9 & 10. Response to Stimuli and Coordination

Question 1

Suggest why, during a period of intensive exercise, the metabolic rate of a nerve cell increases

Answer 1

• More nerve impulses means more Na needs to be removed from the axon
• Active transport of Na requires ATP

Question 2

(Two neurones- one secretes acetyl choline (A), the other secretes GABA (B) - a substance that increases a membranes permeability to chloride ions- both form synapses with a neurone C) Suggest now impulses in neurone B would reduce the chances of an action potential in neurone C

Answer 2

• More negative inside/ hyper polarisation

- Requires more stimulation from acetylcholine/ Na/ more impulses

Question 3

Explain the importance of a nocturnal earthworm moving towards (taxis) from darkness to low light intensities

Answer 3

Allows the worm to come to the surface at night

Question 4

If the worm is touched, it responds by contracting all its segments rapidly. How can this movement be related to the fact that the nerve cord of a worm contains a number of axons which have a very large diameter?

Answer 4

The larger the diameter of the axon, the faster the rate of conduction of a nervous impulse

Question 5

Worms were cultured at three different temperatures with food. They were then put on a plate with a temperature gradient on it. The worms moved towards the temperature at which they had been cultured.

Suggest an explanation for the worms’ behaviour on the plate in the absence of food.

Answer 5

• Hungry so seeking food/ in absence of food respond to temperature
• Move toward temperature they were used to/ cultured in
• Associate this temperature with food
• (Then) stay in this temperature

Question 6

Worms were cultured at three different temperatures with food. They were then put on a plate with a temperature gradient on it. The worms moved towards the temperature at which they had been cultured.

The biologist exposed the plates to dim, even light. Give two reasons why this was important.

Answer 6

• (Dim) Worms live in dark soil/ affected by bright light/ dim light is like normal environment
• (Even) Worms might move towards/ away from bright light/ to avoid creating a light gradient/ prevent worms showing phototaxis
• (Dim light) Ensures that heat from the light not a variable/heat from lamp could kill/ dry out worms

Question 7

When treating patients with dementia, doctors recorded the amount of myelin in the brain for three types of dementia. They used a statistical test to compare the results for AD and LD. They obtained a value for P of 0.047. What does this show about the difference between the means of LD and AD? Use the words probability and chance in your answer.

Answer 7

• Probability of obtaining this difference by chance
• Is less than 5%
• Difference is significant

Allow converse for over 95%

Question 8

*Graph shows myelin levels 
Without dementia - 52 units
With VaD - 25 units
With AD- 35 units 
With LD- 41 units... None of the SE values overlap*

A student concluded that there was a relationship between the amount of myelin in a person’s brain and whether or not they had dementia. Do these data support the conclusion?

Answer 8

• All dementia groups lower than control group
• Error bars do not overlap so the differences are significant
• Dementia may be due to other factors/ not only due to lack of myelin
• (Because) Big/ significant differences in myelin in different dementias
• Only small sample sizes/ only one study/ more data required

Question 9

What is a stimulus and how are they detected?

How does the ability to respond to a stimuli increase the chances of survival for an organism?

Answer 9

A stimulus is a detectable change in the internal or external environment of an organism that produces a response.

They are detected through receptors.

Detecting the stimuli and responding increases the chance of the organism surviving to pass on their alleles and successfully reproduce if the stimulus is a predator, so they can get away from the predator, or extreme temperature so they will not overheat/ get hypothermia etc…

Question 10

How do receptors work as a transducer?

Answer 10

They convert the energy of a stimulus into an electrical impulse that can then be processed by the organism and a response can occur.

Question 11

Give examples of effectors.

Answer 11

-Cells
-Tissues
-Muscles
-Organs
And Systems

Question 12

Compare the hormonal system and the nervous system in terms of

• Method of travel
• Type of transmission
• Origin of each
• Duration of effect caused by hormone/ impulse
• Where the effects of each are seen/ target organs

Answer 12

• Hormones travel through the bloodstream in the plasma whereas nerve impulses travel through the nervous system (neurones and synapses
• Nerves travel as electrical impulses whereas the hormonal system operates through hormones (proteins/ peptides and steroids)
• Hormones are secreted from glands whereas nerve impulses originate at the receptors where a stimulus is converted into an electrical impulse
• The nervous system produces short lived effects whereas the effects caused by the hormonal system are more long lived
• Nervous system a localised effect whereas the hormonal system has effects on any organ that contains the right receptor for the hormone- a more wide-spread response because it travels through the bloodstream.

