OCR Unit 1, Module 1

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A stimulus is any change in the environment that causes a response.  

A response is any change in behaviour or physiology as a result of a change in the environment.

External environments:

  • the external environment may change so putting stress on an organism.
  • For the organism to remain active and survive it must change its behaviour or physiology to reduce stress.

Internal environments:

  • Internal cells and tissues of animals are bather in tissue fluid.
  • As cells undergo metabolic activities they produce waste.
  • The waste could be toxic, the activity of the cells alters their own environment.
  • The waste acts as a stimulus to cause the removal.

Maintaining the internal environment:

  • any waste in the tissue fluid enters the blood and is carried away to be excreted


  • A multicellular organism is more effective as its cells can be differentiated

Cell Signalling

  • This is a process where one cell releases a chemical that is detected by another cell. The second cell then responds.
  • The two major systems of communication that work by cell signalling are:
    • The neuronal system
    • The hormonal system


Homeostasis is the maintenance of the internal environment in a constant state despite external changes.

Negative feedback is a process that brings about a reversal of any change in conditions. it ensures that an optimum steady state can be maintain, as the internal environment is returned to its original set of conditions after any change.

Positive feedback is a process that increases any change detected by the receptor. It tends to be harmful and doesn't lead to homeostasis.


An ectotherm is an organism that relies on external sources of heat to regulate its body temperature.


  • Less food is used in respiration
  • Able to survive long periods without eating
  • A greater proportion of the energy obtained can be used for growth


  • Less active in cooler temperatures therefore, greater risk of predation
  • May not be capable of activity during winter therefore, means they need sufficient stores of energy

When an ectotherm is cold it will change its behaviour or physiology to increase absorption of heat from its environment.

When an ectotherm is hot it will change its behaviour or physiology to decrease absorption of heat and increase loss of heat to its environment


An endotherm is an organism that can use internal sources of heat, such as heat generated from metabolism in the liver, to maintain its body temperature


  • A fairly constant body temperature, regardless of external temperature
  • Activity possible when external temperatures are cool
  • Ability to inhabit cooler parts of the planet


  • A significant part of energy intake is used to maintain body temperature in the cold
  • More food is required
  • Less of the energy from food is used for growth

Control of temperature regulation:

  • Endotherms monitor the temperature of their blood in the hypothalamus.
  • If temperature falls below optimum the hypothalamus sends signals to reverse the change
  • The peripheral temperature receptors in the skin monitor the temperature in the extremities


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