Stimuli and Response (receptors) - A2

Question 1

What are plant growth factors and where are they produced?

Answer 1

• chemicals that regulate plant growth response to directional stimuli
• produced in plant growing regions (apical meristems)
• diffuse from cell to cell / phloem mass transport

Question 2

Explain why shoots show positive phototropism?

Answer 2

• Indoleacetic acid (IAA) diffuses to shade side of root tip.
• as IAA diffuses to shaded side, it causes active transport of H+ ions into cell wall.
• disruption to H bonds between cellulose molecules and action of expansions make cell more permeable to water (acid growth hypothesis)
• cells on shaded side elongate faster due to higher turgor pressure
• shoot bends towards light

Question 3

Explain why roots show positive gravitropism.

Answer 3

1. gravity causes IAA to accumulate on lower side of the root
2. IAA inhibits elongation of root cells
3. cells on upper side of the root elongate faster, so root tip bends downwards.

Question 4

Define taxes and kinesis. Explain their advantage.

Answer 4

Taxes - directional movement in response to external stimulus.
Kinesis - non-directional response to presence and intensity of external stimulus
- Maintain mobile organism in optimum environment eg. to prevent dessicasion

Question 5

Many organisms respond to temperature and humidity via kinesis rather than taxes. Explain why.

Answer 5

• less directional stimuli
• often no clear gradient from one extreme to another

Question 6

How could a student recognise kinesis in an organisms movement?

Answer 6

1. organism crosses sharp division between favourable and unfavourable environment - turning increases
2. if organism moves considerable distance into unfavourable environment - turning slowly decreases - begins to turn in long straight lines (sharper turns lead organisms to new environment)

Question 7

Outline what happens in a simple reflex arc.

Answer 7

receptor detects stimulus - sensory neuron -
relay neuron in CNS coordinate response - motor neuron - response by effector

Question 8

Give the advantage of a simple reflex.

Answer 8

1. rapid response to a potentially dangerous stimuli since only 3 neuron involved
2. instinctive

Question 9

What features are common to all sensory receptors?

Answer 9

• act as energy transducers which establish a generator potential
• respond to specific stimuli

Question 10

Describe the basic structure of a Pacinion corpuscle.

Answer 10

• single nerve fibre surrounded by layers of connective tissues which are separated by viscous gel and are contained by a capsule
• stretch mediated Na+ channels on plasma membrane
• capillary runs along base layer of tissue

Question 11

What stimulus does a Pacinian corpuscle respond to and how?

Answer 11

1. pressure deforms membrane causing stretch-mediated Na+ ion channels to open
2. if influx of Na+ raises membrane to threshold potential, a generator potential is produced
3. action potential moves along sensory neuron

Question 12

Name the 2 types of photoreceptor cell located in the retina.

Answer 12

• cone cells
• rod cells

Question 13

Where are the rod and cone cells located in the retina?

Answer 13

cone - mainly central fovea, no photoreceptors at blind spot
rod - evenly distributed around periphery but not in central fovea

Question 14

Outline the pathway of light from a photoreceptor to the brain.

Answer 14

photoreceptor - bipolar neuron - ganglion cell of optic nerve - brain

Question 15

Define myogenic.

Answer 15

Contraction of heart is initiated within the muscle itself rather than by nerve impulses.

Question 16

State the name and location of the 2 nodes involved in heart contraction.

Answer 16

Sinoatrial node (SAN) - within the wall of the right atrium
Atrioventricular node (AVN) - near lower end of right atrium in the wall that separates the 2 atria

Question 17

Describe how heartbeats are initiated and coordinated.

Answer 17

1. SAN initiated wave of depolarisation (WOD)
2. WOD spreads across both atria = atrial systole
3. layer of fibrous, non-conducting tissue delays impulse whilst ventricles fill and valves close
4. AVN conveys WOD down septum via Bundle of His, which branches into Purkinje fibres along ventricles
5. causes ventricles to contract from apex upwards

Question 18

What is the autonomic nervous system?

Answer 18

• system that controls involuntary actions of glands and muscles
• 2 subdivisions: sympathetic and parasympathetic

Question 19

State the difference between the sympathetic and parasympathetic nervous system.

Answer 19

Sympathetic involved in fight or flight response - stimulates effectors to speed up activity
Parasympathetic involved in normal resting conditions - inhibits effectors to slow down activity

Question 20

Name the receptors involved in changing heart rate and state their location.

Answer 20

Baroreceptors - detect change in blood pressure: carotid body
Chemoreceptors - detect change in PH eg. due to increase in CO2 concentration: carotid body and aortic body

Question 21

How does the body respond to an increase in blood pressure?

Answer 21

1. Baroreceptors send more impulses to cardioinhibitory centre in the medulla oblongata
2. more impulses to SAN down vagus nerve via parasympathetic nervous system
3. stimulates release of acetylcholine, which decreases heart rate

Question 22

How does the body respond to a decrease in blood pressure?

Answer 22

1. Baroreceptors send more impulses to cardio-acceleratory centre in the medulla oblongata
2. more impulses to SAN via sympathetic nervous system
3. stimulates release of noradrenaline, which increases heart rate and strength of contraction

Question 23

How does the body respond to an increase in CO2 concentration?

Answer 23

1. chemoreceptors detect PH decrease and send more impulses to cardio-acceleratory centre of medulla oblongata
2. more impulses to SAN via sympathetic nervous system
3. heart rate increases, so rate of blood flow to lungs increases = rate of gas exchange and ventilation increases