U1 KA4- COMMUNICATION AND SIGNALLING 4) The Vertebrate Eye

Question 1

What is the retina

Answer 1

The area within the eye which detects light / the tissue at the back of the eye which converts light into electrical signals

Question 2

What two types of cells are there within the retina of an animal eye

Answer 2

; about 125 million rods
; 6 million cones

Named like this due to their shape (look at page 104 scholar)

Question 3

How do rods function

- what do they contain

Answer 3

Rod cells function in dim light (they contain one type of light sensitive pigment )
- these cells are sensitive to changes in light intensity and are particularly useful in areas of low light intensity (dim light) BUT do not allow colour perception.

Question 4

How do nocturnal animals have better vision at night

Answer 4

• nocturnal animals have a greater proportion of rod cells in their retina which gives them a better vision at night

Question 5

How do cone cells function / what are they responsible for

Answer 5

• cones are responsible for colour vision in animals
• they only function in bright light
• they are not as sensitive to light as rod cells ; however they are particular sensitive to specific colours (wavelengths) of light : green , red , blue and (in some animals) UV.

Question 6

Colour blind - relation to cone cells

Answer 6

People who are colour blind lack a particular type of cone cell in their retina

Question 7

What do rod cells contain to detect light / all eukaryotes contain
- where is it derived from

Answer 7

• each rod cell in the retina contains photoreceptor proteins that consist of a light absorbing molecule called retinal , to capture light energy
• retinal is derived from vitamin A , which itself can be formed from Carotene in plants

Question 8

Retinal is a __________ group

Answer 8

Retinal is a prosthetic group

Question 9

• What is retinal bound to
• where is this complex in the cell
• what is the complex called

Answer 9

• Retinal is a prosthetic group which is covalently bound to a membrane protein called opsin.
• the retinal - opsin complex is embedded in membranes inside photoreceptor cells.
• in rod cells the retinal - opsin complex is called rhodopsin

Question 10

How is generation of a nerve impulse brought about involving rhodopsin

Answer 10

• retinal absorbs a photon of light and rhodopsin changes conformation to photoexcited rhodopsin. ( photoexcited rhodopsin triggers a cascade of proteins that will amplify a photon signal and generate a nerve impulse)
• the photoexcited rhodopsin activates a G protein called transducin ( a single photoexcited rhodopsin activates hundreds of molecules of transducin)
• transducin activates the enzyme phosphodiesterase (PDE) (each activated G protein activates one molecule of PDE)
• PDE catalyses the hydrolysis of a molecule called cyclic GMP (cGMP). (Each active PDE molecule breaks down thousands of cGMP molecules per second)
• the reduction in cGMP concentration (as a result of its hydrolysis) affects the function of ion channels in the membrane of rod cells. It results in the closure of ion channels.
• the inward leakage of Na and Ca ++ ions is halted so the membrane potential increases( hyperpolarisation - increasing charge - triggers nerve impulses in neurons in the retina.

Question 11

The rod cells in darkness have a mechanism that prevents generation of a _______ _________

Answer 11

The rod cells in darkeness have a mechanism that prevents generation of a nerve impulse

Question 12

How do rod cells prevent generation of nerve impulse

Answer 12

• rode cells in darkness have a mechanism that prevents the generation of a nerve impulse
• the rhodopsin is inactive and the rod cells produce cyclic GMP (cGMP).
• this binds to ligand gated Na+ channels , thus opening the channels so sodium ions leak across the membrane
• the membrane stays depolarised and so no nerve impulse is generated

Question 13

• What is the cGMP handbrake

- what removes this handbrake

Answer 13

The rod could be thought as having a cGMP handbrake.

: activating rhodopsin with light , removes this “handbrake” so a nerve impulse can be generated

Question 14

How do rod cells have high sensitivity at low light intensities

Answer 14

• the protein cascade ( rhodopsin - G protein - enzyme - channel ) provides a high degree of amplification , so stimulation by a single photon results in a large effect on ion movement across the membrane. This gives rod cells high sensitivity at low light intensities . Rods can generate a nerve impulse from as few as ten photons of light

Question 15

Why are cones less sensitive than rods

Answer 15

Cones are less sensitive than rods because they have fewer photoreceptor molecules in their membranes

Question 16

How do cones photoreceptor molecules generate a nerve impulse

Answer 16

• the photoreceptor molecule generate a nerve impulse in exactly the same way as in rods , but the cones have photopsins that are sensitive to diffrent colours

Question 17

How are photopsins made

Answer 17

• different photopsins are made by combining retinal with different forms of opsin.
• these opsin differ by a few amino acids in their primary structure which means their tertiary structures are different when they combine with the retinal.

Question 18

What does the change in structure (the different photopsins) (when retinal combines with different opsins) mean ?

Answer 18

The change in structure means that photopsins absorb light across a different range of wavelengths

Question 19

• how many types of photopsins are there and what do they have

Answer 19

• in humans each cone cell has one of three types of photopsin , each with a MAXIMAL SENSITIVITY To a specific wavelength of light.
• in humans one type of photopsin has a maximal sensitivity to blue , one green and another to red light.
• some species I’m other animals (such as insects / fish / reptiles and birds) have photopsins that have maximal sensitivity to UV light