response to stimuli

Question 1

what is a stimulus

Answer 1

a stimulus is a detectable change in the internal/external environment of organism that leads tp a response in the organism

Question 2

why is responding to a stimuli important

Answer 2

it increases the chance of survival for an organism

e.g. can detect and move away from harmful stimuli e.. predators and extremes of temp, or to detect and move towards a source of food

The organisms that survive have a better chance of raising offspring and of passing their alleles to the next generation - therefore there is a selection pressure favouring organisms with appropriate responses

Question 3

what detects stimuli

Answer 3

receptors which are specific to one type of stimulus

Question 4

what formulates a response to a stimuli

Answer 4

a coordinator formulates a suitable recognise to a stimulus

coordination may be at the molecular level or involve a large organ such as the brain

Question 5

what carries out a response

Answer 5

responses are carried out by an effector

the response may be at a molecular level or involve the behaviour of a whole organism

Question 6

what are hormonal responses

Answer 6

they are relatively slow process found in both plants and animals

Question 7

what is another means of communication that animals have

Answer 7

they also have the more rapid means of communication which is the nervous system

which usually have many different receptors and control effectors

Question 8

what links the receptors and effectors

Answer 8

each receptor and effectors are linked to a central coordinator of some type

the central coordinator connects information from each receptor with the appropriate effector

Question 9

what are the sequence of events involved in either chemical control/ nerve cells

Answer 9

stimulus - receptors - coordinator - effector - response

Question 10

what is a taxes

Answer 10

it is a simple response whose direction is determined by the direction of the stimulus

as a result, a motile organism respond to environmental changes moving its whole body either towards a favourable stimulus or away from an unfavourable one

Question 11

how are taxes classified

Answer 11

taxes are classified according to whether the movement is towards the stimulus - positive taxis

or away from the stimulus - a negative stimulus
and also by the nature of the stimulus

Question 12

what is phototaxis

Answer 12

when an organism moves away/ moves towards the light

this increases their chances of survival of organisms

Question 13

what is chemotaxis

Answer 13

when a species will move towards/away from an area with glucose (a nutrient)

again this will increase their chances of survival because they use glucose as a source of food

Question 14

what is kinesis

Answer 14

it is a form of response in which the organism does not move towards or away from a stimulus

Instead, it changes the speed at which it moves and the rate at which it changes direction
If an organism crosses a sharp dividing line between a favourable and an unfavourable environment, its rate or turning increases

This increases the chances of a quick return to a favourable environment

Question 15

what happens when n organism moves a considerable distance into an unfavourable environment

Answer 15

its rate or turning may slowly decreases ao that it moves in long straight lines before it turns, often very sharply

this type of response tends to bring the organic, into a new region with favourable conditions

Question 16

what is an example of kinesis

Answer 16

woodlice lose water from their bodies in dry conditions

When they move from a damp area into a dry one, they move more rapidly and change direction more often

This increases their chances of moving back into the damp area. Once back in the damp areas, they slow down and change direction less often.

This means they are more likely to stay within the damp area

However. if alter some time spent changing direction rapidly they are still in the dry area, their behaviour changes. Instead, they move rapidly in straight lines, which increases their chances of moving through the dry area and into a new damp one

In this way, they spend more time in favourable damp conditions than in less favourable drier ones
This prevents them from drying out and so increases their chances of survival

Question 17

what is tropism

Answer 17

it is the growth of a plant in response to a directional stimulus

it can be a positive response or a negative response depending on whether it is growing towards (positive) a stimulus or away from a stimulus (negative)

Question 18

what is phototropism

Answer 18

this is when the plants’ shoot grow towards or away from light
The response in both cases increases the probability that the roots will gro into the soil, where they are better able to absorb water and mineral ions