Question 13

Describe the sequence that occurs from detecting a stimulus to an action occurring (nervous system).

Answer 13

Stimulus –> receptor –> coordinator –> effector –> response

Question 14

What is a taxis? What is a positive taxis vs a negative taxis?

Answer 14

A response in which the direction is determined by the direction of the stimulus (directional response).

Motile organism moves its whole body either away from an unfavourable stimulus (negative taxis) or towards a favourable one (positive taxis).

Question 15

What are the two types of taxis?
Hint: not positive and negative

Give examples.

Answer 15

• Chemotaxis- bacteria move towards a region where glucose is more highly concentrated- increase their chances of survival as they get more food
• Phototaxis- Eg single celled algae move towards the light as they are photosynthetic and a greater light intensity increases their chances of food manufacture and therefore survival

Question 16

What is a kinesis?

What is the purpose of a kinesis?

Answer 16

A non directional response where the increase in random movement increases with the intensity of the stimulus.

The purpose is to bring the organism back into more favourable conditions.

Question 17

What is a tropism and what are the types of tropism(use examples)?

Answer 17

It is a growth movement of a plant in response to a directional stimulus.

• Phototropism- eg plants shoots growing towards light and plants roots growing away from light
• Geotropism- plants shoots grow away from the direction of gravity and plant roots grow towards the direction of gravity
• Hydrotropism- plant roots go towards water so that root systems develop where there is most water

Question 18

What are the two major divisions of the nervous system? What are they made up from?

Answer 18

• Central nervous system - brain and spinal cord

- Peripheral nervous system - made up of pairs of nerves that originate from either the brain or the spinal cord.

Question 19

How is the peripheral nervous system divided?

Answer 19

• Sensory neurones- nerve impulses from receptors towards the central nervous system
• Motor neurones- carry impulses away from the central nervous system to the effectors

Question 20

What are the two subdivisions of the motor nervous system?

Answer 20

• Voluntary nervous system- carries nerve impulses to the body muscles and is under conscious control
• Autonomic nervous system- carries nerve impulses to glands, smooth muscle and cardiac muscle and is not under conscious control.

Question 21

Describe the structure of the spinal cord.

Answer 21

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 22

Describe the chain of events in a reflex arc for the example of withdrawing our hand from a hot flame.

Answer 22

• Stimulus- heat from the hot object
• Receptor- temperature receptors in our skin create nerve impulses in the sensory neurone
• Intermediate neurone (relay neurone) links the sensory neurone to the motor neurone in the grey matter of the spinal cord.
• Motor neurone- carries nerve impulse from the spinal cord to the effector muscle
• Effector, in this case a muscle is stimulated to contract
• Response- hand pulling away from the hot flame

Question 23

What are the advantages of reflex actions?

Answer 23

• They do not require the brain as they are involuntary so they leave the brain free for more complex responses- therefore the brain is not overloaded with situations in which the response is always the same.
• Protect the body from harmful stimuli- they are not learned, they are innate responses
• They are FAST - pathways are very short with very few synapses (the slowest part because chemical transmission is slower than electrical transmission)

Question 24

What are the two subdivisions of the autonomic nervous system? How are they antagonistic?

Answer 24

• Sympathetic nervous system- stimulates effectors and so speeds up an activity (controls activities in eg strenuous exercise or powerful emotions).
• Parasympathetic nervous system- inhibits effectors and so slows down activity (controls activities under normal resting conditions).

They are antagonistic as their effects usually oppose one another.

