Difficulty: Intermediate | Prerequisites: Part I notes on light, pigments and chloroplast structure; basic understanding of redox reactions and the concept of electrochemical gradients.
The light-dependent reactions use two photosystems (PS II and PS I) working in series to move electrons from water to NADP⁺, producing NADPH. Along the way, the electron transport chain pumps H⁺ ions into the thylakoid lumen, creating a concentration gradient that drives ATP synthase to make ATP. The ATP and NADPH then fuel the Calvin cycle.
Photosystem
A cluster of pigment molecules and proteins embedded in the thylakoid membrane that captures light energy and channels it to a reaction centre. Think of it as a satellite dish that collects light and funnels it to one spot.
Photosystem II (PS II)
The first photosystem in the light-dependent pathway (despite its numbering). Its reaction centre chlorophyll absorbs light best at 680 nm (called P680). It splits water and feeds electrons into the electron transport chain.
Photosystem I (PS I)
The second photosystem in the sequence. Its reaction centre chlorophyll absorbs light best at 700 nm (called P700). It re-energises electrons and passes them to NADP⁺ reductase to make NADPH.
Reaction centre
The specific chlorophyll a molecule (and associated proteins) at the core of a photosystem that transfers an excited electron to the primary electron acceptor. The surrounding antenna pigments funnel energy here.
Primary electron acceptor
The molecule that receives the excited electron from the reaction centre, beginning the electron transport chain. In simple terms, it is the first handoff point after the photosystem captures light energy.
Electron transport chain (ETC)
A series of protein complexes and mobile electron carriers in the thylakoid membrane that pass electrons from one to the next, releasing energy at each step to pump H⁺ ions across the membrane.
Z scheme
The name for the overall path electrons take through the light-dependent reactions, from H₂O through PS II, the ETC, PS I, and finally to NADP⁺. When plotted on an energy diagram it traces a zigzag (Z) shape as electrons are boosted twice by light energy.
Plastoquinone (Pq)
A mobile electron carrier that shuttles electrons from PS II to the cytochrome b6f complex within the thylakoid membrane.
Cytochrome b6f complex
A protein complex in the thylakoid membrane between PS II and PS I. It transfers electrons from plastoquinone to plastocyanin and uses the energy released to pump H⁺ ions into the thylakoid lumen.
Plastocyanin (Pc)
A small, water-soluble protein that carries electrons from the cytochrome b6f complex to PS I.
Ferredoxin (Fd)
An electron carrier on the stromal side of PS I that passes electrons to NADP⁺ reductase.
NADP⁺ reductase
The enzyme that transfers electrons (and H⁺) from ferredoxin to NADP⁺, reducing it to NADPH.
NADPH
The reduced form of NADP⁺. It carries high-energy electrons and hydrogen to the Calvin cycle, where they are used to reduce CO₂ to sugar. Think of it as an electron shuttle bus heading to the sugar factory.
Photophosphorylation
The production of ATP from ADP and inorganic phosphate (Pᵢ), driven by the light-dependent flow of H⁺ through ATP synthase. It is the chloroplast's version of oxidative phosphorylation.
ATP synthase
The enzyme complex spanning the thylakoid membrane through which H⁺ ions flow down their concentration gradient, powering the synthesis of ATP from ADP + Pᵢ.
Chemiosmosis
The process by which a proton (H⁺) gradient across a membrane is used to drive ATP synthesis. The same principle operates in both chloroplasts and mitochondria.
Photolysis (water splitting)
The light-driven splitting of water molecules at PS II, releasing electrons (which replace those lost by P680), H⁺ ions (which contribute to the gradient), and O₂ as a byproduct.
Variegation
Patches of different colour on a leaf, typically green and white, caused by cells with fewer or no functional chloroplasts in the white areas. Variegated leaves have less total photosynthetic capacity.
A photosystem is a multi-protein complex embedded in the thylakoid membrane.
It contains hundreds of antenna (accessory) pigment molecules that absorb light and funnel the energy to a single reaction centre chlorophyll a.
Most photosynthetic plants have two photosystems, PS II and PS I, which work in series.
Contains the reaction centre pigment P680 (absorbs 680 nm light most efficiently).
Inputs: light energy, H₂O.
Outputs: excited electrons (to the primary electron acceptor), O₂ (byproduct of water splitting), H⁺ ions (released into the thylakoid lumen).
PS II is where water is split (photolysis): 2H₂O → 4H⁺ + 4e⁻ + O₂.
The electrons from water replace those that P680 lost when light excited them away.
Contains the reaction centre pigment P700 (absorbs 700 nm light most efficiently).
Inputs: light energy, electrons arriving via plastocyanin from the ETC.
Outputs: high-energy electrons passed to ferredoxin, then on to NADP⁺ reductase.
PS I re-energises the electrons that lost energy travelling through the ETC between PS II and PS I.
