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Biology A-level > 2 Stimuli And Responses > Flashcards

2 Stimuli And Responses Flashcards

1
Q

Define stimulus

A

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

What is the advantage to organisms of being able to respond to stimuli?

A

Animals increase chances of survival by responding to changes in their external environment, eg by avoiding harmful environments.
Also respond to changes in internal environment to make sure conditions are optimal for their metabolism. Plants also increase their chances of survival by responding to changes in environment.

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3
Q

Describe a response.

A

Stimuli detected by receptors. A coordinator formulates a suitable response to a stimulus. Coordination may be at a molecular level or involve a large organ such as the brain. Response produced by an effector. This response may be at a molecular level or involve behaviour of the whole organism.

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4
Q

Describe how the nervous system send information as electrical impulses.

A
  • stimulus detected by receptor cells + an electrical impulse is sent along a sensory neurone
  • when electrical impulse reaches end of neurone, neurotransmitters take information across to next neurone, which sends electrical impulse
  • the CNS (the coordinator) process the information and sends impulses along motor neurone to an effector.
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5
Q

Define taxis.

A

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

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6
Q

Define kinesis.

A

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

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7
Q

Define tropism.

A

The growth of part of a plant in response to a directional stimulus. In almost all cases the plant part grows towards or away from the stimulus.

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8
Q

How does taxis increase an organisms chances of survival?

A

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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9
Q

Give an example of taxis.

A

Single-felled algae move towards light (positive phototaxis). This increases their chances of survival since, being photosynthetic, they require light to manufacture their food.

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10
Q

How does kinesis increase an organisms chances of survival?

A

If an organism crosses a sharp dividing line between favourable and an unfavourable environment, it’s rate of turning increases. This raises its chances of quick return to favourable environment. However, if it moves considerable distance into unfavourable environment it’s rate of turning may slowly decrease so it moves in straight line before it turns sharply. This type of response tends to bring organism into a new region with favourable conditions

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11
Q

Give an example of kinesis.

A

Woodlice lose water from their bodies in dry conditions. When they move from damp area to dry one, they move more rapidly + change direction more often. This increases chance if them moving back to damp area. Once back in damp area, they slow down + change direction less often. This means they’re more likely to stay within damp area. However, if after some time spent changing direction rapidly they’re still in dry area, they instead move rapidly in straight lines. Increase chance of moving through dry area and into new damp one.

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12
Q

How does tropism increase an organisms chances of survival?

A
  • plant shoots grow towards light (positive phototropism) and away from gravity (negative gravitropism) so their leaves are in most favourable position to capture light for photosynthesis
  • plant roots grow away from light (negative phototropism) and towards gravity ( positive gravitropism). In both cases response increases probability that roots will grow into soil, where they’re better able to absorb water and mineral ions.
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13
Q

How do plants respond to light?

A

Shoots grow towards light because light is needed for photosynthesis

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14
Q

How do plants respond to gravity?

A

Plants need to be firmly anchored in the soil. Roots are sensitive to gravity and grow in the direction of its pull.

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15
Q

How do plants respond to water?

A

Almost all plant roots grow towards water in order to absorb it for use in photosynthesis and other metabolic processes, as well as for support.

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16
Q

Describe plant growth factors.

A
  • they exert their influence by affecting growth and, they may be made by cells located throughout the plant rather than in particular organs
  • some affect the tissues that release them rather than acting on a distant target organ.
  • eg indoleacetic acid (IAA) controls plant cell elongation.
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17
Q

Describe IAA.

A
  • important auxin produced at tips of shoots in flowering plants
  • moves around plant to control tropisms- it moves by diffusion and active transport over short distances and via the phloem over long distances.
  • results in different parts of plant having different conc. of IAA
  • uneven distribution of IAA means there’s uneven growth.
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18
Q

Explain phototropism in flowering plants.

A
  • cells in tip of shoot produce IAA, which is transported down shoot
  • IAA is initially transported evenly throughout all regions as it begins to move down shoot
  • light causes movement of IAA from light side to shaded side of shoot
  • greater conc. of IAA builds up on shaded side of shoot
  • so cells on this side elongate more
  • shaded side of shoot elongates faster than light side, causing shoot tip to bend towards the light.
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19
Q

Explain gravitropism in flowering plants.