Question 19

what factors do plants grow in response to

Answer 19

plants do not have a nervous system

plants respond to
-light because light is needed for photosynthesis

• gravity- plants need to be firmly anchored in the soil. Roots are sensitive to gravity and grow in the direction of its pull (positive gravitropism)
• water- almost all plant roots grow towards water (positive hydrotropism) in order for it to be used in phototropism and other metabolic processes as well as for support

Question 20

what are plant growth factors

Answer 20

plant growth factors involve hormone-like substances or, more correctly, plant growth factors

plant growth factors are produced in small quantities e.g.plant factor indoleacetic acid IAA, which belongs to a group of substances called auxin. IAA controls plant cell elongation

Question 21

explain the control of tropism by IAA

Answer 21

tropism is the directional growth of a plant in response to a directional stimulus

In the case of light, we can observe that a young shoot will grow towards light that is directed as it from one side (unilated light). This is known as positive photropism

Question 22

explain phototropism in flowery plants

Answer 22

1. Cells in the top of the shoot produce IAA, which is then transported down the shoot
2. the IAA is initially transported evenly throughout all regions as it begins to move down at 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 on the light side
5. As IAA causes elongation of the IAA on the shaded side of the shoot, the cells on the side alongside more
6. The shaded od the shoot elongates faster than the light side, causing the shoot tip to bend towards thelight

Question 23

what does IAA do

Answer 23

IAA causes bending of roots in response to light

However whereas a high concentration of IAA increases cell elongation in shoots, it inhibits elongation in roots

e.g. an IAA concentration of 10 parts per million increases shoot elongation by 200% but decreases toot elongation by 100%

As a result, in roots, the elongation of cells is greater on the light side than on the shaded and so roots bend away from light, that is they are negatively phototropic

Question 24

describe gravitropism in flowering plants

Answer 24

1. cells in the tip of the root produce IAA, which is then transported along the root
   2) the IAA is intially transported to all sides of the root
   3) gravity influences the movement of IAA form the upper side to the lower side of the root than in the upper side
2. a greater concentration of IAA builds up on the lower side of the root than in the upper side
3. As IAA inhibits the elongation of root cells and there is a greater concentration of IAA on the lower side, the cells on this side elongates less than those in the upper side
4. The relatively greater elongation of cells on the upper side compared to the lower side causes the root to bend downwards towards the force of gravity

Question 25

in shoots the greater elongation(...)

Answer 25

the greater the concentration of IAA on the lower side cell elongation increases causing the side to elongate more than the upper side As a result, the shoot grows upwards away from the force of gravity

Question 26

how is IAA transported

Answer 26

transport if IAA is one direction, namely away from the tip of shoots and roots where it is produced

Question 27

what are the effects of IAA on plants

Answer 27

IAA has a number of effects on plant cells including increasing the plasticity (ability to stretch) of their cell walls The response only occurs on young cell walls where cells are able to elongate

Question 28

what happens as the cell matures

Answer 28

as the cells mature they develop rigity - therefore older parts of the shot / root will not be able to respond to the IAA

Question 29

what is the acid growth hypothesis

Answer 29

the proposed explanation of how IAA increases the plasticity of cells is called the acid growth hypothesis it involves the active transport of hydrogen ions from the cytoplasm into spaces in the cell wall causing the cell wall to become more plastic allowing the cell to elongate

Question 30

what can the elongation of cells on one sire of a stem or root lead to

Answer 30

it leads to the root/ stem to bend This is the means by which plants respond relatively quickly to environmental stimuli like light and gravity. These response can be explained in terms of the stimuli causing uneven distribution of IAA, as described earlier, as it moves away from the tip of the stem or root

Question 31

what is a reflex arc

Answer 31

it is the simplest type of nervous response to a stimulus is a reflex arc

Question 32

what are the two major divisions of the nervous system

Answer 32

the central nervous system (CNS), which is made up of the brain and spinal cord the peripheral nervous system (PNS), which is made up of pairs of nerves that originate from either the brain or the spinal cord

