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Biology > Topic 6: Response to Stimuli > Flashcards

Topic 6: Response to Stimuli Flashcards

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1
Q

What is a stimulus?

A

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

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2
Q

What is a receptor?

A

A cell or organ that detects a specific stimulus

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3
Q

What is a coordinator?

A

An intermediary between a receptor and effector which formulates a suitable response to a stimulus

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4
Q

What is an effector?

A

A cell, organ or tissue that carries out a response to a stimulus

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5
Q

What is the generic sequence showing how responses are formed to stimuli?

A

Stimulus
Receptor
Coordinator
Effector
Response

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6
Q

Why is response to stimuli important?

A

The ability to respond to stimuli affects the organism’s chances of survival, for example by being able to detect and move away from danger (predators etc), or to move towards food.

Those that respond to stimuli better have a better chance of surviving, reproducing and passing on their alleles, so there is a selection pressure favouring organisms with more appropriate responses.

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7
Q

What are the two methods of communication within the body?

A

Endocrine system (hormones) and nervous system (electrical impulses)

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8
Q

What are the two whole-body types of response?

A

Taxis
Kinesis

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9
Q

What is taxis?

A

A simple directional response to stimuli, whose direction is determined by the direction of the stimulus.

Motile organisms move in direct response to environmental change.

Moving towards the stimulus is positive taxis, away from the stimulus is negative

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10
Q

Give some different types of taxis

A
  • Phototaxis (light)
  • Chemotaxis (chemicals)
  • Thermotaxis (temperature)
  • Hydrotaxis (moisture)
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11
Q

What is kinesis?

A

A form of response to stimuli where the organism doesn’t move towards or away from a stimulus.

Changes the speed of movement and rate at which it changes direction.

Involves random, non-directional movement, where the more unpleasant the stimulus, the more rapidly the organism moves.

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12
Q

Describe how kinesis changes depending on the environment of the organism

A
  • If it crosses into an unfavourable environment, rate of turning increases to increase its chances of returning to favourable conditions
  • If it is completely surrounded by unfavourable conditions, rate of turning decreases, so it moves in a straight line out of the area
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13
Q

What is a tropism?

A

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

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14
Q

Describe the tropisms found in plant shoots

A
  • Positive phototropism - grow towards light
  • Negative geotropism - grows away from gravity

So, leaves are in most favourable position to capture light for photosynthesis

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15
Q

Describe the tropisms found in plant roots

A
  • Negative phototropism - grows away from light
  • Positive geotropism - grows towards gravity
  • Positive hydrotropism - grows towards water

So, roots are more likely to grow into the soil to absorb water and mineral ions

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16
Q

What are plant tropisms controlled by? Give an example

A

Controlled by plant growth factors.

Indoleacetic acid (IAA) is a plant growth factor, belonging to a group called auxins. Controls plant cell elongation

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17
Q

How are plant growth factors different to hormones?

A
  • Exert their influence by affecting growth.
  • Can be made by cells throughout the plant, not just particular organs
  • Unlike animal hormones, some affect the tissues that release them, not a distant target organ
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18
Q

Explain the process of phototropism in shoots

A
  • Cells in the tip of the shoot produce IAA, which diffuses downwards, distributing evenly
  • Light causes IAA to move to the shaded side, increasing IAA concentration there more than the light side
  • IAA stimulates cell elongation of shoot cells, and the uneven distribution makes the shaded side elongate more, causing the shoot tip to bend towards the light
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19
Q

Explain the process of phototropism in roots

A
  • Cells in the tip of the root produce IAA, which diffuses downwards, distributing evenly
  • Light causes IAA to move towards the shaded side, increasing IAA concentration there more than the light side
  • IAA inhibits cell elongation of root cells, and the uneven distribution makes the light side elongate more, causing the root tip to bend away from the light
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20
Q

