Light-Dependent Reaction

The light-dependent reaction refers to a series of reactions in photosynthesis that require light energy. Light energy is used for three reactions in photosynthesis to: 

Light-Dependent Reaction Light-Dependent Reaction

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Table of contents
    1. Reduce NADP (nicotinamide adenine dinucleotide phosphate) and H+ ions to NDPH (addition of electrons).
    2. Synthesise ATP (adenosine triphosphate) from inorganic phosphate (Pi) and ADP (adenosine diphosphate).
    3. Split water into H+ ions, electrons, and oxygen.

    The overall equation for the light-dependent reaction is:

    $$\text{2 H}_{2}\text{O + 2 NADP}^{+}\text{ + 3 ADP + 3 P}_{i} \longrightarrow \text{O}_{2}\text{ + 2 H}^{+}\text{ + 2 NADPH + 3 ATP}$$

    The light-dependent reaction is referred to as a redox reaction as substances both lose and gain electrons, hydrogen, and oxygen in the process. When a substance loses electrons, loses hydrogen, or gains oxygen, it is called oxidation. When a substance gains electrons, gains hydrogen, or loses oxygen, it is referred to as reduction. If these happen simultaneously, redox.

    A good way to remember this (in relation to electrons or hydrogen) is through the acronym OIL RIG: Oxidation Is Loss, Reduction Is Gain.

    What are the reactants in the light-dependent reaction?

    The reactants for the light-dependent reaction are water, NADP+, ADP, and inorganic phosphate (\(\text{ P}_{i}\)).

    As you’ll see below, water is an essential part of photosynthesis. Water donates its electrons and H+ ions through a process called photolysis, and both of these things play a big part in the rest of the light-dependent reactions, particularly in the formation of NADPH and ATP.

    Photolysis refers to the reaction, during which the bonds between the atoms are broken by light energy (direct) or radiant energy (indirect).

    NADP+ is a type of coenzyme - an organic, non-protein compound that catalyses a reaction through binding with an enzyme. It is useful in photosynthesis as it can accept and deliver electrons - essential for a process full of redox reactions! It combines with electrons and H+ ions to form NADPH, an essential molecule for the light-independent reaction.

    The formation of ATP from ADP is a vital part of photosynthesis as ATP is often referred to as the cell’s energy currency. Like NADPH, it is used to fuel the light-independent reaction.

    The light-dependent reaction in stages

    There are three stages in light-dependent reaction: oxidation, reduction and generation of ATP. Photosynthesis takes place in the chloroplast (you can refresh your memory on the strcture in the photosynthesis article).

    Oxidation

    The light reaction occurs along the thylakoid membrane.

    When chlorophyll molecules, found in photosystem II (the protein complex) absorb light energy, the pair of electrons within the chlorophyll molecule are raised to a higher energy level. These electrons then leave the chlorophyll molecule, and the chlorophyll molecule becomes ionised. This process is called photoionisation.

    Water acts as an electron donor to replace the missing electrons in the chlorophyll molecule. This leads to water being oxidised, which means it loses electrons. Water is split into oxygen, two H+ ions, and two electrons through this process (photolysis). Plastocyanin (protein that mediates electron transfer) then carries these electrons from photosystem II to photosystem I for the next part of the light reaction.

    They also pass through plastoquinone (molecule involved in the electron transport chain) and cytochrome b6f (an enzyme), as you’ll be able to see in Figure 1, but these are not usually necessary to know about for A-level.

    The equation for this reaction is:

    $$ \text{2 H}_{2}\text{O} \longrightarrow \text{O}_{2} \text{ + 4 H}^{+} \text{ + 4 e}^{-} $$

    Reduction

    The electrons produced in the last stage enter photosystem I and reach the end of the electron transport chain. Using the enzyme NADP dehydrogenase as a catalyst (speeds up the reactions), they combine with an H+ ion and NADP+. This reaction produces NADPH (nicotinamide adenine dinucleotide phosphate hydrogen) and is referred to as a reduction reaction since NADP+ gains electrons. NADPH is sometimes referred to as “reduced NADP.”

    The equation for this reaction is:

    $$ \text{NADP}^{+} \text{+ H}^{+}\text{ + 2 e}^{-}\text{ }\longrightarrow \text{ NADPH} $$

    Ammonium Hydroxide Effect on Photosynthesis

    Various inhibitors can slow this process. One of these is ammonium hydroxide (NH4OH). The toxic effects of ammonia on many photosynthetic organisms have long been known. Ammonium hydroxide inhibits the enzyme NADP dehydrogenase, which subsequently prevents NADP+ from turning into NADPH at the end of the electron transport chain.

    You can learn more about this and other substances that impact the rate of photosynthesis in "investigating the rate of photosynthesis practical" article.

    Generation of ATP

    The final stage of the light-dependent reaction involves generating ATP.

