Coordination

Question 1

What are the two main forms of coordination in mammals?

Answer 1

The nervous system & the hormonal system.

Question 2

Suggest how indoleacetic acid (IAA) caused a root tip to grow downwards?

Answer 2

IAA moves to lower side.
This inhibits growth on this side.
So it grows downwards.

Question 3

Explain how someone becomes aware of the pain of a thorn.

Answer 3

Impulses to brain.
Sensory areas (in brain).
Processing by association area.

Question 4

In which part of the brain is the cardiovascular centre located?

Answer 4

The medulla

Question 5

During an action potential, the membrane potential rises to + 40 mV and then falls. Why?

Answer 5

K+ ions move out by diffusion because potassium voltage gated channels are open.
This is called repolarisation as the P.D begins to go back to resting potential.

Question 6

After exercise ATP is used to re-establish the resting potential in axons. Explain how the resting potential is re-established.

Answer 6

Active transport of sodium out of axon and potassium in.

Question 7

Explain the effect of myelination on the rate of nerve impulse conduction.

Answer 7

Myelin insulates/prevents ion movement and saltation.

Question 8

Why is the rate of conduction of the nerve impulse in myelinated neurones in a cat faster than a lizard?

Answer 8

Cat has a higher body temperature.

Faster diffusion of ions.

Question 9

During an action potential, the permeability of the cell-surface membrane of an axon changes. A graph shows changes in permeability of the membrane to sodium ions (Na+) and to potassium ions (K+) during a single action potential.
Explain the shape of the curve for sodium ions between 0.5 ms and 0.7ms. (steep)

Answer 9

Ion channel proteins open;
Influx of sodium ions;
This makes the inside of axon less negative

Question 10

Different substances are involved in coordinating responses in animals. Hormones are different from local chemical mediators such as histamine in the cells they affect.
Describe how hormones are different in the cells they affect.

Answer 10

Hormones have widespread effect

Question 11

Describe how hormones and local chemical mediators reach the cells they affect.

Answer 11

Hormones in blood;

Local chemical mediators spread by diffusion

Question 12

Synapses are unidirectional. Explain how acetylcholine contributes to a synapse being unidirectional

Answer 12

Acetylcholine released from presynaptic side;

Diffusion is from higher concentration to lower concentration

Question 13

IAA is a specific growth factor. Name the process by which IAA moves from the growing regions of a plant shoot to other tissues.

Answer 13

Diffusion

Question 14

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 14

Causes plant to grow towards light;

Light is required for photosynthesis;

Question 15

A diagram shows the change in the charge across the surface membrane of a non-myelinated axon when an action potential is produced. (Inside of axon goes from being negative to positive) Describe how the change shown in the diagram occurs when an action potential is produced.

Answer 15

Sodium channels open;

Sodium ions enter axon

Question 16

Explain what causes the conduction of impulses along a non-myelinated axon to be slower than along a myelinated axon.

Answer 16

In a myelinated axon there is ion movement only at the nodes.
and the impulse jumps from node to node;
whereas in non-myelinated axons the impulse has to travel along the whole membrane

Question 17

Nervous transmission is delayed at synapses. Explain why.

Answer 17

Chemical rather than electrical;

Process of transmission takes time because of transmitter diffusion

Question 18

The axon of neurone A is myelinated. The axon of neurone B is non-myelinated. Explain why impulses travel faster along the axon of neurone A.

Answer 18

Myelinated - impulse jumps from node to node;

Non-myelinated - impulse travels whole length of axon membrane

Question 19

Enkephalins are neurotransmitters released by the brain and spinal cord in response to harmful stimuli. Enkephalin molecules are similar in shape to acetylcholine.
Enkephalin molecules act as pain killers by inhibiting synaptic transmission. Explain how this inhibition occurs.

Answer 19

Bind to receptors;
On postsynaptic membrane;
Competes with acetylcholine;
Reduces depolarisation

Question 20

Acetylcholine is a neurotransmitter which binds to postsynaptic membranes and stimulates the production of nerve impulses. GABA is another neurotransmitter. It is produced by certain neurones in the brain and spinal cord. GABA binds to postsynaptic membranes and inhibits the production of nerve impulses. The diagram shows a synapse involving three neurones.
Describe the sequence of events leading to the release of acetylcholine and its binding to the postsynaptic membrane.

