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 meristems).
● 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

please check

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

Many organisms respond 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 & unfavourable environment: turning 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 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 coordinates 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

Touch

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 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

check

Question 17

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

Answer 17

● 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 18

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

Answer 18

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

Question 19

Define myogenic.

Answer 19

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

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 (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

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

Answer 23

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 acetylcholine, which
   decreases heart rate.

Question 24

State the difference between the sympathetic and parasympathetic nervous system.

Answer 24

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 25

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

Answer 25

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 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.