Detection and response

Question 1

What is the basic stimulus-response chain?

Answer 1

stimulus - receptor - coordinator - effector - response

Question 2

What are taxes (taxis)?

Answer 2

Movements in a specific direction that are directed by a stimulus such as light or food (movement towards/away from a stimulus)

Question 3

What are kineses?

Answer 3

Random movements that are not directed by a stimulus (not towards/away from a stimulus). The more unfavorable the stimulus, the more rapidly the organism moves and the more frequently it changes direction

Question 4

What is a tropism?

Answer 4

A growth response of part of a plant in response to a directional stimulus

Question 5

What is positive phototropism and how does it occur?

Answer 5

Plant shoot grows towards the light

• unilateral light is detected by receptors in shoot tips and causes IAA to be produced
• IAA diffuses down the shoot evenly
• light causes IAA to redistribute towards the shaded side of the shoot
• higher IAA concentration on shaded side causes more rapid cell elongation on the shaded side, resulting in positive phototropism

Question 6

What is gravitropism and how does it occur?

Answer 6

• cells in root tips produce IAA which is transported along the root
• gravity causes IAA to move from the upper side to the lower side of the root
• a greater concentration of IAA builds up on the lower side of the root
• IAA inhibits cell elongation in plant roots, so cells elongate less on the bottom
• causes the root to bend downwards towards the force of gravity

Question 7

How is the nervous system organised?

Answer 7

CNS - consists of brain and spinal chord
PNS (peripheral nervous system) - consists of sensory nervous system and motor nervous system

• motor nervous system consists of somatic (voluntary) and autonomic (involuntary). Autonomic made up of sympathetic and parasympathetic

Question 8

How is a reflex arc formed through standing on a pin?

Answer 8

• pain is detected by mechanoreceptors in skin of foot
• electrical impulses are sent along the sensory neurones towards the central nervous system
• neurotransmitters cross the synapse and an electrical impulse is sent along the the intermediate neurone in the spinal chord
• electrical impulse along motor neurone, causes muscles in the leg to contract and move away

Question 9

Why are reflex arcs important?

Answer 9

• involuntary
• protect body from harmful stimuli
• fast
• do not involve conscious part of brain
• prevent damage to tissues

Question 10

How are hormonal and nervous responses different?

Answer 10

• hormonal system = transmission in bloodstream, slow, widespread, long-lasting
• nervous system = transmission by nerve impulses, very rapid, localised and specific, short-lived

Question 11

What type of potential do sensory receptors produce and how?

Answer 11

generator potential - change in membrane potential of a receptor cell

Question 12

What is the structure of the pacinian corpuscle?

Answer 12

a neuron ending surrounded by layers of connective tissue with viscous gel between (lamellae)

Question 13

How does the pacinian corpuscle transduce the mechanical energy of a stimulus into a generator potential?

Answer 13

• pressure from a stimulus causes the lamellae to deform
• as the lamellae deforms, this stimulates stretch-mediated sodium ion channels to open, leading to an influx of sodium ions
• this depolarises the membrane, leading to a generator potential

Question 14

How does a greater pressure affect the pacinian corpuscle?

Answer 14

causes more sodium ion channels to open, so there is a greater influx of sodium ions, leading to a greater generator potential

Question 15

How are action and generator potential different when exposed to different intensity stimuli?

Answer 15

generator potential - higher intensity stimulus = greater generator potential

action potential - higher intensity stimulus = greater frequency of action potentials

Question 16

What is the retina?

Answer 16

contains light-sensitive cells, rods for dim light, cones for colour. It sends nerve impulses to the brain

Question 17

What is the blind spot?

Answer 17

where blood vessels and nerves join the eyeball

Question 18

What is the optic nerve?

Answer 18

receives nerve impulses from the retina and sends them to the brain

Question 19

How does the eye change for distant focus?

Answer 19

• ciliary muscles relax
• suspensory ligaments contract
• lens becomes thicker and less curved

Question 20

How does the eye change for close focus?

Answer 20

• ciliary muscles contract
• suspensory ligaments relax
• lens becomes thinner and more curved

Question 21

What is the structure of the retina?

