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A Level Biology AQA > 6. Organisms Respond To Changes In Their Environment > Flashcards

6. Organisms Respond To Changes In Their Environment Flashcards

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1
Q

Define stimulus

A

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

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2
Q

Give the sequence of events of the response to a stimulus

A

Stimulus —> receptor —> coordinator —> effector —> response

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3
Q

What is a taxis

A

A simple response whose direction is determined by the direction of a stimulus.
As a result a motile organism responds directly to environmental changes by moving its whole body either towards a favourable stimulus or away from an unfavourable one

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4
Q

What is meant by positive/negative taxis

A

Movement towards the stimulus = positive taxis
Movement away from stimulus = negative taxis

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5
Q

What is a kinesis

A

A kinesis is a form of response in which the organism does not move toward 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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6
Q

What is a tropism

A

A tropism is the growth of a plant in response to a directional stimulus

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7
Q

What are plant growth factors referring to

A

The hormone like substances involved in the responses of plants to external stimuli

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8
Q

Why do we refer to the hormone-like substances as plant growth factors over hormones ?

A

They exert their influence by affecting growth
They may be made by cells located throughout the plant rather than in particular organs
Unlike animal hormones, some plant growth factors affect the tissue that release them rather than acting on distant target organs

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9
Q

Give an example of a plant growth factor and its role

A

Indoleacetic acid (IAA)
- controls plant elongation

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10
Q

What is growth of a plant towards the light referred to as

A

Positive phototropism
or
Negative gravitropism

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11
Q

Describe the steps that explain positive phototropism

A
  1. Cells in the tip of the shoot produce IAA, which is then transported (initially evenly) down the shoot
  2. Light causes the movement of IAA from the light side to the shaded side of the shoot
  3. A greater conc of IAA builds up on the shaded side compared with the light side
  4. As IAA causes elongation of shoot cells, cells on the shaded side (side with greatest conc of IAA) elongate faster than the light side
  5. Therefore shoot tip bends towards the light
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12
Q

Describe the steps that explain positive gravitropism in plants

A
  1. Cells in the root tips produce IAA which is initially transported evenly to all sides of the root
  2. Gravity influences the movement of IAA from the upper side to the lower side of the root
  3. A greater conc of IAA builds up on the lower side of the root compared with the upper side
  4. As IAA inhibits elongation of root cells, the cells on the lower side of the root (region with greater IAA conc) elongate less than those on the upper side
  5. Therefore root bends downwards towards the force of gravity
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13
Q

Explain the proposed explanation of how IAA increases the plasticity of cells

A
  • acid growth hypothesis
  • involves the active transport of hydrogen ions from the cytoplasm into spaces in the cell wall
  • this causes the cell wall to become more plastic allowing the cell to elongate by expansion
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14
Q

What are the 2 major divisions of the nervous system and what are they made up of ?

A
  • the central nervous system (CNS) : made up of brain & spinal cord
  • the peripheral nervous system (PNS) : made up of a pair of nerves that originate from either the brain or spinal cord
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15
Q

What is the peripheral nervous system divided into

A

Sensory neurones
Motor neurones

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16
Q

Give the role of sensory neurones

A

Carry nerve impulses (electrical signals) from receptors towards the CNS

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17
Q

Give the role of motor neurones

A

Carry nerve impulses (electrical signals) away from the CNS to effectors

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18
Q

What are the 2 subdivisons of the motor nervous system?

A
  • the voluntary nervous system
  • the autonomic nervous system
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19
Q

Describe the voluntary nervous system

A

Carries nerve impulses to body muscles and is under voluntary (conscious) control

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20
Q

Describe the autonomic nervous system

A

Carries nerve impulses to glands, smooth muscle and cardiac muscle and is not under voluntary control (subconscious)

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21
Q

What is the sequence of the reflex arc

A

Stimulus
Receptor - generates impulses in the…
Sensory neurone - passes nerve impulses to spinal cord
Coordinator - links sensory to motor neurone in spinal cord
Motor neurone - carries nerve impulses from spinal cord to muscle
Effector - muscle is stimulated to contract
Response

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22
Q

Why are reflex actions important ?

