Receptors

Question 1

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

Answer 1

Rhodopsin is bleached

Question 2

Explain why the membrane potential was the same whether medium or heavy pressure was applied to the finger tip. Explain why (2)

Answer 2

• The threshold has been reached
• All-or-nothing principle (If that stimulus exceeds the threshold potential, the nerve or muscle fibre will give a complete response; otherwise, there is no response.)

Question 3

Explain why vision using the fovea has high visual acuity (1)

Answer 3

• Each receptor in the fovea is connected to a separate neurone

Question 4

Explain why the resting potential of -70mV is maintained in the sensory neurone when no pressure is applied (2)

Answer 4

• membrane is more permeable to potassium ions and less permeable to sodium ions
• sodium ions are actively transported out and potassium ions in

Question 5

Describe how the number and distribution of rods and cones across the retina would differ in a nocturnal mammal from a human (3)

Answer 5

• more rods in nocturnal mammals
• rods have high sensitivity
• rhodopsin bleached at low light intensities

Question 6

When the image is focused on the fovea, the person sees the object in colour. Explain why (3)

Answer 6

• colour is detected by cone cells
• fovea contains mainly cone cells
• three types of cone cells that are each sensitive to different wavelengths

Question 7

Explain why vision using other parts of the retina has high sensitivity to light (3)

Answer 7

• many rods in other parts of the retina
• rhodopsin (pigment in rod cells) are sensitive to light
• receptors are connected in groups to neurone
• summation

Question 8

Describe how a Pacinian corpuscle produces a generator potential when stimulated (3)

Answer 8

• increased pressure deforms the stretch-mediated sodium ion channel
• sodium channels open and sodium ions flow in
• depolarisation leads to a generator potential

Question 9

Give one similarity and one difference between a taxis and tropism (2)

Answer 9

• similarity = directional response to a stimulus
• difference:
  taxis = whole organism moves
  tropism = growth response

Question 10

Explain the importance of reflex actions (3)

Answer 10

• reflex actions are involuntary
• reduce damage to tissues
• escape from predators
• find food