Photosynthesis Overview and Mechanisms

Oxygenic Photosynthesis

• Oxygenic photosynthesis evolved around 2.4 billion years ago.
• Early photosynthetic organisms likely used H₂S as an electron source.
• Earth's atmosphere was initially anaerobic.
• Oxygenic photosynthesis, using H₂O as the electron source, evolved around 2.4 bya.
• Atmospheric oxygen concentration subsequently rose, requiring the development of oxidative phosphorylation.

Photosynthesis: The Light Reactions

• Photosynthesis utilizes light energy to elevate electrons from a low-energy state to a high-energy, excited state.
• These excited electrons are employed to generate a proton-motive force that drives ATP synthesis.
• High-energy electrons are also used to form NADPH, a crucial biosynthetic reducing agent.
• The reactions powered by sunlight are designated as the light reactions.

Photosynthesis Initiation

• Photosynthesis commences with light absorption by a photoreceptor molecule, also known as a pigment.
• When a photon of appropriate energy is absorbed by a pigment, an electron within the pigment molecule transitions to a higher energy level.
• The excited electron can return to its original state, releasing energy in the form of light or heat.

Resonance Energy Transfer

• An electron can absorb energy from electromagnetic radiation with appropriate wavelength and elevate to a higher energy state.
• When the excited electron returns to a lower energy level, the absorbed energy is released.
• The released energy can be absorbed by an electron in a neighboring molecule, causing it to transition to a higher energy state.

Photoinduced Charge Separation

• Another function of an excited electron is to migrate to a neighboring molecule possessing a lower excited state, a procedure termed electron transfer.
• Electron transfer results in charge separation, with the initial molecule becoming positively charged and the recipient molecule becoming negatively charged.
• Charge separation occurs at sites designated as reaction centers.

Photosynthesis in Eukaryotes

• Photosynthesis in eukaryotes transpires within chloroplasts, which are double-membrane organelles.
• The inner membrane encloses a compartment called the stroma, where glucose synthesis from CO₂ and H₂O occurs using ATP and NADPH produced in the light reactions.
• Within the stroma, there are membranous structures called thylakoid membranes, which serve as the site of the light reactions of photosynthesis.

Chlorophyll

• Chlorophyll is the primary light receptor in most photosynthetic systems.
• It's a substituted tetrapyrrole bound to a large hydrophobic 20-carbon alcohol called phytol.
• Chlorophyll is a highly conjugated molecule whose pyrrole nitrogens coordinate with magnesium.
• Chlorophyll a and b exhibit distinct absorption spectra.

Light Absorption and Charge Separation

• Light energy absorption by chlorophyll results in charge separation.
• Excited chlorophyll donates electrons to an acceptor molecule, empowering the acceptor to function as a reducing agent.

Light Harvesting Molecules

• Light-harvesting molecules and photosynthetic reaction centers are incorporated into the thylakoid membranes.
• These molecules absorb light and transfer energy between molecules until it reaches the reaction center.
• The pigments are arranged in light-harvesting complexes that surround the reaction center and funnel light energy to it through resonance energy transfer.

Photosystems in Plants

• Photosynthesis in green plants involves two photosystems.
• Photosystem I generates NADPH, a biosynthetic reducing agent.
• Photosystem II replenishes the electrons in Photosystem I while simultaneously establishing a proton gradient used to synthesize ATP.
• Water photolysis replaces missing electrons in Photosystem II.

Photosystem I Structure

• Photosystem I is a sizable, membrane-spanning complex composed of a dozen proteins and numerous cofactors, including chlorophyll, quinones, and iron-sulfur clusters.
• The core of photosystem I comprises an electron transfer chain of chlorophyll, phylloquinone, and iron-sulfur clusters.
• The terminal electron acceptor is an iron-sulfur protein known as ferredoxin.

P700 in Photosystem I

• The reaction center in photosystem I is denoted as P700, comprised of a pair of chlorophyll molecules.
• Electrons from excited P700 travel down an electron transport chain to the iron-sulfur protein ferredoxin.
• Ferredoxin-NADP+ reductase facilitates electron transfer from ferredoxin to produce NADPH, a crucial biosynthetic reducing agent.
• NADPH synthesis occurs on the stromal side of the thylakoid membrane.