Question 25

Describe how the heart is controlled by chemoreceptors

Answer 25

• High conc of CO2 in blood reduces the pH
• Chemoreceptors in the walls of the carotid arteries and aorta detect this and increase the frequency of nervous impulses to the centre in the medulla oblongata that increases heart rate
• This centre increases the freq of impulses via the sympathetic nervous system to the SAN which increases the heart rate
• Increased blood flow causes more carbon dioxide to be removed via the lungs
• CO2 level of blood returns to normal
• Chemoreceptors in the walls of the carotid arteries and aorta reduce the freq of impulses to medulla oblongata
• Medulla oblongata reduces freq on impulses to SAN so heart rate reduced to normal level

Question 26

Describe how the heart rate is controlled by pressure receptors when the blood pressure is higher than normal or lower than normal.

Answer 26

• Pressure receptors are in the walls of the carotid arteries and aorta
• When blood pressure is too high- transmit a nervous impulse to centre in medulla oblongata that reduces heart rate. This centre sends impulses through the parasympathetic nervous system to the SAN which decreases the heart rate
• When blood pressure is too low- nervous impulses to the centre of the medulla oblongata that increases heart rate. This centre sends impulses via the sympathetic nervous system to the SAN which increases the heart rate.

Question 27

What are the features of sensory reception as demonstrated by a pacinian corpuscle?

Answer 27

• Specific to a single type of stimulus- in this case, only responding to mechanical pressure
• Produces a generator potential by acting as a transducer- mechanical energy traduced into electrical energy

Question 28

Describe the structure and function of a pacinian corpuscle.

Answer 28

• Respond to mechanical stimulation eg pressure
• Most abundant in soles of feet, fingers and external genitalia
• Sensory neurone in the centre of many layers separated by gel
• Mechanical pressure opens stretch gated sodium ion channels

Question 29

What are the two types of light receptor in the eye and describe their structure, location and what they do, what type of light they respond to, what kind of visual acuity does each give.

Answer 29

-Rod cells
>Many rod cells sharing a single sensory neurone
>Allow us to see in black and white
>Allow us to see in low light intensities
>Concentrated at the periphery of the eye
>Low visual acuity

-Cone cells
>Each cone cell attached to its own sensory neurone
>3 types (red, green and blue) allow us to see in colour
>Allows us to see in high light intensities
>Concentrated at the fovea
>Gives high visual acuity

Question 30

What is it called when many rod cells are attached to a single bipolar cell?

Answer 30

Retinal convergence

Question 31

Why can rod cells allow us to see in low light intensities and what is the pigment that is associated with rod cells?

Answer 31

• Many rod cells attached to one sensory neurone so retinal convergence occurs and there is a greater chance of the stimulus reaching a threshold value and creating a generator potential
• Low light intensity is sufficient to break down RHODOPSIN and create a generator potential.

Question 32

Why can cone cells allow us to see in high light intensities (with high visual acuity) and what is the pigment associated with cone cells?

Answer 32

• Each cone cell attached to its own sensory neurone so if two adjacent cells are stimulated, the brain gets two separate impulses and can distinguish between them and as they only respond to high light intensities, we can only see in high light intensities with cone cells
• IODOPSIN requires a higher light intensity to be broken down

Question 33

How does the nervous system stimulate their target cells?

Answer 33

By secreting neurotransmitters directly onto them

Question 34

How does the hormonal system stimulate their target cells?

Answer 34

By the hormones attaching to complimentary shaped receptors on the membrane of the target cells.

Question 35

What is a chemical mediator?

Answer 35

Chemicals released from certain mammalian cells and have an effect on cells in their immediate vicinity, typically released by injured or infected cells.

Question 36

Describe two chemical mediators:

• where they are released from
• why they are released
• what their release causes to occur
• effects of being released

Answer 36

-Histamine
>stored in certain white blood cells
>released following injury or exposure to a certain allergen
>causes dilation of small arteries and arterioles and increased permeability of capillaries
>leading to localised swelling, redness and itching

-Prostaglandins
>found in cell membranes
>released following injury
>causes dilation of small arteries and arterioles and increases the permeability of capillaries
>affect blood pressure and neurotransmitters (in doing so they affect pain sensation)

Question 37

What do plant growth factors do? Where are they made? What do they affect?

Answer 37

• Exert their influence by affecting growth
• Made by cells located throughout the plant, rather than in particular organs
• Unlike animal hormones, some plant growth factors affect the tissues that release them rather than acting on a distant target organ

Question 38

What is indolacetic acid (IAA) and what does it cause plant cells to do?

