Topic 9

• What is meant by the term homeostasis?

  The maintenance of a constant internal environment
• How does homeostasis affect body temperature?

  If too cold, rate of reaction slows down and you won't have enough energy but too high and cells will denature.
• How does homeostasis affect water levels in the body?

  If you have too much water, the red blood cells can burst. But if your cells have too little water then cells can shrivel.
• What does a co ordinated response require?

  stimulus, receptor, effector
• What is meant by the plant tropism?

  Growth in response to light.
• What is positive plant tropism?

  Growth towards the light meaning more light energy for photosynthesis e.g leaves and stems.
• What is negative plant tropism?

  Growth away from the light meaning less light energy e.g roots
• What is the role of auxin in phototropism response of stems?

  - Auxins are plant hormones which control growth at the tips of shoots and roots.

  - Auxin is produced in the tips and diffuses backwards to stimulate the cell elongation process which occurs in cells just being the tips.
• How does auxin make shoots positively phototropic? (Grow towards light)

  1) when a shooter is exposed to light, it accumulate more oxygen on the side, it's in the shade to the side that is in the light.

  2) this makes the cell grow(elongate) faster on the shaded side, so the shoot bends towards the light
• How does auxin make shoots negatively geotropic? (Grow away from gravity)


  1) When a shoot is growing sideways, gravity produces an unequal distribution of auxin in the tip, with more auxin on the lower side.

  2) This causes the lower side to go faster, bending the shoot upwards.
• How does auxin make shoots positively geotropic? (grow towards gravity)

  1) A root growing sideways will also have more auxin on its lower side.

  2) But in a root the extra auxin inhibits growth. This means the cells elongate faster, and the roots bends downwards.
• How does auxin make roots negatively phototropic? (Grow away from light)

  1) If a root starts being exposed to some light, more auxin accumulates on the more shaded side.

  2) The auxin inhibits cell elongation on the shaded side, so the root bends downwards, back into the ground.
• How do plants respond to stimuli?

  1) plants increase their changes of survival by responding to changes in their environment e.g they sense the direction of light and grow towards it to maximise light absorption for photosynthesis.

  2) plants are more likely to survive if they respond to the presence of predators to avoid being eaten.

  3) plants are more likely to survive if they respond to abiotic stress - anything harmful that's natural but non-living, like drought.
• What are the important features of the reflex arc?

  - fast

  - involuntary

  - protects from harm
• What is the order of the reflex arc?

  stimulus, receptor, sensory neurone, relay neurone, CNS, motor neurone, effector, response
• How does a synapse work?

  - It is a small gap between neurones that an electrical impulse can't pass through, so when an impulse reaches the end of a neuron, it sends a neurotransmitter, which then diffuses across the gap, and reaches the other side, so the electrical impulse can continue.
• Label a diagram of the eye. .
• What is the function of the eye as a receptor?

  Receptors in the eye are sensory cells that respond to light stimuli and that transmit signals via the nerves to the brain (retina).
• How does the eye adjust for bright lights?

  1) very bright lights trigger a reflex that makes the pupil smaller, allowing less light in.

  2) The opposite process happens in dim light. This time of the brain tells the radial muscles to contract, which makes the pupils bigger.
• How does the eye adjust to look at distant objects?

  1) The ciliary muscle relaxers, which allows the suspensory ligaments to pull tight.

  2) this makes the lens go thin (less curved)

  3) so it refracts light by smaller amounts.
• How does the eye adjust it look at near objects?

  1) The ciliary muscle contracts, which slackens the suspensory ligaments.

  2) Lens becomes fat (more curved)

  3) This increases the amount by which it refracts light.
• What is the role of the skin is response to vasodilation?

  1) lots of sweat is produced, When it evaporates it transfers energy from your skin to the environment, cooling you down.

  2) blood vessels close to the surface of the skin widen. It allows more blood flow near the surface, so it can transfer more energy into the surroundings, which cools you down.

  3) Hairs lie flat.
• What is the role of the skin in response to vasoconstriction?

  1) Very little sweat is produced.

  2) blood vessels near the surface of the skin constrict. This means less blood flows near the surface, so less energy is transferred to the surroundings.

  3) you Shiver, which increases your rate of respiration, which transfers more energy to warm, the body. Exercise does the same.

  4) hairs stand on end to trap an insulating layer of air, which helps to keep you warm.