3.6.1 stimuli and response

Question 1

describe a stimulus

Answer 1

Any change in the internal or external environment e.g. change in temperature, light intensity or pressure

Question 2

what are receptors?

Answer 2

receptors are specific to one type of stimulus and detect this stimulus

Question 3

what is an effectors response?

Answer 3

Muscles and glands respond to stimuli to bring about a change

Question 4

how do animals and plants respond to the environment?

Answer 4

Animals = physically move away from harmful environments such as places that are too hot or too cold. ​

Plants = can’t actually move themselves, but they can change the way they grow in an attempt to find better environmental conditions.

Question 5

how to organisms respond to their internal environments?

Answer 5

Organisms also respond to changes in their internal environment to make sure the conditions are always optimal for their metabolism (all the reactions).

Question 6

what is a simple response?

Answer 6

these responses are automatic responses - organisms have no choice

Simple mobile (moving) organisms, e.g. woodlice have simple responses to keep them in a favourable environment.

Question 7

what are tactic responses (taxis)?

Answer 7

if an organism moves towards a stimulus it’s a positive taxis
if it moves away from a stimulus it’s a negative taxis

This is directional movement in response to a stimulus. The organism will move directly towards or away from a stimulus.​

The direction of the stimulus affects the response.

Question 8

give an example of a tactic response (euglena)

Answer 8

Euglena is a single-celled organism that has chloroplasts and lives in ponds.

It responds to light by swimming towards it has an eye spot to detect light, and a flagellum to swim.

This movement towards light is called a positive phototaxis.

Question 9

word before taxi

Answer 9

light = photo

chemicals = chemo

earth/gravity = geo

water = hydro

sound = phono

Question 10

what is a kinetic response (kinesis)?

Answer 10

This is non-directional (random) movement in response to a stimulus. ​

The intensity of the stimulus affects the response.​

Question 11

give an example of a kinetic response (woodlice)

Answer 11

Woodlice show a kinetic response to humidity. ​

In high humidity they move slowly and turn less often, so that they stay where they are. ​

As air becomes drier, they move faster and more randomly, until they move into favourable areas.

This response helps woodlice move from drier to more humid air and then stay put. This improves survival chances – reducing water loss and keeps them concealed.

Question 12

what is the role of plant hormones?

Answer 12

Plants do not have a nervous system so they respond to the environment using plant hormones = plant growth factors

plant growth factors are not made in specialized organs but in many tissues all over the plant. ​

They may be used where they are made, or transported within the plant. They usually exert their influence by affecting growth.

Question 13

types of growth

Answer 13

tropisms are directional growth responses of plants in response to a directional stimulus

If the plant growth is towards the stimulus it is said to be positive if it is away, it is negative.

Question 14

names of types of response

Answer 14

Phototropism – light- shoots ​

Geotropism – gravity - roots​

Chemotropism – chemicals – pollen grain towards ovule​

Thigmotropism – touch – creeping plants wrap around structures.

Question 15

what is a tropism?

Answer 15

gravitropism is sometimes referred to as geotropism

A tropism is the movement of part of a plant in response to, and directed by, an external stimulus. ​

The movement is by growth and is therefore slow.

Question 16

tropism table

Answer 16

light - phototropism - shoots positive, roots negative

gravity - gravitropism - roots positive - shoots negative

chemical - chemotropism - pollen tubes positive

water - hydrotropism - roots positive

oxygen - aerotropism - pollen tubes negative

Question 17

what are growth factors?

Answer 17

Plants respond to directional stimuli using specific growth factors- these are hormone-like chemicals that speed up or slow down plant growth. ​

Plant growth factors are produced by the growing regions of the plant (e.g. shoot and root tips) and they move to where they are needed in the other parts of the plant.

Different growth factors cause different types of growth eg gibberellin stimulates flowering and seed germination.

Question 18

what is auxins?