Answer 38

It is a plant growth factor made in small quantities that causes plant cells to elongate, among other things.

Question 39

Describe the sequence of events that leads to a plant shoot to bend to wards the light.

Answer 39

• IAA produced in the tip of the shoot is transported down the shoot
• IAA is inititially trnasported to all sides as it is transported down the shoot
• Light causes the IAA to move from the light side to the shaded side of the shoot
• Greater conc of IAA builds up on the shaded side of the shoot rather than the light side
• Greater conc of IAA on the shaded side causes the cells on this side to elongate more
• Shaded side of the shoot grows faster and the shoot bends towards the light.

Question 40

How does IAA control the bending of roots in response to gravity?

Answer 40

A high concentration of IAA in the roots decreases growth in the roots. Gravity causes the IAA to gather on the ‘bottom’ side of the root (side closest to the direction of gravity’s ‘pull’) and decreases growth on this side, causing the other side to have a greater rate of growth/ elongation and causing the root to bend downwards in the direction of gravity.

Question 41

In a mammalian neurone, what is the structure and function of

• cell body
• dendrons
• axon
• schwann cells
• myelin sheath
• nodes of Ranvier

Answer 41

• Cell body contains the nucleus and lots of rough ER, associated with the production of neurotransmitters and proteins
• Dendrons are small extentions of the cell body which subdivide into smaller branched fibres called dendrites- they carry nerve impulses TOWARDS THE CELL BODY
• Axon is a single long fibre that carries nerve impulses AWAY FROM THE CELL BODY
• Schwann cells- surround the axon and protect it/ provide electrical insulation. They carry out PHAGOCYTOSIS and play a part in nerve REGENERATION.They wrap themselves around many times so layers of their membranes build up around the axon.
• Myelin shealth made up of the membranes of the Schwann cells and are rich in a lipid called MYELIN and cause nerve impulses to be transmitted more rapidly than if there was no myelin
• Nodes of Ranvier- gaps between schwann cells where there is no myelin sheath

Question 42

Describe the function of a sensory neurone.

Answer 42

Carries nerve impulses from a receptor to an intermediate or motor neurone. They have one dendron that carries the impulse towards the cell body and one axon that carries it away from the cell body.

Question 43

Describe the function of a relay neurone.

Answer 43

Transmit impulses between neurones. They have numerous short processes

Question 44

Describe the function of a motor neurone.

Answer 44

Transmit nerve impulses from a relay neurone or sensory neurone to an effector. They have a long axon and many short dendrites.

Question 45

What is a nerve impulse?

Answer 45

A self propagating wave of electrical disturbance that travels along the surface of an axon membrane

Question 46

How is a resting potential maintained?

Answer 46

Most sodium gated channels are closed whereas most potassium gated channels are open- the membrane has a greater permeability to K+ ions.
Sodium potassium pump maintains a gradient of more Na+ on the outside of the axon by pumping 3 ions out whilst pumping 2 K+ ions in so that there are more K+ ions inside the axon.

Therefore the axon is negatively charged on the inside with respect to the outside which is positive.

Question 47

What is the resting potential of a neurone?

Answer 47

-65mV

Question 48

When the neurone is at resting potential, what is its condition called?

Answer 48

Polarised

Question 49

How is a potential difference established between the outside of the axon and the inside of the axon?

Answer 49

• Na+ actively transported out (NaK pump)
• K+ actively transported in (NaK pump)
• Greater transport of Na than K (3:2)
• Outward movement of Na is greater than inward movement of K
• Na begins to naturally diffuse back into the axon down its chemical gradient
• Most of the gated channels for K are open whereas most of the gates for Na are closed so K diffuse out of the axon faster than Na diffused in which further increases the potential difference
• There is also an electrical gradient- as more K diffuses out of the axon, further outward movement becomes difficult because they are attracted to the overall negative charge in the axon and repelled by the positive charge outside the axon which prevents them moving out of the axon.
• EQUILIBRIUM is established in which the electrical and chemical gradients are balanced and there is no net movement of ions

Question 50

What is an action potential?