A
  • cells in tip of root produce IAA, which transported along root
  • IAA initially transported to all sides of root
  • gravity influences movement of IAA from upper side to lower side of root
  • greater conc. of IAA builds up on lower side of root
  • as IAA inhibits elongation of root cells + there’s greater conc. of IAA on lower side, cells on this side elongate less
  • relatively greater elongation of cells on upper side causes root to Ben downward towards force of gravity.
20
Q

What is a reflex arc?

A

The pathway of neurones linking receptors to effectors in a reflex .

21
Q

List the stages of a reflex arc with examples.

A
  • stimulus, heat from hot object
  • receptor, thermoreceotors in skin heat stimulus which generates nerve impulses in the sensory neurone
  • sensory neurone, passes nerve impulses to spinal cord
  • a coordinator, links sensory neurone to motor neurone in spinal cord
  • a motor neurone, carries nerve impulses from spinal cord to muscle in upper arm
  • an effector, muscle in upper arm, which is stimulated to contract
  • the response, pulling hand away from hot object.
22
Q

What is the importance of reflex arcs?

A
  • they protect body from harm, effective from birth and don’t have to be learnt
  • involuntary + therefore don’t require decision-making powers of brain, thus leaving it free to carry out more complex responses
  • action is rapid because neurone pathway is short with very few synapses.
23
Q

Describe how sensory reception is specific to a single type of stimulus in the case of the pacinian corpuscle?

A

It responds only to mechanical pressure. It won’t respond to other stimuli, such as heat, light, or sound.

24
Q

Describe how sensory reception produces a generator potential by acting as a transducer with the pacinian corpuscle as an example?

A

All stimuli involve change in some form of energy. It’s the role of transducer to convert the change in form of energy by stimulus into a form that can be understood by body. Receptors I’m nervous system convert energy of stimulus into nervous impulse known as generator potential. Eg. Pacinian corpuscle transducer mechanical energy of stimulus into generator potential.

25
Q

What are pacinian corpuscles?

A

Mechanoreceptors, they detect mechanical stimuli. They’re found in your skin.

26
Q

Describe structure of pacinian corpuscles.

A

They contain end of a sensory neurone, called a sensory nerve ending. Sensory nerve ending is wrapped in loads of layers of connective tissue called lamellae.

27
Q

What happens when the pacinian corpuscle is stimulated?

A
  • pressure causes lamellae to stretch and deform
  • stretch mediated sodium ion channels open + Na+ diffuses into neurone
  • greater the stimulus the more Na+ channels open
  • depolarisation of neurone called generator potential
  • if threshold reached then action potential is initiated.
28
Q

Describe structure of the eye.

A
  • light enters eye through pupil, amount of light let in controlled by muscles of the iris.
  • light rays focused by lens onto retina, which lines inside of eye. Retina contains photoreceptor cells that detect light
  • the fovea is an area of retina where there’s lots of photoreceptors
  • nerve impulses from photoreceptor cells carried from retina to brain by optic nerve. Where optic nerve leaves is called blind spot
29
Q

How do photoreceptors convert light into an electrical impulse?

A
  • light enters eye, hits photoreceptors + is absorbed by light-sensitive optical pigments
  • light bleaches pigments, causing chemical change and altering membrane permeability time sodium ions
  • generator potential created + if it reaches threshold, a nerve impulse us sent along a bipolar neurone
  • bipolar neurones connect photoreceptors to optic nerve, which takes impulses to brain.
30
Q

Where are rods found?

A

Mainly in the peripheral parts of the retina

31
Q

Where are cones found?

A

Packed together in the fovea

32
Q

Explain the different optical pigments in rods and cones.

A

Rods on,y give information in black and white, cones give information in colour. 3 types of cones, red-sensitive, green-sensitive, and blue-sensitive. When stimulated in different proportions you see different colours.

33
Q

Describe the sensitivity of rods.

