2.1 Biological macromolecules  (Page 25/4)

Biological macromolecules

A biological macromolecule is one that is found in living organisms. Biological macromolecules include molecules such as carbohydrates, proteins and nucleic acids. Lipids are also biological macromolecules. They are essential for all known forms of life to survive.

Biological macromolecule

A biological macromolecule is a polymer that occurs naturally in living organisms. These molecules are essential to the survival of life.

Carbohydrates

Carbohydrates include the sugars and their polymers. One key characteristic of the carbohydrates is that they contain only the elements carbon, hydrogen and oxygen. In the carbohydrate monomers, every carbon except one has a hydroxyl group attached to it, and the remaining carbon atom is double bonded to an oxygen atom to form a carbonyl group. One of the most important monomers in the carbohydrates is glucose ( [link] ). The glucose molecule can exist in an open-chain (acyclic) and ring (cyclic) form.

Glucose is produced during photosynthesis , which takes place in plants. During photosynthesis, sunlight (solar energy), water and carbon dioxide are involved in a chemical reaction that produces glucose and oxygen. This glucose is stored in various ways in the plant.

The photosynthesis reaction is as follows:

$6{\mathrm{CO}}_2+6{\mathrm{H}}_2\mathrm{O}+\mathrm{sunlight}\to {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2$

Glucose is an important source of energy for both the plant itself, and also for the other animals and organisms that may feed on it. Glucose plays a critical role in cellular respiration , which is a chemical reaction that occurs in the cells of all living organisms. During this reaction, glucose and oxygen react to produce carbon dioxide, water and Adenosine Triphosphate (ATP). ATP is a molecule that cells use for energy so that the body's cells can function normally. The purpose of eating then, is to obtain glucose which the body can then convert into the ATP it needs to be able to survive.

The reaction for cellular respiration is as follows:

$6{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+{60}_2\to 6{\mathrm{CO}}_2+6{\mathrm{H}}_2\mathrm{O}+\mathrm{ATP}$

We don't often eat glucose in its simple form. More often, we eat complex carbohydrates that our bodies have to break down into individual glucose molecules before they can be used in cellular respiration. These complex carbohydrates are polymers, which form through condensation polymerisation reactions ( [link] ). Starch and cellulose are two example of carbohydrates that are polymers composed of glucose monomers.

• Starch Starch is used by plants to store excess glucose, and consists of long chains of glucose monomers. Potatoes are made up almost entirely of starch. This is why potatoes are such a good source of energy. Animals are also able to store glucose, but in this case it is stored as a compound called glycogen , rather than as starch.
• Cellulose Cellulose is also made up of chains of glucose molecules, but the bonding between the polymers is slightly different from that in starch. Cellulose is found in the cell walls of plants and is used by plants as a building material.

  Interesting fact

  It is very difficult for animals to digest the cellulose in plants that they may have been feeding on. However, fungi and some protozoa are able to break down cellulose. Many animals, including termites and cows, use these organisms to break cellulose down into glucose, which they can then use more easily.