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 is a response to a stimuli good?

Answer 2

It increases the chances of survival for an organism.
Organisms that survive have a greater chance of raising offspring and of passing their alleles to the next generation.
So there is a selection pressure favouring organisms with more appropriate responses.

Question 3

What are receptors?

Answer 3

Specific receptors detect one specific type of stimulus.

Question 4

What is the response to stimuli?

Answer 4

Stimulus - receptor - coordinator - effector - response.

Question 5

What is a coordinator?

Answer 5

A coordinator formulates a suitable response to a stimulus.
It may be at the molecular level or involve a large organ - brain.

Question 6

What is an effector?

Answer 6

This produces the response. The response may be at the molecular level or involve the behaviour of the whole organism.

Question 7

How does communication occur?

Answer 7

In large multicellular organisms it occurs via chemicals called hormones, a relatively slow process found in plants and animals.
It also occurs by the more rapid nervous system.

Question 8

What is the nervous system?

Answer 8

Nervous systems have many different receptors and control effectors, each linked to a central coordinator.
This connects information from each receptor with the appropriate effector.

Question 9

What is a taxis?

Answer 9

A simple response whose direction is determined by the direction of the stimulus.
A motile organism responds directly to the environment by moving its whole body towards or away from the (un)favourable stimulus.

Question 10

How are taxis classified?

Answer 10

Positive taxis - the movement is towards the stimulus.
Negative taxis - movement away from the stimulus.
Also by the nature of the stimulus.

Question 11

What are examples of phototaxis?

Answer 11

Algae move towards the light (+). This increases survival rate as they require light to manufacture food by photosynthesis.
Earthworms move away from the light (-). This increases survival as it takes them to the soil, where they are better able to conserve water, find food and avoid some predators.

Question 12

What is an example of chemotaxis?

Answer 12

Some species of bacteria will move towards a region where glucose is more concentrated (+), and increases survival rate as glucose is a source of their food.

Question 13

What is a kinesis?

Answer 13

A form of response where the organism changes the speed at which it moves (orthokinesis) and the rate at which it changes direction (klinokinesis).

Question 14

How do kineses change?

Answer 14

If an organism crosses a sharp change between favourable and unfavourable environments, rate of turning increases, raising its chances of a quick return.
If it moves far into an unfavourable environment the rate of turning may slowly decrease so it moves straight, then turns very sharply. Important for less directional stimulus - humidity and temperature.

Question 15

What is an example of kinesis?

Answer 15

When woodlice move from a damp area to dry, they move more rapidly and change direction more often. This increases the chance of moving back to the damp area.
They then slow down and change direction less.
It prevents them drying out and so increases their chances of survival.

Question 16

What is a tropism?

Answer 16

The growth of part of a plant in response to a directional stimulus.
Either positive response towards the stimulus or negative away from it.

Question 17

What are the types of tropism?

Answer 17

Plant shoots grow towards light (positive phototropism) and away from gravity (negative gravitropism) so their leaves are most able to capture light for photosynthesis.
Plant roots do the opposite to increase the probability that roots will grow into the soil, where they are better able to absorb water and mineral ions.

Question 18

What are plant growth factors?

Answer 18

Hormone-like substances that affect growth and may be made by cells located throughout the plant rather than in particular organs.
Some effect the tissues that release them rather than acting on a distant target organ.

Question 19

What is IAA?

Answer 19

Indoleacetic acid is a plant growth factor that controls plant cell elongation.
It is part of a group of substances called auxins.

Question 20

How does phototropism occur?

Answer 20

Cells in the tip produce IAA which is intially transported evenly down the shoot.
Light then causes the movement of IAA from the light side to the shaded side.
A greater concentration builds up on the shaded side.
IAA causes elongation of shoot cells, and the cells on the shaded side elongate more.
The shaded side elongates faster than the light side, causing the shoot tip to bend towards the light.

Question 21

How does IAA affect roots?

Answer 21

A high concentration of IAA inhibits cell elongation in the roots.
The elongation in the roots is greater on the light side than the shaded side, so the roots bend away from the light.
They are negatively phototropic.

Question 22

How does gravitropism occur in the roots?

Answer 22

Cells in the tip produce IAA, which is initially transported evenly along the root.
Gravity influences the movement of IAA from the upper side to the lower side.
A greater concentration builds up on the lower side, and the cells on this side elongate less than the upper side.
The greater elongation of cells on the upper side causes the root to bend downwards towards the force of gravity.

Question 23

How does IAA work?

Answer 23

IAA is transported away from the tip of shoots and roots where it is produced.
IAA increases the plasticity of cell walls, but only in young cells, which are able to elongate.
As cells mature they are more rigid, so can’t respond.

Question 24

What is the acid growth hypothesis?

