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Biology > 14. Response to Stimuli > Flashcards

14. Response to Stimuli Flashcards

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1
Q

Define stimulus?

A

A detectable alteration in the internal or external environment of an organism that produces some change in that organism.

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2
Q

What is a receptor?

A

A cell adapted to detect changes in the environment.

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3
Q

Why is there always a selection pressure?

A

Those organisms that survive have a greater chance of raising of raising offspring and passing their alleles to the next generation. Therefore a selection pressure favours organisms with more appropriate responses.

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4
Q

What is an effector?

A

An organ that responds to stimulation by a nerve impulse resulting in a change.

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5
Q

What are 2 different types of communication in the body?

A
  • Hormone communication

- The nervous system

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6
Q

What is the coordinator sequence of the nervous system?

A

Stimulus —> Receptor —> Coordinator —> Effector —> Response

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7
Q

What is a ‘taxis’

A

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

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8
Q

What is a positive taxis?

A

Movement towards the stimulus

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9
Q

What is a negative taxis?

A

Movement away from a stimulus

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10
Q

What is kinesis?

A

A form of response in which the organism doesn’t move towards or away from a stimulus. Instead it changes the speed at which it moves and the rate at which it changes direction.

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11
Q

Define tropism?

A

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

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12
Q

Some species of bacteria move away from waste products they produce. Name the response and it’s survival value

A

Negative chemo-taxis - wastes are often removed from an organism because they’re harmful. Moving away prevents the waste harming the organism, increasing its chance of survival.

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13
Q

The speed cells of a moss plant are attracted towards a chemical produced by the female reproductive organ of another moss plant. Name the response and it’s survival value.

A

Positive chemo-taxis- increases the chance of sperm cells fertilising the egg cells of other mosses and so helps to produce more moss plants. Cross fertilisation increases genetic variability, making species better able to adapt to future environmental changes.

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14
Q

The young stems of seedlings grow away from gravity. Name the response and it’s survival value.

A

Negative gravi-tropism, takes the seedlings above the ground and into the light, where they can photosynthesise.

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15
Q

Give 3 factors that plants respond to

A
  • Light
  • Gravity
  • Water
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16
Q

What is a plant growth factor and give an example?

A

Plant growth factors exert their influence by affecting growth and, they may be made by cells located throughout the plant rather than in particular organs.
Unlike animal hormones, some plant growth factors affect the tissues that release them rather than acting on a distant target organ.
Example: Indoleacetic acid (IAA), an auxin which causes plant cell elongation.

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17
Q

Describe the phototrophic response of shoots of flowering plants to unilateral light

A
  • Cells in the tip of the shoot produce IAA, which is then transport down the shoot.
  • The IAA is initially transported evenly throughout all regions as it begins to move down the shoot.
  • Light causes the movement of IAA from the light side to the shaded side of the shoot.
  • As IAA causes elongation of shoot cells and there’s a greater concentration of IAA on the shaded side of the shoot, the cells on this side elongate more.
  • The shaded side of the root elongates faster than the light side, causing the shoot tip to bend towards light.
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18
Q

Describe the gravitropic response of a horizontally-growing toot to gravity as it follows

A
  • Cells in the tip of the root produce IAA, which is then transported along the root.
  • The IAA is initially transported to all sides of the root
  • Gravity influences the movement of IAA 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 than the upper side.
  • As IAA inhibits the elongation of root cells and there’s a greater concentration of IAA on the lower side, the cells on this side elongate less than those on the upper side.
  • The relatively greater elongation of the cells on the upper side compared to the lower side causes the root to bend downwards towards the force of gravity.
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19
Q

How does IAA increase the plasticity of cells (ability to stretch)?

A

The acid growth hypothesis suggests that the active transport of hydrogen ions from the cytoplasm into the spaces in the cell wall casing the cell wall to become more plastic allowing the cell to elongate by expansion.

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20
Q

Suggest how the movement of IAA in shoots helps plants to survive

A

More IAA moves towards the shaded side of shoots than the light side when the light is unidirectional. In response to their uneven distribution of IAA, the cells on the shaded side elongate faster than those on the light side and the shoot bends towards the light. This ensures that the shoot and the leaves attached to it have a greater chance at being well illuminated. As light is essential for photosynthesis, the process producing organic materials for respiration, the plant has a greater chance of survival.

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21
Q

Suggest 2 advantages to a plant of having roots that respond to gravity by growing in the direction of its force

A
  • Response ensures that roots grow down into the soil.