Feature | Photosystem II | Photosystem I |
|---|---|---|
Reaction centre | P680 | P700 |
Electron source | H₂O (photolysis) | Plastocyanin (from ETC) |
Electron destination | Primary electron acceptor → Pq → ETC | Ferredoxin → NADP⁺ reductase |
Key output | O₂ (byproduct), H⁺ ions | NADPH |
Acts first in sequence | Yes | Second |
The Z scheme describes the complete electron journey through the light-dependent reactions. Follow this sequence:
Light strikes PS II. A photon excites an electron in P680 to a higher energy level.
The excited electron is captured by the primary electron acceptor.
The electron passes along the ETC: from the primary electron acceptor to plastoquinone (Pq), then to the cytochrome b6f complex, then to plastocyanin (Pc). Energy is released at each transfer and used to pump H⁺ into the thylakoid lumen.
Light strikes PS I. A photon excites an electron in P700 to a higher energy level.
The electron passes to ferredoxin (Fd), then to NADP⁺ reductase, which combines 2 electrons + H⁺ + NADP⁺ to form NADPH.
Meanwhile, the electron that P700 lost is replaced by the electron arriving from plastocyanin (step 3). The electron that P680 lost is replaced by splitting water.
Occurs at PS II, on the lumen side of the thylakoid membrane.
An enzyme called the oxygen-evolving complex (OEC) catalyses the reaction.
2H₂O → 4H⁺ + 4e⁻ + O₂.
The electrons replace those donated by P680.
The H⁺ ions add to the proton gradient in the lumen.
The O₂ diffuses out of the chloroplast and, eventually, out of the leaf through stomata.
The oxygen released in photosynthesis comes entirely from water, not from CO₂. Classic experiments using isotope-labelled water (H₂¹⁸O) showed that the ¹⁸O appeared in the O₂ gas, confirming water as the source.
Three processes contribute to a high concentration of H⁺ inside the thylakoid lumen (relative to the stroma):
Water splitting at PS II releases H⁺ directly into the lumen.
The cytochrome b6f complex actively pumps H⁺ from the stroma into the lumen as electrons pass through it.
NADP⁺ reductase consumes H⁺ from the stroma (to make NADPH), further lowering stromal H⁺ concentration.
The result is a steep electrochemical gradient: high H⁺ in the lumen, low H⁺ in the stroma.
H⁺ ions flow down their concentration gradient, from the thylakoid lumen back into the stroma, through ATP synthase.
This flow drives the enzyme to phosphorylate ADP + Pᵢ → ATP.
This process is called photophosphorylation because the proton gradient was ultimately established by light energy.
NADPH and ATP are the two products of the light-dependent reactions that carry energy forward.
They travel from the thylakoid membrane into the stroma, where the Calvin cycle uses them to fix CO₂ into sugar (G3P).
NADPH provides the reducing power (electrons and hydrogen). ATP provides the energy to drive the endergonic reactions of carbon fixation.
The ATP produced during the light reactions is used primarily within the chloroplast (for the Calvin cycle). It does not typically leave the chloroplast to power other cellular processes.
Both organelles use chemiosmosis to make ATP. The table below highlights the parallels and differences.
Feature | Mitochondrion | Chloroplast |
|---|---|---|
Similar components | ETC, ATP synthase, membrane-bound compartments | ETC, ATP synthase, membrane-bound compartments |
Location of higher H⁺ | Intermembrane space | Thylakoid lumen (interior of thylakoid) |
Electron source | NADH and FADH₂ (from glucose breakdown) | H₂O (photolysis at PS II) |
How electrons gain energy | Chemical energy from oxidising food molecules | Light energy absorbed by photosystems |
Final electron acceptor | O₂ (forming H₂O) | NADP⁺ (forming NADPH) |
Energy released by ETC used for | Pumping H⁺ from matrix into intermembrane space | Pumping H⁺ from stroma into thylakoid lumen |
Name of ATP-generating process | Oxidative phosphorylation | Photophosphorylation |
Variegated plants have patches of white or pale tissue on their leaves where cells lack functional chloroplasts (or contain far fewer). Because those cells cannot photosynthesise, the total photosynthetic capacity of a variegated leaf is lower than that of an entirely green leaf of the same size.
Data from the tutorial shows that variegated plants tend to produce fewer leaves and vines compared to their fully green counterparts, indicating a reduced growth rate.
A plausible hypothesis: variegated plants fix less carbon overall because a significant portion of their leaf area lacks chlorophyll. With less sugar produced per unit of leaf surface, the plant has less energy and fewer carbon skeletons available for growth, resulting in slower production of new leaves and stems.
Students often think PS I comes before PS II in the electron flow because of the numbering. PS II acts first; the numbering reflects the order of discovery, not the order of function.
Students sometimes believe the ATP made in chloroplasts is exported to power the rest of the cell. The ATP from the light reactions is used mainly within the chloroplast for the Calvin cycle. The cell's general ATP supply comes from mitochondria.
