
4.
Iodine Test: This test is used to
distinguish between different types of carbohydrates. A sample is treated with
iodine solution, and the presence of starch is indicated by a blue-black color,
while the presence of glycogen is indicated by a reddish-brown color.
5.
Seliwanoff's Test: This test is used to
distinguish between aldoses and ketoses. A sample is treated with Seliwanoff's
reagent (resorcinol in concentrated hydrochloric acid), and the presence of a
ketose (e.g. fructose) is indicated by the formation of a cherry-red color
within a few minutes, while an aldose (e.g. glucose) will not give a color
change.
BIOLOGICAL ROLE OF CARBOHYDRATES
1.
Energy Source: Carbohydrates are a
primary source of energy for the human body. Glucose, a simple sugar, is used
by cells to produce ATP (adenosine triphosphate), which is the energy currency
of the body.
2.
Storage of Energy: Carbohydrates can
also be stored in the body for future energy needs. The storage form of carbohydrates
in animals is glycogen.
3.
Immune System Support: Certain
carbohydrates, such as glycoproteins and glycolipids, are involved in the
immune system response. They help the body identify and eliminate foreign
invaders such as viruses and bacteria.
4.
Brain Function: The brain relies heavily
on glucose as an energy source, and carbohydrates are necessary for maintaining
cognitive function. The body has a mechanism to ensure a steady supply of
glucose to the brain, even during periods of fasting or low carbohydrate
intake.
5.
Signal molecules: Certain types of
carbohydrates, such as glycoproteins and glycolipids, play important roles in
cell signaling and communication. They can act as receptors for hormones and
other molecules, or as markers on the surface of cells that help the immune
system identify and target pathogens.
METABOLISM OF CARBOHYDRATES
GLYCOLYSIS
Glycolysis is the metabolic pathway that breaks down
glucose into pyruvate, generating ATP and NADH
in the process. It consists of ten enzymatic reactions, taking place in the
cytoplasm of cells, and is the first step in cellular
respiration, which ultimately produces energy for the cell.
Pyruvate is converted to acetyl CoA before entering
the Krebs cycle. The conversion of pyruvate to acetyl CoA takes place in the
mitochondrial matrix, specifically in the pyruvate dehydrogenase complex. This
complex catalyze the conversion of pyruvate to acetyl CoA and also produce NADH
as a byproduct.
TCA CYCLE/KREBS CYCLE/CITRIC ACID CYCLE
The Krebs cycle, also known as the citric acid cycle
or TCA cycle, is a series of biochemical reactions that occur in the
mitochondrial matrix of eukaryotic cells.
The cycle involves a series of enzyme-catalyzed
reactions that convert acetyl CoA to carbon dioxide, generating ATP, NADH, and
FADH2 in the process. These energy-rich molecules then participate in oxidative
phosphorylation to produce ATP through the electron transport chain.
GLYCOGEN METABOLISM
Glycogen metabolism is regulated by the hormones insulin and glucagon,
which act in opposition to control glycogen synthesis and breakdown.
Insulin, released by the pancreas in response to
high blood glucose levels, promotes glycogen synthesis by stimulating the
activity of glycogen synthase and inhibiting the activity of glycogen
phosphorylase. This leads to an increase in glycogen storage in the liver and
muscle cells, lowering blood glucose levels.
In contrast, glucagon, also released by the
pancreas, promotes glycogen breakdown by activating glycogen phosphorylase and
inhibiting glycogen synthase. This leads to the release of glucose from
glycogen stores, increasing blood glucose levels.
During periods of fasting or exercise, glucagon
levels increase, promoting glycogenolysis and the release of glucose for energy
production. Conversely, after a meal, insulin levels increase, promoting
glycogenesis and the storage of excess glucose as glycogen.
REGULATION OF BLOOD GLUCOSE LEVEL
Blood glucose levels are tightly regulated by a
complex interplay of hormones, including incretins, amylin, insulin, glucagon,
epinephrine, and cortisol.
