LIGHT DEPENDENT REACTIONS OF PHOTOSYNTHESIS

(1) As light falls upon the pigments of the chloroplast, energy causes the electrons in photosystem II (p680) to leave the chlorophyll a.

(2) At the same time, light is absorbed by photosystem I (p700) and electrons are transferred to carrier proteins. Electrons that leave photosystem II, replace electrons leaving photosystem I.

( They move along a series of molecules called cytochromes called an electron transport chain. )

Plastiquinone transfers electrons from photosystem II to the cytochrome chain)

(3) Electrons from photosystem I move through another transport chain, and travel to the “stroma” side of the thylakoid membrane.

There, the electrons combine with protons and NADP+. The NADP+ becomes NADPH, a proton & electron carrier which is needed for the Calvin Cycle.

Note: Moving electrons during the light dependent reactions is really about a series of reduction-oxidation (“redox”) reactions in which compounds lose and gain electrons to maintain the electric current.

Here is a redox reactionin which hydrogen reactswith fluorine. Electronsleave hydrogen for thefluorine. Fluorine is reducedin charge and hydrogen isoxidized. H2 becomes H+ ions.

4) Electrons must replace those in photosystem II. This is accomplished as water decomposes, or splits (with the aid of an enzyme).

2H2O 4H+ + 4e- + O2

(For every 2 water molecules split, four electrons are produced & 2 ATPs are produced)

Here is one reason that plants need water.

watersplitting

5) The passage of electrons from photosystem II to photosystem I causes protons to be actively transported (against the concentration gradient) into the interior of the thylakoid. This is why the energy from absorbed from Photosystem II is responsible for ATP production. (Plastiquinone transports the protons)

H+ H+

H+ H+

H+

H+ H+

H+

Photophosphorylation – Light energy used to make ATP molecules (phosphorylate = adding phosphate to ADP)

PHOSPHORYLATION...   adding a phosphate to ADP

                 ADP   +   P    ------>    ATP

6) Protons move back to the stroma through ATP synthase. Since this occurs with the concentration gradient, energy is released, and is used to make ATP. This is called chemiosmosis.

ATP Synthase: ATP production

ALSO:

A “recirculation” of electrons from photosystem I can provide energy to produce ATP molecules. This is called cyclic phosphorylation. It is present in some plants and some bacterial photosynthesis. (Photosystem II is not involved)

Summary