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A2 Biology Unit 5 > Photoreceptors > Flashcards

Flashcards in Photoreceptors Deck (25):

What are photoreceptors?

Receptors in your eye that detect light.


What contains photoreceptor cells?

The retina


What controls how much light enters through the pupil?

The muscles of the iris.


What is the fovea?

An area of the retina where there are lots of photoreceptors.
The fovea recieves the highest intensity of light therefore the cone cells and not rod cells are found there.


What does the optic nerve do?

It is a bundle of nerves which carries impulses from the photoreceptor cells to the brain.


How do photoreceptors work?

Light enters the eye, hits the photoreceptors and is absorbed by light sensitive pigments.
Light bleaches the pigments, causing a chemical change and altering the membranes permeability to sodium.
A generator potential is created and if it reaches the threshold, a nerve impulse is sent along a bipolar neurone.
Bipolar neurones connect photoreceptors to the optic nerve, which takes impulses to the brain.


What are the two types of receptor in the human eye?

Rods and cones


Where are rods mainly found?

In the peripheral parts of the retina, where light intensity is lowest.


Where are cones mainly found?

Packed together in the fovea.


What colours do rods give information in?

Black and white (monochromatic)


What colours do cones give information in?

Colour (trichromatic)


What are the three types of cones?

Red- sensitive
Green- sensitive
Blue- sensitive


How many rods and cones join neurones?

Many rods join one neurone
One cone joins one neurone


Rods and cons sensitivity to light

Rods = high
Cones = low


Rods and cons visual acuity

Rods = low
Cones = high


What is visual acuity?

The ability to tell apart points that are close together.


Many rod cells share a single sensory neurone. Rod cells can therefore respond to light of very low intensity. Why is this?

This is because a certain threshold frequency has to be exceeded before the generator potential is created in the bipolar cells to which they are attached.
This is made more likely as a number of rod cells are attached to a single bipolar cell.


How is a generator potential created in a rod cell?

The pigment rhodopsin must be broken down (low intensity light is sufficient to do this).


Why do rod cells have low visual acuity?

Many rod cells link to a single bipolar cell.
The light received by rod cells sharing the same neurone will only generate a single impulse regardless of how many neurones are stimulated.
This means that they cannot distinguish between the different sources of light that stimulated them.


Why do cone cells only respond to high intensity light?

Each cone cell often has its own sensory neurone.
This means that they cannot combine to meet the threshold frequency required for the generator potential.
Also, the pigment required for the generator potential iodopsin, requires a higher light intensity to be broken down.


Why do cone cells have a high visual acuity?

Each cone cell has its own connection to a single bipolar cell, meaning if two adjacent cone cells are stimulated, the brain recieves two different signals which it can distinguish between.


Explain the role of the Na+ and K+ channels in producing the membrane resting potential.

More potassium pores than sodium pores
More potassium leave via active transport
Therefore more positive on outside;


How does the Na+ gate allow depolarisation of this membrane.

Sodium ions pumped out (to create diffusion gradient)
Sodium gates open to allow sodium ions (to flood) in;


Explain how the action potential is generated

Sodium ion channels open;
Allowing rapid influx of sodium ions;


Explain how the axon membrane is repolarised

Sodium ion channels close and potassium ion channels open;
Allowing efflux of potassium ions;