Describe What Happens During Light Dependent Reactions of Photosynthesis

Photosystems

• Photosystems are collections of pigments that capture light energy and begin the reactions of photosynthesis. While multiple pigments are arranged within the photosystem, the central pigment is chlorophyll a. Two distinct photosystems are associated with the light-dependent reactions. Bacteria typically possess only Photosystem II which uses a form of chlorophyll a known as P680, due to its optimal absorbance of light with a wavelength of 680 nm. Eukaryotes (plants and algae) possess Photosystem II and Photosystem I. Photosystem I uses chlorophyll a that absorbs light optimally at 700 nm and hence is known as P700.

Chlorophyll a

• Chlorophyll a uses the captured light energy to energize an electron which is then transported out of the photosystem and on to the rest of the light-dependent reactions. Chlorophyll a replaces its lost electron from a water molecule. As electrons are stripped from the water molecules, the oxygen atoms from two water molecules combine to form oxygen gas which is released. The hydrogens, now lone protons, contribute to the proton gradient created in subsequent steps of the light-dependent reactions.

Electron Transport

• The energized electron from Photosystem II is released to a series of carrier molecules on a membrane. As the electron is transferred between these carrier molecules through a series of redox reactions, the energy from the electron is used to pump protons across the membrane, creating a proton gradient across the membrane. Within eukaryotes, the protons are concentrated within the spaces formed by the thylakoid membranes of the chloroplast. Bacteria use specific infoldings of the plasma membrane to create the enclosed spaces needed to concentrate the protons.

Non-Cyclic and Cyclic Photophosphorylation

• In non-cyclic photophosphorylation, after completing the reactions, the electron enters Photosystem I where it is re-energized and completes a separate series of reactions that reduces NADP+ to NADPH, an energy-carrying molecule needed in the light-independent reactions. The electron is used in the end to reduce NADP+ to NADPH, an energy-carrier molecule used in the light-independent reactions. In cyclic photophosphorylation, the electron re-energized by Photosystem I returns to the electron transport chain. Cyclic photophosphorylation allows additional ATP to be generated instead of generating the NADPH at the end of non-cyclic photophosphorylation.

ATP Synthesis

• The protons concentrated on one side of the membrane are allowed to flow across the membrane through specific channels formed by the enzyme ATP synthase. ATP synthase couples the flow of these protons to the formation of the energy molecule ATP (adenosine triphosphate) from ADP (adenosine diphosphate) and a phosphate group. Collectively, the light-dependent reactions are often referred to as photophosphorylation, as the overall effect is to add a phosphate to ADP using light energy. The ATP is then used as the energy fuel for the light-independent reactions where carbon dioxide is fixed into organic molecules.