Vertebrate eye (C&S)

Question 1

Step 1

Answer 1

Retinal absorbs a photon of light and rhodopsin changes conformation to photoexcited rhodopsin

Question 1

Step 2

Answer 1

A cascade of proteins amplifies the signal

Question 2

Step 3

Answer 2

Photoexcited rhodopsin activates a Gprotein, called transducin, which activates the enzyme phosphodiesterase (PDE)

Question 3

A single photoexcited rhodopsin activates

Answer 3

hundreds of molecules of G-protein

Question 4

Each
activated G-protein activates

Answer 4

one molecule of PDE

Question 5

Step 4

Answer 5

PDE catalyses the hydrolysis of a molecule
called cyclic GMP (cGMP)

Question 6

Each active PDE molecule breaks down

Answer 6

thousands of cGMP molecules per second.

Question 7

Step 5

Answer 7

This results in the closure of ion channels in the membrane of the rod cells, which triggers nerve impulses in neurons in the retina

Question 8

What does a very high degree of amplification result in?

Answer 8

rod cells being able to respond to low
intensities of light