9 Response to stimuli

Question 1

EXAM QUESTION

A biologist investigated the behaviour of a species of worm that lives in soil. He cultured three samples of worms in three separate trays of soil for many days.

The biologist concluded that the worms’ behaviour demonstrated taxis. How do these results support this conclusion? (2)

Using the information provided, suggest an explanation for the worms’ behaviour on
the glass surfaces in the absence of food. (3)

Answer 1

1. (Taxis is) movement towards/away from a
   stimulus / a directional response/movement (to
   a stimulus);
2. (Move towards) temperature they were used
   to/cultured in;
3. Hungry, so seeking food / in absence of food
   respond to temperature;
4. Move towards temperature they were used
   to/cultured in;
5. Associate (this temperature) with food;
6. (Then) stay in this temperature;

Question 2

EXAM QUESTION

In each experiment, the biologist exposed the surfaces to light that was dim and even,
so he could see where the worms went.
Apart from seeing where the worms went, suggest two reasons why it was important
that the light was dim and even. (2)

Answer 2

1. (Dim) worms live in soil/dark/ affected by bright
   light / dim light is like normal environment/what
   they are used to;
2. (Even) because worms might move
   towards/away from bright light / to avoid
   creating light gradient / prevent worms showing
   phototaxis/ all parts of surface exposed to same
   light;
3. (Dim light) ensures heat from light not a
   variable/ heat from lamp could kill/dry out
   worms;

Question 3

EXAM QUESTION

Termite B moved towards the light directed at it.

What type of behaviour was shown by the termite from group B? (1)

Give the evidence for your answer. (1)

Answer 3

Taxis

Moves towards stimulus/towards light;

Question 4

EXAM QUESTION

Give one similarity and one difference between a taxis and a tropism. (2)

Answer 4

1. Similarity – directional response (to a
   stimulus)/movement towards/away from
   a stimulus;
2. Difference – taxis (whole) organism
   moves and tropism a growth (response);

Question 5

EXAM QUESTION

Increased intensity of exercise leads to an increased heart rate. Explain how. (3)

Answer 5

1. (Oxygen/carbon dioxide)
detected by chemoreceptors
/ (pressure) detected by
baroreceptors;
2. Medulla/cardiac centre
involved;
3. More impulses to SAN/along
sympathetic nerve;

Question 6

Termite C moved off the sloping board

Suggest one advantage to the termites from group C of the behaviour shown in the
investigation. (2)

Answer 6

Helps them to leave the soil/ground/reach the surface;
Disperse/produce new colonies;
Avoid competition;

Question 7

Define stimulus

Answer 7

a detectable change in the internal or external environment of an organism that produces a response.

Question 8

Define selection pressure

favours?

Answer 8

environmental force altering the frequency of alleles in a population.

organisms with better responses

Question 9

Define receptor

Answer 9

cell/organ that detects stimuli

Question 10

Define effector

Answer 10

cell/organ that carries out response

Question 11

Why do receptors and effectors require communication system?
Hormones (2)
Nervous system (3)

Describe sequence

Answer 11

Slow
Plants and animals

Rapid
Animals
Central coordinator = connects information from each receptor with the appropriate effector.

stimulus —> receptor —> coordinator —> effector —> response

Question 12

Define taxis

positive taxis?

negative taxis?

Answer 12

movement towards/away from stimulus, directional response

= movement towards stimulus
= movement away from stimulus

Question 13

Define kinesis

Importance?

Answer 13

non-directional response to a stimulus, resulting in changed rate of movement/turning

Keeps organism in/moves organism into favourable environment

Question 14

Define tropism

positive tropism?

negative tropism?

Answer 14

growth movement of part of a plant towards/away from a stimulus, a directional response.

= growth towards stimulus
= growth away from stimulus

Question 15

Nervous organisation
2 major divisions?
divided into?
divided into?
divided into?

