Light Reaction of Photosynthesis

The light reactions of photosynthesis contain two photosystems, two electron transport chains, and ATP synthase. These parts are embedded in the thylakoid membranes of a chloroplast. The photosystems contain a collection of chlorophyll molecules. Chlorophyll, which is the green pigment of leaves, absorbs light energy that excites electrons to a higher energy level. Energized electrons from photosystem I are passed down an electron transport chain and added to NADP+ to form NADPH. Electrons from photosystem II are passed down another electron chain. Their energy is used to pump hydrogen ions (H+) from the stroma into the thylakoid compartment, creating a concentration gradient. Electrons leaving this electron transport chain enter photosystem 1, refilling its lost electrons. Photosystem II replenishes its electrons by splitting water. Hydrogen ions and oxygen are released into the thylakoid compartment. This is where the oxygen gas generated by photosynthesis comes from. The hydrogen ion buildup inside the thylakoid compartment stores potential energy, which is harvested by an enzyme called ATP synthase. As hydrogen ions diffuse through ATP synthase, down their concentration gradient, the enzyme uses the energy of the moving ions to make ATP.

The main difference between C3 photosynthesis, C4 and CAM photosynthetic pathways is in the process of carbon dioxide fixation. The carbon dioxide captured by the Rubisco enzyme and added to RuBP inside the chloroplast produces 3-PGA, a three-carbon compound that gives the C3 process its name. In C4 and CAM photosynthesis, carbon dioxide is set in the cytoplasm of the mesophyll cells, not inside the chloroplasts. Carbon dioxide enters these cells through diffusion from the stoma. The oxaloacetate is produced from when phosphoenolpyruvate (PEP), PEP carboxylase, a highly efficient enzyme, reacts with carbon dioxide. The oxaloacetate quickly is converted into aspartic or malic acid (both of which are also 4-carbon molecules). The four-carbon molecules enter chloroplasts where the carbon dioxide will be released to be used in the Calvin cycle. The light reaction and the Calvin cycle are both the same in all types of photosynthesis. As opposed to C3photosynthesis, C4 photosynthesis and CAM photosynthesis first fix carbon dioxide into four-carbon compounds in the cytoplasm of the mesophyll cells, which then lowers the concentration gradient of carbon dioxide inside the leaf. This increases the diffusion of carbon