Table of Contents
Overview of the Light-Dependent Reactions
The light-dependent reactions are the first stage of photosynthesis. They:
- take place in the thylakoid membranes of chloroplasts,
- require light directly,
- convert light energy into chemical energy (ATP and NADPH),
- split water, releasing $O_2$ as a by-product.
They provide the ATP and NADPH needed for the subsequent light-independent (Calvin) cycle.
Key inputs: light, water, ADP + $P_i$, NADP$^+$
Key outputs: $O_2$, ATP, NADPH
Key Components Involved
You will encounter most of the following structures in the “Structure of the Photosystems” and “Fine Structure of Chloroplasts” chapters; here we focus only on their roles in the light-dependent reactions:
- Photosystem II (PSII) – starts the electron transport chain by using light to extract electrons from water.
- Photosystem I (PSI) – uses light to boost electrons to a higher energy level again for NADPH formation.
- Electron transport chain (ETC) – several protein complexes and mobile carriers that transfer electrons and pump protons.
- Plastoquinone (PQ) – mobile electron carrier between PSII and the cytochrome complex.
- Cytochrome b\$_6\$f complex – part of the ETC; pumps protons across the thylakoid membrane.
- Plastocyanin (PC) – mobile electron carrier between cytochrome b\$_6\$f and PSI.
- Ferredoxin (Fd) – mobile electron carrier on the stromal side downstream of PSI.
- Ferredoxin–NADP\(^+\) reductase (FNR) – enzyme that reduces NADP\(^+\)$ to NADPH.
- ATP synthase – membrane enzyme that uses the proton gradient to synthesize ATP.
Stepwise Process: Linear Electron Flow
In the “Sequence of the Light-Dependent Reactions” chapter, this will be laid out narratively. Here we focus on the functional steps and energy changes.
1. Photon Absorption in PSII and Excitation of P680
- A photon of light is absorbed by pigment molecules in the light-harvesting complex of PSII.
- Energy is transferred between pigment molecules until it reaches the reaction center chlorophyll, called P680 (absorbs maximally at 680 nm).
- P680 absorbs this energy and becomes excited (P680\*), with an electron at a higher energy level.
This excited electron is unstable and is immediately transferred to a primary electron acceptor.
Result:
- P680 loses one high-energy electron and becomes P680\(^+\) (strong oxidant).
- The electron has entered the electron transport chain.
2. Photolysis of Water and Oxygen Evolution
The oxidized P680\(^+\) is so strongly oxidizing that it can pull electrons from water:
- On the lumenal side of PSII, the oxygen-evolving complex (OEC), also called the water-splitting complex, catalyzes water oxidation.
- Overall reaction:
$$
2\,H_2O \rightarrow 4\,H^+_{(\text{lumen})} + 4\,e^- + O_2
$$
- The 4 electrons replace those lost from P680 (one electron per photon event, but the full cycle requires four).
- Protons released contribute to the proton gradient inside the thylakoid lumen.
- Molecular oxygen ($O_2$) is released as a by-product (diffuses out of the chloroplast and eventually the leaf).
Result:
- The electron deficit of P680 is replenished by electrons from water.
- $O_2$ production is uniquely tied to PSII activity.
3. Electron Transfer from PSII to Plastoquinone
The high-energy electron from P680\* travels through a short series of acceptors within PSII and then:
- Is passed to plastoquinone (PQ), a lipid-soluble carrier in the membrane.
- PQ is reduced to plastoquinol (PQH\$_2\$).
For each pair of electrons, PQ picks up 2 electrons and 2 protons from the stroma:
$$
PQ + 2\,e^- + 2\,H^+_{(\text{stroma})} \rightarrow PQH_2
$$
Result:
- Electrons are now in PQH\$_2$, ready to be delivered to the cytochrome b\$_6\$f complex.
- Two protons are “picked up” from the stroma and will be released into the lumen later, contributing to the proton gradient.
4. Cytochrome b\$_6\$f Complex and Proton Pumping
PQH\$_2$ delivers electrons to the cytochrome b\$_6\$f complex:
- PQH\$_2$ is oxidized back to PQ, releasing:
- its 2 electrons into the complex, and
- its 2 protons into the thylakoid lumen.
- Inside cytochrome b\$_6\$f, electrons move through a series of redox centers and exit on plastocyanin (PC).
Functionally important points:
- For each pair of electrons passing through, protons are moved from the stroma to the lumen:
- directly when PQH\$_2$ releases protons into the lumen,
- and via proton uptake on the stromal side during PQ re-reduction (details belong more in a biochemistry course).
Result:
- The proton gradient (difference in [H\^+] across the thylakoid membrane) increases.
- Electrons move “downhill” in energy from PQH\$_2$ to plastocyanin.
5. Electron Transport to PSI via Plastocyanin
- Plastocyanin (PC) is a small copper-containing protein located in the lumen.
- It accepts electrons from cytochrome b\$_6\$f and diffuses to PSI.
- PC donates electrons to the oxidized reaction center chlorophyll of PSI (P700\(^+\)) when needed.
Result:
- Electrons are delivered to PSI.
- PC cycles between reduced and oxidized forms, shuttling electrons within the lumen.
