Life Chemistry

There is a .pdf version of this page: DOWNLOAD

Life is an emergent phenomenon which cannot be reduced to chemistry. It is possible, however, to describe the processes going on within living creatures in terms of chemical structures and interactions. Within the context of this Resource Pack, it is quite impossible to provide more than a very sketchy and simple outline of the fantastic complexity of the chemistry of life. All that will be attempted is:
1. To indicate how complex chemical compounds found in living organisms can be built up from relatively simple ones.
2. To describe, in simple terms, the structure of DNA
3. To provide a basic description of the three main groups of chemicals involved in living things ― proteins, sugars and fats.

The structure and function of living organisms involves a relative small number of chemical elements the most important of which are carbon, hydrogen, oxygen, nitrogen and phosphorus. In addition, calcium and silicon are involved in some hard structures such as shells and bones and a number of other elements such as iron and magnesium play key roles but occur in relatively few specialised chemical structures.
The key factor in the reactivity of the atoms of the various chemical elements is the number of bonds (in the form of shared electrons) they can form with other atoms. The following table presents this information for the five key elements, together with examples of the structures of simple compounds involving the elements in which the bonds are represented by lines linking the atoms together.

The element carbon has a special place in the chemistry of life because, with its four bonds it can link with other carbon atoms to form chains, loops and networks providing the structural basis for complex compounds that may contain many thousands of carbon atoms.

The figure (a) shows just one example of the almost infinite number of possible carbon compounds. It is a very small part of a molecule of a compound called polyethylene, which is better known as the plastic polythene. It is formed by the linking together of molecules of the compound ethylene (b).
The linking together of simple carbon compounds to form more complicated ones, known as polymers, played an important role in the origins of life. Fairly simple carbon compounds existed on the early earth and under suitable conditions these can form polymers. The following figure shows two examples: (a) formaldehyde (H₂CO) can polymerise to form a pentose sugar and (b) hydrogen cyanide (HCN) can form a complex compound called adenine.

These examples were chosen because adenine and pentose sugars are involved in two other key chemical complexes. The first of these is combines adenine with pentose sugar together with three phosphate groups (see above) to form adenosine triphosphate.

This compound, known as ATP, is used to transport energy around inside living cells. Breaking the bond to the third phosphate group and reducing ATP to adenosine diphosphate (ADP) provides the chemical energy used in all the processes that go on inside cells.
Adenine, a form of pentose sugar and phosphate groups are also involved in DNA

DNA stands for Deoxyribose Nucleic Acid, the deoxy- element refers to fact that in the sugar ring the OH groups present in ATP are replaced by simple hydrogen atoms. In addition to adenine three other bases are involved, thymine which always links with adenine, cytosine and guanine which always link with each other. It is the order of these bases linked together in a long molecule that provide the basis for the genetic code. The diagram shows just two of the polymer links in a DNA molecule which may contain millions of them arranged in the famous double helix. The grey lines represent a weaker form of chemical bonding known as hydrogen bonds. The genetic code provides the information which initiates a complex process that leads to the production of proteins.

Proteins consist of amino acids linked together to form long chains. There are 25 different amino acids involved in proteins of which only three are shown in the diagram which also shows how they are linked together. The chains fold into complex shapes depending on the types and the order of the amino acids in the molecule. A typical protein may contain many hundreds of amino acid groups.

In addition to proteins, two other families of carbon compounds are involved in living organisms. These are carbohydrates and fats.

Carbohydrates
The pentose sugar in DNA and ATP is a carbohydrate. Glucose is also a carbohydrate with 6 carbon atoms:

Glucose is used as a source of energy. It can also polymerise to form long chain molecules of cellulose:

Cellulose molecules can be arranged together to form fibrils that have great tensile strength. These fibrils are the main structural element in the cell walls of plants.

Fats

These are long chain hydrocarbon compounds with an acid group –COOH at one end

Palmitic and Linoleic acids are the commonest fatty acids in animals. Palmitic acid is classed as saturated because the carbon chain has no double bonds compared with the unsaturated Linoleic acid which has two double bonds. Fatty acids are used as sources and stores of energy.
Fatty acids also play an important role in forming membranes which form part of the cell walls. Pairs of fatty acid chains come together with other compounds to form lipids:

The hydrocarbon chains exhibit the fatty characteristic of not mixing with water, while the phosphate group (on the right of the diagram) does mix with water.
When mixed with water, lipids can organised themselves into double layers with the phosphate groups in contact with the water and the hydrocarbon 'tails' in contact with each other, forming small vesicles:

One of the more likely theories about the origins of life suggests that such 'bubbles' provided opportunities for compounds in the water inside to react with each other in a coherent way in isolation from the surrounding water.

Links to sites providing further information:

Wikipedia
More Advanced