Question 33

what is the peripheral nervous system divided into

Answer 33

it is divided into: -sensory neurones -motor neurones which

Question 34

what are sensory neurones

Answer 34

carry nerve impulses (electrical signals) from receptors towards the central neurones

Question 35

what are motor neurones

Answer 35

which carry nerve impulses away from the central nervous system to effectors

Question 36

what are the subdivisions of the motor neurones

Answer 36

1. the voluntary nervous system | 2. the automatic nervous system

Question 37

what is the voluntary nervous system

Answer 37

it carries nerve impulses to body muscles and is under voluntary (conscious control)

Question 38

what is the autonomic nervous system

Answer 38

it carries nerve impulses to glands, smooth muscle and it s not under voluntary control, that is involuntary

Question 39

what is the spinal cord

Answer 39

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

Question 40

what are the features of a reflex arc

Answer 40

it is: - rapid - short - lived - localised - totally involuntary - involving response to sensory stimulus is called a reflex. The pathway of neurones involved in a reflex is known as a reflex arc

Question 41

how many neurones are involved in a reflex arc

Answer 41

3 neurones

Question 42

describe the reflex arc

Answer 42

one of the neurones is in the spinal cord and so this type of reflex is also called a spinal reflex the main stages of a spinal reflex arc is: 1. the stimulus is detected by a receptor 2. a sensory neurone passes nerve impulse to the spinal cord 3. a coordinator (intermediate neurones) -links the sensory neurone to the motor neurone in the spinal cord 4. the nerve impulse passes along a motor neurone from the spinal cord to the effector (e.g. a muscle in the upper arm) 5. the effector is stimulated to contract if it is a muscle or secrete if it is a hormone 6. the response is carried out

Question 43

why are reflex arcs important

Answer 43

any action made for survival is clearly of value -they are voluntary and therefore do not require the decision making powers of the brain, this leaving it free to carry more complex responses. In this way, this brain is not overlooked with situations in which the response is always the same - they protect the body from harm - they are fast, because the neurone pathway is short with very few typically one or two synapses - the absences of any decision - making process also means the action is rapid

Question 44

what are receptors

Answer 44

the central nervous system receives sensory information from its internal and external environment through a variety of receptors each type responding of stimulus (this is primary the function of the brain)

Question 45

what is sensory perception

Answer 45

sensory perception involves making sense of the info from the receptors

Question 46

what is the pacinian corpsucle

Answer 46

the pacinian corpuscle responds to changes in mechanical pressure As with all sensory receptors, a Pacinian corpsucle is specific to a single type of stimulus - in this case it is mechanical pressure e.g. heat, light or sound

Question 47

what is a transducer

Answer 47

receptors produces a generator potential by acting as a transducer All stimuli involves a change in some form of energy. It is the role of the tranducer to convert the change in form of energy by the stimulus into a form - namely a nerve impulses

Question 48

what are the nerve impulse in the form of

Answer 48

the nerve impulse is also a form of energy Receptors in the nervous system convert the energy of the stimulus into a nervous impulse known as a generator potential

Question 49

how do receptors function

Answer 49

i1. in its normal (resting) state, the stretch- mediated sodium channels of the membrane around the neurone of the receptor are too narrow to allow sodium ions to pass along them. In this state, the neurone of the receptor has a resting potential 2. when the stimulus is detected by the receptor, it is deformed and the membrane around its neurone becomes stretched 3. This stretching widens the sodium channels in the membrane and the sodium ions diffuse into the neurone 4. The influx of sodium ions changes the potential of the membrane (i.e. it becomes depolarised, thereby producing a generator potential 5. The generator potential in turn creates an action potential (nerve impulse) that passes along the neurone and then, via other neurones, to the central nervous system

Question 50

where are the receptor cells in the mammalian eye

Answer 50

the light receptor cells of the mammalian eye are found on its innermost layer, the retina

Question 51

what are the two main types of receptors in the retina

Answer 51

rod cells cone cells both rod and cone cells act as tranducers by conserving light energy into the electrical energy of a nerve impulse