Answer 24

This explains the effect of IAA on plasticity.
Hydrogen ions are actively transported from the cytoplasm into spaces in the cell wall causing the cell wall to become more plastic allowing the cell to elongate by expansion.

Question 25

What are the divisions of the nervous system?

Answer 25

The central nervous system, made up of the brain and spinal cord.
The peripheral nervous system, made up of pairs of nerves that originate from the brain or spinal cord.

Question 26

What are the divisions of the peripheral nervous system?

Answer 26

Sensory neurones, which carry nerve impulses from receptors towards the CNS.
Motor neurones, which carry nerve impulses away from the CNS to effectors.

Question 27

What are the divisions of motor neurones?

Answer 27

The voluntary nervous system, which carries nerve impulses to body muscles under conscious control.
The autonomic nervous system, which carries impulses to glands, smooth muscle and cardiac muscle subconsciously/ involuntarily.

Question 28

What is a reflex arc?

Answer 28

Shows a rapid, short-lived, localised and involuntary response to a dangerous sensory stimulus such as a hot object.
The pathway of neurones involved in the reflex is the reflex arc.

Question 29

What is the reflex arc of removing the hand from a hot object?

Answer 29

Stimulus - the heat from the object.
Receptor - temperature receptors in the skin on the hand, which generates nerve impulses in the sensory neurone.
A sensory neurone - passes impulses to the spinal cord.
A coordinator (intermediate neurone) links the sensory and motor neurone in the spinal cord.
A motor neurone carries impulses from the spinal cord to a muscle in the arm.
An effector - the muscle is stimulated to contract.
The response of pulling the hand away.

Question 30

What is the spinal cord?

Answer 30

A column of nervous tissue that runs along the back and lies inside the vertebral column for protection.
At intervals along the spinal cord are pairs of nerves.

Question 31

Why is it important that reflex arcs are involuntary?

Answer 31

They do not require the decision-making powers of the brain, leaving it free to carry out more complex responses.
The brain is so not overloaded with situations where the response is always the same.
Some impulses are still sent to the brain, so it is informed and can override the reflex if neccessary.

Question 32

Why are reflex arcs important?

Answer 32

They protect the body from harm, and do not need to be learnt.
They are fast, because the neurone pathway is short with few (1/2) synapses, which are the slowest link in the neurone pathway.
They are also rapid because of the absence of decision making processes.

Question 33

What is a synapse?

Answer 33

A junction between neurones by which they don’t touch but have a narrow gap - the synaptic cleft - which a neurotransmitter can pass.

Question 34

What are the features of sensory receptors - the Pacinian corpuscle?

Answer 34

They respond only to changes in mechanical pressure.
It is specific to a single type of stimulus.
It will not respond to light, heat or sound.

Question 35

What are the features of the Pacinian corpuscle - transducer?

Answer 35

The transducer converts the change in form of energy by the stimulus into nerve impulses, which can be understood by the body.
The stimulus involves a change in the form of mechanical energy.
Receptors transduce one form of energy into an impulse - the generator potential.

Question 36

Where are pacinian corpuscle?

Answer 36

They occur deep in the skin, most abundant on the fingers, soles of feet, and external genitalia.
Also occur in joints, ligaments and tendons, and enable the organism to know which joints are changing direction.

Question 37

What is the structure of the Pacinian corpuscle?

Answer 37

The single sensory neurone is at the centre of layers of tissue, each separated by a gel.
The neurone has a stretch-mediated sodium channel in its plasma membrane.
Their permeability to sodium changes when they are deformed, by stretching.

Question 38

How does the pacinian corpuscle work?

Answer 38

At rest, the stretch-mediated channels of the membrane are too narrow to allow sodium ions to pass along them - neurone has a resting potential.
When pressure is applied the corpuscle is deformed and the membranes stretched.
This widens the sodium channels and sodium ions diffuse into the neurone.
The influx of ions changes the potential of the membrane - depolarised - producing a generator potential.
This creates an action potential that passes along the neurone and to the CNS by other neurones.

Question 39

What are the receptors of the eye?

Answer 39

The light receptor cells are found on the innermost layer, the retina.
They are rod and cone cells, that both act as transducers by conserving light energy into the electrical energy of a nerve impulse.

Question 40

What are the characteristics of rod cells?

Answer 40

They cannot distinguish between wavelengths of light so produce only black and white images.
They are the more numerous - around 120 million in each eye.
They detect light of very low intensity.

Question 41

How do rod cells work?

Answer 41

Many are connected to a single sensory neurone in the optic nerve.
A certain threshold value must be reached before a generator potential is created in the bipolar cells to which they are connected.
As many are connected to one cell, there is a much greater chance that the threshold will be reached than if only a single rod were connected to each bipolar - summation.