Ferredoxin-NADP+ Reductase

• Ferredoxin-NADP+ reductase acts as a mediator between the one-electron transfer from ferredoxin and the two-electron transfer to NADP+.
• It comprises FAD, which mediates electron transfers.

Photosystem II

• Photosystem II transfers electrons to Photosystem I and generates a proton gradient for ATP synthesis.
• The proteins D2 and D1 form a scaffold for more than 20 cofactors and other proteins.
• The water-oxidizing complex performs the crucial task of replacing electrons in Photosystem II through water photolysis, releasing oxygen as a byproduct.

P680 in Photosystem II

• The reaction center in photosystem II is P680, a chlorophyll pair.
• Excited P680 transfers electrons through components including pheophytin, plastoquinone, and the cytochrome b6f complex.
• The cytochrome b6f complex pumps protons into the thylakoid lumen, creating a proton-motive force.
• Plastocyanin delivers electrons to oxidized P700 in photosystem I.

Cytochrome b6f

• Cytochrome b6f releases protons from plastoquinol into the thylakoid lumen and pumps protons.
• The mechanism is analogous to the Q cycle of complex III in cellular respiration.
• Reduced plastocyanin donates electrons to Photosystem I, which is a crucial component of the electron transport chain of photosynthesis.

Water-Oxidizing Complex (WOC)

• The water-oxidizing complex of photosystem II replenishes electrons lost from the special pair.
• This process involves water photolysis at the WOC, using manganese as a crucial component.
  • This process separates water molecules into oxygen, protons, and electrons.

The Z-scheme

• The electron flow from H₂O to NADP⁺ is referred to as the Z-scheme of photosynthesis.

Variations in Electron Donors

• Some organisms employ electron donors other than water.
  • Some bacteria use substances like hydrogen sulfide (H₂S), hydrogen (H₂), or various organic compounds instead of water as electron sources.

Proton-Motive Force

• The flow of electrons from H₂O to NADP⁺ generates a proton-motive force.
• Most of the energy in chloroplasts stems from the proton gradient, with minimal contribution from the membrane potential.
• Electroneutrality is maintained by Mg²⁺ moving into the stroma while H⁺ is pumped into the thylakoid lumen.
• ATP synthase in chloroplasts (CF₁-CF₀ complex) is comparable to mitochondrial F₁-F₀ complex.
• Newly synthesized ATP is released into the stroma, where it's utilized in carbohydrate synthesis.

Herbicides

• Certain herbicides inhibit specific components of the light reactions of photosynthesis.
  • Diuron and atrazine inhibit photosystem II.
  • Paraquat inhibits photosystem I and can generate reactive oxygen species.

Regulation of Chloroplast ATP Synthase

• Chloroplast ATP synthase activity is governed by the availability of reducing agents and the proton-motive force.
• Maximal activity requires a disulfide bond in the γ subunit to be reduced to two cysteines.
• Thioredoxin, formed from ferredoxin in photosystem I, acts as the reductant.
• The δ subunit of ATP synthase is sensitive to changes in proton-motive force, impacting disulfide bond reduction and ATP synthesis regulation.

Cyclic Photophosphorylation

• Cyclic photophosphorylation diverts electron flow to ATP synthesis in the absence of NADPH production or oxygen generation.
• Electrons from Photosystem I flow through the cytochrome b6f complex and plastocyanin to recycle back to Photosystem I.
• Protons pumped by cytochrome b6f are utilized to synthesize ATP.

Photo Yields in Photosynthesis

• Eight photons result in one O₂, two NADPH, and three ATP molecules.
• This process involves the pumping of 12 protons into the thylakoid lumen via electron transport and their subsequent passage through ATP synthase during the synthesis of 3 ATP.
• Cyclic photophosphorylation, using two photons, results in one ATP molecule.

Organization of Photosynthetic Components

• Thylakoid membranes are structured into stacked and unstacked regions.
• Photosystem II is preferentially situated in stacked regions, while Photosystem I and ATP synthase are located in unstacked regions to facilitate easy access to products and substrates.