Lesson Explainer: The Light-Dependent Reactions Biology

In this explainer, we will learn how to describe the reactions that occur in the light-dependent stage of photosynthesis and recall the products formed.

Plants, like humans, are multicellular, eukaryotic organisms that require nutrition to keep themselves alive and functioning. But, unlike humans, plants cannot move around looking for a source of food and do not possess specialized digestive systems to digest food. So, how do plants obtain their food?

Photosynthesis is the process by which plants obtain their nutrition. In a series of biochemical reactions, plants, using light energy, take in carbon dioxide and water and convert them into glucose and oxygen. This reaction is one of the most important reactions on the planet. Without plants photosynthesizing, we—and all other animals—would not have the oxygen that we need to breathe and to stay alive!

Photosynthesis can be divided into two major stages: firstly, the light-dependent stage and subsequently, the light-independent stage (also known as the Calvin cycle). In this explainer, we will look in more detail at the reactions involved in the light-dependent stage.

Definition: Photosynthesis

Photosynthesis is the process by which green plants convert carbon dioxide and water into sugars such as glucose and oxygen in the presence of sunlight.

Reaction: Photosynthesis in Plants

Carbondioxide+waterglucose+oxygenor6CO+6HOCHO+6Olightenergylightenergy2261262

Within plant cells, there is a specialized organelle that acts as the site of both of these stages of photosynthesis called the chloroplast. Figure 1 shows a simple outline of the structure of a chloroplast.

The light-dependent stage of photosynthesis takes place primarily in the thylakoid membrane. The thylakoids are disklike structures that form stacks within the chloroplast of a plant cell, as seen in Figure 1. They are well adapted to this role, as they contain photosystems. These photosystems contain specialized pigments, primarily chlorophyll, that absorb the light energy needed for the light-dependent reactions to take place.

Definition: Chlorophyll

Chlorophyll is a class of green pigments found in the chloroplasts of plants that absorbs the light energy required for photosynthesis.

Key Term: Photosystem

A photosystem is a complex of organic molecules, proteins, and photosynthetic pigments like chlorophyll found in the thylakoid membranes.

Figure 2 is a diagrammatic representation of the thylakoid membrane. As we can see, embedded in the membrane are the photosystems (I and II) that absorb light energy. Also, found within the membrane are electron carrier proteins, proton pumps, and the enzyme ATP synthase. Together, these structures make up the components of the electron transport chain.

As you can see from Figure 2, the main products of the light-dependent reactions are NADPH and ATP. Let’s have a look at the reactions that form these products in more detail.

In the first series of reactions, light falls on photosystem II. You may have noticed that in Figure 2, photosystem II comes before photosystem I. This is because photosystem I was actually discovered first. It was not until later, when photosystem II was discovered, that scientists realized that photosystem II is the photosystem involved in the first reactions of the light-dependent stage!

The chlorophyll pigments within photosystem II absorb this energy, and the electrons within them are excited. The electrons move to a higher energy level and are transferred to the primary electron acceptor within the photosystem.

As these electrons move to an electron acceptor, they need to be replaced. The absorption of light energy by photosystem II not only excites electrons to higher energy levels but is used to split a molecule of water into an oxygen atom and hydrogen ions, releasing electrons in the process. You can see this reaction in Figure 3. This reaction is known as photolysis, which means “breakdown using light.” This splitting of water releases electrons that are used to replace the excited electrons that have been transferred to the primary electron acceptor. It also releases two hydrogen (H+) ions into the interior of the thylakoid.

Definition: Photolysis

Photolysis is the breakdown of a molecule using light energy.

The electrons that have left photosystem II now move down the electron transport chain, releasing energy as they are passed between each component. This energy is used to actively transport more hydrogen ions from the stroma of the chloroplast into the thylakoid interior, as shown in Figure 4.

Example 1: Recalling How Electrons Are Supplied to the Electron Transport Chain in the Light-Dependent Stage of Photosynthesis

The diagram provided shows a basic outline of the light-dependent reactions. As electrons move from photosystem II to photosystem I, how are they replaced?

  1. By the movement of H+ ions
  2. By the photolysis of water
  3. By the absorption of light energy
  4. By the reduction of NADP+

Answer

The light-dependent stage of photosynthesis is the first stage, and it involves a series of reactions that rely on the movement of electrons down an electron transport chain. The initial step is for the pigments in photosystem II to absorb light energy, which excites the electrons contained within the chlorophyll, so they move to a higher energy level. After this, they are passed down proteins and enzymes in the electron transport chain. This means the electrons that were transported from photosystem II need to be replaced.

As we can see from the diagram, the first photosystem in the chain (which is named photosystem II) absorbs light energy. However, this does not provide the photosystem with electrons but instead provides the energy needed to excite electrons. We can also note from the diagram that NADP+ is not involved in the electron transport chain until after the electrons have passed through photosystem I, so NADP+ would not be able to replace these first electrons. The movement of hydrogen (H+) ions will not replace electrons, as electrons are negatively charged particles, and we can see hydrogen ions are positively charged. However, we can see from the diagram that at photosystem II, water (HO2) is split into its components, oxygen and hydrogen. This process, known as photolysis, also produces two electrons. These electrons are used to replace the electrons that left photosystem II to be moved along the electron transport chain.

Therefore, electrons are replaced by the photolysis of water.

As the electrons reach photosystem I, they are excited again, as the chlorophyll pigments in this photosystem absorb light energy. The electrons move to a higher energy level and are transferred to the primary electron acceptor in photosystem I. From here, they are passed to more protein carriers and eventually to the enzyme NADP+ reductase, as you can see in Figure 5.

NADP+ is a coenzyme found in the chloroplast. It acts as a universal electron acceptor. The electrons that have moved down the electron transport chain and have reached NADP+ reductase are used, along with a hydrogen ion, to reduce NADP+ to NADPH. This is an important step as NADPH is a key coenzyme used in the next stage of photosynthesis, the light-independent stage.

Key Term: NADPH (Reduced Nicotinamide Adenine Dinucleotide Phosphate)

NADPH is a coenzyme that can temporarily store electrons produced during light-dependent reactions.

Example 2: Describing the Role of NADP+ in the Light-Dependent Stage of Photosynthesis

What happens to the coenzyme NADP+ in the light-dependent stage of photosynthesis?

  1. It loses electrons and a phosphate ion to become NAD.
  2. It gains an oxygen ion to become oxidized NADP (NADPO+).
  3. It gains electrons and H+ ions to become reduced NADP (NADPH).
  4. It loses electrons and a H+ ion to form NADP+.

Answer

In the light-dependent stage of photosynthesis, electrons move down the electron transport chain located in the thylakoid membrane. Let’s look at a diagram that outlines this process.

These electrons are first excited in photosystem II, and then again in photosystem I. After moving to a higher energy level in photosystem I, they are passed to an enzyme called NADP+ reductase. As the name of this enzyme suggests, it catalyzes the reaction that reduces NADP+. Reduction is the term used to describe the gain of electrons. As NADP is reduced, it gains electrons as well as a hydrogen ion to form NADPH.

Looking at our options, we can see that only option C is correct. Therefore, in the light-dependent stage of photosynthesis, NADP+ gains electrons and H+ ions to become reduced NADP (NADPH).

You may recall that, in Figure 4, we saw how water is split by photolysis to release electrons and hydrogen ions. Alongside this, hydrogen ions are actively transported into the interior of the thylakoid by protein pumps.

The buildup of hydrogen (H+) ions into the interior of the thylakoid makes this space more positively charged than the surrounding stroma. This difference in the charge across the thylakoid membrane, and the concentration of the hydrogen ions, is called an electrochemical gradient.

Definition: Electrochemical Gradient

The electrochemical gradient is a gradient caused by the difference in charge across a membrane and the difference in the concentration of ions across a membrane.

The hydrogen ions will move down their electrochemical gradient by diffusion. Remember that diffusion is the movement of particles, in this case, hydrogen ions, from an area of high concentration to an area of low concentration. However, the hydrogen ions cannot just diffuse through the membrane, but instead, they must travel through an enzyme called ATP synthase—this process is shown below in Figure 6.

You may recognize the molecule ATP—it is the molecule that cells use to store energy! “Synthase” is derived from the term synthesis, and the suffix -ase indicates an enzyme. Therefore, we can derive the function of ATP synthase, which is to synthesize ATP.

