OCR A Level Biology

Revision Notes

5.1.1 The Need for Homeostasis

The Need for Communication Systems in Organisms

  • Animals and plants need to be able to respond to changes in their internal and external environment and to coordinate the activities of their different organs
  • In order to function properly and efficiently, organisms have different control and communication systems that ensure their internal conditions are kept relatively constant
  • Physiological control systems maintain the internal environment within restricted limits through a process known as homeostasis
  • Homeostasis is critically important for organisms as it ensures the maintenance of optimal conditions for enzyme action and cell function
  • Examples of physiological factors that are controlled by homeostasis in mammals include:
    • Core body temperature
    • Metabolic waste (eg. carbon dioxide and urea)
    • Blood pH
    • Concentration of glucose in the blood
    • Water potential of the blood
    • Concentration of respiratory gases (carbon dioxide and oxygen) in the blood
  • Homeostatic mechanisms in mammals require information to be transferred between different parts of the body
  • There are two communication systems in mammals that do this:
    • The nervous system
    • The endocrine system

The nervous system

  • The human nervous system consists of:
    • The central nervous system (CNS) – the brain and the spinal cord
    • The peripheral nervous system (PNS) – all of the nerves in the body
  • It allows us to make sense of our surroundings and respond to them and coordinate and regulate body functions
  • Information is sent through the nervous system as nerve impulses – electrical signals that pass along nerve cells known as neurones
  • A bundle of neurones is known as a nerve
  • Neurones coordinate the activities of sensory receptors (eg. those in the eye), decision-making centres in the central nervous system, and effectors such as muscles and glands

The human nervous system, IGCSE & GCSE Biology revision notesThe human nervous system

The endocrine system

  • A hormone is a chemical substance produced by an endocrine gland and carried by the blood
    • They are chemicals which transmit information from one part of the organism to another and bring about a change
    • They alter the activity of one or more specific target organs
  • Hormones are used to control functions that do not need instant responses
  • The endocrine glands that produce hormones in animals are known collectively as the endocrine system
    • A gland is a group of cells that produces and releases one or more substances (a process known as secretion)

The major endocrine glands in the body, IGCSE & GCSE Biology revision notesThe major endocrine glands in the body

The importance of homeostasis

  • Homeostatic mechanisms help organisms to keep their internal body conditions within restricted limits
  • Three key factors that need to be controlled include:
    • Temperature (thermoregulation is explained in more detail later)
    • pH
    • Blood glucose concentration
  • A stable core temperature and blood pH are vital for enzyme activity
    • If the temperature or pH of the tissue fluid surrounding cells is too high or too low it can negatively affect the rate of important enzyme-controlled reactions


  • All enzymes have an optimum pH or a pH at which they operate best
  • Enzymes are denatured at extremes of pH
    • Hydrogen and ionic bonds hold the tertiary structure of the protein (ie. the enzyme) together
    • Below and above the optimum pH of an enzyme, solutions with an excess of H+ ions (acidic solutions) and OH ions (alkaline solutions) can cause these bonds to break
    • This alters the shape of the active site, which means enzyme-substrate complexes form less easily
    • Eventually, enzyme-substrate complexes can no longer form at all
    • At this point, complete denaturation of the enzyme has occurred
  • Where an enzyme functions can be an indicator of its optimal environment:
    • Eg. pepsin is found in the stomach, an acidic environment at pH 2 (due to the presence of hydrochloric acid in the stomach’s gastric juice)
    • Pepsin’s optimum pH, not surprisingly, is pH 2

Blood glucose concentration

  • Another key factor that must be controlled within mammals is the concentration of glucose in the blood
  • The amount of glucose present in the blood affects the water potential of the blood and the availability of respiratory substrate for cells
  • The normal glucose concentration for human blood is roughly 90mg per 100cm3
  • A sufficient amount of circulating glucose is essential for cellular respiration
    • Brain cells can become rapidly damaged or die if they do not receive a sufficient supply of glucose
  • Alternatively, if the blood glucose concentration is too high then it will have a dramatic effect on the water potential of the blood


  • Plants also carry out homeostasis – just like animals they need to maintain a constant internal environment
    • For example, mesophyll cells in leaves require a constant supply of carbon dioxide for photosynthesis
  • Stomata (specifically the guard cells) control the diffusion of gases in and out of leaves
    • This means stomata control the entry of carbon dioxide into leaves

Response of guard cells & stomata table

Stomata table 1, downloadable AS & A Level Biology revision notes

  • Regulation of stomatal aperture balances the need for carbon dioxide uptake by diffusion, with the need to minimise water loss by transpiration

Advantages & disadvantages of stomatal opening & closure table

Stomata table 2, downloadable AS & A Level Biology revision notes

Exam Tip

A stoma is actually the aperture (hole) between two guard cells, but the term is often used to refer to the whole unit (the two guard cells and the hole between them).

Don’t forget – stoma (singular) refers to one of these units, whereas stomata (plural) refers to many!


Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Already a member?
Go to Top