Answer 21

• pigment cells of choroid at the front of the eye
• cones and rods behind these melanin cells
• nerve impulses travel along rods and cones - connected to bipolar neurones, which are connected to ganglion cells
• ganglion cells connect to sensory neurones, which carry nerve impulses to the optic nerve

Question 22

How do rods and cones differ with bipolar neurones in the retina?

Answer 22

several rod cells synapse with one bipolar cell
one cone cell synapses with one bipolar cell

Question 23

What are the pigments found in rods and cones?

Answer 23

rods - rhodopsin
cones - iodopsin

Question 24

Why is mitochondria needed in large quantities in rod and cone cells?

Answer 24

release energy used to synthesize pigments

Question 25

How are rods and cones distributed?

Answer 25

rods are mainly found at the outer edge of the retina - they are more light sensitive and help to focus in more dim light

cones are mainly found at the fovea (centre of the eye) - they give colour vision and are used to focus in bright light

At the blind spot, there is a complete absence of either photoreceptor

Question 26

How does visual acuity differ in rod and cone cells?

Answer 26

rod cells have a high visual acuity whereas cone cells have a low visual acuity

• several rod cells synapse with one bipolar neurone so fewer impulses are sent along sensory neurones - brain may perceive 2 stimuli as 1
• cone cells synapse with 1 bipolar neurone so a separate action potential is generated for each stimulus - brain perceives each stimulus separately

Question 27

How does sensitivity to light differ in rod and cone cells?

Answer 27

cones have low sensitivity whereas rods have high sensitivity

• several rods synapsing with 1 bipolar neurone leads to summation which makes it more likely that threshold will be reached and an action potential will be generated
• no summation in cones so threshold less likely to be exceeded

Question 28

Why can cones see colour?

Answer 28

they absorb light in different wavelengths

Question 29

Why does it take a long time for eyes to adjust to a dark room?

Answer 29

• in a bright room, most of the rhodopsin in rod cells is broken down
• rhodopsin is resynthesised using energy from ATP produced during respiration
• once the rhodopsin has been resynthesized, the rod cells can be stimulated by the low intensity light in the dim room

Question 30

What is the sequence of events that control the cardiac cycle?

Answer 30

• wave of excitation spreads from the SA node across both atria, causing them to contract and push blood into the ventricles
• wave reaches AV node where there is a short delay to allow the atria to empty and ventricles to fill, then is sent along purkyne fibres of the bundle of His
• atria relax
• wave reaches bottom of ventricles which contract from the bottom upwards. Increased pressure causes AV valves to shut
• this forces blood into the pulmonary artery and aorta through semi-lunar valves
• ventricles relax, pressure goes below aorta, semi-lunar valves shut
• cycle starts again

Question 31

How is heart rate controlled by chemoreceptors?

Answer 31

Increase in CO2:
- decreased pH is detected by chemoreceptors in the aorta and carotid arteries
- nerve impulse sent along sensory neurone to the cardiovascular centre in the medulla
- impulse sent along sympathetic nerve to SAN
- SAN increases frequency of heart rate

Decrease in CO2:
- increased pH is detected by chemoreceptors in the aorta and carotid arteries
- nerve impulse sent along sensory neurone to the cardiovascular centre in the medulla
- impulse sent along parasympathetic nerve to SAN
- SAN decreases frequency of heart rate

Question 32

How is heart rate controlled by pressure receptors?

Answer 32

Increase in blood pressure:
- increased blood pressure detected by pressure receptors (baroreceptors) in aorta and carotid arteries
- nerve impulse sent along sensory neurone to cardiodecceleratory centre in medulla
- nerve impulse sent along parasympathetic nerve to SAN
- heart rate decreases

Decrease in blood pressure:
- decreased blood pressure detected by pressure receptors (baroreceptors) in aorta and carotid arteries
- nerve impulse sent along sensory neurone to cardioacceleratory centre in medulla
- nerve impulse sent along sympathetic nerve to SAN
- heart rate increases

Question 33

How do the radial and circular muscles change in dim/bright light?

Answer 33

In bright light:
- radial muscles relax
- circular muscles contract

In dim light:
- radial muscles contract
- circular muscles relax