Answer 20

Action potential arrives;
Calcium ions enter synaptic knob;
Vesicles fuse with membrane;
Acetylcholine diffuses across synaptic cleft

Question 21

Explain the importance of reflex actions.

Answer 21

Automatic adjustments to changes in environment;
Preventing injury;
Helpful for escaping from predators

Question 22

Describe the sequence of events which allows information to pass from one neurone to the next neurone across a cholinergic synapse

Answer 22

Impulse causes calcium ions to enter axon;
causes vesicles to fuse with presynaptic membrane;
Acetylcholine released;
Acetylcholine diffuses across synaptic cleft;
and binds with receptors on sodium ion channels on postsynaptic membrane;
Sodium ions diffuse into postsynaptic neurone;
Acetylcholinesterase hydrolyses acetylcholine to prevent further action potentials;
ATP from mitochondria used to recombine choline and ethanoic acid into acetylcholine

Question 23

Give two differences between a cholinergic synapse and a neuromuscular junction

Answer 23

Neurone to neurone and neurone to muscle;

Action potential in neurone and no action potential in muscle

Question 24

Acetylcholine is the neurotransmitter at neuromuscular junctions.
Describe how the release of acetylcholine into a neuromuscular junction causes the cell membrane of a muscle fibre to depolarise.

Answer 24

Movement by diffusion;
Binding to receptors on post-synaptic membrane;
Causing sodium channels to open

Question 25

The molecular structure of cobra toxin is similar to the molecular structure of acetylcholine. Explain why the toxin permanently prevents muscle contraction.

Answer 25

Toxin competes for receptors;

The toxin does not cause depolarisation

Question 26

The insecticide DFP combines with the active site of the enzyme acetylcholinesterase. Explain why the muscles stay contracted until the insecticide is lost from the neuromuscular junction.

Answer 26

Acetylcholinesterase is unable to breakdown acetylcholine;

acetylcholine still available to depolarise the membrane

Question 27

What is meant by the refractory period?

Answer 27

No new nerve impulse be produced in this time

Question 28

State three ways in which a response to a hormone differs from a response to a nerve impulse.

Answer 28

Hormone response is slow, widespread and long lasting.

Nervous response is rapid, localised and short lived.

Question 29

Name two chemical mediators and state the effects they each have on blood vessels.

Answer 29

Histamine and prostaglandins - both cause dilation of small arteries and arterioles and increased permeability of capillaries.

Question 30

Suggest two advantages to a plant of having roots that respond to gravity by growing in the direction of its pull.

Answer 30

Response ensures that roots grow downwards into the soil, thus anchoring the plant firmly and bringing it closer to water, needed for photosynthesis.

Question 31

State two differences between animal hormones and plant growth factors.

Answer 31

Animal hormones are made in particular organs and affect other organs some distance away.
Plant growth factors are made by cells located throughout the plant and have localised effects.

Question 32

In a myelinated axon, sodium and potassium ions can only be exchanged at certain points along it.
Explain why ions can only be exchanged at these points.

Answer 32

Because the remainder of the axon is covered by a myelin sheath that prevents ions being exchanged.

Question 33

How is a presynaptic neurone adapted for the manufacture of neurotransmitter?

How is the postsynaptic neurone adapted to receive the neurotransmitter?

Answer 33

It possesses many mitochondria and large amounts of endoplasmic reticulum.

It has receptor molecules on its membrane.

Question 34

Explain why hyperpolarisation reduces the likelihood of a new action potential being created.

Answer 34

As the inside of the membrane is more negative than at resting potential, more sodium ions must enter in order to reach the potential difference of an action potential, i.e. it is more difficult for depolarisation to occur. Stimulation is less likely to reach the threshold level needed for a new action potential.

Question 35

Why is it necessary for acetylcholine to be hydrolysed by acetylcholinesterase?

Answer 35

To recycle the choline and ethanoic acid, to prevent acetylcholine from continuously generating a new action potential in the postsynaptic neurone.

Question 36

Explain how the refractory period ensures that nerve impulses are kept separate from one another.

Answer 36

During the refractory period the sodium channels are closed so no sodium ions can move inwards and no action potential is possible. This means there must be an interval between one impulse and the next.

Question 37

What is the all-or-nothing principle?

Answer 37

There is a particular level of stimulus that triggers an action potential. At any level above this threshold, a stimulus will trigger an action potential that is the same size regardless of the size of the stimulus. Below the threshold, no action potential is triggered.

Question 38

Outline how IAA controls tropisms.

