the eye

Question 1

give 5 characteristics of rod cells

Answer 1

• no colour detected
• several rods connected to 1 bipolar cell and 1 sensory neurone (which is connected to the optic nerve and CNS)
• give low visual acuity (resolution)
• work in low light intensity
• 120 million per eye located across the retina (particularly in the periphery)

Question 2

give 5 characteristics of cone cells

Answer 2

• detect colour (red, blue ,green)
• one cone cell is connected to one bipolar cell which is connected to one sensory neurone (to the optic nerve which is connected to the CNS)
• give high visual acuity
• only work in high light intensity
• 6 million per eye located in forea

Question 3

what is the photosynthetic pigment in rod cells

Answer 3

rhodopsin

Question 4

how does the photosynthetic pigment in rod cells aid vision

Answer 4

rhodopsin is broken down in low light levels which causes a generator potential

Question 5

what is the photosynthetic pigment in cone cells

Answer 5

iopsin

Question 6

what are the 3 types of iopsin

Answer 6

• sensitive to red
• sensitive to green
• sensitive to blue

Question 7

when is the photosynthetic pigment in cone cells broken down

Answer 7

iopsin is broken down in high light intensity

Question 8

Explain how rods and cones work

Answer 8

Pigment in the rod (rhodopsin)/cone (iodopsin) is broken down and this creates a generator potential causing an action potential in the bipolar cell

Question 9

define visual acuity

Answer 9

the ability to distinguish between 2 objects which are close together

Question 10

explain why rod cells enable greater sensitivity to light than cone cells

(4 marks)

Answer 10

• multiple rod cells are connected to 1 bipolar cell and one sensory neurone when stimulated weak generator potentials combine to increase chance threshold is reached (summation) and an action potential is triggered
• one cone cell is connected to 1 bipolar cell and 1 sensory neurone, more light is required to trigger an action potential
• rhodopsin requires low light intensity to break down and cause a generator potential, cone cells pigment (IOPSIN) requires a higher light intensity to break down
• there are many rod cells which are more widely distributed across the retina compared to cone cells so can respond to scattered light as cone cells are only located in the fovea

Question 11

explain why rod cells have a lower visual acuity than cone cells (3 marks)

Answer 11

• light stimulating many rod cells will only trigger an impulse along 1 sensory neurone, more difficult for brain to distinguish between light sources
• when light from 2 points hits 2 cone cells each one connects to a separate sensory neurone, 2 action potentials are generated enables brain to separate activity
• cone cells are positioned closer together on the fovea

Question 12

what is the fovea and why are cone cells concentrated in that region

Answer 12

fovea is a small part of the retina (1 mm squared) on which light is focused by the lens

high concentration of con e cells are located here as light intensity is greatest
fovea provides the greatest visual acuity

Question 13

do rod cells have a high or low sensitivity to light

Answer 13

high

Question 14

do cone cells have a high or low sensitivity to light

Answer 14

low

Question 15

after staring at a red dot for a long time and then looking at a white space why do you see a cyan dot

Answer 15

eyes use R, G, B cone cells to perceive white light

red cone cells are fatigued so you don’t see red

only see with G + B cone cells so see a cyan circle