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2.21: Light Reactions of Photosynthesis

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    f-d:e6774020946ba8dfc3166ead92ab4231656927e5f777952d05a970ec IMAGE_TINY IMAGE_TINY.1

    Oxygen has been described as a ‘‘waste product’’. How is this possible?

    Essentially, oxygen is a waste product of the light reactions of photosynthesis. It is a ‘‘leftover’’ from a necessary part of the process. All the oxygen that is necessary to maintain most forms of life just happens to come about during this process.

    Photosynthesis Stage I: The Light Reactions

    An overview of photosynthesis is available at

    Chloroplasts Capture Sunlight

    Every second, the sun fuses over 600 million tons of hydrogen into 596 tons of helium, converting over 4 tons of helium (4.3 billion kg) into light and heat energy. Countless tiny packets of that light energy travel 93 million miles (150 million km) through space, and about 1% of the light which reaches the Earth’s surface participates in photosynthesis. Light is the source of energy for photosynthesis, and the first set of reactions which begin the process requires light – thus the name, light reactions, or light-dependent reactions.

    When light strikes chlorophyll (or an accessory pigment) within the chloroplast, it energizes electrons within that molecule. These electrons jump up to higher energy levels; they have absorbed or captured, and now carry, that energy. High-energy electrons are “excited.” Who wouldn’t be excited to hold the energy for life?

    The excited electrons leave chlorophyll to participate in further reactions, leaving the chlorophyll “at a loss”; eventually they must be replaced. That replacement process also requires light, working with an enzyme complex to split water molecules. In this process ofphotolysis (“splitting by light”), H2O molecules are broken into hydrogen ions, electrons, and oxygen atoms. The electrons replace those originally lost from chlorophyll. Hydrogen ions and the high-energy electrons from chlorophyll will carry on the energy transformation drama after the light reactions are over.

    The oxygen atoms, however, form oxygen gas, which is a waste product of photosynthesis. The oxygen given off supplies most of the oxygen in our atmosphere. Before photosynthesis evolved, Earth’s atmosphere lacked oxygen altogether, and this highly reactive gas was toxic to the many organisms living at the time. Something had to change! Most contemporary organisms rely on oxygen for efficient respiration. So plants don’t just “restore” the air, they also had a major role in creating it!

    To summarize, chloroplasts “capture” sunlight energy in two ways. Light ‘‘excites’’ electrons in pigment molecules, and light provides the energy to split water molecules, providing more electrons as well as hydrogen ions.

    Light Energy to Chemical Energy

    Excited electrons that have absorbed light energy are unstable. However, the highly organized electron carrier molecules embedded in chloroplast membranes order the flow of these electrons, directing them through electron transport chains (ETCs). At each transfer, small amounts of energy released by the electrons are captured and put to work or stored. Some is also lost as heat with each transfer, but overall the light reactions are extremely efficient at capturing light energy and transforming it into chemical energy.

    Two sequential transport chains harvest the energy of excited electrons, as shown in Figure below.

    (1) First, they pass down an ETC, which captures their energy and uses it to pump hydrogen ions by active transport into the thylakoids. These concentrated ions store potential energy by forming a chemiosmotic or electrochemical gradient – a higher concentration of both positive charge and hydrogen inside the thylakoid than outside. (The gradient formed by the H+ ions is known as a chemiosmotic gradient.) Picture this energy buildup of H+ as a dam holding back a waterfall. Like water flowing through a hole in the dam, hydrogen ions “slide down” their concentration gradient through a membrane protein which acts as both ion channel and enzyme. As they flow, the ion channel/enzyme ATP synthase uses their energy to chemically bond a phosphate group to ADP, making ATP.

    (2) Light re-energizes the electrons, and they travel down a second electron transport chain (ETC), eventually bonding hydrogen ions to NADP+ to form a more stable energy storage molecule, NADPH. NADPH is sometimes called “hot hydrogen,” and its energy and hydrogen atoms will be used to help build sugar in the second stage of photosynthesis.

    The electron transport chain in photosynthesis

    Membrane architecture: The large colored carrier molecules form electron transport chains which capture small amounts of energy from excited electrons in order to store it in ATP and NADPH. Follow the energy pathways: light → electrons → NADPH (blue line) and light → electrons → concentrated H+ → ATP (red line). Note the intricate organization of the chloroplast.

    NADPH and ATP molecules now store the energy from excited electrons – energy which was originally sunlight – in chemical bonds. Thus chloroplasts, with their orderly arrangement of pigments, enzymes, and electron transport chains, transform light energy into chemical energy. The first stage of photosynthesis – light-dependent reactions or simply light reactions – is complete.

    For a detailed discussion of photosynthesis, see (20:16) and (18:51).


    • The light reactions capture energy from sunlight, which they change to chemical energy that is stored in molecules of NADPH and ATP.
    • The light reactions also release oxygen gas as a waste product.

    Explore More

    Use this resource to answer the questions that follow.

    1. How long does it take solar photons of light to reach Earth?
    2. What happens when chlorophyll is struck by sunlight?
    3. What is the immediate fate of the energy absorbed by chlorophyll?
    4. What is a by-product of the light reactions?


    1. Summarize what happens during the light reactions of photosynthesis.
    2. What is the chemiosmotic gradient?
    3. Explain the role of the first electron transport chain in the formation of ATP during the light reactions of photosynthesis.

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