Detecting and amplifying an environmental stimulus

Question 1

Some proteins are light sensitive. What does this mean?

Answer 1

they are capable of sensing and responding to light.

Question 2

What are proteins which are light sensitive called?

Answer 2

photoreceptor proteins

Question 3

What are the 3 domains photoreceptor protein systems are found in?

Answer 3

Photoreceptor protein systems are found in archaea, plants and animals.

Question 4

What is bacteriorhodopsin?

Answer 4

Bacteriorhodopsin is a transmembrane protein which contains three polypeptide chains.

Question 5

What do bacteriorhodopsin molecules have?

Answer 5

a prosthetic group called retinal

Question 6

Describe the changes that occur when a photon is absorbed by retinal in a bacteriorhodopsin.

Answer 6

When a photon of light is absorbed, it causes a change in the conformation (shape) of the retinal molecule.
The retinal changes from a straight form to a bent form; it is this change in shape that allows protons to be pumped across the membrane by bacteriorhodopsin.
The protons which are pumped across the membrane return through ATP synthase, resulting in the production of ATP.

Question 7

What do plants use to convert light energy into chemical energy?

Answer 7

Plants use the photosynthetic pigment chlorophyll to convert light energy into chemical energy.

Question 8

Where are plant photosynthetic pigments found?

Answer 8

within the chloroplasts.

Question 9

Describe what happens when a molecule of chlorophyll absorbs a photon.

Answer 9

When a molecule of chlorophyll absorbs light, one of its electrons is boosted to a higher energy level.
The electron then passes along a series of protein carriers (an electron transport chain).
As the electron moves along the series of proteins, hydrogen ions are pumped across the thylakoid membrane of the chloroplast.
The hydrogen ions which are pumped across the membrane diffuse through ATP synthase, resulting in the production of ATP.

Question 10

In animals, what is the light-sensitive molecule retinal combined with>

Answer 10

a membrane protein opsin to form the photoreceptors of the eye.

Question 11

What does a cascade of proteins do?

Answer 11

A cascade of proteins amplifies the signal.

Question 12

What does the presence of opsin do?

Answer 12

The presence of opsin changes the permeability of sodium which generates a potential.

Question 13

What is the retina?

Answer 13

The retina is a photosensitive layer at the back of the eye.

Question 14

What are the two types of photoreceptors contained within the retina?

Answer 14

The retina contains two types of photoreceptors: Rod and Cone cells.

Question 15

Give 4 characteristic features of rod cells.

Answer 15

• Black & white images only
• mostly for night vision
• Poor visual acuity
• Contains the pigment Rhodopsin

Question 16

Give 4 characteristic features of cone cells.

Answer 16

• Sensitive to colour
• mostly for day vision
• Provides good visual acuity
• Contain the pigment iodopsin

Question 17

What are cone cells sensitive to? Why?

Answer 17

Cone cells are sensitive to high light intensities due to the presence of the pigment iodopsin.

Question 18

Describe what happens when iodopsin is exposed to bright light.

Answer 18

In bright light, iodopsin is broken down into its constituent parts, generating an action potential in the ganglion cell.

Question 19

What do the three different types of cone cell contain?

Answer 19

each types contains a different form of iodopsin.

Question 20

What is different about each form of iodopsin?

Answer 20

Each form absorbs a different wavelength of light – green, blue or red.

Question 21

What kind of light do rod cells allow vision in? Why?

Answer 21

Rod cells allow vision in dim light due to the presence of a pigment called rhodopsin, which is found in the membrane-bound vesicles.

Question 22

What happens to rhodopsin when it absorbs light? What is this known as?

Answer 22

When rhodopsin absorbs light it splits into its constituent parts, opsin and retinal. This is called bleaching.

Question 23

What does the presence of opsin do?

Answer 23

The presence of opsin causes a change in the permeability of the rod cell to sodium, which initiates a generator potential.

Question 24

In the absence of further light stimulation, what can happen to rhodopsin?

Answer 24

it can reform

Question 25

Describe the 4 steps that occur when rhodopsin is stimulated by a photon of light and a nerve impulse can be generated.

Answer 25

1. Excited rhodopsin activates G-proteins which, in turn, activate many enzyme molecules;
2. The enzyme molecules cause the closure of ion channels by catalysing the removal of molecules that keep channels open;
3. The inward leakage of positive ions (Na+ and Ca+) is halted so the membrane potential increases;
4. Hyperpolarisation (increasing charge) stimulates a nerve impulse.