Students often mix up which side of the thylakoid membrane has high H⁺. High H⁺ is in the thylakoid lumen (inside), and H⁺ flows out into the stroma through ATP synthase. This is the opposite direction from mitochondria, where H⁺ is high in the intermembrane space and flows into the matrix.
Students sometimes think that blocking the ETC between PS II and PS I would stop water from being split. Water splitting at PS II still occurs as long as light hits PS II, but the electrons have nowhere productive to go. The real effect of blocking the ETC (as with the herbicide atrazine) is that the proton gradient collapses, ATP production drops, and the plant starves.
⚠️ Be able to trace the full path of electrons from H₂O → PS II → Pq → cytochrome b6f → Pc → PS I → Fd → NADP⁺ reductase → NADPH.
⚠️ Know how to fill in a blank Z-scheme diagram.
⚠️ Understand the isotope-labelling experiment: H₂¹⁸O produces ¹⁸O₂, confirming oxygen comes from water. H₂O + C¹⁸O₂ does not produce labelled O₂.
⚠️ The herbicide atrazine blocks the ETC between PS II and PS I. The result is decreased ATP production (answer A on the tutorial), which kills the plant.
⚠️ Be able to compare chemiosmosis in mitochondria and chloroplasts, including the direction of H⁺ flow and the electron sources.
True or False: PS I acts before PS II in the electron transport chain. (False, PS II acts first)
Fill in the blank: The electrons flowing through the ETC between PS II and PS I create a _______ (proton / H⁺ gradient).
Fill in the blank: Electrons flowing through the ETC after PS I are transferred to NADP⁺, reducing it to _______ (NADPH).
True or False: The ATP produced in the light-dependent reactions is used primarily to power processes outside the chloroplast. (False, it is used mainly for the Calvin cycle within the chloroplast)
Fill in the blank: The flow of H⁺ through ATP synthase drives the photophosphorylation of _______ to _______ (ADP to ATP).
Q: Where do the light-dependent reactions of photosynthesis occur?
A: In and across the thylakoid membranes of the chloroplast. The photosystems, electron transport chain components and ATP synthase are all embedded in or associated with the thylakoid membrane.
Q: What is the source of the electrons that move through the photosystems to NADPH? What is the source of the oxygen produced?
A: Both come from water. Photolysis at PS II splits H₂O into electrons, H⁺ ions and O₂. The electrons replace those lost by P680 and ultimately travel through the chain to NADP⁺, forming NADPH. The O₂ is released as a byproduct.
Q: Atrazine binds to and disrupts electron flow through the ETC between PS II and PS I. How does atrazine kill plants?
A: By decreasing ATP production (answer A). Blocking electron flow prevents the proton gradient from being maintained, so ATP synthase cannot produce sufficient ATP. Without ATP (and without NADPH reaching PS I properly), the Calvin cycle stalls and the plant cannot make the sugars it needs to survive.
Q: Which experiment will produce labelled ¹⁸O₂: Experiment 1 (H₂¹⁸O + CO₂) or Experiment 2 (H₂O + C¹⁸O₂)?
A: Experiment 1. The oxygen in O₂ comes from water, not from CO₂. So only the experiment using isotope-labelled water (H₂¹⁸O) will produce labelled ¹⁸O₂.
Q: Fill in the blanks for the Z scheme summary: A pigment absorbs light energy, increasing the _____ of an electron. This energy is transferred to the reaction centre, which transfers an electron to a _____. NADPH and _____ go to the Calvin cycle.
A: A pigment absorbs light energy, increasing the energy level (or excitation state) of an electron. This energy is transferred to the reaction centre, which transfers an electron to a primary electron acceptor. NADPH and ATP go to the Calvin cycle.
The chemiosmosis mechanism here is the same principle you studied in cellular respiration (oxidative phosphorylation in mitochondria). Comparing the two reinforces how universal the proton-motive force is in biology. The electron transport chain concepts also connect to redox chemistry from earlier in the course. The variegation discussion ties into genetics and gene expression, since variegation often results from mutations affecting chloroplast development.
Photosystem I, photosystem II, PS I, PS II, P680, P700, reaction centre, primary electron acceptor, electron transport chain, Z scheme, noncyclic electron flow, plastoquinone, Pq, cytochrome b6f, plastocyanin, Pc, ferredoxin, Fd, NADP reductase, NADPH, NADP+, photophosphorylation, ATP synthase, chemiosmosis, proton gradient, proton motive force, photolysis, water splitting, oxygen evolution, thylakoid lumen, stroma, atrazine, herbicide, variegation, variegated leaves, growth rate, isotope labelling, H2-18O, labelled oxygen, mitochondria vs chloroplast, oxidative phosphorylation vs photophosphorylation, BIOL 110, Penn State, intro biology, Tutorial 22