Carbohydrates, which mean
hydrated carbon, are composed of C, H, and O in the ratio of 1:2:1. Their
general formula is C(H2O)x, where x is a whole number ranging from
three to thousands. Chemically, carbohydrates are defined as polyhydroxy
aldehydes or ketones, or complex substances that yield polyhydroxy aldehyde or
ketone subunits upon hydrolysis. They are primarily sourced from green plants,
as they are the products of photosynthesis and serve as precursors for other
compounds in plants through various chemical changes.
Carbohydrates can combine
with other molecules to form complex conjugated molecules, such as glycolipids
(carbohydrates + lipids) and glycoproteins (carbohydrates + proteins).
Carbohydrates are classified
into three major classes: monosaccharides,
oligosaccharides, and
polysaccharides.
Monosaccharides, which are
simple sugars, cannot be further hydrolyzed into simpler units. They are sweet
in taste and soluble in water. Chemically, they are either polyhydroxy
aldehydes or ketones. Except for one carbon atom, all other carbon atoms in a
monosaccharide have a hydroxyl group. The remaining carbon atom is either a
part of an aldehyde group or a keto group. Monosaccharides with an aldehyde
group are called aldoses, while those with a keto group are called ketoses.
Examples of aldoses include glyceraldehyde, and examples of ketoses include
dihydroxyacetone. Monosaccharides with 3-7 carbon atoms are found in nature,
and they have a general formula of Cn(H2O)n.
Note:
Glucose and fructose have the same molecular formula of C6H12O6, but they have
different molecular structures or arrangements of atoms.
Oligosaccharides are
sugars that yield 2 to 10 monosaccharides upon hydrolysis. They are less sweet
in taste and less soluble in water.
Disaccharides are
formed when two monosaccharides combine. Examples include maltose (glucose +
glucose), lactose (glucose + galactose), and sucrose (glucose + fructose).
Trisaccharides are
formed when three monosaccharides combine.
The bond formed between two
monosaccharides is called a glycosidic linkage. Water is removed during the
formation of this linkage. Examples of glycosidic linkages include
1,4-glycosidic linkage in maltose and 1,2-glycosidic linkage in sucrose.
Polysaccharides are
composed of many glucose monomers linked by glycosidic bonds, and they are
usually branched. They are tasteless and sparingly soluble in water. Examples
of polysaccharides include starch and glycogen.
Starch is a polymer of
glucose and consists of linear and branched chains. It is insoluble in water,
which allows it to be stored in plant cells. There are two types of starch:
amylose and amylopectin. Amylose has unbranched chains of glucose and is
soluble in hot water, while amylopectin is branched and insoluble in hot or
cold water. Starch gives a blue color with iodine.
Glycogen is
a polysaccharide that animals use to store glucose. It consists of long
branched chains of amylose and is insoluble in water. Glycogen gives a red
color with iodine.
Cellulose: It
is a polymer of glucose. The rings of glucose are arranged in a flip-flop
manner. Cellulose is the building and supporting material of the cell wall. It
is highly insoluble in water and gives no color with iodine. Examples of
cellulose-containing materials are wood, paper, and cotton fibers.
Functions of
Carbohydrates
Carbohydrates exhibit a
variety of functions in living organisms.
Pentose:
Ribose, a type of pentose, is a constituent of RNA (ribonucleic acid) and
deoxyribose is a constituent of DNA (deoxyribonucleic acid). These nucleic
acids are essential for genetic information storage and transfer in cells.
Additionally, ribose is used in the synthesis of some coenzymes, such as NAD
(nicotinamide adenine dinucleotide), which play important roles in cellular
metabolism.
Disaccharide:
Glucose, which is a monosaccharide, is the main source of energy for living
organisms. It is utilized through cellular respiration to produce ATP
(adenosine triphosphate), the energy currency of cells. Maltose, a disaccharide
composed of two glucose units, is used in the production of alcohol, such as in
brewing and fermentation processes. Lactose, another disaccharide composed of
glucose and galactose, is an important source of food for young mammals,
including human infants. Sucrose, commonly known as table sugar, is a
disaccharide composed of glucose and fructose, and it serves as a source of
energy for plants and is also widely used as a sweetener in food and beverages.
Polysaccharides:
Polysaccharides have various functions in living organisms. They can act as
food and energy stores, as well as structural materials.
Starch:
Starch is a polysaccharide composed of glucose monomers and serves as a storage
form of glucose in plants. It consists of both linear and branched chains, and
it is insoluble in water. This insolubility allows starch to be stored in plant
cells without affecting the osmotic balance. There are two types of starch:
amylose, which has unbranched chains of glucose and is soluble in hot water,
and amylopectin, which has branched chains and is insoluble in both hot and
cold water. Starch can be broken down into glucose molecules for energy during
plant metabolism.
Glycogen:
Glycogen is the animal equivalent of starch and serves as a storage form of
glucose in animals, including humans. It is stored in animal cells,
particularly in the liver and muscles, and can be broken down into glucose for
energy during periods of high energy demand, such as during exercise or
fasting. Glycogen has a highly branched structure, which allows for efficient
storage and rapid release of glucose when needed.
Cellulose: Cellulose
is a polysaccharide that forms the structural material of the cell walls in
plants. It provides rigidity and support to plant cells, helping them maintain
their shape and resist external pressures. Cellulose is composed of long chains
of glucose molecules arranged in a flip-flop manner, forming a complex network
of fibers. Unlike starch and glycogen, cellulose is highly insoluble in water
and is not used as an energy source in most animals, including humans. However,
some animals, such as ruminant mammals, can digest cellulose with the help of
specialized microorganisms in their digestive system.
In addition to their roles
as energy sources and structural materials, carbohydrates also play important
roles in cell signaling, cell recognition, and immune response through the
formation of glycolipids and glycoproteins, which are complex carbohydrate-containing
molecules on the cell surface. These glycolipids and glycoproteins are involved
in cell-cell communication, cell adhesion, and recognition processes, playing
critical roles in various physiological and pathological processes in living
organisms.
Furthermore, carbohydrates
also serve as important components of many biological molecules, such as nucleotides,
which are the building blocks of nucleic acids (DNA and RNA), and ATP
(adenosine triphosphate), which is the primary energy currency in cells.
Carbohydrates also play a role in the modification and stabilization of
proteins through processes like glycosylation, where carbohydrates are attached
to proteins to form glycoproteins, regulating their functions and stability.
Carbohydrates also have
important functional roles in the immune system. For example, certain types of
carbohydrates, such as antigens on the surface of pathogens, are recognized by
the immune system as foreign, triggering immune responses to fight against
infections. Carbohydrates also play a role in blood typing, where specific
carbohydrate antigens on red blood cells determine an individual's blood type.
In summary, carbohydrates
are essential biomolecules with diverse functions in living organisms. They
serve as a major source of energy, provide structural support, play roles in
cell signaling and recognition, and are involved in various physiological and
pathological processes. The complex and diverse roles of carbohydrates make
them crucial for the proper functioning of cells and organisms as a whole.
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