6.1 Internal and External Stimuli Are Detected and Lead To A Response

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 meristem)
  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. (acid growth hypothesis)
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

Contrast mammalian hormones and plant growth factors (CASTS)

Answer 4

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Question 5

Define taxis and kinesis. State their advantages

Answer 5

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 desssication

Question 6

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

Answer 6

Less directional 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 and unfavourable environment: turning increases (returns to the original favourable environment)
2. If organisms moves considerable distance into unfavourable environment: turning slowly decreases; begins to move in long, strong lines; sharper turns (lead 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 coordinate response —> motor neuron —> response by effector

Question 9

Give the advantages of a simple reflex

Answer 9

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

Answer 13

1. Pressure deforms membrane, causing stretch-mediated Na+ ion channels to open.
2. If influx of sodium 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 cell located in the retina

Answer 14

1. Cone cells
2. 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 and contrast rod and cone cells. (PACL)

Answer 16

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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 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 21

State the formula for cardiac output

Answer 21

Cardiac output (CO) = 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

State the difference between the sympathetic and parasympathetic nervous system

Answer 23

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 24

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

Answer 24

Barorecptors (detect changes in blood pressure): carotid body

Chemoreceptors(detect changes in pH e.g. due to increase CO2 concentration): carotid body & 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 onlongata
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. Stimulates 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 rate of blood flow to lungs increases = rate of gas exchange and ventilation rate increase