The Light-Dependent Reactions of Photosynthesis (College Board AP® Biology): Study Guide

The light-dependent reactions of photosynthesis

• Photosynthesis occurs in two stages:

  • The light-dependent stage

  • The light-independent stage

• The light-dependent reactions capture light energy and produce:

  • ATP

  • NADPH

• The products of the light-dependent reactions are passed to the Calvin Cycle (light-independent reactions), where they are needed for the production of organic molecules via metabolic reactions

The electron transport chain

• The electron transport chain is a series of electron carrier proteins embedded within the thylakoid membrane of chloroplasts (it also occurs in mitochondria during cellular respiration, and across prokaryotic plasma membranes)

• The events of the electron transport chain are as follows:

  1. Light energy is absorbed by photosystems I and II

  2. Light energy is transferred to electrons

  3. Energised electrons are passed along the electron transport chain

     • The electron carriers are alternately reduced and oxidized as electrons are passed along the chain

  4. Energy is released from the electrons as they pass down the ETC

  5. The energy released is used to generate a proton gradient

  6. Water is split, which provides the electrons to replace those lost from photosystem II

• At the end of the electron transport chain, electrons combine with protons (hydrogen ions) and the carrier molecule NADP to produce NADPH:

2H⁺ + 2e^- + NADP⁺ → NADPH

• Lost electrons are replaced as:

  • electrons from photosystem II pass to photosystem I

  • electrons from photolysis pass to photosystem II

The proton gradient and ATP

• The energy released as electrons pass along the ETC is used to actively transport protons (hydrogen ions) across the thylakoid membrane from the stroma to the thylakoid lumen

• This creates an electrochemical gradient

  • There is a high concentration of protons in the thylakoid lumen and a low concentration in the stroma

  • The thylakoid membrane acts to separate the two regions, maintaining a concentration gradient

• Protons return to the stroma by facilitated diffusion through transmembrane ATP synthase enzymes in a process called chemiosmosis

• This process provides the energy needed to synthesize ATP by adding an inorganic phosphate group (P_i) to ADP

ADP + P_i → ATP

• The process of ATP production described here is known as photophosphorylation