Question 42

Why do rod cells respond to low intensity light?

Answer 42

To create a generator potential, the pigment in the rod cells - rhodopsin - must be broken down.
There is enough energy from low-intensity light to cause this breakdown.

Question 43

What is the visual acuity of rod cells?

Answer 43

Because many rod cells link to a single bipolar cell, the light recieved by rod cells of the same neurone will only generate a single impulse to the brain, regardless of the number of neurones stimulated.
This means that the brain cannot distinguish between the separate sources of light that stimulated them.
2 dots close together will appear as a single blob - low visual acuity.

Question 44

What are cone cells?

Answer 44

The three different types each respond to a different range of wavelengths of light.
They can perceive images in full colour.
There are 6 million in each eye.

Question 45

What is the light intensity for cone cells?

Answer 45

Each cone cell has its own bipolar cell connected to a sensory neurone.
This means that stimulating multiple cone cells cannot be combined to help exceed the threshold value and create a generator potential.
So they only respond to high light intensity.
The pigment iodopsin also requires a high light intensity to be broken down.

Question 46

What are the types of cone cell?

Answer 46

There are three different types of cone cell, each either a specific type of iodopsin.
So each cone cell is sensitive to different specific range of wavelengths.

Question 47

What is the visual acuity of cone cells?

Answer 47

Each cone cell has its own connection, so the brain receives two separate impulses from 2 cone cells.
2 separate sources of light can so be distinguished, so cone cells have good visual acuity.

Question 48

What is the distribution of rod and cone cells?

Answer 48

Light is focused by the lens on the part of the retina opposite the pupil - the fovea.
This therefore receives the highest light intensity.
So only cone cells are found at the fovea, and the concentration reduces further away.
At the edges of the retina, with lowest light intensity, only rod cells are found.

Question 49

What are the differences between rod and cone cells?

Answer 49

Rod - rod shaped, more numerous, distributed at the edges, poor visual acuity, sensitive to low light intensity, only one type.
Cone - cone shaped, less numerous, concentrated at the fovea, good visual acuity, high light intensity, three types.

Question 50

What are the divisions of the autonomic nervous system?

Answer 50

The sympathetic nervous system - stimulates effectors, and helps cope with stressful situations by heightening awareness and preparing us for activity.
The parasympathetic nervous system - inhibits effectors and slows down activity, it conserves energy and replenishes reserves.

Question 51

How are the autonomic nervous systems linked?

Answer 51

The actions of the para and sympathetic systems oppose each other - antagonistic.
If one system contracts a muscle, the other relaxes it.

Question 52

How is heart rate controlled?

Answer 52

It is myogenic, so contraction is initiated from within the muscle, rather than nervous impulses from outside.
In the wall of the right atrium is the sinoatrial node, that originates the initial stimulus and determines the beat of the heart.

Question 53

What is the sequence of heart rate?

Answer 53

A wave of electrical excitation spreads from the SAN across the atria, so they contract.
A layer of non-conductive tissue (AV septum) prevents the wave crossing to the ventricles.
The wave enters the AVN, and after a short delay, this conveys the wave between the ventricles along the Purkyne tissue which makes up the bundle of His.
This conducts the wave through the atrioventricular septum to the base of the ventricles, where the bundle branches.
The wave is released from the Purkyne tissue, so the ventricles contract quickly at the same time, upwards.

Question 54

What is the medulla oblongata?

Answer 54

The region of the brain that controls changes to the heart rate.
It has a centre that increases heart rate, linked to the SAN by the sympathetic system, and a centre that decreases, linked by the parasympathetic system.

Question 55

What are chemoreceptors?

Answer 55

Found in the wall of the carotid arteries (to the brain).
They are sensitive to pH changes in the blood, from CO2 concentration.
CO2 forms an acid and lowers pH.

Question 56

How do the chemoreceptors work?

Answer 56

Chemoreceptors detect the lower pH and increase the frequency of nervous impulses to the centre in the medulla that increases heart rate.
The centre increases the frequency of impulses via the sympathetic nervous system to the SAN, which increases the rate of electrical waves and so heart rate.
The increased blood flow leads to more carbon dioxide being removed and so concentration returns to normal.
The pH rises to normal and chemoreceptors reduce the frequency of impulse.

Question 57

What are baroreceptors?

Answer 57

In the walls of the carotid arteries and the aorta.
When blood pressure is higher, they transmit more nervous impulses to the centre in the medulla that decreases heart rate.
This sends impulses via the parasympathetic nervous system to the SAN, which decreases heart rate.

Question 58

How do pressure receptors increase blood pressure?

Answer 58

When blood pressure is lower, they transmit more nervous impulses to the centre in the medulla that increases heart rate.
This sends impulses via the sympathetic nervous system to the SAN, which increases heart rate.