Internal & External stimuli are detected & lead to a response

Question 1

Explain why shoots show positive phototropism

Answer 1

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 2

Explain why roots show positive gravitropism

Answer 2

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

Question 3

Define taxis and kinesis. State advantage.

Answer 3

• Taxis: directional movement in response to external stimulus
• Kinesis: non-directional response to presence and intensity of external stimulus
• Maintain mobile organism in optimum environment (e.g. to prevent dessication)

Question 4

Outline what happens in a simple reflex arc

Answer 4

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

Question 5

Basic structure of Pacinian Corpuscle

Answer 5

• 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 6

What stimulus does Pacinian corpuscle respond to? How?

Answer 6

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 7

Where are rod and cone cells located in retina?

Answer 7

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

Question 8

Outline pathway of light from photoreceptor to brain

Answer 8

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

Question 9

State name and location of 2 nodes involved in heart contraction

Answer 9

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

Question 10

Describe how heartbeats are initiated and coordinated

Answer 10

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 & valve 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 11

Formula of cardiac output

Answer 11

Cardiac output (CO) = Stroke volume (V) x heart rate (R)

Question 12

State difference between sympathetic and parasympathetic nervous system

Answer 12

Sympathetic involved in ‘fight in flight’ response: stimulates effectors to speed up activity.

Parasympathetic involved in normal resting conditions: inhibits effectors to slow down activity

Question 13

Name receptors involved in changing heart rate & state their location

Answer 13

Baroreceptors (detect changes in blood pressure): carotid body

Chemoreceptors (detect changes in pH e.g. due to increase in CO2 concentration): carotid body & aortic body

Question 14

How does body respond to increase in blood pressure?

Answer 14

1. Baroreceptors send more impulses to cardio-inhibitory centre in medulla oblongata.
2. More impulses to SAN down vagus nerve via parasympathetic nervous system.
3. Stimulates release of acetyl-choline, which decreases heart rate

Question 15

How does body respond to decrease in blood pressure?

Answer 15

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

Question 16

How does body respond to increase in CO2 concentration?

Answer 16

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 & ventilation rate increases.

Question 17

What are plant growth factors & where are they produced?

Answer 17

• 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 18

Features common to all sensory receptors

Answer 18

• Act as energy transducers which establish generator potential
• Respond to specific stimuli