3.6.1 Stimuli, both Internal & External, are Detected and Lead to a Response

Question 1

(Stimuli Lead to Response) 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 meristems).
• Diffuse from cell to cell/phloem mass transport.

Question 2

(Stimuli Lead to Response) Explain why shoots show positive phototropism. (5)

Answer 2

1) Indoleacetic acid (A+IAA) diffuses to shaded side of shoot tip.
2) This 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

(Stimuli Lead to Response) Explain why roots show positive gravitropism. (3)

Answer 3

1) Gravity cause 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 hje root tip bends downwards.

Question 4

(Stimuli Lead to Response) Contrast mammalian hormones (MH) and plant growth factors (PGF): Concentration.

Answer 4

MH: response not always dependent on concentration.

PGF: response proportional to concentration.

Question 5

(Stimuli Lead to Response) Contrast mammalian hormones (MH) and plant growth factors (PGF): Action.

Answer 5

MH: bind to complementary proteins in/on target cells.

PGF: can affect all cells.

Question 6

(Stimuli Lead to Response) Contrast mammalian hormones (MH) and plant growth factors (PGF): Synthesis.

Answer 6

MH: specialised glands.

PGF: various tissues in growing regions.

Question 7

(Stimuli Lead to Response) Contrast mammalian hormones (MH) and plant growth factors (PGF): Transport.

Answer 7

MH: circulatory system.

PGF: diffusion or phloem translocation.

Question 8

(Stimuli Lead to Response) Contrast mammalian hormones (MH) and plant growth factors (PGF): Speed.

Answer 8

MH: faster-acting (homeostasis).

PGF: slower-acting (plant growth).

Question 9

(Stimuli Lead to Response) Define taxis and kinesis. State their advantage.

Answer 9

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 10

(Stimuli Lead to Response) Many organisms respond to temperature and humidity via kinesis rather than taxis. Why?

Answer 10

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

Question 11

(Stimuli Lead to Response) How could a student recognise kinesis in an organism’s movement? (2)

Answer 11

1) Organism crosses sharp division between favourable & unfavourable environment: turing 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 line; sharper turns (lead organism to new environments).

Question 12

(Stimuli Lead to Response) Outline what happens in a simple reflex arc.

Answer 12

Receptor detects stimulus → sensory neuron → relay neuron in CNS coordinates response → motor neuron → response by effector.

Question 13

(Stimuli Lead to Response) Give the advantages of a simple reflex. (2)

Answer 13

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

Question 14

(Stimuli Lead to Response) 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 14

Chi squared

Question 15

(Stimuli Lead to Response) What features are common to all sensory receptors?

Answer 15

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

Question 16

(Stimuli Lead to Response) Describe the basic structure of a Pacinian corpuscle. (3)

Answer 16

• 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 17

(Stimuli Lead to Response) What stimulus does a Pacinian corpuscle respond to? How? (3)

Answer 17

1) Pressure deforms membrane, cause 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 18

(Stimuli Lead to Response) Name the 2 types of photoreceptor cell located in the retina.

Answer 18

1) Cone cells
2) Rod cells

Question 19

(Stimuli Lead to Response) Where are rod and cone cells located in the retina?

Answer 19

Rod: evenly distributed around periphery but NOT in central fovea.

Cone: mainly central fovea no photoreceptors at blind spot.

Question 20

(Stimuli Lead to Response) Compare and contrast rod and cone cells: Pigment

Answer 20

Rod: rhodopsin
Cone: 3 types of iodopsin

Question 21

(Stimuli Lead to Response) Compare and contrast rod and cone cells: Visual acuity

Answer 21

Rod: low res - many rod cells synapse with 1 bipolar neuron
Cone: high res - 1 cone cell synapses with 1 bipolar neuron = no retinal convergence

Question 22

(Stimuli Lead to Response) Compare and contrast rod and cone cells: Colour sensitivity

Answer 22

Rod: Monochromatic - all wavelengths of light detected
Cone: tricolour - red, blue, green wavelengths absorbed by different types of iodopsin

Question 23

(Stimuli Lead to Response) Compare and contrast rod and cone cells: Light sensitivity

Answer 23

Rod: Very sensitive - spatial summation of subthreshold impulses
Cone: Less sensitive = not involved in night vision

Question 24

(Stimuli Lead to Response) Outline the pathway of light from a photoreceptor to the brain. (4)

Answer 24

Photoreceptor → bipolar neuron → ganglion cell of optic nerve → brain

Question 25

(Stimuli Lead to Response) Define myogenic.

Answer 25

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

Question 26

(Stimuli Lead to Response) State the name and location of the 2 nodes involved in heart contraction.

Answer 26

- 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 27

(Stimuli Lead to Response) Describe how heartbeats are initiated and coordinated. (5)

Answer 27

- SAN initates wave of depolarisation (WOD). - WOD spreads across both atria = atria systole. - Layer of fibrous, non-conducting tissue delays impulse while ventricles fill & valves close. - AVN conveys WOD down septum via Bundle of His, which branches into Purkinje fibres along ventricles. - Causes ventricles to contract from apex upwards.

Question 28

(Stimuli Lead to Response) State the formula for cardiac output.

Answer 28

cardiac output (CO) = stroke volume (V) x heart rate (R)

Question 29

(Stimuli Lead to Response) What is the autonomic nervous system? Include information on the subdivisions. (2)

Answer 29

- System that controls involuntary actions of gland and muscles. - 2 subdivisions: sympathetic & parasympathetic.

Question 30

(Stimuli Lead to Response) State the difference between the sympathetic and parasympathetic nervous system.

Answer 30

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 31

(Stimuli Lead to Response) Name the 2 receptors involved in changing heart rate and state their location.

Answer 31

1) Baroreceptors (detect changes in blood pressure): carotid body. 2) Chemoreceptors (detect changes in pH e.g. due to increase in CO2 conc): carotid body & aortic body.

Question 32

(Stimuli Lead to Response) How does the body respond to an increase in blood pressure? (3)

Answer 32

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 acetyl choline, which decreases heart rate.

Question 33

(Stimuli Lead to Response) How does the body respond to a decrease in blood pressure? (3)

Answer 33

1) Baroreceptors send more impulses to cardioinhibitory 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 34

(Stimuli Lead to Response) How does the body respond to an increase in CO2 concentration? (3)

Answer 34

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.