Homeostasis is defined as the processes involved in maintaining a constant internal environment, within tolerance limits, despite changes in the internal and external environment. Tolerance limits simply reference a steady state in cells, and therefore organisms. Homeostasis involves many different systems in multicellular organisms, however it mainly involves:

• The nervous system which is short lasting but provides fast responses, and

• The endocrine system which is long lasting but there is a response lag

Factors in Tolerance Limits A number of factors must be kept within a range of tolerance limits, otherwise this may cause some harm to the organism. These factors include:

• Water and dissolved ions

• pH levels of tissues and/or blood fluids

• Gases (carbon dioxide)

• Blood glucose levels

• Blood pressure

• Temperature

Stimulus-Response Model There are two stages to homeostatic regulation:

1. The detection of a change from a stable state (otherwise known as the stimulus)

2. The response to the stimulus, usually counteracting the change.

Therefore, to understand homeostatic regulation, a stimulus-response model can be used. The purpose of homeostatic regulation is to maintain internal factors around a set normal value, and when the factors deviate away from the value, homeostatic adaptations will attempt to bring the factor back to the normal value. The main components of a stimulus-response model include:

• Stimulus

• Receptor

• Modulator (or control centre)

• Effector

• Response

• Feedback

Role of Enzymes Living organisms carry out a series of chemical reactions in order to continue living; a linked series of chemical reactions are collectively termed a biochemical pathway; the sum of these reactions is called metabolism. Metabolism is controlled by enzymes, and without them, chemical reactions would be too slow to sustain the organism's life.

• Enzymes: Reusable biological catalysts which act by lowering the activation energy of chemical reactions, enabling them to proceed faster.

Enzymes are classed as proteins (however, some are ribosomes!), and proteins are highly sensitive to factors such as temperature and pH level, and they have tolerance limits. In their tolerance limits, they can avoid denaturation and function effectively.

• Denaturation: A structural change in a protein that results in the loss of its biological properties; in enzymes, denaturation changes the shape of their active site and they can no longer catalyse reactions with their specific substrate.

Protein shape in general is influenced by salinity, pH level, temperature and a number of different environmental factors. Homeostatic processes aid in maintaining these factors within tolerance limits for enzymes. The key takeaway from this is that homeostasis is a necessity for survival.