- Anchors the plant firmly and brings closer to water and mineral ions (needed for photosynthesis)

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22
Q

Why is IAA a useful herbicide?

A

IAA is readily absorbed, easily synthesised and lethal to plants in low concentrations, making it an effective herbicide. It more readily kills broad leaved plants than narrow leaved plants, which is in advantage because many agricultural crops are narrow leaved while weeds that compete with them are broad leaved. As a result, application of IAA at appropriate concentrations will kill only the weeds with little/no harm to the crop. As IAA is not easily broken down means it will persist in the soil and continue to act as a selective weed killer for sometime. This may prevent a broad leaved crop being grown on the land some time after application of IAA. There’s also a danger that IAA might accumulate along food chains, possibly harming animals.

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23
Q

What are the 2 major divisions of the nervous system?

A
  • Central nervous system: brain & spinal cord

- Peripheral nervous system: the pairs of nerves that originate from either the brain or the spinal cord.

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24
Q

What can the peripheral nervous system be divided into? (2 neurones)

A
  • Sensory neurone: carry impulse from receptors towards CNS
  • Motor neurone: carry impulse away from the CNS to effectors

A coordinator links the 2 neurones

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25
Q

How is the motor nervous system divided?

A
  • The voluntary nervous system: Carries nerve impulses to body muscles and is under voluntary control.
  • The autonomic nervous system: Carries nerve impulses to glands, smooth muscle and cardiac muscle and is involuntary.
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26
Q

What’s the spinal cord?

A

A column of nervous tissue that runs along the black and lies inside the vertebral column for protection.

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27
Q

What is a reflex?

A

An involuntary response to a sensory stimulus

28
Q

Describe the reflex arc

A

Stimulus —> Receptor —> Sensory neurone —> Coordinator —> Motor neurone —> Effector —> Response

29
Q

Why are reflex arcs important?

A
  • Involuntary and therefore do not require brain, leaving it to carry out more complex responses. So brain isn’t overloaded with situations in which the response is always the same. Response is rapid.
  • They protect the body from harm. They’re effective from birth and don’t have to be learnt.
  • They’re fast because the neurone pathway is short with very few synapses.
30
Q

Describe features of sensory receptors, specific to the Pacinian corpuscle

A
  • Is specific to a single type of stimulus- in this case mechanical pressure.
  • Produces a generator potential by acting as a transducer- the PC transducers the mechanical energy of the stimulus into a generator potential.
31
Q

Give examples of where the Pacinian Corpuscle occurs?

A

Occur deep in the skin and are most abundant on the fingers, soles of feed and external genitalia.
Also occur in joints, ligaments and tendons, where they enable the organism to know which joins are changing direction.

32
Q

Describe the structure of a Pacinian Corpuscle

A

A single sensory neurone, insulated my myelin sheath. The neurone is surrounded by layers of connective tissue with viscous gel between. Surrounding the tissue is a blood capillary, which is enclosed inside the capsule.

33
Q

How does the Pacinian corpuscle function?

A
  • In its normal state, the stretch-mediated sodium channels of the membrane around the neurone of a Pacinian corpuscle are too narrow to allow sodium ions to pass along them. In this state, the neurone of the Pacinian corpuscle has a resting potential.
  • When pressure is applied to the Pacinian corpuscle, it’s deformed and the membrane around it’s neurone becomes stretched.
  • This stretching widens the sodium channels in the embrace and spicing ions diffuse into the neurone.
  • The influx of sodium ion changes the potential of the membrane, it becomes depolarised, triggering a generator potential.
  • The generator potential in turn creates an action potential that passes along the neurone to the CNS
34
Q

What is the sympathetic nervous system?

A

Stimulates effectors and so speeds up any activity. It controls effectors when we exercise strenuously or experience powerful emotions.
Helps us cope with stressful situations by heightening our awareness and preparing for activity. (F or F)

35
Q

What is the parasympathetic nervous system?

A

Inhibits effectors and so slows down any activity. Controls activities under normal resting conditions. It is concerned with conserving energy and replenishing the body’s reserves.

36
Q

Why is the sympathetic and parasympathetic nervous system antagonistic?

A

The actions of the 2 nervous systems usually oppose one another. If one system contracts a muscle, the other relaxes it.

