photosynthesis light reactions

• Electron transport chain (ETC)


  A critical component in the light reactions of photosynthesis responsible for converting light energy into chemical energy in the form of ATP and NADPH
• Light reactions of photosynthesis

  1. Photosystems

  2. Excitation of electrons

  3. Electron transport chain

  4. Proton gradient

  5. ATP synthesis

  6. Water splitting
• Photosystems


  • Photosystem II (PSII) and photosystem I (PSI) embedded within the thylakoid membrane of chloroplasts
• Excitation of electrons


  1. Light energy absorbed by chlorophyll molecules excites electrons to higher energy levels in both photosystems

  2. In PSII, excited electrons transferred to primary electron acceptor

  3. In PSI, excited electrons transferred to series of electron carriers after passing through ferredoxin
• Electron transport chain


  • Excited electrons move along series of electron carrier molecules embedded in thylakoid membrane (plastoquinone, cytochrome complex, plastocyanin, ferredoxin)
  • As electrons move through chain, they release energy used to pump protons from stroma into thylakoid lumen
• Proton gradient


  Pumping of protons from stroma into thylakoid lumen creates proton gradient across thylakoid membrane, representing potential energy
• ATP synthesis


  1. Proton gradient drives flow of protons back into stroma through ATP synthase complexes

  2. ADP and inorganic phosphate combined to form ATP through chemiosmosis
• Water splitting


  1. In PSII, water molecules split by water-splitting enzyme (oxygen-evolving complex)

  2. Releases electrons, protons, and oxygen molecules as byproducts
• Cyclic electron flow


  • Excited electrons from PSI passed along ETC to ferredoxin, then transferred back to reaction center of PSI
  • Generates ATP but not NADPH
• Non-cyclic electron flow


  Electrons from both PSII and PSI used to reduce NADP+ to NADPH
• Cyclic electron flow


  Allows plant to generate additional ATP without producing excess NADPH, maintaining balance of energy and reducing power
• This creates an electrochemical gradient that drives ATP synthesis through chemiosmosis.
• The electrons are passed from one carrier to another, with the energy released being used to pump protons across the membrane.
• ATP is produced by coupling the movement of H+ back into the thylakoid space (through ATP synthase) with the formation of ATP.
• The process is called cyclic electron flow because the excited electrons are recycled within the same photosystem.
• 1. scheme


  A model that describes the flow of electrons during the light reactions of photosynthesis
• 1. scheme


  1. Illustrates the flow of electrons through two photosystems, photosystem II (PSII) and photosystem I (PSI), in the thylakoid membrane of chloroplasts

  2. Electrons are initially excited in PSII by the absorption of light energy, leading to their transfer through an electron transport chain (ETC)

  3. As electrons move through the ETC, they release energy used to pump protons into the thylakoid lumen, generating a proton gradient

  4. The electrons ultimately reach PSI, where they are re-energized by another photon of light and transferred through a second ETC

  5. At the end of the Z-scheme, the electrons are used to reduce NADP+ to NADPH, providing reducing power for the Calvin cycle
• The name "Z-scheme" comes from the zigzag pattern formed when plotting the energy levels of electrons as they move through the two photosystems and the electron transport chain
• Gene


  A section of DNA which codes for a specific polypeptide
• DNA


  • The molecule that contains all the instructions or genetic information to develop the organism and create its general structure
  • It codes for proteins
  • It does not code for anything else like sugars or lipids
• Chromosome


  A very long DNA molecule that contains many genes
• Protein synthesis


  1. Transcription: DNA is transcribed into mRNA

  2. Translation: mRNA is translated into a sequence of amino acids to form a polypeptide
• Messenger RNA (mRNA)


  A single-stranded polynucleotide that carries the genetic information from the DNA to the ribosome for protein synthesis
• Genome


  The full set of DNA found in an organism, including all its genes
• Proteome


  The full range of proteins that can be synthesized from the genome
• Genome


  Encodes the entire proteome
• Primary structure of a protein

  The sequence of amino acids that make up the polypeptide chain
• Primary structure of a protein

  Determines the 3D tertiary structure and function of the protein
• DNA indirectly codes for the shape and function of proteins through the primary structure determined by the sequence of nucleotides