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Biology > Section 6 - Organisms respond to changes in their environment: 14. Response to stimuli > Flashcards

Section 6 - Organisms respond to changes in their environment: 14. Response to stimuli Flashcards

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1
Q

What is a stimulus

A

A detectable change to the internal or external environment of an organism, leading to a response

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2
Q

How does an organism’s ability to respond to stimuli increase the chance of survival

A
  • Can move away from harmful stimuli (eg. predators)
  • Can move towards food source
    etc.

∴ Selection pressures favour organisms with the most appropriate responses, as they will survive and pass on their alleles to the next generation

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3
Q

What are the basic stages of an organism’s response to a stimulus

A

Stimulus → Receptor → Coordinator → Effector → Response

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4
Q

What is the role of a coordinator

A

Formulates an appropriate response to a stimulus

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5
Q

What are the two types of coordination for an organism’s response to a stimulus

A
  • Nervous coordination
  • Hormonal coordination
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6
Q

What is the role of an effector

A

Produces the response to a stimulus
(Muscle or gland)

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7
Q

What are Taxes

A

A taxis is a simple response where a motile organism moves in a direction determined by the direction of the stimulus

eg. Bacteria move towards areas of higher glucose conc. (Positive chemotaxis)

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8
Q

What is a ‘positive taxis’

A

Movement towards a favourable stimulus

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9
Q

What is a ‘negative taxis’

A

Movement away from an unfavourable stimulus

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10
Q

What are Kineses

A

A kinesis is a response in which an organism doesn’t move towards or away from the stimulus, but instead changes it’s speed and the rate at which it changes direction

eg. If a woodlouse is in a dry environment, it will move rapidly, changing direction frequently, to increase the chance of re-entering the damp area.
However, if it is in the dry area for a long period, it will begin to move in long straight lines with sharp turns

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11
Q

What are tropisms

A

A tropism is a response where a plant grows in a certain direction as a result of a stimulus

eg. Plant shoots grow up (negative gravitropism) and towards light (positive phototropism)

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12
Q

What is a ‘positive tropism’

A

Plant growth towards a favourable stimulus

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13
Q

What is a ‘negative tropism’

A

Plant growth away from an unfavourable stimulus

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14
Q

What are the main environmental factors that cause tropisms

A
  • Light: Shoots grow towards light (positively phototropic)
  • Gravity: Shoots grow up (negatively Gravitropic) and roots grow down (positively gravitropic)
  • Water: Roots grow towards water in the soil (Positively hydrotropic)
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15
Q

What are plant growth factors

A

Hormone-like substances that result in responses to external stimuli
eg. Indoleacetic Acid (IAA)

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16
Q

How do plant growth factors differ from animal hormones

A
  • Affect plant growth
  • Made by cells located throughout the plant (rather than a particular organ)
  • Affect the tissue they are released from (rather than a distant target)
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17
Q

What is Indoleacetic Acid (IAA) and what does it do

A

IAA is a type of auxin (plant growth factor) that controls cell elongation, coordinating tropisms

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18
Q

How does IAA control cell elongation

A
  • Increases the plasticity of plant cell walls (in shoots)
  • Response occurs in young cells, as mature cells have greater rigidity
  • ∴ Older parts of the plant don’t respond to directional stimuli
  • (Acid growth hypothesis)
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19
Q

What is the ‘Acid growth hypothesis’

A

Proposed explanation for how IAA increases cell plasticity:
- Active transport of hydrogen ions from cytoplasm into spaces in the cell wall, causing them to become more plastic.
- This allows cells to expand (∴ Elongate)

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20
Q

What is the process of the phototropic response of the shoots of a flowering plant to a unilateral light source

A
  • Cells at the tip of the shoot produce IAA, which is transported evenly down the shoot by diffusion
  • Light causes the movement of IAA from the light side, to the shady side of the shoot
  • ∴ A greater conc. of IAA builds up on the shady side
  • IAA causes the elongation of the shoot cells, so due to the uneven distribution, the cells elongate more on the shaded side
  • This causes the shaded side to grow faster, so the tip bends towards the light
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21
Q

What is the process of the gravitropic response of the roots a flowering plant when grown horizontally

A
  • Cells at the tip of the root produce IAA, which is transported evenly along the root by diffusion
  • In horizontal (or slanted) sections of root, IAA is moved from the upper side to the lower side due to gravity
  • ∴ A greater conc. of IAA builds up on the lower side
  • IAA inhibits cell elongation in roots, so due to the uneven distribution, the cells elongate less on the lower side
  • This causes the upper side to grow faster, so the root tip bends down
    (Opposite effect in shoots, as IAA stimulates cell elngation)
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22
Q

