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
• Diffuse from cell to cell

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, causes active transport of H+ ions into cell wall
3. Disruption to H bonds between cellulose molecules and action of expansins make cells 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 root
2. IAA inhibits elongation of root cells
3. Cells on upper side of root elongate faster, so root tip bends downwards

Question 4

Contrast mammalian hormones and plant growth factors

Answer 4

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
• Maintain mobile organism in optimum environment e.g. to prevent dessication

Question 6

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

Answer 6

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

Question 7

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

Answer 7

1. Organism crosses sharp division between favourable and unfavourable environment - turning increases to return to favourable environment
2. If organisms moves considerable distance into unfavourable environment, turning slowly decreases - begins to move in long straight lines, sharper turns

Question 8

What happens in a simple reflex arc?

Answer 8

• Receptor detects stimulus
• Sensory neurone
• Relay neurone in CNS coordinates response
• Motor neurone
• Response by effector

Question 9

Advantages of simple reflex

Answer 9

• Rapid response to potentially dangerous stimuli
• 3 neurones involved
• Instinctive

Question 10

Suitable stat 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

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

Question 12

Describe 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 and how?

Answer 13

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

Question 14

Name the 2 types of photoreceptor cells 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 central fovea
• Cone: mainly central fovea, no photoreceptors at blind spot

Question 16

Compare and contrast rod and cone cells (PACL)

Answer 16

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

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: within the wall of the right atrium
• Atrioventricular node: 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 atria = atrial systole
3. Layers 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 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 and parasympathetic

Question 23

Difference between 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 changes heart rate and state their location

Answer 24

• Baroreceptors: carotid body
• Chemoreceptors: carotid body and aortic body

Question 25

What do baroreceptors do?

Answer 25

Detect changes in blood pressure

Question 26

What do chemoreceptors do?

Answer 26

Detect changes in pH e.g. due to increase in CO2 concentration

Question 27

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

Answer 27

1. Baroreceptors send more impulses to cardioinhibitory centre in medulla oblongata
2. More impulses to SAN down vagus nerve via parasympathetic nervouse 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 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