Photosynthesis Phase I: Light Reactions

By Eeshaal Hassan Khan

Introduction:

Have you ever wondered how plants eat without a mouth or how they turn sunlight into food? Welcome to the incredible world of photosynthesis — nature’s green magic trick that powers nearly all life on Earth by producing oxygen and the sugars in food. This process doesn’t just happen in the sunlight: while the first part of photosynthesis requires light, the second part can occur in the dark.

This article describes the first phase of photosynthesis: the light-dependent reactions. The energy from sunlight is stored in special molecules called ATP and NADPH — think of them as the plant’s rechargeable batteries. The goal of light reactions is to make the energy storing molecules ATP and NADPH using sunlight and water so that they can be utilized in sugar synthesis in the next phase.

An Overview of Light Reactions:

How exactly do plants absorb sunlight energy to be used for sugar synthesis? As soon as sunlight hits the leaf, tiny structures called chloroplasts jump into action (see Figure 1). Chloroplasts are small green structures found in plant cells that absorb sunlight and use it to make food for the plant through photosynthesis. They house coin-like structures called thylakoids that are stacked together to form a structure called granum. The inside space of chloroplasts surrounding the grana is filled with a fluid called the stroma.

Figure 1: Chloroplast: A plant cell organelle containing thylakoids with pigments that absorb light energy for photosynthesis.

Light energy is converted into chemical energy in the thylakoid membranes of thylakoids, which house Photosystem II (PS-II), Photosystem I (PS-1), and two electron transport chains. Inside the photosystems, a green pigment called chlorophyll absorbs the light energy like solar panels charging up. The photosystems harness energy from light to excite their electrons and the electron transport chain (ETC) is what electrons travel across to help generate ATP and NADPH.

Step I: Absorption of light by PS-II and excitation of its electrons

When two photons of light strike PS-II, energy begins to move along the atoms of different pigments within the photosystem (see Figure 2). Ultimately, the absorbed energy reaches a part of a photosystem specialized in converting absorbed light energy into chemical energy, called the reaction center (which, in PS-II, is called P680). The photosystems contain electrons that receive the energy which, as a reminder, originally came from the photons. The electrons are then taken up into a complex called the primary electron acceptor (see Figure 2). 

Figure 2: Photons of light are received by photosystems in the thylakoid membrane. The energy from the photons is moved across pigments into the reaction center, where it excites electrons into the primary electron acceptor. 

As these energized electrons are transferred from the reaction center to the primary electron acceptor, two empty electron slots are left behind in the photosystem. The electron holes of the photosystem must be filled; thus, water is split so that its electrons are used to fill the "electron holes" of PS-II. Its hydrogen ions are released into the stroma of the chloroplast, and oxygen is released as atmospheric oxygen, making the air fresh for us to breathe in.

Step II: Electron flow from PS-II to PS-I

The energized electrons must be transferred from PS-II to PS-I to generate ATP (which, you’ll remember, is one of the plant’s battery molecules). Henceforth, the electrons which have been released from PS-II now begin to flow to PS-I through an electron transport chain. The energy of these flowing electrons is utilized in synthesising ATP by a mechanism called chemiosmosis. 

When electrons move from PS-II’s primary electron acceptor across the electron transport chain, they pump the protons from the stroma to the thylakoid inner space (see Figure 3). In this way, the energy of flowing electrons creates a proton gradient (a difference in the accumulation of protons between two spaces). The proton gradient activates an enzyme in the thylakoid membrane called ATP synthase which not only moves the protons back into the stroma, but also catalyzes a reaction called photophosphorylation, in which ADP and a phosphate group are combined to form ATP (see Figure 3) . This ATP, generated by light reactions, will provide chemical energy for the synthesis of sugar during the next steps of photosynthesis. 

Figure 3: The energized electrons from PS-II move across the electron transport chain into PS-I. This process pumps protons into the thylakoid inner space, which then move through the ATP synthase enzyme to form ATP. 

Step III: Absorption of light by PS-1 and excitation of its electrons

After passing through the electron transport chain, the de-energized electrons move into the reaction center of PS-I, called P700. When PS-I absorbs two photons of light, the light energy is used to excite the electrons in P700, and they move through a second electron transport chain. An enzyme called NADP reductase transfers the electrons to NADP+. NADP+ also receives a hydrogen ion from the stroma—originating from the splitting of water in Step 1—to form NADPH, the plant’s other energy molecule in addition to ATP (see Figure 4). The NADPH will provide stored energy for the synthesis of sugar in the next steps of photosynthesis .

Figure 4: The energized electrons in PS-I travel across the second electron transport chain to form NADPH. 

Conclusion:

In summary, plants absorb sunlight using chloroplasts, where chlorophyll captures light energy within the thylakoid membranes. This energy excites electrons in Photosystem II, causing water to split and release oxygen while replacing lost electrons. The energized electrons then move through an electron transport chain, creating a proton gradient that drives ATP production. Finally, the electrons reach Photosystem I, where they help form NADPH, and together ATP and NADPH power sugar synthesis. 

The light reactions of photosynthesis do more than just help plants — they help keep us alive. When plants absorb sunlight, they use that energy to split water molecules, releasing oxygen into the air. Without photosynthesis, the air would run out of oxygen, and we couldn’t survive. But it goes beyond just breathing. The food we eat all starts with plants converting sunlight into energy. Even animals we rely on for meat or dairy exist because they eat plants. In short, when plants thrive, so does our health.

Works cited: 

• Khan Academy. (2015). Light-dependent reactions. Khan Academy; Khan Academy. LibreTexts. (2016, September 20). 2.21: Light Reactions of Photosynthesis. Biology LibreTexts. 

• National Book Foundation. (2024). Biology Textbook for 11th Grade: Federal Board Curriculum Guide. BooksNbooks. 

• Ort, D. R., Yocum, C. F., Heichel, I. F., & Netlibrary, I. (1996). Oxygenic photosynthesis : the light reactions. Kluwer. 

• Wikipedia Contributors. (2020, January 5). Light-dependent reactions. Wikipedia; Wikimedia Foundation.