A

Rods are very sensitive to light( they fire action potentials in dim light). This is because many rods join to one neurone, so many weak generator potentials combine to reach threshold + trigger an action potential.

34
Q

Describe sensitivity of cones.

A

Cones less sensitive (fire action potentials in bright light). This is because one cone joins one neurone, so takes more light to reach the threshold and trigger an action potential.

35
Q

Describe the visual acuity of rods.

A

Rods give low visual acuity because many rods join same neurone, which means light from two points close together can’t be told apart.

36
Q

Describe visual acuity of cones.

A

Cones give high visual acuity because cones are close together and 1 cone joins 1 neurone. When light from 2 points hits 2 cones, 2 action potentials got to brain, so you can distinguish 2 points that are close together as 2 separate points.

37
Q

What is the autonomic nervous system?

A

Controls the involuntary activities of internal muscles and glands and has 2 divisions:

  • the sympathetic nervous system
  • the parasympathetic nervous system.
38
Q

Describe the sympathetic nervous system.

A

This stimulates effectors + so speeds up any activity. It acts like an emergency controller. It controls effectors when we exercise strenuously or experience powerful emotions. Ie. Helps us to cope with stressful situations by heightening our awareness and preparing us for activity.

39
Q

Describe the parasympathetic nervous system.

A

Inhibits effectors and slows down any activity. It controls activities under normal resting conditions. It’s concerned with conserving energy + replenishing body’s reserves.

40
Q

Describe how cardiac muscle controls the regular beating of the heart.

A
  • SAN initiates heart beat
  • sends an electrical impulse across atria
  • causes atria to contract
  • non-conductive tissue prevents the electrical impulse reaching ventricles
  • AVN delays electrical impulse, allows atria to contract and empty before ventricles contract
  • AVN sends the electrical impulse down the bundle of his and along pukyne fibres
  • pukyne tissue carries waves of electrical activity into muscular walls of ventricles, causing them to contract simultaneously from base upwards.
41
Q

Describe how the control of heart rate involves brain and autonomic nervous system.

A
  • SAN generates electrical impulses that cause cardiac muscles to contract
  • rate at which SAN fires is unconsciously controlled by medulla oblongata
  • animals need to be able to alter heart rate to respond to internal stimuli
  • stimuli detected by pressure and chemical receptors
  • electrical impulses from receptors sent to medulla along sensory neurones. Medulla processes info and sends impulses to SAN along sympathetic(increase heart rate) and parasympathetic (decrease heart rate) neurones.
42
Q

What is the resting heart rate of a typical adult human

A

70 bpm

43
Q

What is the effect of exercise on cardiac output?

A
  • increased muscular/metabolic activity
  • more carbon dioxide produced by tissues from 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 sympathetic nervous system.
  • SAN increase heart rate
  • increased blood flow removes CO2 faster
  • CO2 conc. returns to normal.
44
Q

What happens when blood pressure is higher than normal?

A
  • baroreceptors detect high blood pressure
  • impulses sent to medulla oblongata, which sends impulses along parasympathetic neurones. These secrete acetylcholine, which binds to receptors on SAN.
  • heart rate slows down to reduce blood pressure back to normal.
45
Q

What happens when blood pressure is lower than normal?

A
  • baroreceptors detect low blood pressure.
  • impulses sent to medulla oblongata, which send impulses along sympathetic neurones. These secrete noradrenaline, which binds to receptors on SAN.
  • heart rate speeds up to increase blood pressure back to normal.
46
Q

What happens when there’s high blood O2, low CO2 or high pH levels?

A
  • chemoreceptors detect chemical changes in blood
  • impulses sent to medulla oblongata, which sends impulses along parasympathetic neurones. These secrete acetylcholine, which binds to receptors on SAN.
  • heart rate decreases to return O2, CO2 and pH levels back to normal
47
Q

What happens when there’s low blood O2, high CO2 or low pH levels?

A
  • chemoreceptors detect chemical changes in blood
  • impulses sent to medulla oblongata, which sends impulses along sympathetic neurones. These secrete noradrenaline, which binds to receptors on the SAN.
  • heart rate increase to return O2, CO2 and pH levels back to normal.

Biology A-level (15 decks)

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