Answer 18

auxins are produced in the tips of shoots and diffuse backwards to stimulate the cells just behind the tips to elongate.​

When cells elongate the cell walls become loose and stretchy, so the cells can get longer. ​

If the tip of the shoot is removed, no auxin will be available and the shoot stops growing. ​

Auxins stimulate growth in shoots, but high concentrations inhibit growth in the roots

Question 19

IAA - an auxin

Answer 19

indoleacetic acid (IAA)

involved in many important processes, including cell elongation and phototropism.​

IAA can move by diffusion or active transport via carrier proteins, from cell to cell

This results in an uneven distribution of IAA around the plant resulting in uneven growth of the plant in certain conditions.

Question 20

what is a plants response of shoots to unilateral light?

Answer 20

1. Cells in the shoot tip detect light and produce IAA​
2. The IAA begins to move down the shoot​
3. Unilateral light causes the movement of IAA from the light to the shaded side of the shoot​
4. IAA causes greater elongation of cells on the shaded side​
5. The shaded side elongates faster than the light side, so the shoot grows towards light​
6. The opposite occurs in roots. They are negatively phototropic

Question 21

what is a plants response of a horizontally growing root to gravity?

Answer 21

1. Cells at the root tip produce IAA, which is transported along the root​
2. Gravity causes the movement of IAA from the upper to the lower side of the root​
3. IAA INHIBITS elongation of the cells on the lower side​
4. The relatively greater elongation of cells on the upper side causes the root to grow downwards, towards the force of gravity​
5. In horizontal SHOOTS IAA also accumulates on the lower side, where it INCREASES cell elongation, and causes growth upwards, away from the force of gravity

Question 22

what do plants respond to?

Answer 22

1. light = shoots grow towards light because plants need light for photosynthesis
2. gravity = plants need to be firmly anchored into the soil. roots are sensitive to gravity and grow in the direction of its pull
3. water = plant roots grow towards water in order to absorb it to use in photosynthesis

Question 23

describe nervous control

Answer 23

there are two major systems involved with communication between the nervous + endocrine systems

provides the quickest mean of communication

divided into central nervous system (brain + spinal cord) and the peripheral nervous system which links the CNS with the body’s receptors + effectors

Question 24

what is a reflex action?

Answer 24

A reflex action is a rapid, involuntary, automatic response to a sensory stimulus. ​

Stimulation of sensory receptors results in generation of impulses in sensory neurones.​

Impulses travel to the CNS > sensory neurones > intermediate neurones > motor neurones > effectors ​

Within the spinal cord, the impulse will also be passed to other neurones which carry it to the brain.

Question 25

what is the pathway taken by an electrical impulse during a reflex action (reflex arc)?

Answer 25

A stimulus is detected by a receptor cell, which synapses with a sensory neurone. ​

The sensory neurone carries the impulse from site of the stimulus to an intermediate neurone (also called interneurone or relay neurone) in the central nervous system. ​

The intermediate neurone synapses with a motor neurone, which carries the nerve impulse out to an effector, such as a muscle, which responds by contracting.

Question 26

what is the importance of reflex arcs/actions?

Answer 26

Involuntary - do not require decision making leaving the brain free to carry out more complex responses​

Protection from unexpected harmful stimuli​

Fast – neurone pathway is short with very few synapses and no decision making is required. Important in withdrawal reflexes​

Inate – don’t need to be learned​

Control many physiological and homeostatic functions – e.g. pupil reflex, blinking, coughing

Question 27

functional classification of neurones
1. sensory neurones

Answer 27

Conduct impulses towards the CNS from sensory receptors.

Impulses travel from receptors to cell bodies occurring in clusters called ganglia next to the spinal cord. Axons then carry the impulse to the CNS.

Question 28

functional classification of neurones
2. motor neurones

Answer 28

Conduct impulses away from the CNS to effectors (muscles and glands).

Cell bodies are located in the CNS. Impulses travel along axons to effectors.

