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Components of Cells
The Macromolecules
Role of Carbohydrates

Trapped Sunlight

Life on this planet needs a constant supply of energy in order to fight the effects of entropy and the second law of thermodynamics. The most abundant source of this energy is the sun, where vast amounts of radiant energy are created in the nuclear fusion furnaces. A tiny part of this radiant energy reaches this planet in the form of light, where a tiny part, of a tiny part of this energy is absorbed by plants and converted from light energy into chemical energy. This is the process called photosynthesis.

Pigments in special cellular organelles trap quanta of light energy and convert them to high energy electrons. These high energy electrons are in turn used to move electrons in covalent bonds to a higher energy state. In this process atoms and bonds in carbon dioxide and water are rearranged and new molecules are created. Quanta of light energy are used to pull electrons in covalent bonds to higher energy levels where they are stable and stored for future use.

Two important molecular products are produced in this process; oxygen, which is released into the atmosphere, and 3-phosphoglyceric acid, which is kept inside the cells. All plants create 3-phosphoglyceric acid (3PG) as the first stable chemical molecule in this energy trapping mechanism. This simple, 3-carbon molecule is then used to make all the other kinds of carbohydrates the plant needs.

Monosaccharide sugars are made by combining and recombining all those carbon atoms first trapped as 3PG. The most abundant and versatile of these monosaccharides is glucose. This versatile molecule then plays many roles in the life of the plant - and the lives of animals that eat them.

Source of Energy

A primary role for the glucose molecule is to act as a source of energy; a fuel. Plants and animals use glucose as a soluble, easily distributed form of chemical energy which can be 'burnt' in the cytoplasm and mitochondria to release carbon dioxide, water and energy. This energy is then trapped in the ATP molecule and used for everything from muscle contraction to pumping water across cell membranes.

Single sugar molecules can also be attached to proteins and lipids to modify their biological role as enzymes, signaling molecules and as components of membranes. Very often the addition of one or more sugar molecules will make the recipient molecule more soluble. Glucose (and other monosaccharides) are very hydrophilic ("water loving"), and this can be a problem.

Pure monosaccharides, such as glucose, attract water. Any plant (or animal) that tried to store large amounts of glucose would have a serious problem with osmosis. Cells containing large numbers of glucose molecules would be constantly fighting the incessant movement of water from the outside of the cell to the inside. The osmotic pressure would be so great that even behind their protective walls, plant cells would have difficulty functioning.

One way round this problem is to convert the monosaccharides to polysaccharides. These larger molecules do not have such a great osmotic pressure and hence can be stored with greater safety and fewer problems.


Although plant and animal cells make a large number of different polysaccharides, for all kinds of roles, the dominant ones are those made from glucose.

Cellulose is a polymer of glucose monosaccharides that plants use as their primary building material. Threads of cellulose are bound by hydrogen bonds into bundles of great strength and flexibility. These are used by plants to surround each cell in a way that protects them from the effects of osmosis and also gives them shape and form.

Each plant cell wall, however, is more than just an inert box. About 0.5 micometers thick, it is a complex of pure cellulose (40% to 60%), a similar polysaccharide made of pentose sugars, and a special bonding agent called lignin. As the cells grow, expand, shrink or change their shape, the wall is adapted and modified accordingly, and when the cell divides, a new wall is formed between the daughter cells.

A cellulose-like material, called chitin, is used by insects and arthropods to stiffen and give form to their outer exoskeleton, and other complex polysaccharides are used in animals in places where tensile strength is needed.

Starch is a polymer of the alternate anomer of glucose and is used by plants as a way of storing glucose. It is a major reserve of energy that can be quickly mobilized as necessary.

Most plants cells have stored starch reserves in the form of tiny granules. Within these granules are two kinds of starch; amylose and amylopectin, which differ from one another in the amount of branching taking place in the molecule.

Many plants also have specialized regions of starch storage in which parenchymatous cells process and package starch molecules for long-term use. Tubers, such as potatoes, and seeds with their valuable embryos, are both plant structures with high concentrations of stored starch.

Mobile animals, such as humans, need energy reserves in much the same way. A small amount of these reserves is in the form of an amylopectin-like molecule called glycogen, which is found in the liver and some muscles. However, carbohydrates like starch or glycogen only produce about 4 kilocalories of energy per gram of weight, about the same as that for protein.

While this kind of efficiency is fine for plants (which don't have to move), it is not enough for animals with their higher metabolic needs. Lipids store about 9 kilocalories of energy per gram, almost twice that of carbohydrates, so they are the preferred fuel in the animal body.

Glucose has one great advantage, however, it is soluble in water and blood and thus easy to distribute around the body. Animals use this simple monosaccharide as a portable source of instant energy, adding and releasing it from the liver if and when it is required.

Humans need about 2-3,000 kilocalories of energy per day (24 hours). When possible, humans try to eat and digest meals with high caloric value, such as meat and lipids. But food of this sort is rare and hard to find (or catch!). Plants are a much more readily available (and easy to catch!) source of food, and hence the energy we need. Carbohydrates from plants, therefore, provide up to 80% of our energy needs every day.

Depending on the diet of the person, starch can account for 30-50% of this carbohydrate, but in some regions of the world where rice is the prime source of starch, it can account for up to 100% of the carbohydrates consumed.

Interestingly, cellulose cannot be digested by most animals, including humans. Grass eating animals, such as cows, must therefore enter into a partnership with micro-organisms that can break the bonds between the glucose molecules in the cellulose. If it was not for this partnership, they would starve.

© 2004, Professor John Blamire