Homeostasis (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award)): Revision Note

Defining Homeostasis: Extended

Extended Tier Only

• Homeostasis is defined as the maintenance of a constant internal environment

The Concept of Negative Feedback: Extended

Extended Tier Only

• Negative feedback occurs when conditions change from the ideal or set point and returns conditions to this set point

• It works in the following way:

  • if the level of something rises, control systems are switched on to reduce it again

  • if the level of something falls, control systems are switched on to raise it again

• Negative feedback mechanisms are usually a continuous cycle of bringing levels down and then bringing them back up so that overall, they stay within a narrow range of what is considered ‘normal’

The negative feedback cycle

Blood Glucose Control: Extended

Extended Tier Only

• Blood glucose levels are controlled by a negative feedback mechanism involving the production of two hormones - insulin and glucagon

• Both hormones which control blood glucose concentration are made in the pancreas

If the blood glucose concentration gets too low

• Cells in the pancreas detect the decreased blood glucose levels

• The pancreas produces the hormone glucagon and secretes it into the blood

• Glucagon causes the glycogen stored in the liver to be converted into glucose and released into the blood

• This increases the concentration of glucose in the blood back to normal levels, at which point the pancreas stops secreting glucagon

If the blood glucose concentration gets too high

• Cells in the pancreas detect the increased blood glucose levels

• The pancreas produces the hormone insulin, secreting it into the blood

• Insulin stimulates muscles and the liver to take up glucose from the bloodstream and store it as glycogen (a polymer of glucose)

• This reduces the concentration of glucose in the blood back to normal levels, at which point the pancreas stops secreting insulin

Negative feedback regulation of blood glucose levels