6. Photon Absorption in PSI and Excitation of P700
- Light is absorbed by PSI’s light-harvesting complex, and the energy is transferred to the reaction center chlorophyll P700 (absorbs maximally at 700 nm).
- P700 becomes excited (P700\*), and an electron is boosted to a higher energy level.
- This high-energy electron is transferred to PSI’s primary electron acceptor, leaving behind oxidized P700\(^+\).
At roughly the same time:
- P700\(^+\) is re-reduced by an electron coming from plastocyanin (PC).
- Thus, in linear flow, electrons have traveled: water → PSII → PQ → cytochrome b\$_6\$f → PC → PSI.
Result:
- PSI has created another high-energy electron, now at a higher energy level than after PSII alone.
- This electron is used to form NADPH.
7. Electron Transport from PSI to NADP\(^+\)
The excited electron from P700\* moves through a chain of acceptors:
- It passes through several iron–sulfur centers within PSI.
- Then it is transferred to ferredoxin (Fd), a small iron–sulfur protein on the stromal side.
Ferredoxin passes the electron to ferredoxin–NADP\(^+\) reductase (FNR):
- FNR catalyzes the reduction of NADP\(^+\)$ to NADPH using 2 electrons and a proton from the stroma:
$$
NADP^+ + 2\,e^- + H^+_{(\text{stroma})} \rightarrow NADPH
$$
Result:
- NADPH is produced, carrying high-energy electrons for use in the Calvin cycle.
- The electrons that originated from water now reside in NADPH.
Proton Gradient and Photophosphorylation (ATP Formation)
So far, several processes have contributed to a proton gradient ($\Delta pH$) across the thylakoid membrane:
- Protons released into the lumen from water splitting at PSII.
- Protons released into the lumen when PQH\$_2\$ is oxidized at cytochrome b\$_6\$f.
- Protons consumed in the stroma when NADP\(^+\)$ is reduced to NADPH.
This leads to:
- High [H\^+] in the thylakoid lumen,
- Low [H\^+] in the stroma.
Additionally, a small membrane potential (charge difference) also forms. Together they make the proton-motive force.
ATP Synthase and Chemiosmosis
The only major route for protons to flow back to the stroma is through ATP synthase, a large protein complex spanning the thylakoid membrane:
- Protons flow “downhill” from the lumen to the stroma through ATP synthase.
- This flow drives a rotational mechanism within ATP synthase.
- The mechanical energy is converted to chemical bond energy in ATP:
$$
ADP + P_i \xrightarrow[\text{ATP synthase}]{\text{proton flow}} ATP
$$
This light-driven ATP production is called photophosphorylation.
Result:
- ATP is produced on the stromal side, where it will be available for the Calvin cycle and other anabolic processes.
Linear vs. Cyclic Electron Flow Around PSI
So far we have described linear (non-cyclic) electron flow, where electrons travel:
$$
H_2O \rightarrow PSII \rightarrow PQ \rightarrow \text{cyt } b_6f \rightarrow PC \rightarrow PSI \rightarrow Fd \rightarrow NADP^+ \rightarrow NADPH
$$
This:
- produces both ATP and NADPH,
- releases $O_2$,
- is the main pathway under normal conditions.
However, chloroplasts can also use cyclic electron flow around PSI (covered in more detail in some treatments of photosynthesis; here just the essentials relevant to light reactions):
- In cyclic flow, electrons from ferredoxin (Fd) are diverted back to the cytochrome b\$_6\$f complex instead of going to FNR.
- They then return via plastocyanin to PSI.
Consequences:
- No NADPH is produced from these electrons, and no $O_2$ is evolved (water is not split specifically for cyclic flow).
- Additional protons are pumped via cytochrome b\$_6\$f, so more ATP is produced.
This allows the chloroplast to adjust the ATP/NADPH ratio to match the demands of the Calvin cycle and other metabolic needs.
Stoichiometry (Simplified)
Exact numbers can vary and are usually treated in more advanced courses, but a simplified picture for linear electron flow is:
- For every 2 water molecules oxidized:
- 4 electrons pass through the chain,
- 1 $O_2$ molecule is released.
Often, to produce one molecule of $O_2$ via the light reactions:
- 4 photons are absorbed by PSII and 4 photons by PSI (8 total, at minimum).
- This leads approximately to the synthesis of:
- 2 NADPH,
- ~3 ATP (exact value depends on assumptions about proton/ATP coupling).
These ATP and NADPH amounts are then used to fix and reduce $CO_2$ in the light-independent reactions.
Summary of the Light-Dependent Reactions
- Light energy is captured by PSII and PSI to boost electrons to higher energy states.
- Water is split at PSII, providing electrons and releasing $O_2$ and protons.
- Electrons travel through an electron transport chain to ultimately reduce NADP\(^+\)$ to NADPH.
- Electron movement and proton pumping create a proton gradient across the thylakoid membrane.
- ATP synthase uses this gradient to produce ATP (photophosphorylation).
- Linear electron flow produces $O_2$, ATP, and NADPH; cyclic flow produces extra ATP without $O_2$ or NADPH.
- The resulting ATP and NADPH power the subsequent light-independent reactions (Calvin cycle).