Photosynthesis: Light Dependent Stage

Light Dependent Stage of Photosynthesis

• Occurs in the thylakoids of chloroplasts.
• Involves capture of light energy for two purposes:
  • Photophosphorylation: adding an inorganic phosphate molecule to ADP to make ATP.
  • Photolysis: splitting water into protons, electrons, and oxygen.
• Photoactivation of chlorophyll: light energy raises the energy level of electrons in chlorophyll molecules.
• Excited electrons are taken up by electron carriers/acceptors, oxidizing the chlorophyll molecule and reducing the carrier.
• Electrons are passed along a series of electron carriers in redox reactions.
• Energy released is used to transfer protons across the thylakoid membrane into the thylakoid space (lumen).
• Proton gradient drives ATP production via chemiosmosis.

Cyclic Photophosphorylation

• Uses photosystem I only.
• Excited electrons are taken up by an electron acceptor and passed back to the same chlorophyll molecule via electron carriers (recycling).
• Produces ATP, but not reduced NADP.
• ATP is used in the light independent stage or for specific functions like stomatal opening.

Non-Cyclic Photophosphorylation

• Uses both photosystem I and II.
• Excited electrons from PSII are passed to PS I via electron carriers.
• Excited electrons from PSI are taken up by NADP+ to form reduced NADP (NADPH + H+).
• Electrons lost in PSII are replaced by electrons from water molecules split by photolysis.
• Photolysis also contributes protons to the thylakoid lumen, increasing the proton gradient.
• Reduced NADP is used in the light-independent stage to reduce carbon dioxide.

Photolysis

• Splitting of water molecules using light energy.
• Occurs only in PSII, associated with the oxygen-evolving complex.
• Produces protons, electrons, and oxygen (O2).
• Electrons replace those lost in PSII, while protons contribute to the proton gradient.

The Z-Scheme

• Illustrates the zig-zag energy levels of electrons during the light-dependent stage.
• Light energy is trapped by PSII, boosting electrons to a higher energy level.
• Electrons are passed to an electron acceptor and then along a chain of carriers to PSI, releasing energy for ATP production.
• Light energy absorbed by PSI further excites the electrons.
• Excited electrons from PSI are taken up by another electron acceptor and then used to reduce NADP+.
• Some electrons may return to PSI via electron carriers, resulting in cyclic photophosphorylation and ATP production.

Light Independent Stage of Photosynthesis (Calvin Cycle)

• Occurs in the stroma of chloroplasts.
• Does not require light directly, but needs the products from the light-dependent stage.
• Details worked out by Melvin Calvin and colleagues.
• Steps:
  • CO2 diffuses into the leaf and dissolves in water around palisade cells.
  • CO2 combines with RuBP (ribulose bisphosphate) using the enzyme rubisco, forming an unstable 6-carbon compound.
  • The unstable compound splits into two molecules of GP (glycerate 3-phosphate).
  • ATP from the light-dependent stage is used to convert GP to TP (triose phosphate).
  • Reduced NADP (from the light-dependent stage) provides hydrogen for the reduction of GP to TP.
  • TP molecules combine in pairs to form 6-carbon (hexose) sugars.
  • Glucose can be further polymerized into starch or cellulose.
  • Five out of six TP molecules are used to regenerate RuBP, using ATP from the light-dependent stage.

Formation of Other Substances in Plants

• Plants synthesize carbohydrates, lipids, and proteins.
• Carbohydrates:
  • Sucrose (transport sugar) is made from glucose and fructose.
  • Starch (storage) and cellulose (cell wall) are made by polymerizing glucose.
• Lipids:
  • Glycerol is made from triose phosphate.
  • Fatty acids are made from GP.
  • Lipids are used for storage and phospholipids in cell membranes.
• Proteins:
  • Amino acids are produced from GP via acetyl coenzyme A and Krebs cycle intermediates.
  • Proteins are essential components of cell membranes and enzymes.