37
Q

Describe the sequence of events that controls the basic heart rate

A
  • A wave of electrical excitation spreads out from the sinoatrial node (SAN) across both atria, causing them to contract.
  • A layer of non-conductive tissue prevents the wave crossing to the ventricles.
  • The wave of excitation enters the atrioventricular node (AVN).
  • The atrioventricular node, conveys a wave of electrical excitation between the ventricles along the Purkyne tissue muscle fibres, collectively making up the bundle of His.
  • The bundle of His conducts the wave through the atrioventricular septum to the base of the ventricles, where the bundle branches into smaller fibres of Purkyne tissue.
  • The wave of excitation is released from the Purkyne tissue, causing the ventricles to contract quickly at the same time, from the bottom of the heart upwards.
38
Q

What part of the brain is concerned with changing heart rate?

A

Medulla oblongata.

Has 2 centres:

  • A centre that increases heart rate, linked to the sinoatrial node by the sympathetic nervous system
  • A centre that decreases heart rate. linked to the sinoatrial node by the parasympathetic nervous system
39
Q

Where are chemoreceptors found?

A

In the wall of the carotid arteries in the brain.

40
Q

How do chemoreceptors control heart rate?

A
  • When blood has a high concentration of carbon dioxide, the pH is lowered.
  • Chemoreceptors detect this pH change and increase the frequency of nervous impulses to the centre in the medulla oblongata that increases heart rate.
  • This centre increases the frequency of impulses via the sympathetic nervous system to the sinoatrial node. This increases the rate of increases the rate of production of electrical waves by the sinoatrial node and therefore increases the heart rate.
  • The increased blood flow that this causes leads to more carbon dioxide being removed by the lungs and so the carbon dioxide concentration of the blood returns to normal
  • As a consequence the pH of blood rises to normal and the chemoreceptors in the wall of caratoid arteries and aorta reduce the frequency of nerve impulses to the medulla oblongata.
  • The medulla oblongata reduces the frequency of impulses to the sinoatrial node, therefore leading to reduction in heart rate.
41
Q

How does excerisce effect cardiac output?

A
  • Increased muscular activity
  • More carbon dioxide produced by tissues by increased respiration
  • Blood pH lowered
  • Chemical receptors in carotid arteries increase frequency of impulses to medulla oblongata.
  • Centre in medulla oblongata that speeds heart rate, increases frequency of impulses to SA node via the sympathetic nervous system.
  • SA node increases heart rate.
  • Increased blood blow removed carbon dioxide faster.
42
Q

Where do pressure receptors occur?

A

Within the walls of carotid arteries and the aorta

43
Q

How do pressure receptors control heart rate when blood pressure is high?

A

Pressure receptors transmit more nervous impulses to the centre in the medulla oblongata that decreases heart rate. This centre sends impulses via the parasympathetic nervous system to the sinoatrial node of the heart which leads to decrease in heart rate.

44
Q

How do pressure receptors control heart rate when blood pressure is low?

A

Pressure receptors transmit more nervous impulses to the centre of the oblongata that increases heart rate. This centre sends impulses via the sympathetic nervous system to the sinoatrial node, increasing heart rate.

45
Q

Describe the function of the autonomic nervous system

A

Autonomic nervous system controls the involuntary activities of internal muscles and glands

46
Q

Suppose the parasympathetic nerve connections from the medulla oblongata to the sinoatrial node were out. Suggest what might happen if a persons blood pressure increases above normal

A

Blood pressure remains high because the parasympathetic system is unable to transit nerve impulses to the SA node, which decreases heart rate and so lowers blood pressure.

47
Q

The nerve connecting the carotid artery to the medulla oblongata of a person is cut. This person then undertakes some strenuous exercise. Suggest what might happen to the persons heart rate?

A

Heart rate remains as it was before taking exercise, blood pressure increases and carbon dioxide concentration of blood rises (lowering pH). The changes are detected by pressure and chemical receptors in the wall of the carotid arteries. As the nerve from here to the medulla oblongata is cut, no nerve impulse can be sent to centres that control heart rate.

48
Q

The nerve connecting the carotid artery to the medulla oblongata of a person is cut. This person then undertakes some strenuous exercise. Suggest what might happen to the persons carbon dioxide concentration?

A

Blood CO2 concentration increased as a result of increased respiration during exercise.

49
Q

What is a transducer cell?

A

Cells that concert a non electrical signal, such as light, into an electrical nervous system.

50
Q

How do rod and cone cells act as transducers?

A

Converts light energy into the chemical energy of a nerve impulse

51
Q

Differences between rod and cone cells?