A
  • they are involuntary and therefore don’t require the brain to make decisions leaving it to carry out more complex responses = not overloaded
  • protect body from harm
  • effective from birth so don’t have to be learnt
  • fast withdrawal reflexes as neurone pathway is short (has very few synapses)
  • action is rapid due to absence of decision making process
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23
Q

What are the features of sensory reception as illustrated by the Pacinian corpuscule

A

Specific to a single type of stimulus
Produces a generator potential by acting as a transducer

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24
Q

How do receptors act as transducers in the nervous system

A

All stimuli involve a change in some form of energy.
A transducer converts the change in form of energy by the stimulus into a form that can be understood by the body
Receptors, convert (transduce) the energy of the stimulus into a nervous impulse known as a generator potential

25
Q

Describe briefly the structure and function of a Pacinian corpuscule

A

Respond to mechanical stimuli such as pressure
Occur deep in the skin as well is in joints, ligaments and tendons
The single sensory neurone of the PC is at the centre of layers of tissue, each separated by gel

26
Q

What feature of the Pacinian corpuscule enables it to transduce the mechanical energy of the stimulus into a generator potential ?

A

The sensory neurone ending at the centre of the pc has a special stretch-mediated sodium channel in its plasma membrane.
Permeability to sodium changes when they are deformed (stretched)

27
Q

Give the step by step functioning of the Pacinian Corpuscule

A
  • in resting state, the stretch-mediated sodium channels are too narrow to allow Na+ ions to pass along them. (Neurone in the pc has a resting potential)
  • when pressure is applied to the pc it becomes deformed and the membrane around its neurone becomes stretched
  • stretching widens the channels and so Na+ ions diffuse into the neurone
  • influx of Na+ ions changes the membrane potential (becomes depolarised) thereby producing a generator potential
  • generator potential in turn creates an action potential
28
Q

Both rod and cone cells act as…

A

Transducers by converting light energy I to the electrical energy of a nerve impulse

29
Q

Why can rod cells only lead to images in black and white?

A

Because they cannot distinguish between different wavelengths of light

30
Q

Explain how rod cells allow us to see in low light intensities ?

A

As a number of rod cells are connected to a single bipolar cell there is a much greater chance that the threshold value for a generator potential to be produced is exceeded compared to if a single rod cell was connected to each bipolar cell.

31
Q

What needs to occur for rod cells to respond to low-intensity light ?

A

In order to create a generator potential the pigment in the rod ells (rhodopsin) must be broken down.
There is enough energy from low intensity light for this breakdown

32
Q

Why do rod cells give a low visual acuity?

A

As a result of many rod cells linking to a single bipolar cell, the light received by the rod cells sharing the same neurone will only generate a single impulse travelling to the brain regardless of how many neurones are stimulated
Means that the brain cannot distinguish between the separate sources of light that stimulated them

33
Q

Explain how cone cells give good visual acuity

A

Each cone cell has its own connection to a single bipolar cell which means that if 2 agacent cone cells are stimulated, the brain receives 2 separate impulses.
Therefore the brain can distinguish between the 2 separate sources of light that stimulated the 2 cone cells

34
Q

Why do cone cells respond only to high intensity light ?

A

Each cone cell is connected to their own serpente bipolar cell connected to a sensory neurone in the optic nerve.
Means that the stimulation of a number of cone cells cannot be combined to help exceed threshold value and so create generator potential
Also the pigment in cone cells, iodopsin, requires a higher light intensity for its breakdown

35
Q

There are 3 types of cone cell - why is each type sensitive to a specific range of wavelengths ?

A

Each contain a specific type of iodopsin requiring different levels of light intensities for its breakdown to produce generator potentials

36
Q

Why is the distribution of rod and cone cells uneven?

A

Light is focused by the lens onto the fovea (part of retina opposite pupil)
Therefore the fovea receives the highest intensity of light = cone cells found here
Concentration of cone cells diminishes further from the fovea
At the peripheries of the retina, where light intensity is at its lowest, rod cells are found

37
Q

Rod cells vs cone cells

A

Rod:
- rod shaped
- greater number
- distributed nearer periphery of retina
- poor visual acuity
- sensitive to low light intensities
- one type only
Cone cells opposite

38
Q

What does the autonomic system control ?

A

Controls the involuntary (subconscious) activities of internal muscles and glands

39
Q

What are the 2 divisions of the autonomic nervous system?

A

sympathetic nervous system
- stimulates effectors and so speeds up any activity
parasympathetic nervous system
- inhibits effectors and so slows down any activity
- concerned with conserving energy and replenishing the body’s reserves

40
Q

If we say the sympathetic and parasympathetic nervous systems are antagonistic what do we mean?

