CHM4116 - Photosynthesis

• Photosynthesis - a process by which energy from sunlight is harnessed and is converted into chemical energy
• Photosynthesis can be involved in plants, cyanobacteria and algae
• The two stages of photosynthesis are traditionally referred as the light reactions and dark reactions
• Light reactions are light dependent

  • Specialized pigments capture light energy
  • A series of electron transfer reactions occurs
  • Reduction of NADP+ to NADPH
  • ATP synthesized from ADP to Pi
  • O2 forms from oxidation of H2O
• Dark reactions 

  • Light dependent or independent reactions
  • Utilizes the products of the light reaction
  • Uses NADPH and ATP to reduce CO2 and incorporate it into the three-carbon precursors of carbohydrates
• Autotrophs - also called photosynthetic organisms transform solar energy into carbohydrates

  Heterotrophs - Consumes autotrophs for energy needs

  Solar energy - All life on earth depends on a star 93 million miles away
• The site of photosynthesis happens in

  • Prokaryotes - in granules bonded to the plasma membrane
  • Eukaryotes - chloroplasts
• Parts and function of a chloroplast

  • Inner, outer, and thylakoid membranes - disk like structure
  • Grana - stacks of thylakoid disks
  • Lumen - the space between thylakoid disks
  • Stroma - colorless liquid in the matrix of the cell
• Label the parts

  A) Outer Membrane

  B) Granum

  C) Lumen

  D) Thylakoids

  E) Stroma

  F) Inner Membrane

  G) Chloroplast

  
• The chloroplast includes some functions

  • Thylakoid membrane
  • Traps light and produces O2
  • Pigment molecules, some arranged in light harvesting complexes, absorb visible light
• The chloroplast includes some functions

  • Stroma
  • Dark reaction occurs by producing carbohydrates and H2O
• Thylakoid membrane

  • A single highly folded vesicle, although in most organisms, it consist of stacks of disklike sacs named grana
  • Arise from invaginations in the inner membrane of developing chloroplasts and therefore resemble mitochondrial cristae
  • Contains protein complexes involved in harvesting light energy, transporting electrons, and synthesizing ATP
• Chlorophyll - main pigments that absorb plants

  A) Mg

  B) Fe

  C) Chlorophyl

  D) Iron-Protoporphyrin-IX

  
• An electronically excited pigment molecule can dissipate its excitation energy in several ways

  1. Internal conversion - heat is released
  2. Fluorescence - light is released
  3. Exciton Transfer/resonance energy transfer - transfers energy from one pigment molecule to another until it reaches the reaction center chlorophyll
  4. Photooxidation - converts light energy to chemical energy where the molecule is oxidized
• Light energy is transformed to chemical energy by the following steps

  1. Light absorbs the molecule
  2. The valence electron of the molecule excites into a higher electronic state
  3. The electronic state at higher energy levels are unstable; the energy goes back to its ground state by four ways
• Reaction center chlorophyll - the primary reactions of photosynthesis
• The energy of an absorbed photon randomly migrates among the molecules of an antenna complex until it reaches a reaction center chlorophyll, or, less frequently, is reemitted (fluorescence)
• Antenna chlorophylls - Gathers light from molecule to molecule until it reaches a photosynthetic reaction center
• Light harvesting complexes - Contains multiple pigments
• Accessory pigments - Fill in the absorption spectra of the antenna complexes, covering the spectral regions where chlorophylls do not absorb light strongly for increased survivability
• Examples of accessory pigments include

  1. Beta Carotene - found in carrots (orange color)
  2. Phycoerythrobilin and Phycocyanobilin - water dwelling photosynthetic organisms that absorb almost completely by passage through more than 10 m of water
• Light reactions

  • Produces O2, NADPH, and ATP
  • The special pair of the purple bacterial photosynthetic reaction center undergoes photooxidation, and an electron transport chain returns to the special pair
  • Z scheme - In plants and in cyanobacteria, two photosystems, cytochrome b6f and mobile electron carriers form an ETC
  • Photosystem II - Reduces its photooxidized special pair O2 with electrons derived from H2O
• • Electrons traveling from photosystem II through the cytochrome b6f complex undergo a Q cycle that generates a transmembrane proton gradient used for the production of ATP.