Answer 50

A temporary reversal of charge of the axon membrane in which the negative charge of -65mV is switched to +40mV- in this condition the membrane is said to be depolarised.

This occurs when voltage gated channels open or close depending on the voltage across the membrane

Question 51

Describe the sequence from resting potential to repolarisation.

Answer 51

• Resting potential maintained by increased permeability to K and decreased permeability to Na & the KNa pump
• Energy of the stimulus causes some Na voltage gated channels to open- Na diffuses in down an electrochemical gradient, being positively charged, they trigger a reversal in membrane potential (pd)
• As Na diffuse in, they cause more influx of Na as more Na channels open
• Once an action potential of +40mV has been established, voltage gated sodium ion channels close and voltage gated K channels open.
• With some K gates open, the electrical gradient that was preventing outward movement is now reversed, causing more K to diffuse out, causing repolarisation of the axon
• Outward diffusion of K causes a temporary overshoot of the electrical gradient- hyperpolarisation- gates on K channels close and the KNa pump repolarises the axon.

Question 52

Describe the passage of an action potential along a myelinated neurone vs a non myelinated neurone.

Answer 52

Myelin acts as an electrical insulator, preventing action potentials occurring. Localised circuits arise between adjacent nodes of Ranvier and the action potentials ‘jump’ from node to node in what is known as SALTATORY CONDUCTION.

In non myelinated neurones, depolarisation occurs along the whole length of the axon, making the transmission of impulses slower.

Question 53

What factors affect the speed of an action potential?

Answer 53

• Myelin sheath- saltatory conduction makes nervous transmission more rapid
• Diameter of the axon- Greater diameter of axon means impulses travel faster as there is less ion leakage from the large axon (leakage makes membrane potentials harder to maintain

-Temperature- higher temperatures mean a faster rate of diffusion so the a faster transmission of a nerve impulse
>Energy for active transport comes from respiration which involves enzymes- higher temperatures means enzymes are working at a faster rate
>Temperatures that are too high mean no nervous transmission at all because the enzymes have been denatured

Question 54

What is a refractory period and what three purposes does it serve?

Answer 54

It is the period after an action potential where inward movement of sodium ions is prevented because the sodium voltage gated channels are closed so any further action potential cannot be generated in this period

• Ensures the action potential is propagated in ONE DIRECTION ONLY as an action potential can only pass from an active region to a resting region. An action potential cannot be propagated in a region that is refractory so therefore it can only move in a forward direction
• Produces DISCRETE IMPULSES- due to the refractory period, a new action potential cannot be formed directly behind the other so action potentials are separated from one another
• It LIMITS THE NUMBER of action potentials- as action potentials are separated from one another, this limits the number of action potentials that can pass along the axon in a given time.

Question 55

What is the effect on stimulus intensity to impulse frequency?

Answer 55

As the intensity of the stimulus increases (provided it exceeds the threshold value) the action potentials are more frequent (although action potentials are all the same size)

Question 56

Why are nerve impulses described as ‘all or nothing’ responses?

Answer 56

There is a certain level of stimulus called the threshold value which triggers an action potential- below the threshold value, there is no action potential

Any stimulus above the threshold will generate an action potential, no matter how much above the threshold it is, it will only generate one action potential.

Question 57

Given the all or nothing principle concerning nervous impulse transmission, how can an organism perceive the size of a stimulus?

Answer 57

• Number of impulses passing in a given time- larger stimulus= more impulses in a given time
• Different neurones with different threshold values- brain interprets the number and type of neurones that pass impulses as a result of a given stimulus and thereby determines the size

Question 58

What are the functions of a synapse?

