Concept 8

A Closer Look at the Light Reactions

The light reactions use light energy to produce ATP and NADPH. The molecules that function in the light reactions are built into the thylakoid membrane. This equipment includes two complexes called photosystem I (PSI) and photosystem II (PSII). Each photosystem has many chlorophyll molecules along with other pigment molecules and proteins. One particular chlorophyll molecule of each photosystem is located in a region called the reaction center, where the energy absorbed from light initiates a transfer of electrons to other molecules.

Together, PSI and PSII move electrons from water to NADP+, forming NADPH. Absorption of light excites an electron from the reaction-center chlorophyll of PSI to a higher energy level. A molecule called a primary electron acceptor captures that high-energy electron and passes it, via an electron transport chain, to NADP+. The electron lost from PSI is replaced by an electron transferred from PSII via another electron transport chain. PSII replaces its electrons by extracting electrons from water molecules, releasing O2 as a byproduct.

The thylakoid membrane uses the flow of electrons through the transport chain between PSII and PSI to pump hydrogen ions (protons) from the stroma into the thylakoid space. This generates a store of potential energy in the form of a hydrogen ion gradient, with the H+ concentration in the thylakoid space exceeding that in the stroma. The ATP synthase, which harnesses the energy to produce ATP from ADP and inorganic phosphate. This mechanism of ATP synthesis is called photophosphorylation because it is driven by light.

When PSI and PSII both participate in the light reactions, this is called noncyclic photophosphorylation because electrons flow continuously from water to NADP+ without being recycled. Chloroplasts may sometimes perform a variation on the light reactions called cyclic photophosphorylation, in which PSI and the electron transport chain recycle electrons without participation by PSII. The cyclic electron flow generates a H+ gradient for ATP synthesis but produces no NADPH.

All of this will seem very confusing until you activate the noncyclic and cyclic animations and follow the electrons (red balls). (The animations simplify by showing one H+ transported for each ATP synthesized; the actual number is 2-3 H+ per ATP.)