9.1 Light reactions and photosystems

Photosynthesis: Light Reactions and Photosystems

Photosynthesis converts light energy into chemical energy, and the light reactions are where that conversion begins. These reactions take place in the thylakoid membranes of chloroplasts, where two photosystems work together to capture photons and drive electron transport. The end products, ATP and NADPH, provide the energy and reducing power that fuel the Calvin cycle.

Photosystems in Light Capture

Photosystems are large protein complexes embedded in the thylakoid membrane. Their job is to absorb light energy and convert it into a form the cell can use.

Each photosystem contains pigments organized into light-harvesting complexes (LHCs). These pigments include:

• Chlorophyll a — the primary pigment that directly participates in the reaction center
• Chlorophyll b and carotenoids — accessory pigments that absorb wavelengths chlorophyll a misses, then transfer that energy inward toward the reaction center

Think of the LHC as a funnel: dozens of pigment molecules absorb photons across a range of wavelengths and pass that energy, molecule to molecule, until it reaches the reaction center chlorophyll.

Two photosystems drive the light reactions, and they're numbered in the order they were discovered, not the order they function:

• Photosystem II (PSII) — reaction center pigment is P680, absorbing light most effectively at 680 nm
• Photosystem I (PSI) — reaction center pigment is P700, absorbing light most effectively at 700 nm

When light hits the reaction center chlorophyll, it excites an electron to a higher energy state. That energized electron is then captured by a nearby primary electron acceptor, which kicks off electron transport. In PSII, the first acceptor is pheophytin (which hands electrons to plastoquinone). In PSI, the first acceptor passes electrons toward ferredoxin.

Electron Transport Chain Function

The electron transport chain (ETC) is a series of protein complexes and mobile electron carriers in the thylakoid membrane. It connects PSII to PSI and uses the energy from electron transfer to build a proton gradient.

Here's how electrons move through the chain in order:

1. PSII absorbs light and sends an excited electron to plastoquinone (PQ), a mobile carrier in the membrane.
2. PQ shuttles electrons to the cytochrome b6f complex, a large protein that acts as a proton pump.
3. As electrons pass through cytochrome b6f, energy is released and used to pump H⁺ ions from the stroma into the thylakoid lumen, building a proton gradient.
4. Plastocyanin (PC), a small soluble protein in the lumen, carries electrons from cytochrome b6f to PSI.
5. PSI absorbs light and re-energizes the electrons, sending them through ferredoxin to the enzyme ferredoxin-NADP⁺ reductase (FNR).
6. FNR catalyzes the reduction of NADP⁺ + H⁺ → NADPH.

The proton gradient created by this process powers ATP synthase, an enzyme that lets H⁺ flow back into the stroma. That flow drives the phosphorylation of ADP + Pi into ATP. This mechanism is called chemiosmosis, and it's the same basic principle that mitochondria use during cellular respiration.

Photosystem I vs. Photosystem II

Even though PSII is numbered "II," it acts first in the linear electron flow. Here's a side-by-side comparison:

Water splitting (photolysis) happens at PSII. A cluster of manganese ions in the oxygen-evolving complex (OEC) strips electrons from water:

$2H_2O \rightarrow 4H^+ + 4e^- + O_2$

Those electrons replace the ones PSII lost when light excited them away. The oxygen you breathe is a byproduct of this reaction.

Both photosystems must absorb light simultaneously for linear electron flow to work. PSII feeds electrons into the chain, and PSI re-energizes them so they have enough reducing power to form NADPH.

Z-Scheme in Linear Electron Flow

The Z-scheme is a diagram that plots the redox potential (energy level) of electron carriers on the y-axis against the sequence of the light reactions on the x-axis. It gets its name from the Z-shaped path electrons trace as they zigzag between energy levels.

Reading the Z-scheme from left to right:

1. Water donates electrons to PSII at a very positive (low-energy) redox potential.
2. PSII absorbs light, boosting those electrons to a much more negative (high-energy) redox potential.
3. Electrons lose energy as they travel through the ETC (PQ → cytochrome b6f → PC), and that released energy pumps protons to create the gradient for ATP synthesis.
4. Electrons arrive at PSI at a lower energy level. PSI absorbs light and boosts them again to an even more negative redox potential than before.
5. These high-energy electrons reduce NADP⁺ to NADPH via ferredoxin and FNR.

The Z-scheme makes two things visually clear. First, two separate light-driven boosts are needed to move electrons from water all the way to NADPH, because no single photosystem provides enough energy to do it alone. Second, water is the ultimate electron donor and NADP⁺ is the terminal electron acceptor, with oxygen released as a byproduct along the way.