Question 29

functional classification of neurones
3. intermediate/relay neurones

Answer 29

Receive impulses from sensory or other relay neurones. They relay impulses to motor neurones or other relay neurones. Situated mainly in CNS.

Question 30

what can the motor nervous system be subdivided into?

Answer 30

voluntary nervous system = carries nerve impulses to bodily muscles and is under voluntary (conscious) control

autonomic nervous system = carries nerve impulses to glands, smooth muscle + cardiac muscle and is not under voluntary control
subconscious and involuntary

Question 31

what is a spinal cord?

Answer 31

spinal cord is a column of nervous tissue that runs along the back and lies inside the vertebral column for protection

emerging at intervals along the spinal cord are pairs of nerves

Question 32

describe the process of a reflex arc involved in the withdrawal of a hand from a heat stimulus

Answer 32

1. stimulus eg heat from a candle
2. receptor eg nerve ending in skin sensitive to heat
3. sensory neurone eg passes nerve impulses to spinal cord
4. intermediate neurone eg passes impulses across
5. motor neurone eg passes impulses to the muscle
6. effector contracts
7. response eg hand is moved quickly away from flame

Question 33

describe receptors

Answer 33

The CNS receives sensory information about its internal and external environment via RECEPTORS

Each type responds to a specific stimulus

They all convert a particular form of energy into electrical energy (nerve impulse/action potential)

Question 34

what is sensory reception?

Answer 34

the function of our receptors and their individual responses to different types of stimuli.

Question 35

what is sensory perception?

Answer 35

Making sense of the information from the receptors, largely a function of the brain.

Question 36

what do all sensory receptions have in common?

Answer 36

Are specific to one type of stimulus

Produce a generator potential by acting as a transducer

Transduce a change in some form of energy (the stimulus) such as heat, light, sound or mechanical energy, into the energy of a generator (receptor) potential, which if large enough, can lead to an action potential (nerve impulse)

Question 37

what is a resting potential?

Answer 37

When a nervous system receptor is in its resting state (not being stimulated), there is a difference in charge between the inside and the outside of the cell membrane.

The inside is negatively charged relative to its outside.

This means there is a voltage or potential difference across the membrane.

The potential difference when a cell is resting is called the resting potential.

The resting potential is generated by ion pumps and ion channels.

Question 38

what is a generator’s potential?

Answer 38

When a stimulus is detected, the cell membrane is excited and becomes more permeable, allowing positive ions to move into the cell- making it less negative inside.

The change in the potential difference due to a stimulus is called DEPOLARISATION and is known as a GENERATOR OR RECEPTOR POTENTIAL.

A bigger stimulus excites the membrane more, causing a bigger movement of ions and so a greater change in the potential difference.

Question 39

what is an action potential?

Answer 39

If the generator potential is big enough it will trigger an action potential – an electrical impulse along a neurone.

An action potential is only triggered if the generator potential reaches a certain level called the threshold level.

Action potentials are all one size, so the strength of a stimulus is measured by the frequency of action potentials (the number of action potentials in a time).

If the stimulus is too weak the generator potential won’t reach the threshold, so there is no action potential.

Question 40

what is a pacinian corpuscle (receptor)?

Answer 40

Specific to a single type of stimulus.

Responds only to mechanical pressure. It will not respond to other stimuli, such as heat, light or sound.

Produces a generator potential by acting as a transducer (converts energy from one type to another).

Converts energy picked up by the receptors into a generator potential, leading to a nerve impulse.

Question 41

what is the function of a pacinian corpuscle?

Answer 41

Occur deep in the skin and are most abundant on the fingers, the soles of the feet and the external genitalia.

Also occur in joints, ligaments and tendons, where they enable the organism to know which joints are changing direction.

Question 42

what is the structure of a Pacinian corpuscle?

Answer 42

Single sensory neurone is at the centre of layers of tissue, each separated by a gel.

