3.6.1.1 Survival and response (A-level only)

Question 1

How does response help organisms survive?

Answer 1

1. Can avoid harmful environments - places that are too hot, too cold etc.
2. Respond to changes in their internal environment to ensure optimal conditions for their metabolism.

Question 2

How do receptors use stimuli to effect a response?

Answer 2

Stimulus = any change in the internal / external environment.

1. Receptors detect SPECIFIC stimuli - can be cells or proteins on cell-surface membranes.
2. Effectors = cells that bring about a response to a stimulus, to produce an effect. Include muscle cells and cells found in glands (pancreas etc.).
3. Receptors communicate with effectors via the nervous / hormonal system / both.

Question 3

Outline the different systems of the nervous system.

Answer 3

1. Central Nervous System (CNS) - made up of the brain and the spinal cord.
2. Peripheral Nervous System (PNS) - made up of the neurons connecting the CNS to the rest of the body.
   a —> autonomic nervous system - controls unconscious activities (digestion). 2 divisions that have opposite effects on the body.
   -> a,s - sympathetic nervous system gets body ready for action - fight or flight.
   -> a, ps - parasympathetic nervous system calms body down - rest and digest.

Question 4

How does the nervous system transmit information?

Answer 4

1. Stimulus detected by receptor cells - electrical impulse sent along a sensory neurone.
2. Sensory neurones transmit electrical impulses from receptors to the CNS.
3. Relay neurones transmit electrical impulses between sensory neurones.
4. Motor neurones transmit electrical impulses from the CNS to effectors.
   When an electrical impulse reaches the end of a neuron, neurotransmitters transmit the information across to the next neurone, triggering an action potential.

Question 5

Define reflex.

Answer 5

Rapid, innate, unlearned response to a stimulus.

—> so quick as information can travel rapidly from receptors to effectors - no need to think.

Question 6

Outline the hand-withdrawal response to heat - reflex arc.Reflex arc = the pathway of neurones linking receptors to effectors in a reflex.

Answer 6

1. Thermoreceptors in the skin detect the heat stimulus.
2. Sensory neurone carries impulses to the relay neurone.
3. Relay neurone connects to the motor neurone.
4. Motor neurone sends impulses to the effector (biceps muscle).
5. Muscle contracts to withdraw hand and stop it being damaged.
   NB => If there is a relay neurone involved in the simple reflex arc, it is possible to override the reflex.

Question 7

Comment on the nature of nervous system communication.

Answer 7

Localised, short-lived and rapid.

• Localised as once an electrical impulse has reached the end of a neurone, the neurotransmitters are secreted directly onto target cells.
• Short-lived as neurotransmitters are quickly removed once they’ve done their job.
• Electrical impulses are fast, so the response is rapid, allowing animals to react quickly to stimuli.

Question 8

How to plants use stimuli response to increase their chances of survival?

Answer 8

• Sense the direction of light and grow towards it to maximise light absorption for photosynthesis.
• Sense gravity, so their roots and shoots grow in the right direction.
• Climbing plants have a sense of touch so can find things to climb up to reach sunlight more easily.

Question 9

Define tropism.

Answer 9

= The response of a plant to a directional stimulus.

• –> plants respond by regulating their growth.
• –> positive tropism = growth towards stimulus.
• –> negative tropism = growth away from stimulus.

Question 10

Comment on phototropisms.

Answer 10

= Growth of a plant in response to light.

• > shoots are positively phototropic and grow towards light.
• > roots are negatively phototropic and grow away from light.

Question 11

Comment on gravitropisms.

Answer 11

= Growth of a plant in response to gravity.

• > shoots are negatively gravitropic and grow upwards.
• > roots are positively geotropic and grow downwards.

Question 12

How do growth factors bring about responses in plants?

Answer 12

Growth factors = chemicals similar to hormones that speed up or slow down plant growth.

1. Growth factors produced in the growing regions of the plant (shoot tip / leaves) and move to where they’re needed in other parts of the plant.
2. Growth factors called auxins stimulate growth of shoots by cell elongation —> where cell walls become loose and stretchy so cells get longer.
3. High [auxin] inhibit growth in roots.

Question 13

What is IAA? How does its distribution vary?

Answer 13

= Indoleacetic acid - an important auxin produced in the tips of shoots of flowering plants.

1. IAA is moved around the plant to control tropisms - diffusion and active transport over short distances, and via the phloem for longer distances.
2. Different parts of the plant have different [IAA]
   - –> uneven distribution of IAA means there is uneven growth of the plant.

Question 14

What effect does IAA have in shoots?

Answer 14

Higher [IAA] in shoots (higher on dark side for phototropisms or higher on lower side for gravitropisms) —> increased cell elongation.

Question 15

What effect does IAA have in roots?

Answer 15

Higher [IAA] in roots (on dark side for phototropisms or higher on lower side for gravitropisms) —> inhibited cell elongation.

Question 16

How does IAA work in regards to phototropisms?

Answer 16

1. IAA moves to more shaded parts of the shoots and roots.
2. In shoot, [IAA] increases on the shaded side - cells elongate and shoot bends towards the light.
3. In root, [IAA] increases on the shaded side —> growth is inhibited so the root bends away from the light.

Question 17

How does IAA work in regards to gravitropisms?

Answer 17

1. IAA moves to underside of the shoots and roots.
2. In shoot, [IAA] increases on the lower side - cells elongate and shoot grows upwards.
3. In root, [IAA] increases on the lower side —> growth is inhibited so the root grows downwards.

Question 18

What is the overall effect of taxes and kineses for simple organisms?

Answer 18

Overall aim and effect is to keep simple organisms in a favourable environment.

Question 19

How do woodlice show a tactic response?

Answer 19

Taxis = Moving towards or away from a directional stimulus.

1. Woodlice show a phototactic response —> they move away from a light source.
2. Helps them survive as it keeps them concealed under stones during the day (safe from predators) and keeps them in damp conditions (reducing water loss).

Question 20

How do woodlice show a kinetic response?

Answer 20

Kinesis = Organism’s movement is affected by a non-directional stimulus.
1. Woodlice show a kinetic response to humidity —> in high humidity they move slowly and turn less often so they stay where they are.
2. As the air gets drier, they move faster and turn more often, so they will move into a new area.
3. The response increased the chance that a woodlouse will move into an area with higher humidity.
=> improves the survival chances of the organism - reduces their water loss and it helps to keep them concealed.