Stimuli Are Detected + Lead To A Response

Question 1

What are plant growth factor and where are they produced?

Answer 1

• chemicals that regulate plant growth response to directional stimuli
• produce in apical meristems
• diffuse from cell to cell/phloem mass transport

Question 2

Explain why shoots show positive phototropism

Answer 2

1. IAA diffuses to shaded side of shoot tip
2. As IAA diffuses down shaded side it causes active transport of H+ ions into cell wall
3. Disruption to H-bonds between cellulose molecules & action of expansins make cell more permeable to water
4. Cells on shaded side elongate faster due to higher turgor pressure
5. 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 the upper side of the root elongate faster so the root tip bends downwards

Question 4

What is taxis?

Answer 4

Directional movement in response to external stimulus

Question 5

What is 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 organism’s movement?

Answer 8

1. Organism crosses sharp division between favourable & unfavourable environment: turning increases
   (Return to the original favourable environment)
2. If organism moves considerable distance into unfavourable environment: turning slowly decreases; begins to move in long, straight lines; sharper turns
   (Lead organism to new environment)

Question 9

Outline what happens in a simple reflex arc

Answer 9

Receptor detects stimulus—>sensory neuron—>relay neuron in CNS coordinates response —> motor neuron —> response by effector

Question 10

What are the advantages of a simple reflex?

Answer 10

1. Rapid response to potentially dangerous stimuli since only 3 neurons involved
2. Instinctive

Question 11

Suggest a suitable statistical test to determine whether a factor has a significant effect on the movement of an animal in a choice chamber

Answer 11

Chi squared

Question 12

What features are common to all sensory receptors?

Answer 12

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

Question 13

Describe the basic structure of a pacinian corpuscle

Answer 13

• 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 14

What stimulus does a pacinian corpuscle respond to and how?

Answer 14

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 15

What are the 2 types of photoreceptor cell located in the retina?

Answer 15

• cone cells

- rod cells

Question 16

Where are rod and cone cells located in the retina?

Answer 16

Rod: evenly distributed around periphery but NOT in central fovea
Cone: mainly central fovea no photoreceptors at blind spot

Question 17

What are some properties of rod cells?

Answer 17

• rhodopsin pigment
• low visual acuity since many rod cells synapse with 1 bipolar neuron
• monochromatic: all wavelengths of light detected
• very sensitive to light: spatial summation of subthreshold impulses

Question 18

What are some properties of cone cells?

Answer 18

• 3 types of iodopsin pigment
• high visual acuity: 1 cone cell synapses with 1 bipolar neuron so no retinal convergence
• tricolour: red, blue, green wavelengths absorbed by different types of iodopsin
• less sensitive to light so not involved in night vision

Question 19

Outline the pathway of light from a photoreceptor to the brain

Answer 19

Photoreceptor—>bipolar neuron—>ganglion cell of optic nerve—>brain

Question 20

Define myogenic

Answer 20

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

Question 21

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

Answer 21

• 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 22

Describe how heartbeats are initiated and coordinated

Answer 22

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 & 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 23

What is the formula for cardiac output?

Answer 23

Cardiac output = stroke volume (v) x heart rate (r)

Question 24

What is the autonomic nervous system?

Answer 24

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

Question 25

What is the difference between sympathetic and parasympathetic nervous system?

Answer 25

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 26

What and where are the receptors involved in changing heart rate?

Answer 26

Baroreceptors: detect change in blood pressure/found in carotid body

Chemoreceptors: detect changes in pH/found in carotid & aortic body

Question 27

How does the body respond to increase in blood pressure?

Answer 27

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 28

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

Answer 28

1. Baroreceptors send more impulses to cardioacceleratory 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 29

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

Answer 29

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 rate increase