A

Rod- rod shaped, Cone- cone shaped.
Rod- greater numbers than cone cells, Cone- fewer numbers than rod cells.
Rod- distribution more at the periphery of the retina, Cone- at the fovea,
Rod- give poor visual acuity, Cone- give good visual acuity.
Rod- sensitive for low-intensity light, Rod- Sensitive to low intensity light, Cone- not sensitive to low intensity light.

52
Q

How many rod and cone cells in each eye?

A

120 million rod cells

6 million cone cells

53
Q

How do rod cells allow us to see in low light intensity?

A

Many rod cells are connected to a single sensory neurone in the optic nerve. A certain threshold value has to be exceeded before a generator potential is created in bipolar cells to which they are connected. A number of rod cells are connected to a single bipolar cell, creating a higher chance that the threshold value will be exceeded than if only a single rod were connected to each bipolar cell. Due to summation. This allows rod cells to see low light intensity, in black and white.

54
Q

How is a generator potential created in rod cells?

A

The pigment in the rod cells must be broken down. There’s enough energy from low-intensity light to cause their breakdown, explaining why rod cells respond to low-intensity light.

55
Q

Why do rod cells have low visual acuity?

A

As many rod cells link to a single bipolar cell, 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. This means that the brain can’t distinguish between the separate sources of light that stimulated them. Two dots close together therefore can’t be resolved and will appear as a single blob.

56
Q

How many different types of cone cells are there?

A

3, each responding to different wavelengths of light. Depending on the proportion of the type that is stimulated, we can perceive images in full colour.

57
Q

How many cone cells are there in each human eye?

A

6 million.
This means that the stimulation of of a number of cone cells can’t be combined to help exceed threshold value and create a generator potential. Therefore cone cells only respond to high light intensity not low light intensity.

58
Q

Why is each cone cell sensitive to a different specific range of wavelengths?

A

Cone cells contain different types of pigment found in rod cells. Cone cells contain iodopsin. Only light of high intensity will therefore provide enough energy to break it down and create a generator potential. There are 3 different types of cone cells, each containing a a specific type of iodopsin. As a result, each cone cell is sensitive to a different specific range of wavelengths.

59
Q

Why do cone cells have good visual acuity?

A

Each cone cell has its own connection to a single bipolar cell, which means that, if 2 adjacent cone cells are stimulated, the brain can therefore distinguish between the 2 separate sources of light that stimulated the 2 cone cells. This means that 2 dots close together can be resolved and will appear as 2 dots. Therefore cone cells have good visual acuity.

60
Q

Where are cone cells found on the retina?

A

The fovea.
Light is focussed by the lens on the part of the retina opposite the pupil- the fovea. The fovea therefore receives the highest intensity of light, and cone cells are found here.

61
Q

Where are rod cells found in the retina?

A

At the peripheries of the retina, where light intensity is at its lowest, only rod cells are found.

62
Q

Describe a stretch-mediated sodium channel.

A

A special type of sodium channel that changes its permeability to sodium when it is stretched.

63
Q

Describe the sequence of events by which pressure on a Pacinian corpuscle results in the creation of a generator potential.

A 
Pressure on Pacinian corpuscle,
Corpuscle changes shape,
Stretches membrane of neurone,
Widens stretch mediated sodium ion channels,
Allows sodium ions into neurone,
Depolarises the membrane,
Produces a generator potential
64
Q

Explain why brightly coloured objects often appear grey in dim light.

A

Only rod cells are stimulated by low intensity light. Rod cells can’t distinguish between different wavelengths, therefore the object is perceived only in a mixture of black and white.

65
Q

At night it is often easier to see a star in the sky by looking at it slightly to the side of it rather than directly at it.
Suggest why this is so.

A

Light reaching Earth from a star is of low intensity. Looking directly at a star, light is focused on the fovea, where there are only cone cells. Cone cells respond only to light of of high intensity so they’re not stimulated from the low intensity from the star and it cannot be seen. Looking to on side of the star means that light from the star is focussed towards the outer regions do the retina, where there are mostly rod cells. These are stimulated by low intensity and therefore the the star is seen.

66
Q

How do photoreceptors convert light into an electrical impulse?

A
  • Light enters the eye, hits the photoreceptors and is absorbed by light-sensitive optical pigments.
  • Light bleaches the pigments, causing a chemical change and altering the membrane permeability to sodium ions. Pigments in rods sensitive to low intensity light, pigments in cones sensitive to high intensity light.
  • A generator potential is created, and if it reaches the threshold, a nerve impulse is sent along a bipolar neurone.
  • Bipolar neurones connect photoreceptors to the optic nerve, taking impulses to the brain.

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