Answer 15

Central nervous system (CNS) - brain and spinal cord.
Peripheral nervous system (PNS) - pairs of nerves branching off

Sensory neurones - NI towards CNS
Motor neurones - NI away from CNS

Voluntary nervous system - NI to body muscles
Autonomic nervous system - NI to glands and smooth/cardiac muscle

Sympathetic nervous system - stimulates effectors/speeds up activity when body exercises strenuously
Parasympathetic nervous system - inhibits effectors/slows activity under normal resting conditions

Question 16

Define reflex

Answer 16

Rapid, involuntary response to a stimulus.

Question 17

Define reflex arc

Does not involve?

Answer 17

the nerve pathway taken by action potential that leads to reflex.

Does not involve brain

Question 18

Describe reflex arc (7)

Answer 18

Stimulus - detected by receptor
Receptor - creates NI in sensory neurone
Sensory neurone - carries NI to spinal cord
Intermediate neurone - links sensory neurone to motor neurone in spinal cord
Motor neurone - carries NI to effector
Effector - stimulated by NI
Response - carried out by effector

Question 19

3 advantages of reflex arc?

Answer 19

Do not have to be learned/effective from birth
Ensures brain not overloaded - free to carry out complex responses (some NI sent to inform/can override if necessary)
Fast because short neurone pathway/few synapses.

Question 20

Define Medulla oblongata

2 centres (4)

Answer 20

Region of the brain which controls changes to the heart rate.

One increases heart rate - linked to SAN by sympathetic nervous system.
One decreases heart rate - linked to SAN by parasympathetic nervous system.

Question 21

Where are chemoreceptors and pressure receptors found?

Answer 21

Walls of carotid arteries and aorta

Question 22

Describe the effect of exercise on heart rate (4)

Answer 22

Carbon dioxide/oxygen detected by chemoreceptors
Chemoreceptors increase frequency of NI to Medulla oblongata (centre that increases)
Medulla oblongata increases frequency NI to SAN along sympathetic nervous system
SAN increases heart rate

Question 23

Describe what happens if blood pressure too high (4)

Answer 23

Pressure increase detected by pressure receptors
Pressure receptors increase frequency of NI to Medulla oblongata (centre that increases)
Medulla oblongata increases frequency NI to SAN along parasympathetic nervous system
SAN decreases heart rate

Question 24

Describe what happens if blood pressure too low (4)

Answer 24

Pressure decrease detected by pressure receptors
Pressure receptors increase frequency of NI to Medulla oblongata (centre that decreases)
Medulla oblongata increases frequency NI to SAN along sympathetic nervous system
SAN increases heart rate

Question 25

Define Pacinian corpuscle

2 features?

Structure? (3)

Found where? (4)

Answer 25

A receptor that responds to changes in mechanical pressure.

Specific to single type of stimulus
Transducer - mechanical energy to generator potential

One sensory neurone, amongst layers of tissue, separated by gel.

Deep in skin - fingers, soles, genitalia,

Question 26

Pacinian corpuscle

At resting potential?

Describe how an action potential is generated. (5)

Answer 26

Sretch-mediated sodium channels too narrow to allow sodium ions through

Pressure applied 
Stretch-mediated sodium channel widens
Sodium ions diffuse into neurone = depolarisation
Generator potential established
Action potential generated

Question 27

Rod and cone cells

For each:
Shape?
Amount?
Bipolar cell? convergence?
Visual acuity?
Light sensitivity?
Pigment?

Answer 27

Rod-shaped
Greater number
Share bipolar cell (retinal convergence)
Low visual acuity
Sensitive to low-intensity light
Pigment rhodopsin must be broken down (by exceeding threshold value)
Found at the periphery of the retina, absent at the fovea.

Cone-shaped
Fewer number
One bipolar cell each (no retinal convergence)
High visual acuity
Not sensitive to low-intensity light
Pigment iodopsin must be broken down (by exceeding threshold value)
Found at fovea, absent at periphery of retina