6.1 response to stimuli

Question 1

define the term ‘stimulus’.

Answer 1

a detectable change in the internal or external environment of an organism that leads to a response within that organism.

Question 2

give the two forms of communication found within organisms.

Answer 2

• hormonal communication.
• nervous communication.

Question 3

summarise the sequence of events that would occur within an organism following contact with a stimulus.

Answer 3

stimulus → receptor → coordinator → effector → response

Question 4

define the term ‘taxis’.

Answer 4

a simple response whose direction is determined by the direction of a stimulus.

Question 5

explain the difference between a ‘positive taxis’ and a ‘negative taxis’.

Answer 5

positive taxis - movement towards the stimulus.

negative taxis - movement away from the stimulus.

Question 6

define the term ‘kinesis’.

Answer 6

• a form of response in which an organism does not move towards or away from a stimulus.
• instead, it changes the speed at which it moves, and the rate at which it changes direction.

Question 7

define the term ‘tropism’.

Answer 7

the directional growth of a plant in response to a directional stimulus.

Question 8

explain the differences in the tropisms of a plant shoot as opposed to a plant root.

Answer 8

• plant shoots are positively phototropic and negatively gravitropic, meaning that their leaves are in the most favourable position to capture light for photosynthesis.
• plant roots are negatively phototropic and positively gravitropic.
• as a result, the roots grow into the soil, increasing their uptake of water and mineral ions.

Question 9

give two ways in which plant growth factors may instigate a response to an external stimulus.

Answer 9

• plant growth factors affect plant growth, so influence the phototropic, gravitropic, and hydrotropic potential of a plant.
• some plant growth factors affect the tissues that release them as opposed to acting on a distant target organ, such as in animals.

Question 10

give an example of a plant growth factor, and what this factor controls.

Answer 10

indoleacetic acid (IAA), which controls plant cell elongation.

Question 11

IAA belongs to which group of substances?

Answer 11

auxins.

Question 12

explain how IAA can result in positive phototropism in flowering plants.

Answer 12

• the position of a light source causes the movement of IAA from the light side to the shaded side of a plant shoot.
• a greater concentration of IAA builds up on the shaded side of the shoot.
• this increased concentration of IAA on the shaded side causes the cells on this side to elongate more rapidly than those on the light side.
• as a result, the shoot tip grows in the direction of the light source.

Question 13

a high concentration of IAA increases cell elongation in plant shoots. explain how a high concentration of IAA affects plant roots.

Answer 13

• a high concentration of IAA inhibits cell elongation in plant roots.
• in roots, the elongation of cells is greater on the light side than the shaded side, so the roots are negatively phototropic.

Question 14

explain how IAA can result in positive gravitropism in flowering plants.

Answer 14

• gravity influences the movement of IAA from the upper side of the root to the lower side of the root.
• there is a greater concentration of IAA on the lower side, therefore, these cells elongate less than those on the upper side.
• the increased elongation of cells on the upper side of the root as opposed to the lower side causes the root to grow downwards, towards the centre of gravity.

Question 15

explain the effect of a greater concentration of IAA on the lower side of a plant shoot.

Answer 15

• the greater concentration of IAA on the lower side increases cell elongation, causing the lower side to elongate more rapidly than the upper side.
• as a result, the shoot grows upwards, away from the force of gravity.

Question 16

describe how the ‘acid growth hypothesis’ can be used to explain how IAA increases the plasticity of plant cell walls, leading to cell elongation.

Answer 16

• the acid growth hypothesis involves the active transport of hydrogen ions from the cytoplasm of the plant cell into the spaces in the cell wall.
• this causes the cell wall to become more plastic, allowing the cell to elongate by expansion.

Question 17

give the two main components of the nervous system.

Answer 17

the central nervous system (CNS) - composed of the brain and spinal cord.

the peripheral nervous system (PNS) - composed of pairs of nerves that originate from either the brain or the spinal cord.

Question 18

give the two types of neurones found within the PNS, and their roles.

Answer 18

sensory neurones - carry nerve impulses from receptors towards the CNS.

motor neurones - carry nerve impulses away from the CNS to effectors.

Question 19

describe the two main divisions of the motor nervous system, and explain the role of each of these systems.

Answer 19

the voluntary nervous system - carries nerve impulses to body muscles, under voluntary (conscious) control.

the autonomic nervous system - carries nerve impulses to glands, smooth muscle, and cardiac muscle, under involuntary (subconscious) control.

Question 20

the spinal cord is a column of nervous tissue that runs along the length of the back. where is the spinal cord located?

Answer 20

the vertebral column.

Question 21

what is a reflex?

Answer 21

an involuntary response to a sensory stimulus.

Question 22

what is a reflex arc?

Answer 22

the pathway of neurones involved in stimulating a reflex.

