Light Reactions

Two photosystems, the electron transport chain, and ATP synthase are the key components of the light reactions of photosynthesis. These parts are embedded in the thylakoid membranes of a chloroplast. The photosystems consist of arrays of chlorophyll molecules. Chlorophyll-- the green pigment of leaves-- absorbs light energy. The absorbed energy excites electrons to a higher energy level. Energized electrons from photosystem I are added to NADP+ to form NADPH, while energized electrons from photosystem II are passed through the electron transport chain. Their energy is used to pump hydrogen ions from the stroma into the thylakoid space, creating a concentration gradient. Electrons leaving the electron transport chain enter photosystem I, replenishing its lost electrons. Photosystem II replenishes its electrons by splitting water. Hydrogen ions and oxygen are released into the thylakoid space. This is where the oxygen gas comes from. The buildup of hydrogen ions inside the thylakoid space stores potential energy. This energy is harvested by an enzyme called ATP synthase. As hydrogen ions diffuse through the membrane, down their concentration gradient, ATP synthase uses the energy of the moving ions to make ATP. ATP and NADPH are later used in the sugar-making process of the Calvin cycle.

This diagram shows the transfer of energy.

The above graph shows where the absorption levels are greatest for chlorophyll. You notice that the peak is in the area of blue-violet light while the low point is at green (which is reflected).

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