As hydrogen ions move through ATP synthase from the thylakoid membrane to the stroma, the enzyme couples this movement to the phosphorylation of ADP. When ADP is phosphorylated, it has a phosphate group added to produce ATP; this is outlined in a simple diagram shown in Figure 7. The synthesis of ATP using an electrochemical gradient in this way is known as chemiosmosis.

Reaction: Phosphorylation

Phosphorylation is a biochemical reaction that involves the addition of a phosphate group to an organic compound.

Key Term: Chemiosmosis

Chemiosmosis is the movement of hydrogen ions across a membrane down their electrochemical gradient. In photosynthesis, this movement of ions is used to form ATP.

Example 3: Describing the Role of ATP Synthase in the Light-Dependent Reactions

What is the primary role of ATP synthase in the light-dependent reactions?

  1. To actively transport H+ ions from the stroma and into the thylakoid lumen
  2. To phosphorylate ADP to create ATP
  3. To actively transport electrons along the electron transport chain
  4. To absorb wavelengths of light to provide the energy needed for the light-dependent reactions

Answer

The light-dependent reactions are the first stage of photosynthesis. The main products of the light-dependent reactions are ATP and NADPH. This requires energy released from the movement of electrons down an electron transport chain and movement of hydrogen ions down their electrochemical gradient.

As electrons move down the electron transport chain, they release energy. This energy is used by proteins in the thylakoid membrane to actively transport hydrogen ions from the stroma and into the thylakoid space. This creates an electrochemical gradient of hydrogen ions as they are in a higher concentration within the internal space of the thylakoid than in the surrounding stroma.

The hydrogen ions then diffuse down this concentration gradient. However, they do not just pass through the membrane, but instead, they move through a specialized membrane enzyme. This enzyme is ATP synthase, which is responsible for catalyzing the production of ATP. The movement of hydrogen ions through ATP synthase is coupled to the phosphorylation of ADP, which forms ATP.

Therefore, the primary role of ATP synthase is to phosphorylate ADP to create ATP.

We have discussed the details of the series of reactions in the light-dependent stage of photosynthesis; so, let’s recap the key reactants and products in Table 1.

The products NADPH and ATP are very important for the continuation of photosynthesis. Both ATP and NADPH are coenzymes crucial for the light-independent stage. Oxygen is not used in further steps of photosynthesis but is incredibly useful for us, as this is how plants produce the oxygen we breathe!

Example 4: Recalling the Reactants and Products of the Light-Dependent Reactions

Which of the following tables correctly summarizes what happens to substances involved in the light-dependent reactions?

  1. SubstanceWaterADPNADP+
    EffectSynthesizedSynthesizedReduced
  2. SubstanceOxygenADPNADP+
    EffectPhotolyzedSynthesizedOxidized
  3. SubstanceWaterATPNADP+
    EffectPhotolyzedSynthesizedReduced
  4. SubstanceWaterATPNADP+
    EffectOxidizedSynthesizedOxidized

Answer

The light-dependent stage of photosynthesis is made up of a series of reactions that use light as an energy source. All of these reactions occur in the thylakoid membrane of a chloroplast, which is adapted for this function as it contains photosynthetic pigments to absorb and utilize light energy.

Let’s have a look at a diagram that summarizes the light-dependent reactions.

As we can see, an early step of these reactions is the splitting of water into its components, oxygen and hydrogen ions. This process, known as photolysis, also releases electrons from the water molecule.

As these electrons move down the electron transport chain, they release energy. This energy is used to actively transport hydrogen ions across the thylakoid membrane. Once there is a high concentration of hydrogen ions within the thylakoid space, they diffuse down their concentration gradient through ATP synthase. ATP synthase couples this movement of ions with the phosphorylation of ADP to produce ATP.

After the electrons in photosystem have been excited, they move onto a complex of proteins and enzymes that use the energy to reduce NADP+. This process produces the coenzyme NADPH.

Therefore, we can see from our options that the only correct answer is C; water is photolyzed, ATP is synthesized, and NADP+ is reduced.

