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CARBOHYDRATES
Carbohydrates have the general molecular formula (CH 2 O)n.
Sugars
Monosaccharides
These are simple sugars that serve as fuel molecules as well as fundamental constituents of living organisms, are the simplest carbohydrates, and are required as energy sources. The most commonly known ones are perhaps glucose and fructose.
Three common sugars share the same molecular formula: C 6 H 12 O 6. Because of their six carbon
atoms, each is a hexose.
They are: glucose , "blood sugar", the immediate source of energy for cellular respiration galactose , a sugar in milk (and yogurt), and fructose , a sugar found in honey.
Although all three share the same molecular formula (C 6 H 12 O 6 ), the arrangement of atoms differs in each case. Substances such as these three, which have identical molecular formulas but different structural formulas, are known as structural isomers.
Carbohydrates were once thought to represent "hydrated carbon". However, the arrangement of atoms in carbohydrates has little to do with water molecules.
Starch and cellulose are two common carbohydrates. Both are macromolecules with molecular weights in the hundreds of thousands. Both are polymers (hence " polysaccharides "); that is, each is built from repeating units, monomers, much as a chain is built from its links.
The monomers of both starch and cellulose are the same: units of the sugar glucose.
Glucose, galactose, and fructose are "single" sugars or monosaccharides. Two monosaccharides
can be linked together to form a "double" sugar or disaccharide.
Disaccharides Three common disaccharides:
sucrose — common table sugar = glucose + fructose lactose — major sugar in milk = glucose + galactose maltose — product of starch digestion = glucose + glucose
Although the process of linking the two monomers is rather complex, the end result in each case is the loss of a hydrogen atom (H) from one of the monosaccharides and a hydroxyl group (OH)
from the other. The resulting linkage between the sugars is called a glycosidic bond. The
molecular formula of each of these disaccharides is
C 12 H 22 O 11 = 2 C 6 H 12 O 6 − H 2 O
All sugars are very soluble in water because of their many hydroxyl groups. Although not as concentrated a fuel as fats, sugars are the most important source of energy for many cells.
Carbohydrates provide the bulk of the calories (4 kcal/gram) in most diets, and starches provide the bulk of that. Starches are polysaccharides.
Polysaccharides
Polysaccharides are complex carbohydrate polymers consisting of more than two
monosaccharides linked together covalently by glycosidic linkages in a condensation reaction.
Being comparatively large macromolecules, polysaccharides are most often insoluble in water.
Polysaccharides are extremely important in organisms for the purposes of energy storage and
structural integrity.
There are two types of polysaccharides: homo-polysaccharides and hetero-polysaccharides. A
homo-polysaccharide is defined to have only one type of monosaccharide repeating in the chain;
whereas, a hetero-polysaccharide is composed of two or more types of monosaccharides. In both
types of polysaccharide, the monosaccharide can link in a linear fashion or they can branch out
into complex formations. It should also be noted that for a polysaccharide to be considered acidic
it must contain one or more of the following groups: phosphate, sulfuric, or carboxyl.
amylopectin differs from amylose in being highly branched. At approximately every thirtieth residue along the chain, a short side chain is attached by a glycosidic bond to the #6 carbon atom (the carbon above the ring). The total number of glucose residues in a molecule of amylopectin is several thousand.
Starches are insoluble in water and thus can serve as storage depots of glucose. Plants convert excess glucose into starch for storage. The image shows starch grains (lightly stained with iodine) in the cells of the white potato. Rice, wheat, and corn (maize) are also major sources of starch in the human diet.
Before starches can enter (or leave) cells, they must be digested. The hydrolysis of starch is done by amylases. With the aid of an amylase (such as pancreatic amylase), water molecules enter at
the 1 -> 4 linkages, breaking the chain and eventually producing a mixture
of glucose and maltose. A different amylase is needed to break the 1 -> 6 bonds of amylopectin.
Glycogen
Animals store excess glucose by polymerizing it to form glycogen. The structure of glycogen is similar to that of amylopectin, although the branches in glycogen tend to be shorter and more frequent.
Glycogen is broken back down into glucose when energy is needed (a process called glycogenolysis).
In glycogenolysis ,
Phosphate groups — not water — break the 1 -> 4 linkages The phosphate group must then be removed so that glucose can leave the cell.
The liver and skeletal muscle are major storage depots of glycogen.
There is some evidence that intense exercise and a high-carbohydrate diet ("carbo-loading") can increase the reserves of glycogen in the muscles and thus may help marathoners work their muscles somewhat longer and harder than otherwise. But for most of us, carbo loading leads to increased deposits of fat.
Cellulose
Cellulose is probably the single most abundant organic molecule in the biosphere. It is the major structural material of which plants are made. Wood is largely cellulose while cotton and paper are almost pure cellulose.
Like starch, cellulose is a polysaccharide with glucose as its monomer. However, cellulose
differs profoundly from starch in its properties.
Because of the orientation of the glycosidic bonds linking the glucose residues, the rings of glucose are arranged in a flip-flop manner. This produces a long, straight, rigid molecule. There are no side chains in cellulose as there are in starch. The absence of side chains allows these linear molecules to lie close together. Because of the many -OH groups, as well as the oxygen atom in the ring, there are many opportunities for hydrogen bonds to form between adjacent chains.
The result is a series of stiff, elongated fibrils — the perfect material for building the cell walls of plants.
Many organisms store energy in the form of polysaccharides, commonly homopolymers of
glucose. Glycogen, the sugar used by animals to store energy, is composed of alpha-1,4-
glycosidic bonds with branched alpha-1,6 bonds present at about every tenth monomer. Starch,
used by plant cells, is similar in structure but exists in two forms: amylose is the helical form of
starch comprised only of alpha-1,4 linkages, and amylopectin has a structure like glycogen
except that the branched alpha-1,6 linkages are present on only about one in 30 monomers.
These polysaccharides often contain tens of thousands of monomers, and each type is
synthesized in the cell and broken down when energy is needed.
Glycogen metabolism is an intricate process involving many enzymes and cofactors resulting in
the regular release and storage of glucose. This metabolic process is in turn broken down to
glycogen degradation and synthesis. Glycogen synthesis is carried out by the enzyme glycogen
synthase in which the activated form of glucose, UDP-glucose (uridine diphosphate), is formed
by way of the reaction between UTP and glucose-1 phosphate. From this synthesis two outer
phosphoryl groups are released from UTP producing the pyrophosphate compound.
Pyrophosphate becomes an important aspect in this portion of the synthesis as the reaction to
produce UDP-glucose is readily reversible. What allows the reaction to be driven forward is the
hydrolysis of the pyrophosphate to orthophosphate in an irreversible reaction thus allowing the
production of UDP-glucose to continue unhindered. The UDP-glucose is then attached to the
non-reducing ends of glycogen. How this is accomplished is through an alpha-1,4-glycosidic
linkage at the C-4 terminal with the terminal hydroxyl group ready to bind on glycogen. At this
point the enzyme glycogen synthase plays the important role of catalyzing the attachment of
UDP. Since an oligomer of at least four monomers is required for glycogen synthase to extend a
chain, the process uses a primer that is itself provided by another enzyme, glycogenin. After
several units of UDP have been attached to the glycogen by way of alpha-1,4 linkages, branching
begins to take place by breaking an alpha-1,4 link and forming a alpha-1,6-link.A number of
other enzymes, including insulin, play important roles in glycogen's synthesis. The breakdown of
glycogen is completed through an entirely different biochemical pathway. Epinephrine and
glucagon are signaling molecules whose binding to certain 7TM receptors activate the
degradation, which is carried out in the cells by glycogen phosphorylase. This enzyme breaks up
the polysaccharide chain by replacing the glycosidic bond with a phosphate group. As with its
synthesis, glycogen's degradation requires numerous enzymes besides those mentioned here.
Starch is a good storage of carbohydrates because it is an intermediate compared to ATP and
lipids in terms of energy. In plants, starch storage folds to allow more space inside cells. It is also
insoluble in water, making it so that it can stay inside the plant without dissolving into the
system. Starch can also be used as a back up source of energy when plants cannot obtain carbon
dioxide, light, or nutrients from the surrounding soil.
Discussion on plants' metabolism of starch....
Cellulose is the major polysaccharide found in plants responsible for structural role. It is one of
the most naturally abundant organic compounds found on the planet. Cellulose is an unbranched
polymer of glucose residues put together via beta-1,4 linkages, which allow the molecule to form
long and straight chains. This straight chain conformation is ideal for the formation of strong
fibers.
Although mammals cannot digest cellulose, it and other plant forms are necessary soluble fibers
that mammals can digest. Pectin, for example, slows down the movement of food molecules in
the digestive tract, which thereby allows for more necessary nutrients to be absorbed by the body
instead of being quickly passed through as waste. Likewise, insoluble fibers like cellulose
expedite the digestive movement of food molecules, which is imperative in the quick removal of
harmful toxins.
Although human can't digest cellulose because we lack cellulases that allow us to cleave the beta
1,4 linkages. Some animals do eat and obtain energy from cellulose. One example of that is
termites. These animals digest cellulose in a stepwise manner, using a combination of their own
cellulases (produced in the foregut) and those of a microbial community resident in the distal
parts of their digestive tract. This is a great example of symbiosis relationship.
Cellulose is insoluble in water and aqueous solutions. It forms crystals and hydrogen bonds with
amino acids. This quality of using intra and intermolecular hydrogen bonds to make crystals
renders cellulose excessively insoluble in water and aqueous solutions. However, individual
strands of cellulose aren't very hydrophobic as compared to other polysaccharides. It is the
property of forming crystals that makes cellulose so insoluble.
Structure of Cellulose
currently too slow to be used in industry) and apply its use to producing "green" energy sources.
In this way, the most abundant source of bioenergy on Earth, cellulose, can become a part of the
world's accessible energy supply. Some types of cellulase already find uses in industry, for
example in food production and the textile industry.
Useful Web Links and sources of content:
- http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Carbohydrates.html
Likely examination questions
i) Discuss plants' metabolism of starch. ii) Describe the recycling of glucose in humans. iii) Discuss the occurrence of cellulases in nature.