Question 52

how are images seen in rods

Answer 52

rod cells can't distinguish different wavelengths of light and therefore lead to images being seen only in black and white

Question 53

how many rod cells are there

Answer 53

20 million in each eye

Question 54

what are rod cells connected to

Answer 54

many rod cells are connected to a single sensory neurone in the op[tic nerve

Question 55

what are rod cells used for

Answer 55

rod cells are very sensitive to light (they work well in dim light) This is because many rod cells join one single bipolar neurone and so many weak generator potentilas combine to reach the threshold and trigger and action potential

Question 56

what are cone cells

Answer 56

cone cells are three different types, each responding to a different range of wavelengths of light Depending upon the proportion of each type that is stimulated can perceive images

Question 57

how many cone cells are there in each eye

Answer 57

around 6 million cone cells often with their separate bipolar cell connected to a sensory neurone in the optic nerve

Question 58

why do cone cells respond only to high intensity

Answer 58

cone cells often have their own separate bipolar cell connected to a sensory neurone in the optic nerve This means that the stimulation of a number of cone cells cannot be combined to help exceed the threshold value and create a generator potential As a result, cone cells only respond to high light intensity and not to low light intensity

Question 59

why do rods have low visual acuity

Answer 59

visual acuity is the abiltity to tell apart points that are close together Rods give low visual acuity because many rods join the sam bipolar neurone which means light from two points close together can't be told apart

Question 60

what are the different pigment that cone cells contain

Answer 60

cone cells contain different types of pigment from that found in rod cells The pigment in cone cells (iodopsin) requires a higher light intensity for its breakdown. Only light at high intensity will involve provide enough energy to break it down and create a generator potential

Question 61

how many different types of cone cells are there

Answer 61

3 different types of cone each containing a specific type of iodopin As a result, each cone cell is sensitive to a different specific range of wavelengths these pigments are: red sensitive green sensitive blue sensitive when they are simulated in different proportions you see different colours

Question 62

how is the distribution of rod and cone cells on the retina

Answer 62

the distribution of rod and cone cells on the retina is uneven Light is focused by the lens on the part of the retina opposite the pupil. This point is known as the fovea. This fovea, therefore, receives the highest intensity of light - reason why there are cone cells, not rod cells at the fovea

Question 63

how is the concentration of cone cells distributed in the fovea

Answer 63

the concentration of 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 all this shows how the distribution of rod and cone cells, and the connections they make in the optic nerve, can explain the differences in sensitivity and visual activity in mammals

Question 64

what is the autonomic nervous system

Answer 64

the autonomic nervous system controls the involuntary (subconscious) activities of internal muscles and glands it has two divisions

Question 65

what are the two subdivisions of the nervous system

Answer 65

the sympathetic nervous system the parasympathetic nervous system

Question 66

what is the sympathetic nervous system

Answer 66

in general, this stimulates effectors and so speedup any activity It acts rather like an emergency controller, it controls effectors when exercising extraneously/ or experiencing powerful emotions In other words, it helps us to deal with stressful situations

Question 67

what is the parasympathetic nervous system

Answer 67

this inhibits effectors and so slows down any activity, it controls activities under normal resting conditions It is concerned with conserving energy and replenishing the body's reserves

Question 68

what do we mean when we say that the action of the sympathetic and parasympathetic systems are antagonist

Answer 68

they oppose each other

Question 69

what are the muscles of the heart called

Answer 69

the cardiac muscle

Question 70

what do we mean when we say that the cardiac muscles are myogenic

Answer 70

it is myogenic which means that its contractions are initiated from within the muscle itself, rather than by nervous impulses from outside (neurogenic)

Question 71

what are the sinoatrial node (SAN)

Answer 71

within the walls of the right atrium of the heart is a group of cells known as the sinoatrial (SAN) form here, the initial stimulus for contractions originates The sinoatrial node has a basic rhythm of stimulus that determines the beat of the heart hence reformed to as the pacemaker