The process we have reviewed so far is known as noncyclic photophosphorylation. If we break this down, “noncyclic” means that the process is linear—it acts in one direction only and does not cycle around. You may have noticed that once the electrons reached NADP+ reductase, they were used to reduce NADP+ and could not be reused. “Photophosphorylation” means that substances, ATP in this case, are phosphorylated using energy supplied by light.

Reaction: Photophosphorylation

Photophosphorylation is the process of generating ATP from ADP and an inorganic phosphate using energy provided by light.

A process called cyclic photophosphorylation can also occur in the light-dependent stage of photosynthesis and is outlined in Figure 8. The term cyclic indicates that electrons are reused to initiate the movement of hydrogen ions and the subsequent phosphorylation of ADP.

In this process, only photosystem I is used. No NADPH is produced, but the electrochemical gradient is still set up by the H+ ions. They will still move down their electrochemical gradient, through ATP synthase. This means ADP can still be phosphorylated to produce ATP.

Cyclic photophosphorylation may occur when the level of ATP within the chloroplast drops to a level that cannot sustain the light-independent reactions of photosynthesis. This way, plants can use a less efficient method of photophosphorylation to make sure all the reactions of photosynthesis still take place.

Example 5: Describing Photophosphorylation and Identifying the Difference between Cyclic and Noncyclic Processes

The diagram provided shows the process of cyclic photophosphorylation, which only uses one photosystem.

  1. What is meant by photophosphorylation?
    1. The addition of a phosphate group to ATP using photons
    2. The splitting of water using light energy
    3. The creation of ATP from the phosphorylation of ADP using light
    4. The movement of protons through proton pumps to generate energy
  2. Which of the following correctly compares cyclic and noncyclic photophosphorylation?
    1. Noncyclic photophosphorylation uses photosystems II and I, whereas cyclic photophosphorylation only uses photosystem II.
    2. Noncyclic photophosphorylation requires the movement of electrons, whereas cyclic photophosphorylation requires the movement of protons.
    3. Noncyclic photophosphorylation produces ATP and NADPH, whereas cyclic photophosphorylation only produces ATP.
    4. Cyclic photophosphorylation produces ATP and NADPH, whereas noncyclic photophosphorylation only produces ATP.

Answer

Part 1

To help us answer this question, let’s break down the word itself. “Photo” means “light”—you may recognize it from “photosynthesis,” which means “to make using light.” “Phosphorylation” refers to the addition of a phosphate group to a molecule or substance. In the process of photosynthesis, ADP is phosphorylated to produce ATP, going from a molecule with two phosphate groups to a molecule with three phosphate groups.

Therefore, photophosphorylation is the creation of ATP from the phosphorylation of ADP using light.

Part 2

The light-dependent stage of photosynthesis involves photophosphorylation of ADP to form ATP. This is catalyzed by the enzyme ATP synthase, which couples this reaction with the movement of hydrogen (H+) ions through the enzyme. This can happen in a cyclic process or a noncyclic process. Noncyclic photophosphorylation occurs as electrons move down the electron transport chain, from photosystem II to photosystem I. As they move down the chain, they transfer energy that is used to actively transport hydrogen ions and eventually, to reduce NADP+ to NADPH. Cyclic photophosphorylation, as shown in the diagram, only uses photosystem I. It also does not produce NADPH via the reduction of NADP+, but it does produce ATP via the phosphorylation of ADP.

So, our correct answer is C. Noncyclic photophosphorylation produces ATP and NADPH, whereas cyclic photophosphorylation only produces ATP.

Let’s recap the main points of this explainer.

Key Points

  • The light-dependent stage of photosynthesis occurs primarily in the thylakoid membrane.
  • The series of reactions involved in this stage are initiated by the absorption of light by photosystem II.
  • Electrons are excited and move down the electron transport chain, releasing energy as they move.
  • This energy is used to actively transport hydrogen ions into the thylakoid space, which creates an electrochemical gradient.
  • Hydrogen ions then move down this electrochemical gradient, through ATP synthase, which aids the phosphorylation of ADP to form ATP.
  • This process is known as noncyclic photophosphorylation, but cyclic photophosphorylation can also occur.
  • The products of the light-dependent reactions are oxygen, NADPH, and ATP.

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