When establishing the role of IAA in tropisms, what experiments were carried out to determine that is is the tip of the shoots that cause the phototropic response

A

1) Shoot placed in light, and bends towards the source
∴ Shoot is positively phototropic, and bending occurs behind the tip

2) The tip is removed from a shoot before placing it in light, and no response occurred
∴ Tip must either detect the stimulus or produce the messenger (or both)

3) Shoot tip is covered then placed in light, and no response occurred
∴ Light stimulus must be detected by the tip

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23
Q

When establishing the role of IAA in tropisms, what experiments were carried out to determine that is is a chemical response rather than an electrical signal that causes the response

A

1) A barrier of ‘mica’ is placed beneath the tip on the light side, and the shoot bent towards the light
2) A barrier of ‘mica’ is placed beneath the tip on the shaded side and no response occurred
(mica = Thin and impermeable, conducts electricity but won’t allow chemicals to pass)
∴ Messenger must be chemical, as it is stopped by mica, which would allow electric signals to pass.
+ Response can only occur if the messenger is on the shaded side, so the messenger causes growth

3) Gelatine barrier placed below the tip across the whole shoot, and it still bent towards the light
∴ Must be a chemical messenger, as the gelatine would stop any electrical signals, but allow chemicals to pass.

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24
Q

When establishing the role of IAA in tropisms, what experiments were carried out to determine how the chemical messenger in the plant shoot causes a response

A

1) Tip is removed and placed on one side, and the shoot bends towards the side with no tip
∴ Chemical messenger causes growth as the side it is released into elongates faster

2) One shoot is placed in the dark while another is placed in light, and only the shoot in the light had a phototropic response, but the total IAA in each shoot after was the same
∴ Light doesn’t cause IAA to be broken down, as there was the same amount in each, it was just unevenly distributed in the light

3) A thin glass plate is placed vertically between each side of the shoot, and no response occurred in light conditions, with the same amount of IAA collected afterwards from each side.
4) The glass is slightly lowered so lateral movement of IAA is possible, and the shoot bent towards the light, with 30% of the total IAA collected from the light side and 70% from the shaded side
∴ IAA must move from the sunny side to the shaded side to become unevenly distributed

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25
Q

What are the two main divisions of the nervous system

A
  • Central nervous system: Brain and Spinal cord
  • Peripheral nervous system: Made up of pairs of nerves that link to either the brain or spinal cord
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26
Q

What type of neurone is within the central nervous system

A

Relay/intermediate neurone

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27
Q

What are the two types of neurones within the peripheral nervous system

A
  • Sensory neurone
  • Motor neurone
28
Q

What is the role of the Relay (intermediate) neurone

A

Relays impulses from the sensory neurone to the motor neurone within the central nervous system

29
Q

What is the role of the sensory neurones

A

Carries nerve impulses from receptors towards the central nervous system

30
Q

What is the role of the Motor neurones

A

Carries nerve impulses away from the central nervous system, to effectors

31
Q

What are the two subdivisions of the motor nervous system

A
  • Voluntary nervous system
  • Autonomic nervous system
32
Q

What is the role of the voluntary nervous system

A

Carries impulses to body muscles, and is under conscious control

33
Q

What is the role of the autonomic nervous system

A

Carries impulses to glands, smooth muscles and cardiac muscles, and isn’t under conscious control

34
Q

What are the two subdivisions of the autonomic nervous system

A
  • Sympathetic nervous system
  • Parasympathetic nervous system
35
Q

What is the role of the sympathetic nervous system

A

Stimulates effectors, so speeds up any activity
(Acts like an emergency controller, aiding the fight or flight response)

36
Q

What is the role of the parasympathetic nervous system

A

Inhibits effectors, so slows down any activity
(Controls activities during rest, conserving energy and replenishing reserves)

37
Q

What is a reflex

A

An involuntary response to a sensory stimulus, which is rapid, short lived and localised

38
Q

What is a reflex arc

A

The pathway of neurones involved in a reflex

eg. Withdrawing hand from hot object:
- Stimulus: Heat from object
- Receptor: Sensory receptors on skin that generate nerve impulse
- Sensory neurone: Passes nerve impulse to spinal cord
- Coordinator (relay neurone): Links the sensory neurone to the motor neurone
- Motor neurone: Carries impulse from the spinal cord to the muscle (upper arm)
- Effector: Muscle is stimulated to contract
- Response: The hand is pulled away from the hot object