Question 23

give the main components involved in a reflex arc.

Answer 23

stimulus → receptor → sensory neurone → coordinator → motor neurone → effector → response

Question 24

give three reasons why reflex actions are important in contributing to an organism’s survival.

Answer 24

• they are involuntary and do not require the decision-making powers of the brain, leaving it free to carry out more complex responses.
• they protect the body from harm.
• they are effective from birth, and do not need to be learned.

Question 25

the Pacinian corpuscle is a sensory receptor specific to the CNS. state the type of change that the Pacinian corpsucle responds to.

Answer 25

changes in mechanical pressure.

Question 26

what is a generator potential?

Answer 26

a generator potential occurs when the receptors in the nervous system convert the energy of a stimulus into a nervous impulse.

Question 27

the Pacinian corpsucle produces a generator potential by acting as a what?

Answer 27

a transducer - the Pacinian corpsucle transduces the mechanical energy of the stimulus into a generator potential.

Question 28

the Pacinian corpsucle is composed of layers of tissue which surround a single sensory neurone. give the type of sodium channel present in the plasma membrane of the sensory neurone found within the Pacinian corpsucle.

Answer 28

stretch-mediated sodium channels.

Question 29

in its resting state, the stretch-mediated sodium channels found throughout the membrane of the sensory neurone within the Pacinian corpsucle are too narrow to allow sodium ions to pass along them. explain what happens to these channels when pressure is applied to the Pacinian corpsucle.

Answer 29

• when pressure is applied to the Pacinian corpsucle it becomes deformed, and the membrane around its sensory neurone stretches.
• this stretching causes the widening of the sodium ion channels in the membrane, allowing sodium ions to diffuse into the neurone.
• this influx of sodium ions depolarises the membrane, producing a generator potential.
• this generator potential creates an action potential, which passes along adjacent neurones to the CNS.

Question 30

state where the photoreceptor cells of the mammalian eye are found.

Answer 30

in the retina.

Question 31

give the two main types of photoreceptor cells found in the mammalian retina.

Answer 31

• rod cells.
• cone cells.

Question 32

explain the role of rod cells and cone cells in the mammalian retina, including what they act as.

Answer 32

both rod and cone cells act as transducers, by conserving light energy into the electrical energy of a nerve impulse.

Question 33

give three differences between rod cells and cone cells.

Answer 33

rod cells

• more present at the periphery of the retina, and absent at the fovea.
• poor visual acuity.
• sensitive to low-intensity light.

cone cells

• fewer present at the periphery of the retina, concentrated at the fovea.
• good visual acuity.
• sensitive to high-intensity light.

Question 34

define the term ‘retinal convergence’.

Answer 34

retinal convergence refers to the sharing of a single nerve fibre by several rod cells in the retina of the eye.

Question 35

explain how retinal convergence leads to the stimulation of a generator potential in the bipolar cells of the retina.

Answer 35

• a certain threshold value has to be exceeded before a generator potential is created in the bipolar cells, to which the rod cells are connected.
• as a number of rod cells are connected to a single bipolar cell, there is a much greater chance that the threshold value will be exceeded than if only a single rod cell were connected to each bipolar cell.

Question 36

explain why rod cells respond to low levels of light intensity, including the pigment involved.

Answer 36

• in order to create a generator potential, the rhodopsin pigment in the rod cells must be broken down.
• there is enough energy from low-intensity light to cause this breakdown, which explains why rod cells respond to low levels of light intensity.

Question 37

give a consequence of retinal convergence, and explain how this leads to the low visual acuity of rod cells.

Answer 37

• a consequence of many rod cells linking to a single bipolar cell is that light received by rod cells sharing the same neurone will only generate a single impulse travelling to the brain, regardless of how many neurones are stimulated.
• the brain is therefore unable to distinguish between the separate sources of the light that stimulated the rod cells, leading to low visual acuity.

Question 38

rod cells cannot distinguish between different wavelengths of light and therefore lead to images being seen in only black and white. explain how the existence of cone cells allow us to perceive images in full colour.

Answer 38

• cone cells are of three different types, each responding to a different wavelength of light.
• cone cells can distinguish between red, blue, and green wavelengths.
• depending upon the proportion of each type that is stimulated, we can perceive images in full colour.

Question 39

explain why cone cells cannot stimulate a generator potential within a sensory neurone of the optic nerve.

Answer 39

• each cone cell has its own separate bipolar cell connected to a sensory neurone in the optic nerve.
• this means that the stimulation of a number of cone cells cannot be combined to help exceed the threshold value, therefore a generator potential cannot be stimulated.

Question 40

explain why cone cells respond to high levels of light intensity, including the pigment involved.

Answer 40

• cone cells contain the pigment iodopsin.
• only light of a high intensity can provide enough energy for the breakdown of iodopsin, in order to stimulate a generator potential.
• as a result, cone cells respond to high levels of light intensity.

