Redox Reactions in Photosynthesis
6CO2 + 6H2O —> C6H12O6
-polar covalent bonds in the reactants are broken (C=O, O-H)
-non-polar covalent bonds in the products are formed (C-C, O=O)
-bonding electrons between the carbon and oxygen in carbon dioxide have moved closer to the C atoms in glucose, therefore CO2 is reduced
-bonding electrons between the oxygen and hydrogen in H2O have moved farther away from the O atoms in oxygen, therefore H2O is oxidized
The Chloroplast
-triple lipid bilayer organelle
- has an outer membrane
-inside is inter membrane space
-inside is inner membrane
-inside is stroma
-inside is thylakoid membrane
- inside is thylakoid lumen ( space)
The Electromagnetic Spectrum
Shorter wavelength- have more energy
Longer wavelength - have less energy
Potential Photon Pathways
When a photon strikes an object it can be:
- Reflected- bounced back
- Transmitted- pass through
- Absorbed - photons are absorbed by electrons in the molecule they strike
Pigment
-molecules efficient in absorbing photons
-their chemical structure allows their electrons to absorb photons of specific wavelengths
- the wavelength must match the energy needed to raise an electron up an entire orbital
-if not exact…nothing happens
Pigment Electron Excitation
-photon is absorbed by an electron that moves from a low-energy level to a higher energy level
Photosynthetic Pigments
-photosynthetic pigments are embedded in the thylakoid membrane
-chlorophylls are the main pigment in most photoautotrophs
-capture most energy for photosynthesis
-carotenoids act as accessory pigments
-extend range of photons absorbed
-prevent sunburn - direct excess energy away
Pigment Organization
Pigment molecules are organized into photosystems
-light harvesting complexes
Hundreds of antenna pigments are grouped around a reaction centre (RC)
-a collection of proteins that interact with the pigments
When excited, the antenna pigments channel energy to the reaction centre
-energy transfer, NOT electron transfer
Inductive Resonance
-the transfer of energy from one electron to another
Reaction Centres
-reduce a primary e- acceptor
What Occurs in Reaction Centres
-light is absorbed by the antenna pigments
-the energy is transferred to the reaction centre
-an excited electron is passed to a primary electron acceptor
-the first redox reaction
Proton Motive Force in PSII
-energy comes from light in the photosystems
- P680* reduces its primary electron acceptor, which passes the e- to plastoquinane (PQ).
- PQ is a hydrophobic e- taxi that takes e- from PSII and some H+ from the stroma
- PQ passes the e- to cytochrome b6f and releases the H+ into the thylakoid lumen
- b6f passes the e- to plastocyanin (PC) - a hydrophobic e- taxi
-PC delivers e- to P700+
- H2O is split (photolysis), forming O2, e-, and H+
Photosystem I
- energy from photons going to P700
- P700* passes e- to a primary e- acceptor, which passes the e- to ferradoxin (protein)
- Ferradoxin is a hydrophilic e- taxi that passes e- to NADP+ - reductase
- NADP+ - reductase will pass the e- to NADP+, which is reduced to NADPH in the stroma
-notice P700* replaces the e- by taking some from PC
Electrochemical Gradient Across the Thylakoid Membrane
- H+ are released into the thylakoid lumen, when water is oxidized at PS II
-H+ are moved from the stroma into the thylakoid lumen by PQ
-H+ are removed from the stroma as NAD+ is reduced to NADPH
Photophosphorylation
-uses solar energy to generate PMF to power ATP Synthase
- ATP is generated on the stroma side of the thylakoid membrane
Requirements For Redox in Photosynthesis
- Lots and lots of energy to oxidize water and reduce NADP+
Cyclic Electron Transport
- Calvin Cycle requires more ATP than NADPH
- Ferradoxin can reduce PQ instead of NADP+ reductase
- More e- will move through PQ=more H+ moved across the membrane = increased PMF and increase ATP
G3P
-G3P is a 3C sugar
-formed in the Calvin Cycle and converted to glucose for:
1.used in glycolysis and aerobic respiration (mitochondria)
2.linked into polymers of starch (storage) or cellulose (cell walls and growth)
3.used in the synthesis of aā, nucleic ā and lipids
Oxygenic Photosynthesis
-prokaryotic organisms do not have membrane bound organelles (no chloroplasts)
-all metabolism occurs in the cytosol and on the cell membrane
-Cyanobacteria have a photosynthetic mechanism very similar of chloroplasts ( PSII, PSI, Calvin Cycle)