    In the thylakoid membrane of the chloroplasts, ATP is generated by combining ADP with inorganic phosphate. This is done using an enzyme called ATP synthase. In previous stages of the light-dependent reaction, H+ ions have been produced through photolysis. This means there is a high concentration of protons in the thylakoid lumen, behind the membrane that separates this space from the stroma.

    Chemiosmotic theory

    The production of ATP can be explained by something called the chemiosmotic theory. Proposed in 1961 by Peter D. Mitchell, this theory states that most ATP synthesis comes from an electrochemical gradient established over the thylakoid disc membrane. This electrochemical gradient is established through the high concentration of H+ ions in the thylakoid lumen, and the low concentration of H+ ions in the stroma. These H+ ions can only cross the thylakoid membrane through ATP synthase as it is a channel protein - meaning it has a channel-like hole in it that protons can fit through. As these protons pass through ATP synthase, they cause the enzyme to change in structure. This catalyses the production of ATP from ADP and phosphate.

    The equation for this reaction is:

    $$ \text{ADP + P}_{i}\longrightarrow \text{ATP} $$

    What does the light-dependent reaction look like on a diagram?

    Figure 1 will help you visualise the light-dependent reaction. You’ll be able to see the electron flow from photosystem II to photosystem I, as well as the flow of H+ ions from the thylakoid lumen into the stroma via ATP synthase.

    What are the products of the light-dependent reaction?

    The products of the light-dependent reaction are oxygen, ATP, and NADPH.

    Oxygen is released back into the air after photosynthesis, whilst ATP and NADPH fuel the light-independent reaction.

    As discussed earlier, ATP is considered a transporter of energy. ATP is a nucleotide, made up of an adenine base which is attached to a ribose sugar and three phosphate groups (Figure 2). These three phosphate groups are linked to each other by two high-energy bonds, referred to as phosphoanhydride bonds. When one phosphate group is removed by breaking a phosphoanhydride bond, energy is released. This energy is then used in the light-independent reaction. NADPH functions as both an electron donor and energy source for various stages of the light-independent reaction.

    Light-Dependent Reaction - Key takeaways

    • The light-dependent reaction is a series of reactions in photosynthesis that require light energy.
    • The light-dependent reaction has three functions: to produce NADPH from NADP+ and H+ ions, to synthesise ATP from inorganic phosphate and ADP, and to break water into H+ ions, electrons, and oxygen.
    • The overall equation for the light-dependent reaction is: \( \text{2 H}_{2}\text{O + 2 NADP}^{+}\text{ + 3 ADP + 3 P}_{i} \longrightarrow \text{O}_{2}\text{ + 2 H}^{+}\text{ + 2 NADPH + 3 ATP} \)
    • The reactants of the light reaction are oxygen, ADP, and NADP+. The products are oxygen, H+ ions, NADPH and ATP. NADPH and ATP are both essential molecules for the light-independent reaction.
    Frequently Asked Questions about Light-Dependent Reaction

    Where does a light-dependent reaction take place?

    The light-dependent reaction takes place along the thylakoid membrane. This is the membrane of the thylakoid discs, which are found in the structure of the chloroplast. The relevant molecules for the light-dependent reaction are found along the thylakoid membrane: these are photosystem II, photosystem I, and ATP synthase.

    What happens in the light-dependent reactions of photosynthesis?

    The light-dependent reaction can be split into three stages: oxidation, reduction, and ATP synthesis. 


    In oxidation, water is oxidised through photolysis, meaning that light is used to split water into oxygen, H+ ions, and electrons. Oxygen is produced as a result, and the H+ ions go into the thylakoid lumen in order to facilitate the conversion of ADP to ATP. The electrons are produced and transferred down the membrane in an electron transfer chain, and the energy is used to power other stages of the light-dependent reaction.

    How is oxygen produced in light-dependent reactions?

    In the light-dependent reaction, oxygen is produced through photolysis. This involves the use of light energy to split water into its basic compounds. The end products of photolysis are oxygen, 2 electrons, and 2H+ ions.

    What do the light-dependent reactions of photosynthesis produce?

    The light-dependent reactions of photosynthesis produce three essential molecules. These are oxygen, NADPH (or reduced NADP), and ATP. Oxygen is released back into the air, whilst NADPH and ATP are used up in the light-independent reactions.

    How does ammonium hydroxide affect the light-dependent reaction?

    Ammonium hydroxide has an adverse effect on the light-dependent reaction. Ammonium hydroxide inhibits the enzyme that catalyses the reaction that turns NADP into NADPH, NADP dehydrogenase. This means that NADP cannot be reduced to NADPH at the end of the electron chain. Ammonium hydroxide also accepts electrons, which further slows the electron transport chain as fewer electrons will be carried along the thylakoid membrane. 


    Ammonium hydroxide also has a highly alkaline pH (around 10.09), which further inhibits the rate of the light-dependent reaction. Most of the light-dependent reactions are enzyme-controlled, so if the pH is too acidic or too alkaline, they will denature, and the reaction rate will sharply decrease.

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