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Coordination and Response
The Nervous System and Responding to Stimuli:
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Animals increase their chances of survival by responding to changes in their external
environment
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Plants also increase their chances of survival by responding to changes in their
environment
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Receptors:
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Receptors in the sense organs are groups of cells that detect external stimuli
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They include muscle cells
found in glands
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Receptors communicate with effectors via the nervous system, the hormonal
system, or sometimes both
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There are three
main types of neurone – sensory, relay and motor
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When receptors in a sense organ detect a stimulus, they send electrical impulses
along sensory neurones to the CNS
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The
effector then responds accordingly
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Coordinated responses always
need a stimulus, a receptor and an effector
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Reflexes:
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Reflexes are automatic responses to certain stimuli – they can reduce the chances of
being injured
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If you get a shock, your body releases the hormone adrenaline automatically
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Reflex Arcs:
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The neurones in reflex arcs go through the spinal cord
or through an unconscious part of the brain
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In the CNS the sensory neurone passes on the message to
another type of neurone passes on the message to
another type of neurone – a relay neurone
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Messages are
passed
across synapses using chemicals
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The impulse then travels along the motor neurone to the effector
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Because you don’t have to think about the response it’s quicker than normal
responses
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Cornea – Refracts the light into the eye
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Iris – Controls the diameter of the pupil
and therefore how much light enters
the eye
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Rods – More sensitive in dim light but can’t sense colour
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Cones are found all over the retina, but there are
loads of them at the fovea
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Hormones:
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Hormones are chemicals released directly into the blood
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Hormones control things in organs and cells that need constant adjustment
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They travel quite slowly and tend to have
relatively long-lasting effects
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Hormones:
o Slow messages that act for a long time that act in a general way
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Some information needs to be passed to effectors really quickly
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If the response lasts for a long time, it’s probably hormonal
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Homeostasis:
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Conditions in your body to be kept steady so that cells can function properly
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Water Content – You need to keep a balance between the water you gain and the
water you lose
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Homeostasis – The maintenance of a constant internal environment
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On a hot day, or when you’re exercising, you sweat a lot
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More water will be lost in breath when you exercise
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You produce more, less concentrated urine
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The enzymes in the
human body work best at about 37oC
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It’s sensitive to the blood
temperature in the brain, and it receives messages from temperature receptors in
the skin that provide information about skin temperature
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Smaller Organisms:
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Smaller organisms have bigger surface are to volume ratios
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This allows small organisms to lose body heat more easily in hot climates and
reduces the chance of them overheating
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Organisms with smaller SA:V ratios gain or lose heat more slowly because there is
less area for the heat to transfer across
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Responses in Plants:
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Plants increase their chances of survival by responding to changes in their
environment:
o They sense the direction of light and grow towards it to maximise light
absorption for photosynthesis
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o Climbing plants have a sense of touch, so they can find things to climb and
reach the sunlight
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Cattle start to eat the clover when field overgrazed
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Auxins:
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Auxins are plant hormones which control growth at the tips of shoots and roots
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Auxin is produced in the tips and diffuses backwards to stimulate the cell elongation
process which occurs in the cells just behind the tips
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Auxins are involved in the growth responses of plants to light and gravity
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o When a shoot tip is exposed to light, it accumulates more auxin on the side
that’s in the shade than the side that’s in the light
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Shoots are negatively geotropic
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o This causes the lower side to grow faster, bending the shoot upwards
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o A root growing sideways will also have more auxin on its lower side
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This means the cells on top
elongate faster, and the root bends downwards

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