Coordination

Question 1

How does the hormonal system work?

Answer 1

It produces chemicals (hormones) that are transported in the blood plasma to the target cells.

Question 2

What are cell mediators?

Answer 2

Chemicals that are released from a specific cell and have an effect on cells in their immediate vicinity. They’re typically released by infected or damaged cells and cause small arteries to dilate.

Question 3

Examples of chemical mediators

Answer 3

• histamine - stored in certain white blood cells and released in response to an allergen. It causes increased permeability of capillaries leading to swelling and redness.
• prostaglandins - found in cell membranes causing dilation of small arteries. They also effect the blood pressure and neurotransmitters.

Question 4

An example of a plant growth factor and how it causes plant cells to elongate.

Answer 4

IAA initially transported to all sides of the shoot.
Light causes the movement of IAA to the shaded side of the shoot.
A greater concentration on the shaded side than the light side.
IAA causes elongation of cells causing the shaded side to grow faster causing the shoot to bend upwards.

IAA in high concentrations in roots stops growth.

Question 5

The structure of a neurone

Answer 5

• cell body has large amounts of rough endoplasmic reticulum
• dendrons and dendrites carry nerve impulses to the cell body
• axon the single long fibre that carries the impulse away from the cell body
• Schwann cells surround the axon providing electrical insulation
• myelin sheath forms a covering to the axon made up of membranes of the Schwann cells
• nodes of Ranvier gapes between Schwann cells where there’s no myelin sheath

Question 6

Explain how a resting potential is maintained

Answer 6

Resting potential is -65mV and considered polarised

• sodium ions actively transported out of the axon by the Na K pump
• potassium ions actively transported into the axon by the Na K pump
• three sodium out two potassium in
• sodium ions diffuse back in naturally so the gates are closed to Na ions

Question 7

Explain how an action potential is reached

Answer 7

Positively charged+40mV and considered depolarised

• Stimulus causes Na voltage-gates channels to open
• More Na gates open and ions diffuse in
• Action potential is established and Na channels close and potassium channels open causing repolarisation
• Potassium channels stay open (hyper polarisation)
• Potassium channels close and resting potential is re-established (repolarised)

Question 8

What is the refractory period?

Answer 8

A period when inward movement of Na ions is prevented and it is impossible for a further action potential to be generated

Question 9

What factors affect speed of an action potential

Answer 9

Myelin sheath - increases the speed of conductance

Diameter of the axon - greater the diameter, the faster the conductance because there is less leakage of ions

Temperature - affects rate of diffusion of ions so a higher temperature means faster impulse

Question 10

What does unidirectionality mean?

Answer 10

Synapses can only pass impulses in one direction - from the presynaptic to the postsynaptic

Question 11

What are the two types of summation?

Answer 11

Spatial - a number of different presynaptic neurones together release enough neurotransmitter to exceed the threshold value

Temporal - a single presynaptic neurone releases enough neurotransmitter over a short period to exceed the threshold value

Question 12

Transmission across a synapse

Answer 12

Action potential causes calcium ion channels to open allowing calcium ions to enter the synaptic knob
Causes synaptic vesicles to fuse with the membrane releasing acetylcholine into the synaptic cleft
Acetylcholine binds to a receptor site on the sodium ion channel opening the channel
Sodium influx generates a new action potential
Acetylcholine is broken down by acetylcholinesterase to choline and ethanoic acid and diffuse back into presynaptic
ATP recombines them to form acetylcholine