The Need for Homeostasis (OCR A Level Biology)

• 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

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

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

pH

• 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

Stomata

• 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

• 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