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(6) Organisms respond to changes in their internal and external environments > Detection of Stimuli > Flashcards

Detection of Stimuli Flashcards

1
Q

What is a stimulus?

A

A detectable change in the external or internal environment of an organism.

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2
Q

What is an receptor?

A

A cell or organ that detects a stimulus.

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3
Q

What is a tropism?

A

A type of growth towards an external stimulus.
Gravitropism = gravity
Phototropism = light

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4
Q

Where is IAA synthesised?

A

The tips of roots and shoots.

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5
Q

What does IAA control?

A

Cell elongation.

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6
Q

How does IAA cause cell elongation?

A
  • IAA molecules bind to a receptor protein on the cell surface membrane
  • IAA stimulates ATPase proton pumps to pump hydrogen ions from the cytoplasm into the cell wall
  • This lowers the pH of the cell wall
  • This activates proteins known as expansins, which loosen the bonds between cellulose microfibrils
  • Potassium ion channels are stimulated to open → increase in potassium ion concentration in the cytoplasm lowers the water potential of the cytoplasm
  • Cell absorbs water by osmosis through aquaporins which is then stored in the vacuole
  • Increases the internal pressure of the cell, causing the cell wall to stretch
  • The cell elongates
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7
Q

Describe the mechanism of phototropism.

A
  • The higher concentration of IAA on the shaded side of the shoot causes a faster rate of cell elongation
  • This causes the shoot to bend towards the light
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8
Q

Describe the mechanism of gravitropism.

A
  • The larger concentration of IAA at the lower side of the root inhibits cell elongation
  • As a result, the lower side grows at a slower rate than the upper side of the root
  • This causes the shoot to bend downwards
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9
Q

What is taxis?

A

Taxis is a directional response to a stimulus.

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10
Q

What is kinesis?

A

Kinesis is a non-direction response to a stimulus.

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11
Q

Describe a reflex arc.

A

Stimulus → Receptor → Sensory Neuron → Relay Neuron → Motor Neuron → Effector → Response

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12
Q

Importance of reflex arcs.

A
  • doesn’t involve conscious decision making
  • doesn’t need to be learnt
  • immediate protection
  • extremely fast
  • controls everyday actions (i.e. digestion)
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13
Q

Receptors will only respond to…

A

a specific stimulus.

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14
Q

What does the pancinian corpuscle respond to?

A

Changes in pressure.

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15
Q

Describe the structure of the pacinian corpuscle.

A
  • layers of membrane separated by a gel containing Na+
  • contains stretch-mediated sodium ion channels which open when sufficient pressure is applied
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16
Q

How is a generator potential established?

A
  1. When pressure is applied to the Pacinian corpuscle, the membrane is stretched.
  2. The sodium channels open.
  3. The influx of sodium ions will depolarise the membrane, leading to a generator potential
17
Q

What are rod cells?

A

Cells that are sensitive to light intensity.

18
Q

What are cone cells?

A

Cells that are sensitive to different wavelengths of visible light

19
Q

What pigment is in rod cells?

A

Rhodopsin

20
Q

What pigment is in cone cells?

A

Iodopsin

21
Q

Sensitivity of rod cells

A
  • Rods are very sensitive to even low light intensities
    _ They allow humans to distinguish between light and dark objects when light is very dim
  • They do not allow humans to see in colour
22
Q

Sensitivity of cone cells

A
  • Cones are less sensitive to light
  • They are sensitive to different wavelengths of light
    There are three cone types:
    - Red
    - Blue
    - Green
  • The combined effect of all three pigments allows humans to observe all the other colours that are on the visible spectrum
23
Q

Visual acuity of rod cells?

A

Lower visual acuity
- Multiple rod cells synapse with a single bipolar cell
- General understanding of the fields of vision that are light or dark

24
Q

Visual acuity of cone cells?

A

Higher visual acuity
- A single cone cell synapses with a single bipolar cell
- If two cones are stimulated to send an impulse the brain is able to interpret these as two different spots of light

25
Q

Describe the importance of summation of rod cells.

A
  • Group of rods are stimulated at the same time the combined generator potentials are sufficient to reach the threshold and stimulate the bipolar cell for the conduction of nerve impulses onwards towards the optic nerve
  • Summation produces a less sharp image but enables organisms to see in much dimmer light than cones allow
26
Q

Outline the stages of the cardiac cycle.

A
  1. The SAN initiates a wave of depolarisation
  2. Causes the atria to contract
  3. Non-conducting tissue prevents the depolarisation spreading straight to the ventricles
  4. After a slight delay, the AVN is stimulated and passes the stimulation along the bundle of His (This delay means that the ventricles contract after the atria)
  5. Purkinje fibers spread around the ventricles and initiate the depolarization of the ventricles from the apex of the heart
  6. Ventricles contract
27
Q

Where are pressure and chemoreceptors located?

A
  • Aorta
  • Carotid Arteries
28
Q

Describe the acceleratory centre.

A
  1. Once acceleratory centre has been activated impulses are sent along the sympathetic neurones to the SAN
  2. Noradrenaline is secreted at the synapse with the SAN
  3. Noradrenaline causes the SAN to increase the frequency of the electrical waves that it produces
  4. This results in an increased heart rate
29
Q

Describe the inhibitory centre.

A
  1. Once the inhibitory centre has been activated impulses are sent along the parasympathetic neurones to the SAN
  2. Acetylcholine is secreted at the synapse with the SAN
  3. This neurotransmitter causes the SAN to reduce the frequency of the electrical waves that it produces
  4. This reduces the elevated heart rate towards the resting rate

(6) Organisms respond to changes in their internal and external environments (4 decks)

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