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:

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.

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.

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.

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 (${\mathrm{NH}}_4\mathrm{OH}$). 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.

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.