Organisms Respond to Changes in Environments

Describe the myogenic stimulation of the heart and how the regular contraction of the atria and ventricles is coordinated.

Do not include the autonomic nervous system in your answer. (5 marks)

• SAN releases wave of electrical activity

• So atria contract at the same time

• AVN relays/passes electrical activity after a short delay

• Via Purkyne tissue and bundle of His

• So ventricles contract at the same time from bottom upwards

Describe how stimulation of a Pacinian corpuscle produces a generator potential. (3 marks)

• There are stretch-mediated sodium ion channels in the membrane

• Increased pressure deforms sensory neurone membrane/lamella

• Sodium ion channels open

• Sodium ions diffuse in

• Depolarisation leading to generator potential

Rod cells allow us to see objects in dim light. Explain how the connections of rod cells to neurones in the retina make this possible. (3 marks)

• Several rod cells have connections with one neurone

• Summation of generator potentials exceed threshold

• Individual generator potentials do not exceed threshold

Explain how the connections of rod and cone cells with neurones in the retina give rise to differences in sensitivity and acuity. (3 marks)

• Several rod cells to each neurone/bipolar cell, additive effect of light striking several rod cells

• Each cone is connected to a specific neurone, light striking cone cells generating separate action potential

Explain why it takes time for the rod cells to recover their sensitivity to light after moving into darkness. (2 marks)

• Rhodopsin is broken down by light

• Time needed for resynthesis

Explain how a rise in blood pressure results in a decrease in the rate of heartbeat. (6 marks)

• Pressure receptors in aorta detect change in blood pressure

• Impulse passes to cardiovascular medulla via sensory neurone

• Impulses pass along the parasympathetic neurone in cardiac nerve

• Acetyl Choline (ACh) released from ends of parasympathetic neurone in SAN

• Decreases impulses to AVN

A woman takes moderate exercise. Explain what causes her heart rate to increase while she exercises. (6 marks)

• Rate of respiration increases in muscle cells

• Carbon dioxide concentration increases/pH falls

• Chemoreceptors in aorta detect change

• Impulses passed to cardiovascular medulla

• Increased frequency of impulses

• Along sympathetic pathway to sinoatrial node (SAN)

Describe how a heartbeat is initiated and contracted. (5 marks)

• SAN sends a wave of electrical activity across atria causing atrial contraction

• Non-conducting tissue prevents immediate contraction of ventricles

• AVN delays impulse while ventricles fill

• AVN sends wave of electrical activity down bundle of His

• Causing ventricles to contract from base up

Explain how the heart muscle and the heart valves maintain a one-way flow of blood from the left atrium to the aorta. (5 marks)

• Atrium has higher pressure than ventricle causing atrioventricular valves to open

• Ventricle has higher pressure than atrium due to filling causing atrioventricular valves to close

• Ventricle has higher pressure than aorta causing semilunar valve to open

• Higher pressure in aorta than ventricle causing semilunar valve to close

• Muscle contractions cause increase in pressure

Describe the sequence of events involved in transmission across a cholinergic synapse.

Do not include details on the breakdown of acetylcholine in your answer. (5 marks)

• Depolarisation of presynaptic membrane

• Calcium channels open and calcium ions enter the presynaptic knob

• Calcium ions cause synaptic vesicles to move to and fuse with presynaptic membrane and release acetylcholine

• Acetylcholine diffuses across synaptic cleft

• Acetylcholine attaches to receptors on postsynaptic membrane

• Sodium ions enter postsynaptic neurone leading to depolarisation

Explain how insulin lowers the concentration of blood glucose. (3 marks)

• Bind to receptor on liver/muscle/target cell

• Causes more carrier proteins to become active and move to plasma membrane

• Glucose diffuses into cells and lowers blood glucose

• Enzymes in cells convert glucose to glycogen (glycogenesis)

• Stimulates lipid formation from glucose

• Raises rate of respiration in cells using more glucose