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

Cara Head

Written by: Cara Head

Reviewed by: Naomi Holyoak

Updated on

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 light-dependent reactions where they are needed for production of organic molecules

The electron transport chain

  • The electron transport chain is a series of electron carrier proteins embedded within the thylakoid membrane of chloroplasts

  • 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. energized 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

  • At the end of the electron transport chain electrons combine with 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

  • 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 (Pi) to ADP

ADP + Pi → ATP

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

Diagram of the light-dependent reactions in photosynthesis, showing PSII, PSI, electron transport, ATP synthesis, and chemiosmosis within the thylakoid.
The light-dependent reactions of photosynthesis in eukaryotes involve a series of coordinated reaction pathways that capture energy present in light to yield ATP and NADPH

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Cara Head

Author: Cara Head

Expertise: Biology Content Creator

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding

Naomi Holyoak

Author: Naomi Holyoak

Expertise: Biology

Naomi graduated from the University of Oxford with a degree in Biological Sciences. She has 8 years of classroom experience teaching Key Stage 3 up to A-Level biology, and is currently a tutor and A-Level examiner. Naomi especially enjoys creating resources that enable students to build a solid understanding of subject content, while also connecting their knowledge with biology’s exciting, real-world applications.