Light-dependent reactions Knowledge-Based Study

Study notes

1. The Purpose and Location of Light Reactions

   Photosynthesis occurs in two interconnected stages: the light-dependent reactions and the Calvin cycle (light-independent reactions). The primary objective of the light-dependent reactions is to convert solar energy into chemical energy stored in two high-energy molecules: ATP and NADPH. These molecules subsequently power the synthesis of carbohydrates in the stroma.

   The light-dependent reactions take place within the thylakoid membranes of chloroplasts. Embedded in these lipid bilayers are multiprotein complexes known as photosystems, electron transport carriers, and ATP synthase complexes. The internal compartment enclosed by the thylakoid membrane is the thylakoid lumen, while the fluid surrounding the thylakoids is the stroma.

   Photosystems and Light-Harvesting Complexes

   Each photosystem consists of two functional parts:

   • Light-harvesting complex: Contains hundreds of pigment molecules (chlorophyll a, chlorophyll b, and carotenoids) bound to proteins. These pigments act as an antenna, absorbing photons and transferring the excitation energy toward the reaction center via resonance energy transfer.
   • Reaction-center complex: Contains a specialized pair of chlorophyll a molecules and a primary electron acceptor. When the special pair absorbs energy, an electron is elevated to a higher energy level and transferred directly to the primary electron acceptor, initiating redox reactions.

   Linear Electron Flow: Starting at Photosystem II

   Linear (non-cyclic) electron flow is the predominant pathway of the light reactions. It involves both photosystems and generates both ATP and NADPH in equal measure along with molecular oxygen.

   Activation of Photosystem II (PS II)

   PS II contains a reaction-center chlorophyll a pair known as P680, which has an absorption peak at 680 nm. The sequence begins as follows:

   1. A photon excites an antenna pigment, transferring energy sequentially to P680.
   2. An electron in P680 is boosted to an excited state and captured by the primary electron acceptor, converting P680 into its oxidized form, P680+. P680+ is one of the strongest known biological oxidizing agents.
   3. An enzyme complex on the luminal side of PS II catalyzes photolysis—the splitting of water:
      2 H2O → 4 H+ + 4 e− + O2
   4. The electrons generated from water replace those lost by P680, returning it to its ground state. The hydrogen ions ($H^+$) are released directly into the thylakoid lumen, contributing to a proton gradient, while $O_2$ diffuses out as a metabolic byproduct.

   The Intermediary Electron Transport Chain

   The energized electron leaves the primary electron acceptor of PS II and passes through an electron transport chain consisting of:

   • Plastoquinone (Pq): A lipid-soluble mobile carrier that shuttles electrons through the membrane.
   • Cytochrome $b_6f$ complex: A membrane-bound protein complex that uses the energy from passing electrons to pump protons from the stroma into the thylakoid lumen.
   • Plastocyanin (Pc): A water-soluble, copper-containing peripheral protein that delivers electrons to Photosystem I.

   Photosystem I (PS I) and Reduction of NADP+

   Photosystem I functions downstream of Photosystem II in linear electron flow. Its reaction-center chlorophyll a pair is designated P700, reflecting its optimal absorption at 700 nm.

   Electron Excitation in PS I

   1. Simultaneously with PS II, light energy absorbed by the antenna complex of PS I excites P700, sending an electron to its primary electron acceptor and yielding P700+.
   2. The electron hole in P700+ is filled by an electron arriving from the cytochrome $b_6f$ complex via plastocyanin (Pc).
   3. The photoexcited electron from PS I is passed down a second, shorter electron transport chain via the iron-sulfur protein ferredoxin (Fd) located in the stroma.

   Formation of NADPH

   The enzyme NADP+ reductase catalyzes the transfer of two electrons from two molecules of reduced ferredoxin along with a proton from the stroma to $NADP^+$:

   NADP+ + 2 e− + H+ → NADPH

   This reaction removes a free proton from the stroma, which further enhances the electrochemical proton gradient across the thylakoid membrane. The resulting NADPH stores reducing power required for the sugar-building reactions of the Calvin cycle.

   Photophosphorylation via Chemiosmosis

   The synthesis of ATP during the light reactions occurs by chemiosmosis, driven by a proton-motive force established across the thylakoid membrane. Three mechanisms establish and maintain this gradient:

   • Water photolysis: Splits in the thylakoid lumen, depositing $H^+$ ions directly into the interior space.
   • Cytochrome $b_6f$ proton pumping: Translocates $H^+$ ions from the stroma into the lumen against their concentration gradient as electrons traverse the chain.
   • NADPH synthesis: Consumes $H^+$ ions on the stromal side, keeping stromal $H^+$ concentration low.

   As a result, the lumen reaches a significantly lower pH (higher acidity) than the stroma. Because the lipid bilayer is impermeable to ions, protons can only flow back down their concentration and electrical gradient through the central channel of ATP synthase. The exergonic flow of protons drives the catalytic rotation of the enzyme, phosphorylating ADP and inorganic phosphate to yield ATP in the stroma.

   Cyclic Electron Flow

   Under certain physiological conditions, chloroplasts can switch from linear to cyclic electron flow:

   • This pathway involves only Photosystem I.
   • Electrons cycle from ferredoxin back to the cytochrome $b_6f$ complex, and return via plastocyanin to P700.
   • No NADPH is produced, and no oxygen is evolved because photolysis does not occur.
   • However, proton pumping by cytochrome $b_6f$ continues, allowing the continued production of ATP.

   Cyclic electron flow satisfies the excess ATP demand of the Calvin cycle and protects photosynthetic machinery from light-induced damage (photoprotection).