Response To Stimuli

Question 1

What is a stimulus?

Answer 1

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

Question 2

How can response to a stimulus increase an organisms chances of survival?

Answer 2

Organisms can detect harmful stimuli and move away from them.

Question 3

Describe the response pathway:

Answer 3

Detection by receptors that are specific to one type of stimulus=>co-ordination=> response produced by an effector

Question 4

What is taxis?

Answer 4

A directional response to a directional stimulus

Question 5

Differentiate between positive and negative taxis

Answer 5

Positive - movement towards a stimulus

Negative - movement away from a stimulus

Question 6

What is kinesis?

Answer 6

A change in the speed at which an organism moves and the rate at which it changes direction.

Question 7

What is tropism?

Answer 7

Growth of a plant in response to a directional stimulus.

Question 8

Describe the tropism of plant shoots

Answer 8

Positive phototropism and negative gravitropism

Question 9

Describe the tropism of plant roots

Answer 9

Negative phototropism

Positive gravitropism

Question 10

What are the three main things plants respond to?

Answer 10

Light, water and gravity.

Question 11

What controls plants response to external stimuli?

Answer 11

Plant growth factors (hormone-like substances)

Question 12

Why are plant growth factors NOT hormones?

Answer 12

The influence growth and are made by cells located throughout the plant rather than one organ

Some growth factors affect the tissue that releases them as opposed to only distant target organs.

Question 13

What is the main role of IAA?

Answer 13

Cell elongation

Question 14

Describe how positive phototropism happens in the shoot of a plant

Answer 14

In the condition of unilateral light;

1) cells in the top of the shoot produce IAA
2) this is initially transported evenly through all regions as it moves down the shoot
3) light causes the movement of IAA from the light side to the shaded side of the shoot
4) a greater concentration of IAA builds up on the shaded side of the shoot than the light side causing the elongation to be greater on the shaded side.
5) since the shaded side grows faster than the light side, the shoot tip bends towards the light l

Question 15

What does IAA do in plant roots?

Answer 15

IAA inhibits cell elongation in roots so elongation of roots would occur on the light side causing the roots to bend away from the light (negative phototropism)

Question 16

Explain gravitropism in plants

Answer 16

In the conditions of a horizontally growing root;

1) cells in the tip of root produces IAA which is then transported to all sides of the root.
2) initially, the IAA is transported to all sides of the root
3) gravity influences the movement of IAA from the upper side to the lower side of the root.
4) a greater concentration of IAA builds up on the lower side of the root than the upper side. This makes the cells on the lower side elongate less than the upper side.
5) this causes the root to bend downwards towards the force of gravity

Question 17

Explain how IAA causes cell elongation

Answer 17

IAA increases the plasticity (ability to stretch) of cell walls.

Question 18

What type of cells does cell elongation work on and why?

Answer 18

Young cells as mature cell walls are more rigid and do not respond

Question 19

Explain acid growth hypothesis

Answer 19

Hydrogen ions are actively transported from the cytoplasm into spaces into the cell wall to become more plastic allowing the cell to elongate

Question 20

What are the two divisions of the nervous system?

Answer 20

The central nervous system (brain and spinal cord)

The peripheral nervous system (pairs nerves connected to the brain and spinal cord)

Question 21

What are the subdivisions of the peripheral nervous system?

Answer 21

Sensory neurones and motor neurones

Question 22

What can the motor nervous system be divided into?

Answer 22

• The voluntary nervous system which carries nerve impulses to muscles under conscious thought
• The autonomic nervous system that carries nerve impulses to glands, smooth muscle and cardiac muscle and is subconscious.

Question 23

Describe a reflex arc/spinal reflex

Answer 23

A simple response consisting of only 3 neurones- one being the spinal cord.

Question 24

Why are reflexes an aid to survival?

Answer 24

Responses are involuntary so they require little thought.
They protect the body from harm.
They’re fast because the pathway is so short- important for withdrawal reflexes.
It is effective from birth and does not have to be learnt.
Since there is no decision making the response is rapid.

