The Eye

Question 1

What is the structure of the human eye?

Answer 1

Retina - inner layer of the eye containing rods/cones and provides the brain information to understand an image by passing nerve impulses to the optic nerve (contains rod cells)

Fovea - only contains cones, area of highest visual acuity

Blind spot - area with no cones or rod cells so no vision

Cornea - bends light to focus an image onto the retina

Question 2

Why do cones have greater acuity and rods don’t?

Answer 2

Cones are tightly packed together and each has its own SENSORY NEURONE so there’s a higher RESOLUTION (detect separate light stimulus so brain can distinguish between different points of light)
Cones have 3 types of iodopsin meaning it can produce an image in colour

Rods are further away from each other and share synpases with 1 SENSORY NEURONE so there’s a lower resolution (multiple light stimuli summated to create an impulse so brain can’t differentiate).

Question 3

What is the pigment for rod cells?

Answer 3

Rhodopsin, which is made of opsin and retinal.
- When light hits CIS-retinal the shape of the retinal changes forcing rhodopsin to break into opsin and TRANS retinal (bleaching) as opsin can no longer attach tightly.

Question 4

What happens when light is absorbed by the rod cell?

Answer 4

1. Rhodopsin is bleached when a photon of light hits the molecule.
2. This triggers sodium gates to become less permeable to sodium ions and the gates close.
3. Mitochondria provides ATP for the sodium pump to move Na ions out.
4. The rods inside becomes more negative and causes hyperpolarisation (the generator potential) and this reduces the release of glutamate.
5. The bipolar cell is no longer inhibited so becomes depolarised, allowing acetylcholine to diffuse across the synapse which sets up an action potential in a sensory neurone (sends message to brain to understand an image).

Question 5

Why do rod cells not generate an action potential in the dark?

Answer 5

• Na channels open so Na ions diffuse into the rod cell, depolarising the cell.
• Depolarisation causes glutamate to be released by the rod cell which inhibits hyperpolarisation of the bipolar cell so no acetylcholine is released to the ganglion/sensory cell (no action potential is generated).

Question 6

What is the absorption spectrum

Answer 6

Shows how much light from a wavelength is absorbed by a pigment

Question 7

What is the action spectrum

Answer 7

Shows the effect of light wavelength on the rate of photosynthesis