3.2 a level biology wjec

What is a pigment?

A molecule that absorbs specific wavelengths of light.

Why do many photosynthetic organisms have a mixture of pigments?

This allows the organism to absorb energy from a wider range of visible light wavelengths and so increases the efficiency of photosynthesis

Absorption spectra

a graph showing how much light is absorbed at different wavelengths

Action spectra

A graph showing the rate of photosynthesis at different wavelengths

Where do photosystems lie

In the plane of the thylakoid membrane

What does each photo system comprise of

an antenna complex and reaction centre

What constitutes a reaction centre

two molecules of chlorophyll a

absorption peak of photo system I

700nm

absorption peak of photo system II

680nm

primary pigment

chlorophyll a

accessory pigments

chlorophyll b and carotenoids

what does the light dependent stage produce

ATP, reduced NADP, and oxygen

What is photophosphorylation?

The addition of a phosphate ion to ADP.

What is non-cyclic photophosphorylation?

ATP can be produced by electrons that take a linear pathway from water, through PSII and PSI to NADP, which they reduce.

What is cyclic photophosphorylation?

ATP can be produced by electrons that take a cyclical pathway and are recycled back into the chlorophyll a in PSI.

Photolysis definition

The splitting of water molecules by light, producing hydrogen ions, electrons and oxygen.

What happens to the products of photolysis?

• the electrons replace those lost from PSII in non-cyclic photophosphorylation

• the protons from water and electrons from PSI reduce NADP

• oxygen diffuses out of the chloroplast and cell into intercellular air spaces, then out through the stoma as a waste product.

The passage of protons and phosphorylation

• As electrons pass through a proton pump in the thylakoid membrane, they provide energy to pump protons from the stroma into the thylakoid space. The protons join H+ ions from photolysis of water and accumulate, generating an electrochemical gradient.

• Chemiosmosis occurs. The H+ ions diffuse down their electrochemical gradient throigh ATP synthetase in the thylakoid membrane into the stroma. As they pass through ATP synthetase, ADP is phosphorylated into ATP.

• Once in the stroma, H+ ions are passed to oxidise NADP, reducing it.

What three factors maintain the proton gradient between the thylakoid space and the stroma?

• the proton pump associated with the electron transport chain pushing protons into the thylakoid space.

• The photolysis of water in the thylakoid space

• The removal of protons from the stroma, reducing NADP.