INCRETINS
Incretins, such as GLP-1 (glucagon-like peptide-1)
and GIP (gastric inhibitory polypeptide), are hormones released from the gut in
response to food intake. They stimulate insulin release from the pancreas,
inhibit glucagon release, and slow down the rate at which the stomach empties
its contents into the small intestine. Incretins also activate the satiety
center in the hypothalamus to decrease appetite and food intake.
AMYLIN
Amylin is a hormone co-secreted with insulin by beta
cells in the pancreas. It slows down the rate at which food empties from the
stomach, which helps to regulate blood glucose levels by reducing the amount of
glucose entering the bloodstream after a meal. Amylin also promotes satiety and
reduces food intake. Additionally, it helps to regulate blood glucose levels by
inhibiting the secretion of glucagon from the pancreas and promoting insulin
release.
INSULIN
In the liver, Insulin stimulates the conversion of
glucose into glycogen, a process called glycogenesis, thereby promoting the
storage of glucose in the liver.
In muscle cells, insulin promotes the uptake of
glucose through the GLUT4 transporter, a protein that facilitates the transport
of glucose across the cell membrane.
Insulin triggers the translocation of GLUT4 from
intracellular compartments to the plasma membrane, allowing glucose to enter
the cell and be used for energy or stored as glycogen.
GLUCAGON
Glucagon is a hormone secreted by the alpha cells of
the pancreas, and it plays a crucial role in raising blood glucose levels in
the body. It acts in opposition to insulin, which lowers blood glucose levels.
When blood glucose levels are low, glucagon is
released to stimulate the liver to convert stored glycogen into glucose and
release it into the bloodstream, a process known as glycogenolysis.
Glucagon also promotes the production of glucose
from non-carbohydrate sources, such as amino acids and fatty acids, a process
called gluconeogenesis.
EPINEPHRIN
Epinephrine, also known as adrenaline, is a hormone
produced by the adrenal glands.
In the liver, epinephrine stimulates the breakdown
of glycogen into glucose, a process known as glycogenolysis. This glucose is
then released into the bloodstream, raising blood glucose levels. Epinephrine
also stimulates the production of glucose from non-carbohydrate sources, such
as amino acids and fatty acids, a process known as gluconeogenesis.
In muscle tissue, epinephrine promotes the breakdown
of glycogen into glucose, which is then used by the muscles for energy. At the
same time, epinephrine reduces glucose uptake by peripheral tissues such as
adipose tissue, helping to ensure that glucose is available to the muscles
where it is needed most.
CORTISOL
Cortisol is a hormone produced by the adrenal glands
in response to stress. One of its primary functions is to increase blood
glucose levels in the body, particularly during periods of stress or fasting.
Cortisol acts on several organs and tissues to
promote the breakdown of glycogen into glucose, increase gluconeogenesis, and
decrease glucose uptake by peripheral tissues.
In the liver, cortisol stimulates the breakdown of
glycogen into glucose, a process known as glycogenolysis. This glucose is then
released into the bloodstream, raising blood glucose levels. Cortisol also
stimulates the production of glucose from non-carbohydrate sources, such as
amino acids and fatty acids, a process known as gluconeogenesis.
Cortisol also reduces glucose uptake by peripheral
tissues such as muscle and adipose tissue, which allows glucose to be available
for other vital organs such as the brain, heart, and liver.
DISEASES RELATED TO ABNORMAL METABOLISM OF CARBOHYDRATES
DIABETES MELITUS
Diabetes mellitus is a metabolic disorder
characterized by hyperglycemia (high blood glucose levels) due to defects in
insulin secretion, insulin action, or both.
Type 1 diabetes mellitus
(T1DM) is an autoimmune disorder in which the immune system attacks and
destroys the beta cells in the pancreas that produce insulin. This results in a
lack of insulin production and requires lifelong insulin replacement therapy.
Type 2 diabetes mellitus (T2DM) is a metabolic disorder characterized by insulin resistance and relative insulin deficiency.