Answer 58

• Allow a single impulse to be transmitted to a number of neurones at a synapse so that a single stimulus can create a number of simultaneous responses
• A number of different impulses can be combined at a synapse to interact and produce a single response

Question 59

Describe the features of a synapse:

• Unidirectionality
• Summation (two types)
• Inhibition

Answer 59

-Synapses can only pass impulses in one direction- from the pre synaptic membrane to the post synaptic membrane, so they act as ‘valves’

-Low frequency action potentials rarely produce sufficient neurotransmitter to produce a new action potential.
>SPATIAL summation- in which a number of different presynaptic neurones release enough neurotransmitter collectively to reach the threshold
>TEMPORAL summation- in which a single presynaptic neurone releases neurotransmitter many times over a short period so reaching the threshold is more likely

-On the post synaptic membranes of some synapses, chloride channels can be made to open which hyper polarises the post synaptic neurone, making it less likely that an action potential will be generated. (inward diffusion of cl- ions means the inside of the axon is even more negative so a new action potential is less likely)

Question 60

Describe the chain of events before and after the transmission across a synapse

Answer 60

• Action potential arrives at pre S.M and causes calcium ion channels to open and calcium ions to diffuse in
• Diffusion of Ca causes the pre synaptic vesicles to fuse to the pre S.M and release their neurotransmitter through exocytosis into the synaptic cleft
• Neurotransmitter (acetylcholine) diffuses across the synaptic cleft and binds to receptors on the post S.M
• This causes sodium ion channels to open and there is an influx of sodium ions which creates a new action potential
• Acetylcholinesterase hydrolyses acetylcholine into ethanoic acid (acetyl) and choline which diffuse back across the synaptic cleft and into the pre synaptic neurone (benefits: recycling; prevents continuous generation of action potentials in the post synaptic neurone)
• ATP used by mitochindria to recombine ethanoic acid (acetyl) and choline to form acetylcholine which is stored in presynaptic vesicles
• Sodium ion channels close in the absence of acetylcholine from the receptors

Question 61

What are the two ways that drugs can act on synapses?

Answer 61

-Creating more action potentials in the post S.N
>mimicking a neurotransmitter
>stimulating the release of more neurotransmitter
>inhibiting the enzyme that breaks down the neurotransmitter

-Creating fewer action potentials in the post S.N
>inhibiting the release of neurotransmitter
>blocking the receptors on KNa channels on the post synaptic neurone

Question 62

What are endorphins and what do they do?

Answer 62

They are neurotransmitters used in certain sensory nerve pathways. Endorphins block the sensation of pain by binding to pain receptor sites

Question 63

What is GABA?

Answer 63

It is a neurotransmitter that reduces the chances of a new action potential forming by increasing the post synaptic membrane’s permeability to chlorine ions, hyperpolarising the membrane and antagonising the effect of sodium ions influxing.

Question 64

(2 marks)
When a young shoot is illuminated from one side, IAA stimulates growth on the shaded
side. Explain why growth on the shaded side helps to maintain the leaves in a
favourable environment.

Answer 64

1. Causes plant to bend/grow towards light / positive
   phototropism;
2. (Light) required for photosynthesis;

Question 65

(2 marks)
NAA is a similar substance to IAA. It is used to control the growth of cultivated plants.
Plant physiologists investigated the effect of temperature on the uptake of NAA by
leaves. They sprayed a solution containing NAA on the upper and lower surfaces of a
leaf.

Predict and explain the effect of temperature on the rate at which NAA is taken up by the lower
surface of the leaf

Answer 65

1. More kinetic energy;
2. Faster movement of molecules;
3. More diffusion;
4. So therefore quicker rate of uptake in higher temperatures

Question 66

(2 marks)
NAA is a similar substance to IAA. It is used to control the growth of cultivated plants.
Plant physiologists investigated the effect of temperature on the uptake of NAA by
leaves. They sprayed a solution containing NAA on the upper and lower surfaces of a
leaf.

There are differences in the properties of the cuticle on the upper and lower surfaces
of leaves.

Suggest how these differences in the cuticle might explain the differences in rates of
uptake of NAA by the two surfaces.

Answer 66

1. Thick cuticle on upper surface / thin cuticle on lower
   surface / few stomata on upper surface / no stomata on
   upper surface;
2. More diffusion / shorter diffusion pathway (on lower
   surface);

Question 67

Suggest how indoleacetic acid (IAA) could have caused caused a downwards growth of a root tip when the top half of the root tip was removed. This experiment was carried out in the absence of light in order to measure geotropisms ONLY

Answer 67

1. Less IAA/auxin (produced);
2. Lower side grows
   more/faster / less inhibition
   of growth on lower side;