Survival & Response (AQA A Level Biology)

• Organisms must respond to changes in their environment in order to survive
• They can only survive if they are successful at:
  • Finding favourable conditions for living
  • Finding food
  • Avoiding being eaten

• If these vital requirements are not met then a species will die out or go extinct
  • For example, a red robin must find worms and insects to feed on and at the same time, they must also be watching out for predators such as crows

Detecting and responding to change

• Responses to change can vary in complexity depending on the type of organism involved and the specific circumstances they are responding to
• Responding to change requires detection
  • Detection involves a stimulus being detected by a receptor cell

• There are different types of receptors
  • Some receptor cells produce electrical activity in nerve cells in response to stimuli
  • Other receptor cells secrete substances in response to stimuli

• The nerve impulses sent by receptor cells travel to a coordinator
  • This is either the brain or the spinal cord

• From the coordinators, the impulse is conducted to the specific effector that will produce the appropriate response
• Using the earlier example of the red robin staying alert to predators:
  • A sudden movement by a crow (the stimulus) is detected by the receptors in the robin’s eye
  • The receptor cells send an impulse along the nerves and to the brain (coordinator)
  • The brain sends an impulse to the wing muscles (effectors) of the red robin so it can fly away (response)

The sequence of detection, coordination and action results in a response that saves the robin’s life

• This kind of fast response is necessary for organisms to survive
  • An initial three-stage reflex action as demonstrated in the example allows for the initial response to be rapid

• The response, however, must not be completely fixed, it needs to be flexible
  • After the initial reflex action, the organism can take control of the response and display much more complex behaviour

• In the case of the red robin, its reflex reaction to seeing a crow is to begin to fly, it is then able to control its direction of flight so that it flies away from the crow