Looks like an onion cut vertically.

plasma membrane:
contains proteins
some have channels through which ions can be transported. Some are specific to one type of ion, e.g. Sodium channels.

Question 43

what neurone is centre of the pacinian corpuscle?

Answer 43

Sensory neurone at the centre of thePC has a special type of sodium ion channel called
stretch-mediated sodium ion channels

Their permeability to sodium changes when they are deformed by e.g. stretching

Question 44

PC functions

Answer 44

1. In a normal (resting) state, the stretch mediated sodium channels of the membrane around the neurone of a Pacinian corpuscle are too narrow to allow sodium ions to pass through them. In this state, the neurone of the PC has a resting potential.
2. When pressure is applied to the PC, it changes shape and the membrane around the neurone becomes stretched.
3. This stretching deforms/widens the sodium channels in the membrane and sodium ions diffuse into the neurone.
4. The influx of sodium ions changes the potential of the membrane , depolarising it, producing a generator potential.
5. If the generator potential reaches a particular size (called the threshold), then an action potential (nerve impulse) is triggered. This passes along the neurone and then, via other neurones, to the central nervous system.
6. The greater the pressure, the more frequent the action potentials

Question 45

what structures are present in the eye?

Answer 45

1. Control the amount of light that enters = iris
2. Refract light to focus it = cornea and lens
3. Transduce light energy to the electrical energy of a nerve impulse = rods and cones

Light is a form of electromagnetic radiation. It is emitted and absorbed in discrete packets called photons in the wavelength range 400 – 700nm.

Question 46

what does the retina contain?

Answer 46

Contains photoreceptor cells (rods and cones) and associated bipolar neurones/cells, which convey information to the optic nerve.

Energy in light is converted to energy in action potentials, which are carried along the optic nerve to the brain.

Note that light has to pass through these cell layers (which are transparent) before reaching the rods and cones

Question 47

summarise rods and cones

Answer 47

The human retina contains about 125 million rod cells and about 6 million cone cells.

Rods = more sensitive to light but cannot distinguish colour as they contain only one kind of pigment and cannot distinguish different wavelengths. They are useful in dim light. They are found in greatest density peripherally and are absent from the fovea.

Cones = less sensitive to light but can distinguish colour in daylight. Sharpest daylight vision is achieved by looking directly at an object as cones are most dense in the fovea which is the centre of the field of vision.

Question 48

what is the structure of rods and cones?

Answer 48

Rods and cones are modified neurones.

Detection of light is carried out on the stacks of membranes in the outer segment, formed from invaginations of the plasma membrane.

The discs provide a large surface area of visual pigment. Contain molecules of the visual pigment rhodopsin (photoreceptor molecule).

three types of visual pigment – collectively called iodopsin.

The inner and outer segments of rods and cones are connected by a narrow region which contains microtubules.

Question 49

what are the segments of rods and cones?

Answer 49

In both RODS and CONES, the visual pigments break down when LIGHT hits them.

OUTER SEGMENT – visual pigment in membrane invaginations

INNER SEGMENT – contains cell organelles e.g. mitochondria, RER, nucleus.

SYNAPTIC BULB – contains vesicles with neurotransmitter.

Question 50

what is the formation of an action potential?

Answer 50

When visual pigments are broken down by light, a generator potential is created which may result in action potentials being sent to the brain along the OPTIC NERVE

Rod & Cone cells are TRANSDUCERS – transfer LIGHT energy into electrical energy in ACTION POTENTIALS.

Question 51

how are rods stimulated?

Answer 51

Rhodopsin is the visual pigment found in rods.

It consists of a light-absorbing molecule (retinal – a vitamin A derivative), bound to a membrane protein (opsin)

Question 52

how does rhodopsin create a generator potential?

