OCR A Level Biology

Revision Notes

5.5.11 Coordination of Responses

Coordination of Responses

Responding to change and stimuli

  • 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
  • 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 messenger chemicals such as hormones in response to stimuli
  • The impulses or hormones sent by receptor cells travel to a coordinator
  • From the coordinators, the impulse is transported to the specific effector that will produce the appropriate response

The ‘fight-or-flight’ response

  • An animal may produce a ‘fight-or-flight’ response in situations where there is a high level of stress, fear or aggression induced by environmental stimuli
  • ‘Fight-or-flight’ responses are rapid and can be crucial for preserving life
  • 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)

Response to Change Robin, downloadable AS & A Level Biology revision notes

The sequence of detection, coordination and action results in a ‘fight-or-flight’ response that saves the robin’s life

Coordination of the nervous system and endocrine system in the ‘fight-or-flight’ response

  • There are two important coordination systems in the body, the nervous system and the endocrine system
  • Both of these systems are involved in the ‘fight-or-flight’ response
  • The nervous and endocrine systems work together in a complementary manner to coordinate this fast response
  • The sympathetic nervous system is responsible for coordinating many of the responses to danger
  • Its actions are supported by the effect of two hormones: adrenaline and cortisol (both secreted from the adrenal glands
  • The initial part of the response is controlled by the nervous system, the response is continued by the endocrine system

Mechanism of the ‘fight-or-flight’ response

  • Sensory neurones detect environmental stimuli associated with danger and send impulses to the brain
  • The amygdala (a small region of the brain located in the cerebrum) sends impulses to various other parts of the brain, including the hypothalamus
  • The hypothalamus is stimulated to send impulses via the sympathetic nerves to the adrenal glands
    • This causes the adrenal medulla to secrete the hormone adrenaline
    • Adrenaline stimulates target organs and tissues to increase sensory awareness, making the organism more alert and so improving its ability to respond to danger
  • At the same time, the hypothalamus also releases a peptide hormone that stimulates the anterior pituitary gland to release ACTH (adrenocorticotropic hormone)
  • ATCH is transported to the adrenal glands via the bloodstream
    • This causes the adrenal cortex to secrete the hormone cortisol
    • Cortisol stimulates target organs and tissues to increase blood pressure, blood glucose ensuring the tissues have sufficient glucose and oxygen needed for rapid response
    • Cortisol also suppresses the immune system

The effects of adrenaline

  • The hormone adrenaline is secreted from the adrenal glands (and sometimes the medulla oblongata)
    • It is often during times of stress or aggression that this hormone is secreted, which is why it is sometimes referred to as the ‘fight-or-flight’ hormone
  • Adrenaline is transported via the bloodstream and it has a rapid effect on cells
  • It can have a range of effects on a number of different cell types:
    • In the eyes it stimulates the muscles in the irises to contract, causing the pupils to dilate
    • It increases the diameter of the bronchioles by relaxing smooth muscle. This helps to increase the airflow to the alveoli
    • Adrenaline decreases the amount of blood flowing to the gut and skin via vasoconstriction
      • A higher blood pressure occurs due to increased resistance from vasoconstriction
    • It increases the amount of blood flowing to the brain and muscles via vasodilation
    • Heart rate and stroke volume (volume of blood pumped per beat) increase as a result of adrenaline
    • Adrenaline stimulates the breakdown of glycogen into glucose in the liver cells via enzymes, causing the blood glucose concentration to increase

The second messenger model and adrenaline

  • When adrenaline is secreted it increases the concentration of blood glucose
  • It does this by binding to different receptors on the surface of liver cells that activate the same enzyme cascade that occurs when glucagon binds to its specific receptors
  • Adrenaline binds to specific receptors on the membrane of liver cells
  • This causes the enzyme adenylyl cyclase to change shape and become activated
  • Active adenylyl cyclase catalyses the conversion of ATP to the second messenger, cyclic AMP (cAMP)
  • cAMP binds to protein kinase A enzymes, activating them
  • Active protein kinase A enzymes activate phosphorylase kinase enzymes by adding phosphate groups to them
  • Active phosphorylase kinase enzymes activate glycogen phosphorylase enzymes
  • Active glycogen phosphorylase enzymes catalyse the breakdown of glycogen to glucose
    • This process is known as glycogenolysis
  • The enzyme cascade described above amplifies the original signal from adrenaline and results in the releasing of extra glucose by the liver to increase the blood glucose concentration back to a normal level

Effect of adrenaline, downloadable AS & A Level Biology revision notes

The effect of adrenaline is amplified so that each molecule can stimulate many molecules of cAMP, which in turn activate many enzymes molecules

  • Adrenaline also stimulates the breakdown of glycogen stores in muscle during exercise
  • The glucose produced remains in the muscle cells where it is needed for respiration

Exam Tip

The adrenal medulla and adrenal cortex are both regions of the adrenal glands!


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