Biological Reactions

Background

Photosynthesis

Photosynthesis is vital to all life on Earth, because it is the first link in the transfer of solar energy (light) to living things. Thus we rely on photosynthesis for food and to help maintain the atmospheric balance. The use of controlled glasshouse conditions has helped maximise the production of many crops and allowed the supply of 'seasonal' crops outside of their normal season. The crucial factors that commonly control the rate of photosynthesis are the availability of carbon dioxide and light. A factor is called limiting if any lack of it causes the rate of photosynthesis to drop below the maximum.

In a greenhouse, additional carbon dioxide may be supplied - up to a level of 1.0%, above which there is no further increase in photosynthesis and in fact the gas becomes toxic. Chlorophylls absorb light at different wavelengths, particularly at the blue and red ends of the visible spectrum. Artificial lighting takes this into account. Water is not a limiting factor as the water content of living cells is so high. Clearly, the greater the rate of photosynthesis, the greater the rate of increase in biomass. The net primary productivity is the assimilation of dry biomass per unit area per day. Other factors (such as the length of light and dark periods) influence whether there is a lot of leaf, flower or fruit growth. The health of plants is also affected by the presence of parasites that cause diseases.

Respiration

This is the next link in energy transfer through ecosystems, and takes place in all living things. The universal energy store in plants and animals is ATP (adenosine triphosphate) which is produced by the process of respiration. It acts as an intermediary, transferring energy on to other chemicals and allowing reactions to take place. The activity level of an organism determines the rate of respiration required and the demand for food. Respiration is also important in maintaining levels of oxygen and carbon dioxide in the atmosphere.

Fermentation is a form of anaerobic respiration which is important commercially because it yields alcohol.

Enzymes

These are biological catalysts which provide a reaction site for molecules and lower the activation energy required for a reaction to proceed. Without enzymes, biological reactions would be too slow at the temperature at which cells exist. Enzymes can be used commercially to help in tenderising meat, to give soft centres to chocolates and other sweets, and in washing detergents to aid stain removal.