A

(Normally) they oppose eachother
The activities of internal glands and muscles are regulated by a balance of the 2 systems

41
Q

Describe the role of the nervous system

A
  • The nervous system uses nerve cells to pass electrical impulses along their length.
  • They stimulate target cells by secreting neurotransmitters directly onto them
  • Results in rapid communication
  • Responses produced are often short lived and localised
42
Q

Describe the hormonal system

A
  • produces chemicals (hormones) that are transported int he blood plasma to target cells
  • the receptors of the target cells detect change in the concentration of hormones and this stimulates them
43
Q

Describe the difference between the hormonal and nervous systems

A
  • hormonal system is a slower, less specific form of communication
  • responses of the hormonal system are often long lasting and wide spread whereas those of the nervous system are often short lived and localised
44
Q

What are dendrons

A

Extensions of the cell body which subdivide into smaller branched fibres called dendrites that carry nerve impulses towards the cell body

45
Q

What are shwann cells ?

A

Surround the axon, protecting it and providing electrical insulation
Also carry out phagocytosis and play a part in nerve regeneration

46
Q

What is the axon ?

A

A single long fibre that carries nerve impulses away from the cell body

47
Q

What is the myelin sheath ?

A

Forms a covering to the axon and is made up of the membranes of shwann cells
These membranes are rich in a lipid called myelin

48
Q

What are nodes of ranvier

A

Constrictions between adjacent shwann cells where there is no myelin sheath
Occur every 1x3mm in humans

49
Q

Describe sensory neurones

A

Transmit nerve impulses from a receptor to a intermediate/motor neurone.
They have one, often very long, dendron that carries nerve impulse towards the cell body and the axon carries it away

50
Q

Describe motor neurones

A

Transmit nerve impulses from a relay/intermediate neurone to an effector such as a gland or muscle
Have a long axon and many short dendrites

51
Q

Describe intermediate/relay neurones

A

Transmit impulses between neurones eg. from sensory to motor neurones

52
Q

How can a nerve impulse be described ?

A

A self propagating wave of electrical activity that travels along the axon membrane
It is a temporary reversal of electrical potential difference across the axon membrane

53
Q

What are the ways in which the movement of ions across the axon membrane is controlled ?

A
  • phospholipid bilayer of axon plasma membrane prevents Na+ and K+ diffusing across
  • channel proteins span the phospholipid bilayer
  • carrier proteins acting as sodium potassium pumps
54
Q

What is the resting potential

A

The inside of the axon is negatively charged relative to the outside - the axon is said to be polarised

55
Q

Give the steps to establishing the potential difference (resting potential)

A
  • Na+ ions are actively transported out of the axon by sodium-potassium pumps
  • K+ ions are actively transported into the axon by sodium-potassium pumps
  • Active transport of Na+ ions is greater than K+ ions
  • As a result there are more Na+ ions in the tissue fluid surrounding the axon than the cytoplasm and more K+ ions in the cytoplasm than the tissue fluid surrounding the axon, thus creating an electrochemical gradient
  • Na+ ions diffuse naturally back into axon and K+ ions begin to diffuse out of axon
  • However, most of the gates allowing K+ ions through are open whilst those for Na+ ions are mostly closed
56
Q

What is the action potential ?

A

When a stimulus of sufficient size is detected by a receptor in the nervous system its energy causes a temporary reversal of the charges either side of this part of the axon membrane.
This part of the axon membrane is said to become depolarised

57
Q

Why does depolarisation occur

A

Because the channels in the axon membrane change shape and hence open or close depending on the voltage across the membrane

58
Q

Describe the step by step process of the action potential

A
  • the energy of the stimulus causes some sodium voltage-gated channels in the axon membrane to open and therefore Na+ ions diffuse along the electrochemical gradient into the axon
  • being positively charged, they trigger a reversal in the potential difference across the membrane
    - as the Na+ ions diffuse into the axon more sodium channels open causing an even greater influx of Na+ ions by diffusion
  • once the action potential of around +40mV is established (preventing further influx of Na+) the voltage gates on the K+ ion channels begin to open
  • as a result the electrical gradient that was preventing further outward movement of K+ ions is now reversed causing more K+ ion channels to open so more K+ diffuses out, starting the depolarisation of the axon
  • outward diffusion of K+ causes a temporary overshoot of the electrical gradient with the inside of the axon being more negative than usual.
  • gates on K+ ion channels close - resting potential can be re-established

A Level Biology AQA (9 decks)

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