Photosynthesis: Light Reactions and Calvin Cycle

Linear Electron Flow

  • Light drives ATP and NADPH synthesis through photosystems in the thylakoid membranes.
  • Linear electron flow involves electron movement through photosystems and molecular components within the thylakoid membrane during the light reactions of photosynthesis.
  • A photon strikes a pigment molecule in PS II, exciting an electron to a higher energy level.
  • The energy is relayed to other pigment molecules until it reaches the P680 pair of chlorophyll aa molecules in the PS II reaction-center complex, exciting an electron.
  • The excited electron is transferred from P680 to the primary electron acceptor, creating P680$^+$.
  • An enzyme splits a water molecule into two electrons, two hydrogen ions (H+H^+), and an oxygen atom.
  • Electrons are supplied to P680$^+$ one by one, replacing those transferred to the primary electron acceptor.
  • P680 is a strong oxidizing agent, facilitating electron transfer from water.
  • H+H^+ ions are released into the thylakoid space.
  • Oxygen atoms combine to form O2O_2.
  • Photoexcited electrons move from PS II to PS I via an electron transport chain including plastoquinone (Pq), a cytochrome complex, and plastocyanin (Pc).
  • Redox reactions occur as electrons move down the electron transport chain, releasing energy to pump protons (H+H^+) into the thylakoid space, creating a proton gradient.
  • The potential energy in the proton gradient is used to make ATP via chemiosmosis.
  • Light energy is transferred to the P700 pair of chlorophyll aa molecules in the PS I reaction-center complex, exciting an electron.
  • The excited electron is transferred to PS I's primary electron acceptor, creating P700$^+$, which accepts electrons from the electron transport chain from PS II.
  • Electrons are passed from the primary electron acceptor of PS I down a second electron transport chain through ferredoxin (Fd).
  • This chain does not create a proton gradient or produce ATP.
  • NADP$^+$ reductase catalyzes the transfer of electrons from Fd to NADP$^+$, requiring two electrons to form NADPH.
  • Electrons in NADPH have a higher energy level than in water, making them readily available for the Calvin cycle.
  • This process also removes H+H^+ from the stroma.
  • The light reactions use solar power to generate ATP and NADPH, providing chemical energy and reducing power for the Calvin cycle.

Cyclic Electron Flow

  • Cyclic electron flow is an alternative path where photoexcited electrons cycle back from ferredoxin (Fd) to the cytochrome complex and then to a P700 chlorophyll in PS I.
  • No NADPH is produced, and no oxygen is released.
  • Cyclic flow does generate ATP.
  • Some photosynthetic bacteria have only one photosystem and use cyclic electron flow as their only means of ATP production.
  • Cyclic electron flow can occur in species with both photosystems and may be photoprotective.
  • Plants unable to carry out cyclic electron flow grow well in low light but not in intense light.

Chemiosmosis

  • ATP synthesis is driven by either linear or cyclic electron flow via chemiosmosis.
  • Chemiosmosis: membranes couple redox reactions to ATP production.
  • Electron transport chains pump protons (H+H^+) across a membrane, creating a proton-motive force.
  • ATP synthase uses the diffusion of hydrogen ions down their gradient to phosphorylate ADP, forming ATP.
  • Chloroplasts and mitochondria both use chemiosmosis, but chloroplasts obtain high-energy electrons from water, while mitochondria extract them from organic molecules.
  • Mitochondria transfer chemical energy from food to ATP, while chloroplasts transform light energy into chemical energy in ATP.
  • Electron transport chain proteins pump protons from the mitochondrial matrix to the intermembrane space.
  • In chloroplasts, protons are pumped from the stroma into the thylakoid space.
  • The thylakoid space and intermembrane space are comparable, as are the mitochondrial matrix and chloroplast stroma.
  • In mitochondria, protons diffuse from the intermembrane space to the matrix through ATP synthase.
  • In chloroplasts, hydrogen ions diffuse from the thylakoid space back to the stroma through ATP synthase, forming ATP in the stroma.
  • The proton gradient across the thylakoid membrane is substantial; when chloroplasts are illuminated, the pH in the thylakoid space drops to about 5, while the pH in the stroma increases to about 8.
  • This creates a thousandfold difference in H+H^+ concentration.
  • NADPH and ATP are produced on the side of the membrane facing the stroma.
  • Electron flow moves electrons from water to NADPH, storing them at high potential energy.
  • Light-driven electron flow generates ATP and produces oxygen as a by-product.

Calvin Cycle

  • The Calvin cycle is anabolic, building carbohydrates from smaller molecules and consuming energy.
  • Carbon enters as CO2CO_2 and leaves as sugar.
  • ATP is used as an energy source, and NADPH is consumed as reducing power.
  • The carbohydrate produced is glyceraldehyde 3-phosphate (G3P), a three-carbon sugar.
  • The net synthesis of one G3P molecule requires the cycle to occur three times, fixing three molecules of CO2CO_2.

Phases of the Calvin Cycle

Phase 1: Carbon Fixation
  • Each CO2CO_2 molecule is attached to ribulose bisphosphate (RuBP), a five-carbon sugar.
  • Rubisco catalyzes this step.
  • The product is an unstable six-carbon intermediate that splits into two molecules of 3-phosphoglycerate.
Phase 2: Reduction
  • Each molecule of 3-phosphoglycerate receives a phosphate group from ATP, becoming 1,3-bisphosphoglycerate.
  • Electrons from NADPH reduce 1,3-bisphosphoglycerate, which loses a phosphate group, becoming glyceraldehyde 3-phosphate (G3P).
  • For every three CO2CO_2 molecules, six G3P molecules are formed, but only one is a net gain.
  • The cycle begins with 15 carbons in three RuBP molecules and results in 18 carbons in six G3P molecules. One G3P exits, and the other five are recycled to regenerate RuBP.