Question 25

Name three types of receptors:

Answer 25

Pacinian corpuscle- mechanical stimuli receptor
Rod cells- light receptors
Cone cells- light receptors

Question 26

What is the role of receptors?

Answer 26

To act as transducers- converting a stimulus’ energy into a generator potential (nerve impulse).

It is always specific to one type of stimulus.

Question 27

Where are pacinian corpuscles most abundant?

Answer 27

Fingers, soles of feel and external genitalia.

They can also occur on joints, ligaments and tendons where their role is to alert organisms which joints are changing direction.

Question 28

Describe the structure of a pacinian corpuscle:

Answer 28

A single sensory neurone enclosed within layers of tissue, each separated by a layer of gel.

Question 29

What feature of the pacinian corpuscle allows mechanical energy to be converted into a generator potential?

Answer 29

The sensory neurone at the centre of the PC has stretch mediated sodium channels on its surface- the permeability to sodium to these channels changes when they are deformed.

Question 30

Explain the process in which mechanical energy is converted into a generator potential.

Answer 30

In a resting state, stretch-mediated sodium channels on the membrane of the neurone of a PC are too narrow to allow sodium ions to pass through them. (This allows a resting potential to develop)
When pressure is applied, the membrane around the PC’s neurone becomes stretched. This widens the sodium ion channels in the membrane and allows an influx of sodium ions. This changes the resting potential (it becomes depolarised) and therefore produces a generator potential. This then produces an action potential that passes along the neurone and other neurones, eventually reaching the central nervous system.

Question 31

What are the features of cone cells?

Answer 31

Cone shaped.
There are fewer cone cells than rod cells (6 million in each eye)
More concentrated at the fovea, fewer at the periphery of the retina.
It gives good visual acuity.
There are three types of cone cells that respond to different wavelengths of light.
They are not sensitive in low light intensities.

Question 32

What are the features of rod cells

Answer 32

Rod shaped.
More abundant that cone cells (120 million in each eye)
More concentrated at the periphery of the retina. It is absent in the fovea.
It gives poor visual acuity.
It is sensitive to low light intensities.
There is only one type of rod cell.
Multiple rod cells are connected to a single sensory neurone in the optic nerve.

Question 33

What pigment is in rod cells?

Answer 33

Rhodopsin (a pigment in the rod cells) is broken down by light energy.

Question 34

How do rod cells allow us to see in the dark?

Answer 34

Low light intensities is enough to break down rhodopsin. Since there are many rod cells connected to one sensory neurone, the threshold value is more likely to be exceeded due to summation making it more likely for a generator potential to be created.

Question 35

Why do rod cells give low visual acuity?

Answer 35

Since multiple rod cells are linked to a single bipolar cell, all light received to each rod cell will generate only one nerve impulse. This prevents the brain from being able to distinguish between different sources of light if they stimulate rod cells attached to the same bipolar cell. This prevents some separate objects from being resolved.

Question 36

Why don’t cone cells work in low light?

Answer 36

Each cone cell is connected to its own bipolar cell, therefore that one cell has to be stimulated with enough light to reach the threshold value.

Question 37

What pigment is in cone cells?

Answer 37

Iodopsin which requires a higher light intensity to break down than rhodopsin.

Question 38

What determines which wavelengths of light are detected by the cone cells?

Answer 38

The type of iodopsin- different types have different sensitivities to different wavelengths.

Question 39

Why do cone cells result in high visual acuity?

Answer 39

Since each cone cell is connected to a single bipolar cell, different cone cells being stimulated will result in multiple nerve impulses being generated. This allows the brain to distinguish between separate sources of light.

Question 40

Why are only cone cells present in the fovea?

Answer 40

Light is focused by the lens to the fovea. This means the fovea will receive the highest intensity of light. This is why only cone cells are found here as they are sensitive to high light intensities.

Question 41

Why are rod cells in higher abundance on the periphery of the retina?

Answer 41

Light intensities are the lowest here. Rod cells are usually found here as light can still be broken down in them and used to produce generator potentials.