Answer 52

1. To create a generator potential, rhodopsin must be broken down. Low intensity light is sufficient to do this, which is why rod cells respond to low level light.
2. When rhodopsin changes shape it breaks down into opsin and retinal. This leads to a change in permeability of the rod cell membrane (generator potential). If the threshold is reached, this can result eventually in impulses being sent along the optic nerve to the brain via bipolar cells.
3. Opsin and retinal recombine to form rhodopsin which can be used again.

Question 53

how is rhodopsin adapted?

Answer 53

dark adaptation

In bright light, most of the rhodopsin molecules break down = bleaching .

rod cells are not useful in bright light. If we move into dim light, regeneration of rhodopsin takes time and our ability to see gradually increases over several minutes.

Question 54

rod cells: high sensitivity

Answer 54

Many rods share one bipolar neurone (retinal convergence) so they can respond to low level light intensity.

This is because the combined generator potentials of many rods make it more likely that the threshold value will be exceeded to create an action potential in a bipolar cell (summation).

Question 55

rod cells: low visual acuity

Answer 55

rod cells share one bipolar neurone, only one impulse will be produced, regardless of which of these rods are stimulated.

Two dots close together will therefore appear as a single blob - low visual acuity

Question 56

what are cone cells?

Answer 56

There are 3 types of Cone cell – each contains a DIFFERENT TYPE of pigment called IODOPSIN.

Usually have their own separate bipolar neurones connected to a sensory neurone.

Each type responds to a different range of wavelengths of light

Question 57

cone cells and colour vision - trichromatic theory

Answer 57

1. Iodopsin B – absorbs blue light most strongly.
2. Iodopsin G – absorbs green light most strongly.
3. Iodopsin R – absorbs longer wavelengths including red light.

The brain interprets the colour of an object by comparing the relative frequency of the impulses from the 3 types of cone. All colours of the visible spectrum can be seen.

Question 58

cone cells: low sensitivity

Answer 58

cone cells require more light before the pigment breaks down.

stimulation of cone cells cannot be combined to help exceed the threshold value to create an action potential (as they each only stimulate one bipolar neurone). As a result, they can only respond in high light intensities which is why we can’t see colour in dim light.

Question 59

cone cells: high visual acuity

Answer 59

Each cone sends its own impulse via a bipolar cell to the brain so the brain receives separate impulses from all of the cones, enabling it to distinguish between the separate sources of light.

Two dots close together will therefore appear as two dots = HIGH VISUAL ACUITY.

Question 60

what is a fovea/macula?

Answer 60

receives highest intensity of light

Where NO rods are present at all and where the CONE cells are most tightly packed.

This is the part of the retina where light is focussed when we look directly at an object, and from where the brain gets the most detailed information which it uses to form an image.

The concentration of the cone cells diminishes further away from the fovea .

At the peripheries of the retina, where light intensity is at its lowest, only rod cells are found.

Question 61

rods summary

Answer 61

Outer segment is rod shaped

More cells per eye, distributed throughout the retina, so used for peripheral vision.

Good sensitivity

Only 1 type = monochromatic vision

Many rods connected to one bipolar cell = poor acuity = poor resolution

Question 62

cones summary

Answer 62

Outer segment is cone shaped

Less cells per eye, found mainly in the fovea, so can only detect images in centre of retina.

Poor sensitivity

3 types (R, G & B) = colour vision

Each cone is connected to one bipolar cell = good acuity = good resolution

Question 63

how is the cardiac cycle controlled?

Answer 63

the heart initiates its own electrical activity automatically at the sinoatrial node - SAN

myogenic

basic rhythm is initiated by the SAN, but the rate this happens can be altered by the brain

Question 64

describe the autonomic nervous system

Answer 64

self governing = controls the involuntary activities of internal muscles and glands

opposing antagonistic

activities of internal glands + muscles

Question 65

what is the effect of sympathetic stimulation?

