The light-dependent reaction is a stage in photosynthesis, where light energy is absorbed by chlorophyll molecules in the thylakoid membranes of chloroplasts. Understanding the light-dependent reaction process in plants and the role of photosystems in the light-dependent reaction highlights the complex mechanisms by which plants harness solar energy for biological processes.

In this article, we will cover the light-dependent reaction of photosynthesis in detail.

Light-Dependent Reaction in Photosynthesis

The light-dependent reaction, also known as the light reaction, is the first stage of photosynthesis. It’s where plants, algae, and some bacteria capture the energy from sunlight and convert it into a usable form to fuel the next stage. In this reaction, chlorophyll and other pigments capture light energy. This energy splits water molecules into oxygen, protons, and electrons, releasing oxygen as a result.

Meanwhile, the energized electrons move through a series of carriers, producing ATP and NADPH, which are vital energy molecules. These products are then used in the Calvin cycle to make carbohydrates from carbon dioxide. In summary, the light-dependent reaction uses light energy to create chemical energy for the plant’s growth and development.

Where does Light-Dependent Reaction Take Place?

The light-dependent reaction occurs within the thylakoid membranes of chloroplasts, which are tiny structures found in plant cells. These membranes contain specialized molecules called chlorophyll and other pigments, which absorb light energy from the sun. This absorbed energy initiates a series of chemical reactions that split water molecules into oxygen, protons, and electrons.

As a result, oxygen is released as a byproduct. The energized electrons then move through a chain of carriers, generating ATP and NADPH, which are essential energy molecules used in the later stages of photosynthesis.

Role of Photosystems in Light-Dependent Reaction

There are two types of photosystems in the thylakoid membrane. Photosystem 2 (PS2) functions first (the number reflects the order of discovery) and is best at absorbing a wavelength of 680 nm. The reaction center chlorophyll of PS2 is called P680. Photosystem 1 (PS1) absorbes a wavelength of 700 nm. The reaction center chlorophyll of PS1 is called P700.

The thylakoid membranes of chloroplasts have two kinds of Photosystems, each with its own molecules. The pigments are organized into two discrete photochemical light-harvesting complexes (LHC) within Photosystem 1 (PS1) and Photosystem 2 (PS2). These are names in the sequence of their discovery, and not in the sequence in which they function during the light reaction. The LHC is made up of hundreds of pigment molecules bound to proteins.

Each Photosystem has all the pigments (except one molecule of chlorophyll a) forming a light-harvesting system also called antennae. These pigments help to make photosynthesis more efficient by absorbing different wavelengths of light. The single chlorophyll molecule forms the reaction center. The reaction center is different in both the photosystems. In PS1 the reaction center chlorophyll a had an absorption peak at 700 nm, hence is called P700, while in PS2 it has absorption maxima at 680 nm, and is called P680. All oxygen-evolving photosynthetic organisms contain Chl-a and two types of photosystems (PS1 and PS2). Photosynthetic bacteria which do not release oxygen lack Chl-a and PS2.

Red Drop and Emerson’s Effect 

Emerson and his co-workers exposed chlorella plants to only one wavelength of light at a time and measured the quantum yield. Such light with one wavelength is called monochromatic light. He plotted a graph of the quantum yield in terms of O² evolution at various wavelengths of visible light. He made this study to determine the wavelength of visible light. Furthermore, he made this study to determine the wavelength at which the photochemical yield of oxygen was maximum. The yield was almost constant in the region of 600-680 nm (red region), they fall in photosynthetic yield beyond the red region of spectrum is called Red Drop or Emerson’s first effect.

This is assumed due to non-functioning PS-2. Emerson and his co-workers modified the previous experiments by supplying shorter wavelengths of light (red light) along with longer wavelengths of light (far-red light beyond 680 nm). They found that the monochromatic light of longer wavelength (far-red light) supplemented with a shorter wavelength of light (red light), enhanced the photosynthetic yield in comparison to sum of yield in comparison to the sum of the yield when two photosystems operate independently. This led to the concept of two photosystems. This enhancement of photosynthetic yield is referred to as Emerson’s enhancement effect or Emerson’s second effect.

The number of photons (quanta) required to release one molecule of oxygen during photosynthesis is called the quantum requirement (8 quanta). The number of oxygen molecules released per photon in photosynthesis is called quantum yield. One oxygen molecule is released per eight photons of light absorbed, hence quantum yield is 1/8=0.025.

Key Points 

• The light response traps the energy from the sun and converts it into compound energy that is put away in NADPH and ATP.
• Oxygen is delivered as a side effect.

Light Dependent Reaction Process

Let’s discuss the light dependent reaction process occurs:

• Light Absorption: Chlorophyll and other pigments absorb light energy in the thylakoid membranes of chloroplasts.
• Water Splitting: This absorbed light energy initiates the splitting of water molecules into oxygen, protons (H+), and electrons (e-).
• Photosystem Activation: Light energy is utilized in both Photosystem I (PSI) and Photosystem II (PSII) within the thylakoid membranes.
• Electron Transport: Energized electrons move through a series of electron carriers, including cytochrome b6f, between PSII and PSI.
• ATP Synthesis: As electrons pass through the electron transport chain, ATP is generated through chemiosmosis and photophosphorylation.
• NADPH Production: In PSI, electrons are re-energized and used to reduce NADP+ to NADPH, a high-energy molecule.
• Oxygen Release: Oxygen is released as a byproduct of the water-splitting reaction.
• Electron Replenishment: Electrons lost from PSII are replenished by the splitting of water molecules, ensuring continuous electron flow.

Light Dependent Reaction

Overall function of light dependent reaction is that they convert light energy into chemical energy in the form of ATP and NADPH, which are used in the Calvin cycle to synthesize carbohydrates.

Light-Dependent Reaction Products

The products of light-dependent reaction are:

• ATP (Adenosine Triphosphate): Generated through photophosphorylation during the electron transport chain in the thylakoid membranes. ATP serves as a primary energy currency in cells.
• NADPH (Nicotinamide Adenine Dinucleotide Phosphate): Formed through the reduction of NADP+ by electrons from Photosystem I (PSI). NADPH acts as a reducing agent in biochemical reactions, transferring high-energy electrons to drive the synthesis of organic molecules.
• Oxygen (O2): Released as a byproduct of the photolysis of water molecules during the light-dependent reaction. Oxygen is essential for cellular respiration in plants and other organisms.
• Protons (H+): Released during the splitting of water molecules. These protons contribute to the formation of a proton gradient across the thylakoid membrane, which drives ATP synthesis.
• Electrons (e-): Transferred through the electron transport chain between Photosystem II (PSII) and Photosystem I (PSI). These electrons carry energy that is used to synthesize ATP and reduce NADP+ to NADPH, ultimately contributing to the production of carbohydrates in the Calvin cycle.

Difference Between Light Reaction and Dark Reaction

The difference between light reaction and dark reaction are given below: