Light-Dependent Reaction

Light-Dependent Reaction

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The light-dependent reaction is an important part of photosynthesis, the process by which plants and other organisms convert light energy into chemical energy. This reaction requires light energy to reduce NADP (nicotinamide adenine dinucleotide phosphate) and H+ ions to NDPH, synthesise ATP (adenosine triphosphate) from inorganic phosphate (Pi) and ADP (adenosine diphosphate), and split water into H+ ions, electrons, and oxygen. The overall equation for the light-dependent reaction is: Energy + 6CO2 + H2O → C6H12O6 + 6O2.

The light-dependent reaction is a redox reaction, meaning that substances both lose and gain electrons, hydrogen, and oxygen in the process. Oxidation is when a substance loses electrons, loses hydrogen, or gains oxygen, while reduction is when a substance gains electrons, gains hydrogen, or loses oxygen. A helpful acronym to remember this is OILRIG: Oxidation Is Loss, Reduction Is Gain.

What are the reactants in the light-dependent reaction?

In the light-dependent reaction, the reactants are water, NADP+, ADP, and inorganic phosphate (Pi). Water is a crucial part of photosynthesis, as it donates its electrons and H+ ions through a process called photolysis. This process plays a big role in the formation of NADPH and ATP, which are vital for the rest of the light-dependent reaction.

Photolysis refers to the process in which light energy (direct) or radiant energy (indirect) breaks the bonds between atoms.

NADP+ is a type of co that binds with catal a reaction. It is important in photosynthesis because it can accept and deliver electrons, which are essential for the redox reactions in the process. It combines with electrons and H+ ions to form NADPH, a molecule that is necessary for the light-independent reaction.

ATP is often called the cell's energy currency, and its formation from ADP is a critical part of photosynthesis. Like NADPH, ATP plays a significant role in fueling 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).


The light reaction takes place along the thylakoid membrane.

When light energy is absorbed by chlorophyll molecules in photosystem II, the electrons within the molecule are raised to a higher energy level. These then leave the chlorophyll molecule, causing it to become ionised. This process is called photoionisation.

To replace the missing electrons in the chlorophyll molecule, water acts as an electron donor. This leads to the oxidation of water, where it loses electrons. Through a process called photolysis, water is split into oxygen, two H+ ions, and two electrons. These electrons are carried from photosystem II to photosystem I by plastocyanin, a protein thatates electron transfer reaction electrons also pass through plastoquinone, a molecule involved in the electron transport chain, and cytochrome b6f, an enzyme. However, these are not typically necessary to know for A-levels.


The electrons produced in the previous stage enter photosystem I and travel through the electron transport chain. With the help of the enzyme NADP dehydrogenase, they combine with an H+ ion and NADP+. This reaction results in the production of NADPH (nicotinamide adenine dinucleotide phosphate hydrogen) and is called a reduction reaction since NADP+ gains electrons. NADPH is also known as "reduced NADP."

The equation for this reaction is:

Various inhibitors can slow down this process, such as ammonium hydroxide. Ammonium hydroxide inhibits the enzyme NADP dehydrogenase, which prevents NADP+ from being converted into NADPH at the end of the electron transport chain. You can read more about this and other substances that affect the rate of theInvestigating the Rate of Photosynthesis Practical" article.

Generation of ATP

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

In the thylakoid membrane of chloroplasts, ATP is created by combining ADP with inorganic phosphate using an enzyme called ATP synthase. During earlier stages of the light-dependent reaction, H+ produced through photolysis, creating a high concentration of protons in the thylakoid lumen behind the membrane that separates this space from the stroma.

The production of ATP can be explained by the chemiosmotic theory proposed by Peter D. Mitchell in 1961. According to this theory, most ATP synthesis is generated from an electrochemical gradient established across the thylakoid disc membrane. This gradient is created by the high concentration of H+ ions in the thylakoid lumen and the low concentration of H+ ions in the stroma. ATP synthase is a channel protein that allows H+ ions to move across the thylakoid membrane. As protons pass through ATP synthase, they cause the enzyme to change shape, catalyzing the production of ATP from ADP and phosphate.

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.

Light-dependent reaction in the chloroplast
Light-dependent reaction in the chloroplast.

What are the products of the light-dependent reaction?

The light-dependent reaction produces oxygen, ATP, and NADPH.

Oxygen is released back into the atmosphere during photosynthesis, while ATP and NADPH are used in the light-independent reaction.

As mentioned earlier, ATP is an energy transporter. It is composed of a nucleotide, which consists of an adenine base attached to a ribose sugar and three phosphate groups (Figure 2). These three phosphate groups are connected by two high-energy bonds, known as phosphoanhydride bonds. When one phosphate group is removed by breaking a phosphoanhydride bond, energy is released. This energy is utilized in the light-independent reaction. NADPH serves as both an electron donor and an energy source for various stages of the light-independent reaction.

The ATP molecule
The ATP molecule

In summary, the light-dependent reaction is a crucial in photosynthesis that requires light energy. It serves three functions: producing NADPH, synthesizing ATP, and breaking down water. The reactants of the light-dependent reaction are oxygen, ADP, and NADP+, while the products are oxygen, H+ ions, NADPH, and ATP. NADPH and ATP are both essential molecules for the light-independent reaction.

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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