Answer 65

heart = increases rate and force of contraction

eye = pupil dilates, ciliary muscle relaxes
lens thin for distant vision

digestive system = little/no effect on glands
sphincter muscles contract and liver releases glucose

skin = sweat glands release more sweat
hair erector muscles make hair stand up
arterioles constrict

Question 66

what is the effect of parasympathetic stimulation?

Answer 66

heart = reduces rate and force of contraction

eye = pupil constricts
ciliary muscle constructs - lens become thick for near vision

digestive system = stimulates secretion from glands
sphincter muscles relax
liver increase glycogen production slightly

skin = sweat glands in general not affected
hair erector muscles not affected
arterioles not affected

Question 67

how is heart rate modified by the brain (control of heart rate)?

Answer 67

cardio regulatory centre (cardiac control centre) is found in the medulla oblongata of the brain

1. acceleratory centre = increases HR by sending impulses via sympathetic neurones to the SAN
2. inhibitory centre = slows HR by sending impulses via parasympathetic neurones to the SAN

BOTH LOCATED IN THE AORTA AND CAROTID ARTERIES

Question 68

how is heart rate controlled by chemoreceptors?

Answer 68

chemoreceptors are found in the wall of carotid arteries

sensitive to changes in pH of blood that result from changes in carbon dioxide concentration

in solution, CO2 forms an acid and lowers pH

Question 69

describe the process of chemoreceptors

Answer 69

1. When the blood has a higher than normal concentration of carbon dioxide, its pH is lowered.
2. The chemoreceptors in the wall of the carotid arteries and the aorta detect this and increase the frequency of nervous impulses to the centre in the medulla oblongata that increases heart rate.
3. This centre increases the frequency of impulses via the sympathetic nervous system to the sinoatrial node. This, in turn, increases the rate of production of electrical waves by the sinoatrial node and therefore increases the heart rate.
4. The increased blood flow that this causes leads to more carbon dioxide being removed by the lungs and so the carbon dioxide concentration of the blood returns to normal.
5. As a consequence the pH of the blood rises to normal and the chemoreceptors in the wall of the carotid arteries and aorta reduce the frequency of nerve impulses to the medulla oblongata.
6. The medulla oblongata reduces the frequency of impulses to the sinoatrial node, which therefore leads to a reduction in the heart rate

Question 70

how is heart rate controlled by pressure receptors?

Answer 70

When blood pressure is higher than normal, pressure receptors transmit more nervous impulses to the centre in the medulla oblongata that decreases heart rate. This centre sends impulses via the parasympathetic nervous system to the sinoatrial node of the heart, which leads to a decrease in the rate at which the heart beats.

When blood pressure is lower than normal, pressure receptors transmit more nervous impulses to the centre in the medulla oblongata that increases heart rate. This centre sends impulses via the sympathetic nervous system to the sinoatrial node, which increases the rate at which the heart beats.

Question 71

what is required practical 10?

Answer 71

investigating simple animal responses

Question 72

How can a choice chamber be used to measure the favourable environment of a small organism?

Answer 72

By setting up chambers in different quadrants with different environmental conditions: dark + dry, dark + damp, light + dry, light + damp

Organisms will move to the quadrant they find favourable

Question 73

what factors must be controlled when repeating the experiment?

Answer 73

number of animals
environmental conditions
time allowed for animals to choose

Question 74

which statistical test is used to analyse the results of this practical and why?

Answer 74

Chi squared test

It compares the expected and observed values and tests if there is a significant difference

Question 75

what is the conclusion drawn if the calculated value is greater than the critical value?

Answer 75

Null hypothesis is rejected.

Less than 5% probability that the difference is due to chance alone.

There is a statistically
significant difference between the expected and observed values.

Question 76

what do animals do when they are in unfavourable environments?

Answer 76

they move faster and change direction more frequently to increase their chances of survival

Question 77

state the hazard and precaution involved in this practical

Answer 77

the live organisms used are a biohazard

wash hands after handling