Question 72

what is the sequence of events that controls the basic heart rate is:

Answer 72

1. a value of electrical spreads out from the sinoatrial across both atria, causing them (the atrium) to contract 2. a layer of non-conductive tissue (atrioventricular septum) prevents the wave from crossing the ventricles 3. the wave of excitation enters the second group of cells called the atrioventricular node (AVN), which lies between the atria 4. the atrioventricular node, after a short delay, conveys a wave of electrical excitation between the ventricular along with a series of specialised muscle fibres called Purkyne tissue which collectively made up a structure called the bundle of His 5. the bundle of His conducts the wave through the atrioventricular septum to the base of the ventricles where the bundle branches into smaller fibres of Purkyne tissue 6. the wave of excitation is released from the Purkyne tissue, causing the ventricles to contract quickly at the same time, from the bottom of the heart upwards

Question 73

what controls changes to the heart rate

Answer 73

changes to the heart rate are controlled by a region of the brain called the medulla oblongata this has two centres concerned with heart rate: -a centre that increases heart rate, which is linked to the sinoatrial node by the sympathetic nervous system -a centre that decreases heart rate, which is linked to the sinoatrial node by the parasympathetic nervous system

Question 74

what are chemoreceptors

Answer 74

chemoreceptors are found in the wall of the carotid arteries, the aorta and in the medulla the arteries that serve the brain) these are sensitive to changes in the pH of the blood that results from changes in carbon dioxide concentration

Question 75

what is the process of chemoreceptors control

Answer 75

1. when the blood has a higher than normal concentration its pH is lowered 2. the chemoreceptors in the wall of the carotid arteries and the aorta detects this and increase the frequency of nervous impulses to the centre in the medulla oblongata increasing heart rate 3. the centre increases the frequency of impulses vai 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 CO2 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 76

what happens when the blood pressure is higher than normal

Answer 76

the pressure receptors transmit more nerve impulses to the centre in the medulla oblongata decreasing 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 heartbeats

Question 77

what happens when the blood pressure is lower than normal

Answer 77

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

Question 78

how do receptors work

Answer 78

when a nervous system receptor is in its resting state ( not stimulated) there is a difference of charge inside and outside of the cell the inside is negatively charged relative to the outside This means there is voltage across the membrane. The voltage is also known as the potential difference. The potential difference when a cell is resting is called the resting potential The resting potential is generated by ions pumps and ion channels

Question 79

what is a generator potential

Answer 79

When a stimulus is detected, the cell membrane is excited and becomes more permeabele This allows more ions to move in and outof the cell altering the potential difference This change in dofference is due to a stimulus called the generator potential A bigger stimulus excited the membrane more, causing a bigger movement of ions ad a bigger change in potential difference - so a bigger generator potential is produces

Question 80

what is the action potenital

Answer 80

if the generator potential is big enough it will trigger an action potential which is an electrical implies that runs along a neurone Sn action potential is only triggered if the generator potential reached a certain level called the threshold level Action potentials are all one size, so the strengthofg the stimulus is measured by the frequency of action potentials ( the number of action potentials triggered during a certain period of time). If the stimulus is too weak the generator potential won's reach the threshold so there is no action potential

Question 81

what are the pacinian corpuscles

Answer 81

these are pacinian corpuscles are mechorecptors - they detect mechanical stimuli e.g. pressure and vibrations. They're found in your skin. Pacinian corpuscles contain the end of a sensory nerve ending is wrapped in loads of layers of connective tissue called lamellae

Question 82

what happens when the pacinian corpuscle is stimulated

Answer 82

When a Pacinian corpuscle is stimulated e.g. by a tap on the arm, the lamellae are deforemd and press on the sensory nerve ending. This causes the sensory neurone's cell membrane to stretch, deforming the stretch - mediated sodium ion channels The channel open and sodium ions diffuse into the cell, creating a generator potential If the generator potential reaches the threshold, it triggers an action potential