  • Electrons liberated by photooxidation of photosystem I reduce NADP+ or return to the cytochrome b6f complex, whose activity contributes to the proton gradient.

  • ATP is produced by photophosphorylation.
• The light reaction sequence happens on two photosystems with corresponding processes

  1. Photosystem II
  2. Mobile electron carrier
  3. Cytochrome b6f complex
  4. Mobile electron carrier
  5. Photosystem I
• Photosynthesis I and II and Light reaction

  • In the light reactions, H2O is oxidized to O2 and NADP+ is reduced to NADPH
  • Phosphorylation - The series of redox reactions is coupled to the phosphorylation of ADP to ATP
  • The light reactions are accomplished by two distinct photosystems; photosystem I and photosystem II
• The electrons from photosystem I can be transferred to an enzyme that catalyzes the reduction of NADP + to NADPH
• • The reaction is endergonic for a gibbs free energy of +220 kJ/mol
  • The reaction is driven by the light energy absorbed by the chlorophylls of the two photosystems

  A) 1/2 O2

  B) NADP+

  C) NADP+

  D) NADPH

  E) NADPH

  F) 2e-

  
• PbRC - Photosynthetic bacteria reaction center ; in a prokaryotic
• PbRC- reactions where photosynthesis take place in a bacteria and is a transmembrane protein
• During the electron-transfer process, cytoplasmic protons are translocated across the plasma membrane. Dissipation of the resulting proton gradient drives ATP synthesis
• Electron transport in photosynthetic bacteria follows a circular path

  PbRC

  • Contains three hydrophobic subunits known as H, L and M
  • L and M subunits collectively bind
  • four molecules of bacteriochlorophyll - contains Mg2+ ion
  • two molecules of bacteriopheophytin - replaces Mg2+ by two protons
  • one Fe(II) ion
  • two molecules of the redox coenzyme ubiquinone or one molecule of ubiquinone and one of the related menaquinone
• What is the name of this electron acceptor?

  A) Menaquinone

  
• Pseudotwofold axis - The transmembrane portions of M and L subunits that passes through the Fe(II) ion and that the prosthetic groups are sandwiched between the two subunits
• The H, M, and L subunits of the protein are viewed from within the plane of the plasma membrane
• Label the parts of during the disposition of prosthetic groups in the PbRC

  A) Special pair

  B) Accessory BChl b

  C) BPheo b

  D) Menaquinone (Qa)

  E) Fe(II)

  F) Ubiquinone (QB)

  G) BPheo b

  H) hv

  
• Disposition of prosthetic groups in photosynthetic reaction center

  1. Special pair - First photooxidation reaction with two of the BChl that are closely associated and are nearly parallel with an Mg—Mg distance of ∼7 Å
  2. Accessory BChl b - the intervening BChl group probably plays a role in conveying electrons, although it is not itself reduced
• Disposition of Prosthetic groups in photosynthetic reaction center

  
  

  3. BPheo b - Energy was transferred from Accessorry BChl b

  4. Menaquinone - Energy was transferred from BPheo b where it is not fully reduced

  5. Ubiquinone - Energy was transferred from Menaquinone by a series of Q reactions and it is reduced
• 3. During the next 200 ps, the electron migrates to the menaquinone (or in the second ubiquinone) designed QA, to form the anionic semiquinone radical Q-A. All these electron transfers, are to progressively lower energy states, which makes the process almost irreversible
• Photon Absorption Rapidly Photooxidizes the Special Pair.
• 1. The primary photochemical event of bacterial photosynthesis is the absorption of a photon by the special pair (e.g., P960). The excited electron is delocalized over both its BChl molecules.
  2. P960*, the excited state of P960, has but a fl eeting existence. Within ∼3 picoseconds (ps; 10−12 s), P960* transfers an electron to the BPheo on the right in Fig. 19-9 to yield P960+ BPheo b− (the intervening BChl group probably plays a role in conveying electrons, although it is not itself reduced; it is therefore known as the accessory BChl).