8.2 The Light-Dependent Reactions of Photosynthesis

Light Capture and Energy Conversion

Light capture by photosystem pigments

Photosystems are protein complexes embedded in the thylakoid membrane that capture light energy and convert it into chemical energy. Two photosystems work together to drive the light-dependent reactions: Photosystem II (PSII) and Photosystem I (PSI). Despite the numbering, PSII acts first in the sequence.

Each photosystem has a reaction center containing a specialized chlorophyll a molecule:

• PSII contains P680 (absorbs light best at 680 nm)
• PSI contains P700 (absorbs light best at 700 nm)

Surrounding each reaction center are antenna pigments that broaden the range of light the photosystem can use:

• Chlorophyll a is the primary pigment. It absorbs mainly blue (~430 nm) and red (~662 nm) light while reflecting green light, which is why plants look green.
• Accessory pigments like chlorophyll b and carotenoids (beta-carotene, lutein) absorb wavelengths that chlorophyll a misses. They pass the captured energy to chlorophyll a through resonance energy transfer, funneling it toward the reaction center.

When light energy reaches the reaction center chlorophyll, it excites an electron to a higher energy state. That excited electron is immediately grabbed by a primary electron acceptor, which kicks off the electron transport chain.

Electron transport for energy production

Once the primary electron acceptor captures the excited electron from PSII, that electron enters the electron transport chain (ETC). Here's how the process unfolds:

1. The excited electron from PSII passes to plastoquinone (Pq), a mobile electron carrier in the thylakoid membrane.
2. Plastoquinone delivers the electron to the cytochrome b6f complex, another key carrier in the chain.
3. As electrons move through the cytochrome b6f complex, energy is released and used to pump protons (H⁺) from the stroma into the thylakoid lumen. This builds a proton gradient across the thylakoid membrane, with H⁺ concentration much higher inside the lumen than in the stroma.
4. From the cytochrome b6f complex, electrons pass to plastocyanin (Pc), which delivers them to PSI.
5. PSI re-energizes the electrons using light, then passes them to ferredoxin (Fd).
6. The enzyme ferredoxin-NADP⁺ reductase uses those electrons (plus H⁺ from the stroma) to reduce NADP⁺ to NADPH.

The proton gradient built during electron transport doesn't go to waste. ATP synthase, an enzyme spanning the thylakoid membrane, lets protons flow back down their concentration gradient from lumen to stroma. This flow drives a rotational mechanism in ATP synthase that catalyzes the synthesis of ATP from ADP and inorganic phosphate (Pi). This process is called chemiosmosis.

The two key products of the light-dependent reactions, ATP and NADPH, then supply the energy and electrons needed for the Calvin cycle.

The Z-scheme and Photophosphorylation

The Z-scheme is a diagram that maps the energy level of electrons as they travel through the light reactions. It gets its name from the Z-shaped path electrons trace when you plot their energy changes on a graph:

• Electrons start at a low energy level in water.
• PSII boosts them to a high energy level, then they lose energy as they move through the ETC.
• PSI boosts them a second time, and they end up in NADPH.

The two "upward jumps" in energy correspond to the two photosystems absorbing light, while the "downhill" stretches represent energy being harvested for ATP production.

Photophosphorylation is the production of ATP using light energy. It comes in two forms:

• Non-cyclic photophosphorylation involves both PSII and PSI. It produces both ATP and NADPH, and it requires water as an electron source. This is the main pathway described above.
• Cyclic photophosphorylation involves only PSI. Electrons excited from PSI cycle back through the cytochrome b6f complex instead of going to NADP⁺. This generates additional ATP without producing NADPH. Cells use this pathway to fine-tune the ATP:NADPH ratio when the Calvin cycle demands more ATP than non-cyclic flow provides.

Photolysis and Oxygen Generation

Every time PSII donates an excited electron to the ETC, it needs a replacement. That replacement comes from water, through a process called photolysis (literally "light-splitting").

Photolysis occurs at the oxygen-evolving complex (OEC) of PSII, which contains a cluster of manganese ions that catalyze water oxidation. The overall reaction is:

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

Each product of this reaction has a specific role:

• Electrons (e⁻) replace the ones lost by P680, keeping the ETC running continuously.
• Protons (H⁺) are released into the thylakoid lumen, adding to the proton gradient that drives ATP synthase.
• Oxygen ($O_2$) is released as a byproduct. This is the source of nearly all atmospheric oxygen.

The oxygen generated by photolysis is essential for aerobic respiration in animals, plants, fungi, and many microbes. Over billions of years, photosynthetic oxygen production transformed Earth's atmosphere during the Great Oxidation Event, making the evolution of complex aerobic life possible.