39
Q

Why are reflex arcs important

A
  • They are involuntary
    ∴ No decision is required, so brain can focus on more complex response
    + FAST!
  • They protect the body from harm
    (Are effective from birth and don’t need to be learned)
  • Neurone pathway is short (less synapses)
    ∴ Rapid response to avoid danger
40
Q

What is the role of a receptor

A

Sensory reception (rather than sensory perception) as they only detect the stimulus and trigger the events that cause a response, rather than making sense of the information received (done by the brain)

41
Q

What are the main features of sensory recepors

A
  • Specific to s single type of stimulus (eg. light)
  • Produce a generator potential by acting as a transducer (Will convert the change in energy resulting from the stimulus into a nerve impulse)
42
Q

What is the Pacinian corpuscle

A

Sensory receptor that responds to changes in mechanical pressure

43
Q

What is the structure of the Pacinian corpuscle

A
  • The end of the sensory neurone is at the centre of layers of tissue, separated by a gel
  • The end of the neurone has Stretch-mediated protein channels (channel proteins that open when pressure is applied), which lead to the formation of an action potential
44
Q

Where are Pacinian corpuscles located

A
  • Deep in the skin
  • More abundant on fingers, soles of feet and external genitalia
  • Present in joints, ligaments and tendons, enabling the organism to know which joints are changing direction
45
Q

How does the Pacinian corpuscle cause an electrical impulse

A
  • Stretch-mediated sodium channels are permeable to Na+ when the membrane is deformed
  • When at rest (no pressure applied), these channels are closed, allowing a resting potential to be maintained
  • When pressure is applied to the Pacinian corpuscle, these channels open, depolarising the membrane, resulting in the formation of an action potential that travels along the sensory neurone
46
Q

What are the main structural features of the mammalian eye

A
  • Retina: Back of the eye containing light sensitive cells
  • Macula: Centre of the retina
  • Fovea: Most sensitive part of the retina, located in the macula
  • Choroid: Black layer behind the retina to prevent internal reflection
  • Optic nerve: Carries impulses from receptor cells
  • Blind spot: Point on the retina where the optic nerve begins, so no receptor cells are present
  • Lens: Focuses light on the retina
  • Ciliary muscle: Alters the thickness of the lens for focusing
  • Iris: Controls the amount of light entering the eye
  • Cornea: Bends light onto the lens
  • Conjunctiva: Layer on the front of the eye to protect the cornea
  • Vitreous humour: Transparent jelly within the main body of the eye
  • Sclera: Protective layer surrounding the outside of the whole eye
47
Q

What are the different layers within the retina of the eye

A

↓ Light ↓
Optic Nerve
Ganglion cells
Bipolar neurones
Receptor cells (Cones and Rods)

48
Q

What are Rod cells

A

Photoreceptor cells that make up most of the receptor cells in the retina
- Can’t distinguish colour
- Allow for vision in low light intensity
- Low visual acuity
- More located around the edge of the retina

49
Q

How do rod cells create a generator potential

A

The pigment ‘Rhodopsin’ is broken down into opsin by light energy, which leads to the hyperpolarisation of the rod cells, depolarising the sensory neurone

50
Q

How do Rod cells allow for vision at low light intensity

A
  • There is enough energy in low-intensity light to break down Rhodopsin
  • Retinal convergence: There are multiple rod cells connected to a single bipolar cell, so there is a greater chance that the threshold potential is reached
51
Q

What is retinal convergence and how does it affect rod cells

A

Multiple rod cells are connected to a single bipolar cell:
- ∴ Rod cells allow for vision in low light, as this convergence means there is a greater chance that the threshold potential is reached
- ∴ Rod cells have low visual acuity, as light received by two separate cells will form just one action potential, so two light sources many appear as one.

52
Q

What are Cone cells

A

Photoreceptor cells that make up less of the receptor cells in the retina
- Different types can distinguish different Wavelengths/colours
(Red-sensitive, Blue-sensitive, Green-sensitive)
- Can only respond to high-intensity light
- High visual acuity

53
Q

How do cone cells create a generator potential

A

The pigment ‘Iodopsin’ is broken down by light energy, which leads to the hyperpolarisation of the cone cells, depolarising the sensory neurone

54
Q

How do cone cells allow for colour vision

A

There are three types of cone cells, each with a different type of Iodopsin that is broken down by different wavelength of light (R/G/B)

55
Q

Why can cone cells only respond to high light intensity

A
  • The limited wavelengths of light absorbed by one cone cell provides less energy, so a higher intensity id required to breakdown the iodopsin
  • Each cone cell has it’s own bipolar cell, so there are no low-light benefits of retinal convergence
56
Q

Why do cone cells have high visual acuity

A

Each cone cell is connected to it’s own bipolar cell (no retinal convergence), so light detected by 2 cells will result in 2 action potentials and can be seen as separate sources

57
Q

What are the main differences between Rod and Cone cells

A
  • Rod cells outnumber cone cells 20:1
  • Rod and cone cells are distributed unevenly across the retina (more rod cells at the periphery, and more cone cells at the fovea)
  • Rod cells only produce black and white images, while cone cells allow colour to be detected
  • Rod cells have poor visual acuity, whereas cone cells have high visual acuity
  • There is only one type of rod cells, but there are 3 types of cone cells (Red/Blue/Green-sensitive)
58
Q

What are ‘Neurogenic muscles’

A

Muscles stimulated by external nervous impulses

59
Q

What are ‘Myogenic muscles’

A

Muscles where the contractions are initiated from within the muscle itself (eg. Cardiac muscle)

60
Q

What is the Sinoatrial node (SAN) and what is it’s function

A
  • Group of cells in the right atrium that cause the initial stimulation for a contraction
  • Has a basic rhythm that determines the heart beat, so acts as a pacemaker
61
Q

What is the sequence for the control of the resting heart rate

A
  • Wave of excitation spreads out from the Sinoatrial node (SAN) across both atria, causing them to contract
  • A layer of non-conductive tissue (Atrioventricular septum) prevents the wave reaching the ventricles
  • The wave of excitation then enters a second group of cells known as the atrioventricular node (AVN), located between the atria
  • After a short delay, the AVN conveys a wave of excitation down between the ventricles, along a series of specialised muscle fibres called ‘Purkynĕ Tissue’, which collectively make up a structure called the ‘Bundle of His’
  • The wave of excitation travels down the ‘Bundle of His’ until it reaches the base of the ventricles where it branches into smaller fibres
  • The wave of excitation is then released from the Purkynĕ tissue, causing the ventricles to contract quickly from top to bottom
62
Q

Why is it important that there is a delay before the wave of excitation leaves the AVN and travels down the Purkynĕ tissue, provided by the Atrioventricular septum

A

This allows there to be a delay between the contractions of the atria and the contractions of the ventricles, so blood can flow between them

63
Q

Why is it important that the contraction of the ventricles occurs bottom to top

A

This allows blood to be forced up and out of the ventricles, into the pulmonary artery / aorta

64
Q

How are changes to the heart rate controlled

A

The ‘Medulla Oblongata’ in the brain has 2 centres that control heart rate (stimulated by several receptors)
- One centre that increases the heart rate, linked to the SAN by the sympathetic nervous system
- One centre that decreases the heart rate, linked to the SAN by the parasympathetic nervous system

65
Q

How is the heart rate controlled by chemoreceptors, allowing for the regulation of CO(2) in the blood

A
  • Chemoreceptors are located in the wall of the carotid arteries (going to the brain), and are sensitive to pH changes that result from the CO(2) concentration
  • During periods of intense metabolic activity, blood CO(2) increases
  • This change is detected by the chemoreceptors, which then increase the frequency of nerve impulses sent to the medulla oblongata
  • The centre of the medulla oblongata that increases heart rate then increases the frequency of impulses to the SAN via the sympathetic nervous system
  • This increases the production of electrical waves from the SAN, increasing heart rate
  • This increases the rate of blood flow, so more CO(2) is removed at the lungs
  • This decreases the CO(2) levels so pH returns to normal
  • This is detected by the chemoreceptors, so the heart beat is returned to it’s resting rate
66
Q

How is the heart rate controlled by Pressure receptors, allowing for the regulation of blood pressure

A

Pressure receptors are located in the wall of the carotid arteries (going to the brain) and in the aorta

When the blood pressure is higher than normal:
- Receptors increase freq. of impulses to the medulla oblongata
- The centre of the medulla oblongata that decreases heart rate then increases the frequency of impulses to the SAN via the parasympathetic nervous system
- This decreases the heart rate, so blood pressure is reduced

When the blood pressure is lower than normal:
- Receptors increase freq. of impulses to the medulla oblongata
- The centre of the medulla oblongata that increases heart rate then increases the frequency of impulses to the SAN via the sympathetic nervous system
- This increases the heart rate, so blood pressure is increased

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