Explain the process of geotropism in roots

A
  • Cells in the root tip produce IAA, which evenly diffuses along the root
  • Gravity causes IAA to accumulate on the lower side, increasing IAA concentration there more than the upper side
  • IAA inhibits cell elongation in roots, so cells on the lower side elongate less than the upper side, causing the root to bend downwards towards gravity
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21
Q

Explain the process of geotropism in shoots

A
  • Cells in the shoot tip produce IAA, which evenly diffuses along the shoot
  • Gravity causes IAA to accumulate on the lower side, increases IAA concentration there more than the upper side
  • IAA stimulates cell elongation in shoots, so cells on the lower side elongate less than the upper side, causing the shoot to bend upwards against gravity
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22
Q

What cells does IAA work on?

A

IAA is transported away from root/shoot tips where it is produced.

It increases the plasticity (ability to stretch) of cell walls, only occurring on young cells that can still elongate.

As cells mature, they become more rigid, so older parts of the plant can’t respond

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23
Q

What is the acid growth hypothesis?

A

The proposed explanation of how IAA increases cell plasticity.

It involves the active transport of H+ from the cytoplasm into the spaces in the cell wall, letting it elongate by expansion

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24
Q

Describe the subdivisions of the nervous system

A
  • The nervous system is split into the peripheral and central nervous systems
  • The CNS is made from the brain and spinal cord
  • The PNS is split into the sensory and motor nervous systems
  • The motor nervous system is split into the voluntary and autonomic nervous systems
  • The autonomic nervous system is split into the sympathetic and parasympathetic nervous systems
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25
Q

Describe what makes up the peripheral nervous system

A

Made from pairs of nerves originating from either the brain or the spinal cord.

These are either sensory neurones (carry nerve impulses from receptors to the CNS) or motor neurones (carry nerve impulses from CNS to effectors)

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26
Q

What is the voluntary nervous system?

A

Carries nerve impulses to body muscles under conscious control

27
Q

What is the autonomic nervous system?

A

Carries nerve impulses to glands, smooth muscle and cardiac muscle. Is not under voluntary control

28
Q

What are some properties of reflex responses?

A
  • Rapid
  • Short-lived
  • Localised
  • Involuntary
29
Q

What is a reflex arc?

A

The pathway of neurones involved in a reflex

30
Q

What is the sequence of a reflex arc?

A
  • Stimulus
  • Receptor
  • Sensory neurone
  • Coordinator
  • Motor neurone
  • Effector
  • Response
31
Q

What is the coordinator in a reflex arc?

A

The intermediate/relay neurone found in the spinal cord. The arc never involves the conscious part of the brain, so is under the control of the peripheral nervous system.

This is why reflex responses are also called a spinal reflex

32
Q

Why are reflex responses important?

A
  • Protect body from harm, increasing survival chances
  • Effective from birth and don’t have to be learnt
  • Fast because neurone pathway is short with few synapses (slowest link in the pathway), important in withdrawal reflexes
  • No decision-making process, so are rapid, and free to carry out more complex responses without overloading the brain when no thought is required
  • Some impulses are still sent to the brain, so the reflex can be overridden if necessary
33
Q

Give some properties of all receptors

A
  • Are specific to a single type of stimulus
  • Produce a generator potential by acting as a transducer
34
Q

In simple terms, how do receptors produce a generator potential?

A

All stimuli are a change in some form of energy. The transducer (receptor) converts this change into a nerve impulse that can be understood by the body (convert one form of energy into another). The nervous impulse produced is called a generator potential

35
Q

What are Pacinian corpuscles and where are they found?

A

Receptors that respond to mechanical pressure.

Are mostly found in the skin dermis, as well as joints, tendons and ligaments to provide information about how and when we move

36
Q

Describe the structure of a Pacinian corpuscle

A

Made from a sensory neurone surrounded by layers of connective tissue with viscous gel in between them.

The sensory neurone has stretch-mediated sodium channels in its membrane, as their permeability to sodium changes when they are deformed as a result of pressure

37
Q

How does a Pacinian corpuscle work?

