6.1 - Internal and external; stimuli are detected and lead to a response

Question 1

What are the 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 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.
5. Shoots bend 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 in hibits elongation of root cells
3. Cells on the upper side of the root elongate faster so the root tip bends downwards

Question 4

Contrast mammalian hormones and plant growth factors.
- Concentration
- Action
- Synthesis
- Transport
- Speed

Answer 4

Mammalian hormone:
- Concentration: response not always dependent on concentration.
- Action: bind to complementary proteins in or on target cells
- Synthesis: specialised glands
- Transport: circulatory system
- Speed: faster-acting
Plant Growth Factor:
- Concentration: response is directly proportional to concentration
- Action: can affect all cells
- Synthesis: various tissues in growing regions
- Transport: diffusion or phloem translocation
- Speed: slower-acting in plant growth

Question 5

Define taxis and kinesis. State their advantage.

Answer 5

Taxis: directional movement in response to external stimulus
Kinesis: non-directional response to presence and intensity of external stimulus
Advantage: maintains mobile organism in optimum environment.

Question 6

Many organisms respond to temperature and humidity via kinesis than taxis. Why?

Answer 6

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

Question 7

How could a student recognise kinesis in an organism’s movement?

Answer 7

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

Question 8

Outline what happens in a simple reflex arc.

Answer 8

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

Question 9

Advantages of a simple reflex.

Answer 9

• rapid response to potentially dangerous since only 3 neurons involved
• Instinctive

Question 10

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 10

Chi squared

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 basic 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 capsule.
• Stretch-mediated Na+ channels on plasma membrane

Question 13

What stimulus does a Pacinian corpuscle respond to? How?

Answer 13

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 14

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

Answer 14

• Cone cells
• Rod cells

Question 15

Where are rod and cone cells located in the retina?

Answer 15

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

Question 16

Compare + contrast rod and cone cells.
Pigment
Visual acuity
Colour sensitivity
Light sensitivity

Answer 16

ROD:
- Pigment: rhodopsin
- Visual acuity: low res - many rod cells synapse with 1 bipolar neuron
- Colour sensitivity: Monochromatic - all wavelengths of light detected
- Light sensitivity: Very good - spatial summation of subthreshold impulses
CONE:
- Pigment: 3 types of iodopsin
- Visual acuity: high res- 1 cone cell synapses with 1 bipolar neuron = no retinal convergence
- Colour sensitivity: tricolour - red, blue and green wavelengths absorbed by different types of iodopsin
- Light sensitivity: less sensitive - not involved in night vision

Question 17

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

Answer 17

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

Question 18

Define myogenic

Answer 18

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

Question 19

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

Answer 19

• 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 20

Describe how heartbeats are 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 & 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 21

State the formula for cardiac output.

Answer 21

Cardiac output = stroke volume (V) x heart rate (R)

Question 22

What is the autonomic nervous system?

Answer 22

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

Question 23

Sate the difference between the sympathetic and parasympathetic nervous system

Answer 23

• Sympathetic is 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 24

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

Answer 24

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

Question 24

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

Answer 24

• Baroreceptors (detect changes in blood pressure): carotid body
• Chemereceptors (detect changes in pH e.g. due to increase in CO2 concentration): carotid body & aortic body.

Question 25

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

Answer 25

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 26

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

Answer 26

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 rate of blood flow to lungs increases = rate of gas exchange and ventilation rate increase.

Question 27

Exercise causes an increase in heart rate. Describe the role of receptors and of the nervous system in this process.

Answer 27

• Sympathetic nervous system increases heart rate.
• Due to chemoreceptors detecting a rise in Co2 levels in the blood.
• It then sends impulses to the cardiac centre / medulla oblongata.
• More impulses are sent to SAN.