Response To Stimuli

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

Diffuse from cell to cell/ phloem mass transport

Question 2

Explain why shoots show positive phototropism

Answer 2

1. Indoleacetic acid (IAA) diffuses to shaded side of shoot tip
2. As IAA diffuses down shaded side it causes active transport of H+ ions into the cell wall
3. Disruption to H-bonds between cellulose molecules make cells more permeable to water
4. Cells on shaded side elongate faster due to higher turgor pressure
5. Shoot bends towards the light

Question 3

Explain why roots show positive gravitropism

Answer 3

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

Question 4

Define taxis

Answer 4

Directional movement in response to external stimulus

Question 5

Define Kinesis

Answer 5

Non directional response to presence and intensity of external stimulus

Question 6

What is the advantage of taxis and kinesis?

Answer 6

Maintain mobile organism in optimum environment

Question 7

Why do many organisms respond to temperature and humidity via kinesis rather than taxis?

Answer 7

Less directional stimuli; often no clear gradient from one extreme to the other

Question 8

How could a student recognise kinesis in an organisms movement?

Answer 8

1. The organism crosses sharp division between favourable and unfavourable environment and turning increases, in favourable environment
2. If an organism is in the unfavourable environment turning decreases and it begins to move in long straight lines

Question 9

Outline what happens in a simple reflex arc

Answer 9

Receptor detects stimulus - Sensory neurone - Relay neurone - Motor neurone - response by effector

Question 10

Give 2 advantages of a simple reflex

Answer 10

Rapid response to potentially dangerous stimuli

Instinctive

Question 11

What features are common to all sensory receptors?

Answer 11

Act as energy transducers which establish a generator potential

Respond to specific stimuli

Question 12

Describe the structure of a Pacinian corpuscle

Answer 12

Single nerve fibre surrounded by layers of connective tissue which are separated by viscous gel and contained by a capsule

Stretch mediated Na+ channels on plasma membrane

Capillary runs along base layer of tissue

Question 13

What stimulus does a pacinian corpuscle respond. to and how?

Answer 13

1. Pressure deforms membrane causing stretch mediated Na+ ion channels to open
2. If the influx of Na+ raises the membrane to the threshold a generator potential is produced
3. Action potential moves along the sensory neurone

Question 14

Name the 2 types of photoreceptor cell located in the retina

Answer 14

Cone cells and rod cells

Question 15

Where are rod and cone cells located in the retina?

Answer 15

Rod- Evenly distributed around the periphery but not in the central fovea

Cone- Mainly central fovea, no photoreceptors at blind spot

Question 16

Give 3 differences between rod and cone cells

Answer 16

Rod has rhodopsin and cones have 3 types of iodopsin

Rods have a low visual acuity- many attach to one bipolar neurone and cones have a high visual acuity- one attaches to one bipolar neurone

Rods are very sensitive to light- spatial summation (impulses combine from each cell to exceed threshold) and cones are less sensitive to light

Question 17

Outline the pathway of light from a photoreceptor to the brain

Answer 17

Photoreceptor - bipolar neurone - ganglion cell of optic nerve - brain

Question 18

What does myogenic mean?

Answer 18

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

Question 19

What nodes are involved in heart contraction?

Answer 19

Sinoatrial node (SAN)- in the wall of the right atrium

Atroventricular node (AVN) near lower end of right atrium in the wall that separates the 2 atria

Question 20

How are heartbeats initiated and coordinated?

Answer 20

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

Question 21

What is the formula for cardiac output?

Answer 21

Cardiac output = stroke volume x heart rate

Question 22

What is the autonomic nervous system?

Answer 22

System that controls involuntary actions of glands and muscles

Divides into the sympathetic and parasympathetic

Question 23

What is the difference between the sympathetic and parasympathetic nervous system?

Answer 23

Sympathetic - Involved in fight or flight, speeds up activity

Parasympathetic - Involved in normal resting conditions, slows down activity

Question 24

What are the receptors involved in changing heart rate?

Answer 24

Baroreceptors, detect changes in blood pressure - in carotid body

Chemoreceptors, detect changes in pH e.g due to increase in CO2 concentration - in carotid body and aortic body

Question 25

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

Answer 25

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 26

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

Answer 26

1. Baroreceptors send more impulses to cardioacceleratory centre in the medulla oblongata
2. More impulses to SAN via sympathetic nervous system
3. Stimulate release of noradrenaline which increases heart rate and strength of contraction

Question 27

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

Answer 27

1. Chemoreceptors detect pH decrease and send more impulses to cardioacceleratory centre of medulla oblongata
2. More impulses to SAN via sympathetic nervous system]
3. Heart rate increases so the rate of blood flow to the lungs increases = rate of gas exchange and ventilation rate increase