Answer 38

Cells in the tip of a shoot produce IAA, which is transported down the shoot;
Light causes IAA to move to shaded side of shoot;
Greater concentration of IAA on shaded side than light side;
IAA causes elongation of cells, so cells on shaded side elongate more;
Shaded side grows faster, causing shoot to bend towards light

Question 39

Describe the structure of neurones.

Answer 39

Cell body containing a nucleus and lots of RER;
Dendrites which carry nerve impulses towards cell body;
Axon, long fibre which carries nerve impulses away from cell body;
Schwann cells which surround axon, protect it and provide electrical insulation;
Myelin sheath, made of membranes of Schwann cells, rich in the lipid ‘myelin’ which transmits nerve impulses faster;
Nodes of ranvier which are gaps between Schwann cells

Question 40

Describe how a resting potential is established in an axon.

Answer 40

Inside of axon negatively charged relative to outside;
Because active transport of sodium ions out of the axon by sodium-potassium pumps is faster than active transport of potassium ions into the axon;
Potassium ions diffuse out of axon but few sodium ions diffuse in as sodium gates are closed;
Potassium ions stop diffusing out because they are attracted to negative state of axon and repelled by outside;
Further increasing potential difference;
More positive ions outside than inside, creating a chemical gradient;
Equilibrium is reached, no net movement of ions

Question 41

How is the movement of ions controlled?

Answer 41

Phospholipid bilayer of axon membrane prevents ions diffusing across it;
Intrinsic proteins contain channels which open and close accordingly;
Some intrinsic proteins actively transport ions in and out (sodium-potassium pump).

Question 42

Describe how an action potential is produced in an axon.

Answer 42

A stimulus causes a temporary reversal of charge and inside of axon becomes positive (depolarisation);
Energy of stimulus causes some sodium channels to open;
So sodium diffuses into axon;
Once an action potential of +40mV has been established, channels close and there is no more influx of sodium;
Potassium channels open and potassium ions diffuse out, causing repolarisation of axon;
Axon becomes more negative than usual, so potassium gates close and sodium-potassium pump again actively transports sodium ions out and potassium ions in

Question 43

Describe how an action potential moves across an axon.

Answer 43

At resting potential, high concentration of sodium ions outside membrane compared to inside, so inside is more negative;
Stimulus causes some sodium channels to open so sodium diffuses into axon and membrane is depolarised;
Localised electrical circuits cause sodium channels further along to open;
Influx of sodium ions in this region causes depolarisation;
Behind this new region of depolarisation, sodium channels close and potassium channels open and membrane is repolarised;
Action potential continues in this way along the neurone

Question 44

Why does an action potential pass along a myelinated neurone faster than an unmyelinated one?

Answer 44

Saltatory conduction;
Myelin sheath acts as an insulator;
Prevents action potentials from forming;
Action potentials jump between nodes of Ranvier

Question 45

How does the diameter of an axon affect the speed of a nerve impulse?

Answer 45

The greater the diameter of an axon, the faster the speed of conductance;
due to less leakage of ions from a large axon

Question 46

How does temperature affect the speed of a nerve impulse?

Answer 46

The higher the temperature the faster the nerve impulse;
Sodium-potassium pump uses active transport;
Which requires energy from ATP from respiration;
Respiration is enzyme-controlled;
More kinetic energy so more enzyme-substrate complexes formed and faster diffusion

Question 47

Why do synaptic knobs possess many mitochondria and large amounts of endoplasmic reticulum?

Answer 47

Mitochondria provides ATP for synthesis of proteins and neurotransmitters;
Endoplasmic reticulum synthesises proteins and neurotransmitters

Question 48

How can a synapse prevent a new action potential being created?

Answer 48

Chloride ion channels can be kept open;
Influx of chloride ions;
Making postsynaptic membrane more negative than at resting potential

Question 49

How may drugs effect synapses?

Answer 49

They stimulate the nervous system by creating more action potentials in postsynaptic neurones;
They inhibit the nervous system by creating fewer action potentials in postsynaptic neurones

Question 50

How may a drug stimulate the nervous system?

How may a drug inhibit the nervous system?

Answer 50

Mimick a neurotransmitter;
Stimulate the release of more neurotransmitter;
Inhibit the enzyme that breaks down the neurotransmitter

Inhibit the release of the neurotransmitter;
Block the receptors on the ion channels on postsynaptic neurone