Organisms response to changes in their environments

Question 1

Explain the role played by adrenaline in the control of heart rate
(5 marks)

Answer 1

Adrenaline is released by the neurones of the sympathetic nervous system.
Adrenaline binds to receptors on the surface of heart cells, causing the heart rate to increase, in response to impulses received by the medulla.
- chemoreceptors detect low blood pH
- baroreceptors detect low blood pressure

Question 2

Explain why conduction speed in a myelinated neurone is so much faster than in an unmyelinated neurone
(3 marks)

Answer 2

In unmyelinated axons ions diffuse through the cell causing ion channels to open all along the axon length.
Myelinated neurons are insulated by Schwann cells so depolarisation can only take place at the gaps between the Schwann cells (nodes of Ranvier).
The impulse travels by saltatory conduction and this is faster than depolarising all the way along.

Question 3

Suggest how SSRIs can be effective in the treatment of depression
(3 marks)
(drugs, synapses and serotonin)

Answer 3

SSRIs block the serotonin transporter proteins.
The blocked transporter proteins cannot take up serotonin so it is not returned to the presynaptic cell.
The level of serotonin in the synaptic cleft increases.

Question 4

Suggest how acetylcholine acts as an inhibitory neurotransmitter in a synapse
(3 marks)

Answer 4

Acetylcholine binds to complementary receptors on the cell surface membrane of the postsynaptic cell.
Acetylcholine leads to the opening of potassium channels.
Positively charged potassium ions leave the cell preventing the membrane from reaching threshold potential causing the membrane to become hyperpolarised.

Question 5

How do T-tubules help in the initiation of a muscle contraction?
(2 marks)

Answer 5

T-tubules form a connection between the cell surface membrane (sarcolemma) and the myofibril.
The depolarisation of the muscle cell membrane can pass more quickly to the centre of the muscle cell leading to a faster release of calcium ions.

Question 6

Explain the role player by ATP in muscle contraction
(2 marks)

Answer 6

ATP hydrolase is activated by calcium ions so ATP is hydrolysed.
ATP hydrolysis releases energy that is used in the bending of the myosin head to produce the power stroke.
ATP hydrolysis release energy which allows the myosin heads to return to its original shape, detach from the actin myosin binding site allowing the process to be repeated.

Question 7

Describe how glucagon enables the production of glucose inside cells
(4 marks)

Answer 7

Glucagon binds to complementary receptors on the surface of target cells.
Glucagon activates adenylate cyclase.
ATP is converted into cyclic AMP and this is the second messenger molecule.
cAMP activates the enzyme protein kinase A and this sets of a chain of reactions which results in the breakdown of glycogen to glucose.

Question 8

Explain how ultrafiltration would be affected by severe dehydration
(2 marks)

Answer 8

Ultrafiltration takes place at high blood pressure so they will be affected by the drop in blood pressure caused by dehydration.
The rate of ultrafiltration would decrease