Where Does O2 Gas Come From in Photosynthesis?

Light-Dependent Reactions

• The light-dependent reactions involve a series of steps to convert sunlight energy into usable chemical energy. Sunlight is collected in photosystems, which use sunlight energy to energize an electron. This electron passes from the photosystem to a series of redox reactions that create a proton gradient. The protons pass across a membrane through channels formed by the enzyme ATP synthase. As the protons pass through the channels, ATP synthase attaches a phosphate to ADP (adenosine diphosphate) to create ATP (adenosine triphosphate). ATP then provides chemical energy to drive the subsequent light independent reactions.

Photosystems

• The photosystems that harvest the sunlight are complex arrangement of pigments. Each pigment absorbs light at a specific wavelengths. Of all the pigments within the photosystem, chlorophyll is the pivotal pigment providing the electron that is energized. Two types of photosystems are associated with photosynthesis. Eukaryotes -- plants and algae -- have both photosystems, called Photosystem I and Photosystem II. Many photosynthetic bacteria possess only Photosystem II, which is associated with oxygen generation.

Chlorophyll A

• Several types of chlorophyll are found in photosystems, but the type involved in oxygen generation is chlorophyll A, specifically P680, the chlorophyll A found in Photosystem II. As the chlorophyll loses an electron, the electron is replaced from water molecules attached to the manganese ion complex within the pigment. As the electrons are stripped from water molecules, the water molecules split with the oxygen atoms to water molecules, combining to form oxygen gas. The hydrogens -- actually lone protons -- contribute to the proton gradient used for ATP synthesis. One oxygen molecule is generated for every four electrons -- two from each water molecule.

Photosystem I

• After completing the electron transport chain, the electron from Photosystem II becomes the electron for Photosystem I. Photosystem I re-energizes the electron using sunlight energy. The re-energized electron may be used to convert NADP+ to NADPH in non-cyclic phosphorylation or re-enter the electron transport chain during cyclic phosphorylation. In either case, oxygen is not generated since oxygen generation is limited to Photosystem II at the beginning of photosynthesis.