Question 41

explain why cone cells have good visual acuity.

Answer 41

• each cone cell has its own connection to a single bipolar cell, meaning that if two adjacent cone cells are stimulated, the brain receives two separate impulses.
• the brain can therefore distinguish between the two separate sources of light that stimulated the cone cells.
• as a result, cone cells have good visual acuity.

Question 42

explain why only cone cells are found at the fovea of the retina.

Answer 42

• light is focused on the lens on the fovea of the retina, opposite the pupil.
• the fovea therefore receives the highest intensity of light.
• cone cells are sensitive to high intensities of light, so only cone cells are found at the fovea.

Question 43

what does the autonomic nervous system control?

Answer 43

the autonomic nervous system controls the involuntary activities of internal muscles and glands.

Question 44

give the two divisions of the autonomic nervous system, and explain their roles.

Answer 44

the sympathetic nervous system - stimulates effectors, heightening awareness, and speeding up activity.

the parasympathetic nervous system - inhibits effectors and slows down activity, to conserve energy and replenish the body’s reserves. the PNS controls activities under normal resting conditions.

Question 45

explain why the actions of the SNS as opposed to the PNS are described as ‘antagonistic’.

Answer 45

• the actions of the sympathetic and parasympathetic nervous systems normally oppose one another.
• therefore, they are described as ‘antagonistic’.

Question 46

explain why the cardiac muscle of the heart is described as ‘myogenic’.

Answer 46

the cardiac muscle of the heart is described as myogenic, meaning that its contraction is initiated from within the muscle itself, rather than by nervous impulses from outside the muscle.

Question 47

give the name of the group of cells found within the wall of the right atrium, which are responsible for stimulating the contraction of the heart.

Answer 47

the sinoatrial node (SAN)

Question 48

give the name of the group of cells responsible for generating impulses for the contractions of the heart.

Answer 48

the atrioventricular node (AVN)

Question 49

describe the sequence of events that stimulate the contraction of the heart muscle, and control heart rate.

Answer 49

• a wave of electrical excitation spreads out from the SAN across both atria, which causes them to contract.
• this wave of excitation reaches the AVN, which after a short delay, conveys a wave of electrical activity between the ventricles along the bundle of His.
• the bundle of His conducts the wave through the atrioventricular septum to the base of the ventricles, where it branches into smaller fibres of Purkyne tissue.
• the wave is then released at the apex of the heart, causing the ventricles to contract.

Question 50

explain what prevents the wave of electrical activity, which causes the atria to contract, from crossing to the ventricles.

Answer 50

a layer of non-conductive tissue, the atrioventricular septum, prevents this electrical wave from crossing to the ventricles.

Question 51

which region of the brain controls changes to heart rate?

Answer 51

the medulla oblongata.

Question 52

give the two centres of the medulla oblongata concerned with regulating heart rate.

Answer 52

• a centre that increases heart rate, linked to the SAN by the sympathetic nervous system.
• a centre that decreases heart rate, linked to the SAN by the parasympathetic nervous system.

Question 53

where are chemoreceptors found?

Answer 53

within the wall of the carotid arteries.

Question 54

what do chemoreceptors respond to?

Answer 54

changes in the pH of the blood, which result from changes in carbon dioxide concentration.

Question 55

describe how chemoreceptors regulate an increase in carbon dioxide levels in order to restore blood pH levels to optimum conditions.

Answer 55

• chemical receptors in the carotid arteries detect the drop in blood pH, and increase the frequency of impulses to the medulla oblongata.
• the centre of the medulla oblongata that increases heart rate increases the frequency of impulses to the SA node via the sympathetic nervous system.
• the SA node increases heart rate, and ultimately blood flow.
• increased blood flow removes carbon dioxide faster and as a result, the concentration of carbon dioxide returns to optimum levels.

Question 56

where are pressure receptors found?

Answer 56

within the walls of the carotid arteries and aorta.

Question 57

describe how pressure receptors regulate increased blood pressure levels to restore to optimum conditions.

Answer 57

• pressure receptors transmit more nervous impulses to the centre in the medulla oblongata that decreases heart rate.
• this centres sends impulses via the parasympathetic nervous system to the SAN, which leads to a decrease in the rate at which the heart beats.
• as a result, blood pressure levels decrease, and optimum conditions are restored.

Question 58

describe how pressure receptors regulate decreased blood pressure levels to restore to optimum conditions.

Answer 58

• pressure receptors transmit more nervous impulses to the centre in the medulla oblongata that increases heart rate.
• this centres sends impulses via the sympathetic nervous system to the SAN, which leads to a increase in the rate at which the heart beats.
• as a result, blood pressure levels increase, and optimum conditions are restored.