Responding to the environment


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Responding to their environment helps organisms su

  •  Animals increase their chances of survival by responding to changes in their external environment, e.g by avoiding harmful environments such as places that are too hot or too cold.
  • They also respond to changes in their internal environment to make sure that the conditions are optimal for their metabolism (all the chemical reactions that go on inside them).
  • Plants also increase their chances of survival by responding to changes in their environment.
  • Any change in the internal or external environment is called a stimulus.
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Receptors detect stimuli and effectors produce a r

  • Receptors detect stimuli - they can be cells or proteins on cell surface membranes. There are loads of different types of receptors that detect different stimuli.
  • Effectors are cells that bring about a response to a stimulus, to produce an effect. Effectors include muscle cells and cells found in glands, e.g the pancreas.
  • Receptors communicate with effectors via the nervous system or the hormonal system, or sometimes using both.
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The nervous system sends information as electrical

The nervous system is made up of a complex network of cells called neurones. There are three main types of neurone:

1. Sensory neurones transmit electrical impulses from receptors to the central nervous system (CNS).

2. Motor neurones transmit electrical impulses from the CNS to effectors.

3. Relay neurones transmit electrical impulses between sensory neurones and motor neurones.

Electrical impulses are also called nerve impulses.

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A stimulus is detected by receptor cells and an electrical impulse is sent along a sensory neurone.

When an electrical impulse reaches the end of a neurone chemicals called neurotransmitters take the information across to the next neurone, which then sends an electrical impulse.

The CNS processes the information, decides what to do about it and sends impulses along motor neurones to an effector.

So the order goes : Stimulus to receptors through sensory neurones to the CNS down motor neurones to effectors, which produce a response.

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Nervous System communication is short-lived and ra

  • When an electrical impulse reaches the end of a neurone, neurotransmitters are secreted directly onto cells (e.g muscle cells) - so the nervous response is localised.
  • Neurotransmitters are quickly removed once they've done their job, so the response is short-lived.
  • Electrical impulses are really fast, so the response is rapid - this allows animals to react quickly to stimuli.
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The hormonal system sends information as chemical

1. The hormonal system is made up of glands and hormones:

  • A gland is a group of cells that are specialised to secrete a useful substance, such as a hormone e.g the pancreas secretes insulin.
  • Hormones are "chemical messengers". Many hormones are proteins or peptides e.g insulin. Some hormones are steroids e.g progesterone.

2. Hormones are secreted when a gland is stimulated:

  • Glands can be stimulated by a change in concentration of a specific substance (sometimes another hormone).
  • They can also be stimulated by electrical impulses.
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3. Hormones diffuse directly into the blood, then they're taken around the body by the circulatory system.

4. They diffuse out of the blood all over the body but each hormone will only bind to specific receptors.

5. The hormones trigger a response in the target cells (the effectors).

So the order goes : Stimulus, receptors,hormone,effectors,response.

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Hormonal system communications are slower, long-la

  • Hormones aren't released directly onto their target cells- they must travel in the blood to get there. This means that chemical communication (by hormones) is slower than electrical communication (by nerves).
  • They aren't broken down as quickly as neurotransmitters, so the effects of hormones can last for much longer.
  • Hormones are transported all over the body, so the response may be widespread if the target cells are also widespread.
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