A
  • In their normal state, stretch-mediated sodium channels are narrow, not allowing Na+ to pass. Here, the neurone has a resting potential
  • When pressure is applied the corpuscle is deformed, and the membrane around the neurone is stretched
  • This widens the stretch-mediated sodium channels, so Na+ diffuse into neurone
  • Influx of Na+ changes the potential over the membrane (depolarisation), producing a generator potential
  • If the generator potential reaches the threshold, an action potential is produced and passed to the CNS
  • Greater pressure means more deformation of the corpuscle, so more sodium channels open, creating a larger generator potential. This makes it a graded response
38
Q

Describe the difference in the generator and action potentials produced by a Pacinian corpuscle under low and high pressure

A
  • Greater pressure produces a larger generator potential for a longer time
  • Greater pressure produces a higher frequency of action potentials for a longer time, but with the same amplitude as the lower pressure
39
Q

What is adaptation in reference to the nervous system?

A

When pressure is first applied, impulses are transmitted along the neurone.

With continuous pressure, frequency of action potentials decreases and then stops.

Prevents the nervous system being overloaded with insignificant information

40
Q

Where are light receptors found and what do they do?

A

Found on the retina in the eye.

They act as transducers by converting light energy into the electrical energy of a nerve impulse

41
Q

What are the two main types of light receptors?

A

Rod cells
Cone cells

42
Q

What types of light do rod cells respond to?

A

Cannot distinguish between different wavelengths of light so produce black and white images.

Are sensitive to a low light intensity (just one photon), so can produce images in very dim light

43
Q

Why can rod cells have a high light sensitivity?

A

Several rod cells synapse to a single bipolar cell (retinal convergence), which then synapse with several other bipolar cells to a single ganglion cell, connected to a single sensory neurone (to the optic nerve).

This means the generator potentials of the connected rod cells are added together by summation, so there is a higher chance the threshold will be reached, so very dim light can still lead to an action potential.

44
Q

Why do rod cells have a low visual acuity?

A

Light received by rod cells sharing the same neurone only generates one impulse to the brain no matter how many rod cells are stimulated, meaning two close points can’t be distinguished

45
Q

How is a generator potential produced in rod cells?

A

The pigment rhodopsin found in rod cells is broken down

46
Q

Why are there different types of cone cell?

A

There are three different types, each responding to a different range of light wavelengths, in order to distinguish colours

Each type has a different type of iodopsin pigment for this to be possible

47
Q

Why do cone cells have low light sensitivity?

A

Each cone cell connects to a single bipolar cell, which connects to its own sensory neurone in the optic nerve. This means generator potentials can’t be combined by summation like rod cells, so it takes brighter light to create an action potential.

Their pigment (iodopsin) requires more energy and so a higher light intensity to break down than the rhodopsin in rod cells.

48
Q

Why do cone cells have a high visual acuity?

A

Each cone cell connects to its own bipolar cell, connecting to its own ganglion cell and sensory neurone, so if 2 cone cells are stimulated, 2 impulses are sent to the brain, and the two points can be distinguished

49
Q

Explain the distribution of rod and cone cells on the retina.

A

There is an uneven distribution.

Light is focused on the fovea (part of the retina opposite the pupil), so it receives the highest light intensity, so has the highest concentration of cone cells.

At the peripheries of the retina, only rod cells are found, as these are the only receptors that can detect dim light

50
Q

What are the differences between rod and cone cells?

A
  • There are more rod cells than cone cells on the retina
  • Rod cells occur more at the periphery of the retina and are absent at the fovea, but cone cells are concentrated at the fovea
  • Rod cells have poor visual acuity, but cone cells have good visual acuity
  • Rod cells are sensitive to low-intensity light, but cone cells have low light sensitivity
  • There is one type of rod cell, but three types of cone cell
  • Rod cells produce images in black and white, but cone cells can detect colour
51
Q

What are the subdivisions of the autonomic nervous system and how do they interact?

