Principles of Homeostasis (AQA A Level Biology): Revision Note

Homeostasis

• Homeostasis can be defined as:

maintaining the internal environment within restricted limits

• Physiological control systems ensure that internal conditions within the body do not stray beyond the narrow limits required for survival

  • This ensures optimal conditions for enzyme action and cell function

• Receptor cells detect changing conditions inside or outside the body, and then send information to a coordination system, which communicates with effectors to restore conditions to normal

  • The nervous system communicates via nerve impulses in neurones

  • The endocrine system communicates via hormones; chemical signals in the blood

• Examples of conditions that are controlled by homeostasis in mammals include:

  • core body temperature

  • blood pH

  • blood glucose concentration

  • water potential of the blood

Homeostasis: temperature & pH

• Temperature and pH need to be maintained within narrow limits because they affect enzyme activity

  • At low temperatures:

    • molecules have limited kinetic energy and move slowly, so collisions are infrequent and fewer enzyme-substrate complexes form

  • At body temperature:

    • molecules have more kinetic energy and move quickly, so collisions occur frequently and many enzyme-substrate complexes form

  • At high temperatures:

    • the bonds holding the active site together break, so the enzyme denatures and enzyme-substrate complexes can no longer form

  • At extremes of pH:

    • the bonds holding the active site together break, so the enzyme denatures and enzyme-substrate complexes can no longer form

• Cells rely on many enzyme-controlled reactions in order to function, so even a small change in the rate of enzyme activity can have a significant impact on cells

Homeostasis: blood glucose concentration

• The concentration of glucose in the blood needs to be maintained within narrow limits because:

  • glucose is an important respiratory substrate

    • A lack of glucose may slow respiration, resulting in a lack of ATP to fuel cellular processes

  • glucose can affect the water potential of the blood

    • an increase in dissolved glucose will lower the water potential of the blood and cause water to move out of the surrounding cells by osmosis

    • a decrease in dissolved glucose with increase the water potential of the blood and water will move into the surrounding cells by osmosis