Coordination

Question 1

What are Chemical Mediators

Answer 1

Chemical messengers that are released from mammalian cells and effect the cells in their immediate vicinity. Typically, they are released by infected or injured cells.

Question 2

What are the effects of chemical mediators

Answer 2

They cause small arteries and arterioles to dilate. This leads to a rise in temperature and swelling of the affected area.

Question 3

What are some examples of chemical mediators

Answer 3

Histamine- stored in certain white blood cells and released following an injury or in response to an allergen.

Prostaglandins- Found in cell membranes and are released following injury. This affects blood pressure, neurotransmitters and pain sensation.

Question 4

What effect does indoleacetic acid have on plants?

Answer 4

• Cells in the tip of the shoot produce IAA, which is then transported down the shoot
• The IAA is initially transported to all sides of the shoot
• Light causes the movement of IAA from the light side to the shaded side of the shoot
• A great conc. of IAA builds up on the shaded side of the shoot
• As IAA causes elongation of cells, the cells in the shaded side elongate more
• The shaded side grows faster, causing the shoot to bend towards the light.

Question 5

What is meant by resting potential?

Answer 5

The difference in electrical charge maintained across the membrane of the axon of a neurone when not stimulated
-Inside is negatively charged in relation to outside.

Question 6

What is meant by action potential?

Answer 6

The change that occurs in the electrical charge across the membrane of an axon when it is stimulated and a nerve impulse passes.
-Temporary reversal of charges- the inside becomes positively charged.

Question 7

Describe the depolarisation phase

Answer 7

• Stimulus causes ion channels to open, allowing more sodium ions to enter.
• Membrane becomes depolarised
• More sodium channels open allowing more sodium ions to enter.

Question 8

Describe the repolarisation phase

Answer 8

• Potassium ion channels open so these leave the axon

- Sodium ion channels close

Question 9

Describe how the resting potential is established in an axon by the movement of ions across the membrane.

Answer 9

• Sodium ions are transported out of the axon by active transport.
• Potassium ions diffuse out of the axon

Question 10

What is a nerve impulse?

Answer 10

A temporary reversal of the electrical potential difference across the axon membrane.

Question 11

Describe the passage of an action potential

Answer 11

• Once it has been created, an act potential ‘moves’ rapidly along an axon
• Nothing physically ‘moves’ from one place to another but rather the reversal of electrical charge is reproduced at different points along the axon membrane.
• Depolarisation of one region on the axon stimulates depolarisation of the next region
• Sodium ions entering causes the opening of the sodium voltage-gated channels little further along.
• The previous section then returns to its resting potential (repolarisation)
• Sodium channels close, potassium channels open

Question 12

Describe the passage of an act potential along an unmylelinated axon

Answer 12

The fatty sheath of myelin around the axon acts as an electrical insulator, preventing act potentials from forming. At intervals of 1-3mm there are breaks called Nodes of Ranvier. Action potentials can occur at these points and jump from node to node (saltatory conduction). §

Question 13

What factors affect the speed of an action potential?

Answer 13

• The myelin sheath: electrical insulator- increases speed of conductance
• Diameter of the axon: the greater the diameter, the faster the speed of conductance due to less leakage of ions
• Temperature: Affects the rate of diffusion of ions and therefore the higher the temperature, the faster the nerve impulse

Question 14

What is the refractory period and what is it’s purpose?

Answer 14

The delay between the end of one action potential and the beginning of another.

• It ensures that an action potential is propagated in one direction
• It produces discrete impulses
• it limits the number of action potentials

Question 15

What is the ‘all or nothing’ principle?

Answer 15

• 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 regardless of the size of the stimulus.
• Below the threshold, no action potential is triggered.