A
  • Sympathetic nervous system
  • Parasympathetic nervous system

They are antagonistic (oppose each other)

52
Q

What does the sympathetic nervous system do?

A

Stimulates effectors and speeds up activities, often in response to stressful stimuli (exercise, powerful emotions, etc) by preparing the fight or flight response

53
Q

What does the parasympathetic nervous system do?

A

Inhibits effectors, slowing down activities. Controls normal resting conditions. Concerned with conserving energy and replenishing the body’s reserves

54
Q

Cardiac muscle is myogenic. What does this mean?

A

Its contraction is initiated from within its own muscle, not nervous impulses from outside

55
Q

Where is the sinoatrial node (SAN) found and what does it do?

A

Found in the wall of the right atrium of the heart.

it is the origin of the original stimulus for contraction. It controls the basic rhythm of the heart, and so is often called the pacemaker

56
Q

Describe the sequence of impulses that produces a heart beat

A
  • Electrical waves are generated in and sent out of the SAN across both atria, causing them to contract
  • The atrioventricular septum (layer of non-conductive tissue) prevents the wave of excitation crossing into the ventricles
  • The wave enters the atrioventricular node (AVN) between the atria
  • There is a short delay after this to allow the atria to empty and ventricles to fill
  • Impulses are sent from the AVN down the bundle of His (in the ventricular septum) to the Purkinje fibres at the apex (base) of the heart
  • Impulses spread from the apex up the ventricular walls, making them contract quickly at the same time
  • Contraction is from the bottom up to ensure all blood moves out the arteries and doesn’t pool at the apex
57
Q

Why does heart rate increase during exercise?

A

To increase the rate at which oxygen is carried to cells for aerobic respiration (used as the final electron acceptor) to produce ATP for muscle contraction, and to reduce anaerobic respiration (preventing a lactate build-up).

Increases the rate at which carbon dioxide is excreted, as carbon dioxide reduces pH, affecting enzyme action

58
Q

What part of the brain controls heart rate and how?

A

The medulla oblongata.

Has an acceleratory centre connected to the SAN by the sympathetic nervous system, and an inhibitory centre connected to the SAN by the parasympathetic nervous system.

Which one is stimulated depends on nerve impulses received by chemoreceptors or blood pressure receptors (baroreceptors)

59
Q

Where are chemoreceptors found and what do they do?

A

Found in the wall of the carotid arteries (arteries that serve the brain) and the aorta.

They detect changes in blood pH from changes in carbon dioxide concentration, in order to adjust heart rate accordingly

60
Q

Describe how chemoreceptors lead to a response to an increase in blood carbon dioxide concentration

A
  • When blood carbon dioxide concentration increases, pH decreases
  • Chemoreceptors detect this and increase the frequency of nervous impulses to the acceleratory centre of the medulla oblongata
  • Frequency of impulses to the SAN via the sympathetic nervous system increases
  • Increased rate of production of electrical waves from SAN so heart rate increases
  • Increased blood flow leads to carbon dioxide being removed by lungs, so carbon dioxide concentration returns to normal
  • Blood pH rises to normal, so chemoreceptors reduce frequency of impulses to medulla oblongata.
  • Frequency of impulses to SAN via parasympathetic nervous system increases, reducing heart rate back to normal
61
Q

Where are blood pressure receptors (baroreceptors) found?

A

Walls of the carotid arteries and aorta

62
Q

Describe how a response is produced to an increase in blood pressure

A
  • Pressure receptors in artery walls transmit more nervous impulses to the inhibitory centre in the medulla oblongata
  • This sends impulses to the SAN via the parasympathetic nervous system
  • Heart rate decreases to reduce blood pressure
63
Q

Describe how a response is produced to a decrease in blood pressure

A
  • Pressure receptors in artery walls transmit more nervous impulses to the acceleratory centre of the medulla oblongata
  • This sends impulses to the SAN via the sympathetic nervous systenm
  • Heart rate increases to increase blood pressure

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