Photosynthesis Overview

Photosynthesis Overview

• Photosynthesis is a two-step process
• Light-dependent reactions: Convert light energy into chemical energy (ATP) and split water to release electrons and hydrogen atoms.
• Light-independent reactions: Use ATP and NADPH (from light-dependent reactions) to fix carbon (CO2) and synthesize organic molecules.

Interdependence of Reactions

• Light-dependent reactions produce ATP and NADPH (energy carriers) needed for light-independent reactions (Calvin cycle).
• Light-independent reactions require ATP for energy and NADPH to reduce carbon dioxide and form organic molecules.
• NADP+ (unloaded) is critical in the light-dependent stage, and is converted to NADPH in the light-independent stage.

Photosystem Functionality

• Photosystems are complexes of pigments (e.g., chlorophyll) that capture light energy and generate high-energy electrons.
• Two main photosystems:
  • Photosystem II (PSII): Absorbs light (~680 nm) and splits water, releasing electrons to the electron transport chain, generating ATP.
  • Photosystem I (PSI): Absorbs light (~700 nm) and passes excited electrons to NADP+ producing NADPH. Can also recycle electrons in a cyclic pathway for generating more ATP.

Pigment Structure & Function

• Photosynthetic organisms use various pigments (not just chlorophyll) to absorb different wavelengths of light maximizing energy capture.
• Pigments are organized into photosystems to maximize light absorption and efficient energy transfer to the reaction center.

Z Scheme (Electron Flow)

• The Z scheme represents the flow of electrons through photosystems:
  • Photosystem II: Light energizes electrons, which are passed to an electron transport chain generating ATP.
  • Photosystem I: Receives de-energized electrons, uses light to energize them, and uses them to reduce NADP+ to NADPH.

Summary of Photosynthesis

• Light-dependent reactions generate ATP and NADPH, which are essential for light-independent reactions (Calvin cycle) to produce organic compounds.
• Two photosystems (PSII and PSI) work together to capture light energy, split water, generate ATP, and produce NADPH.

Thylakoid Structure & Function

• Light-dependent reactions occur in thylakoids, specialized membrane-bound structures within chloroplasts.
• In plants, thylakoids are arranged in stacks called grana.
• Thylakoids house photosystems, which absorb light energy and convert it into chemical energy.

Role of Photosystems

• Photosystems use light energy to excite electrons in pigments.
• Energized electrons are passed along an electron transport chain (ETC) producing ATP.
• Electron transport chain initiates ATP production and electron transfer.

ATP Production (Chemiosmosis & Photophosphorylation)

• Electron transport chain loses energy, pumping protons (H+) from stroma into thylakoid space.
• Proton gradient created drives protons back through ATP synthase.
• ATP synthase generates ATP from ADP and inorganic phosphate (Pi) (photophosphorylation).

Reduction of NADP+ (Non-Cyclic Photophosphorylation)

• Non-cyclic photophosphorylation uses both Photosystem II (PS II) and Photosystem I (PSI) to produce ATP and NADPH.
• In PS II, light energizes electrons, passed through electron transport chain producing ATP.
• In PS I, light energizes electrons, which reduce NADP+ to NADPH.
• Water molecules split (photolysis) in PS II to replace lost electrons, making this process non-cyclic.

Cyclic Photophosphorylation

• Cyclic photophosphorylation uses only Photosystem I (PS I).
• Electrons are excited and passed to ETC but recycled back to PS I.
• This process produces ATP but not NADPH.
• It balances the production of ATP and NADPH.

Calvin Cycle Overview

• The light-independent reactions (Calvin cycle) occur in the stroma of the chloroplast.
• ATP and NADPH (from light-dependent reactions) are used to convert CO2 into organic molecules.

Step 1: Carbon Fixation (Rubisco)

• The Calvin cycle starts with a 5-carbon compound called ribulose bisphosphate (RuBP).
• The enzyme Rubisco catalyzes the addition of CO2 to RuBP, forming a 6-carbon compound.
• This quickly breaks down into two molecules of glycerate-3-phosphate (GP), each containing 3 carbon atoms.
• For each cycle, 3 molecules of CO2 combine with 3 molecules of RuBP, producing 6 molecules of GP.

Step 2: Reduction of GP to Triose Phosphate (TP)

• GP is converted to triose phosphate (TP) using NADPH and ATP.
• NADPH provides electrons, ATP provides energy.
• Six molecules of GP are converted into six molecules of TP per cycle.

Step 3: Regeneration of RuBP

• Of the six TP molecules produced, one is used to form a sugar molecule (like glucose).
• Two cycles are required to form one glucose monomer.
• The remaining five TP molecules regenerate RuBP.

Carbon Compounds Produced by the Calvin Cycle

• Calvin Cycle fixes carbon from CO2 into organic compounds primarily as triose phosphates (TP).
• These TP molecules can be used to form carbohydrates, lipids, proteins, and nucleic acids.

Linking Carbon Fixation with Organic Molecule Synthesis

• TP from the Calvin cycle can be converted into sugars (e.g., glucose), forming starch or used in metabolic pathways to produce proteins, lipids, and nucleic acids.
• These organic compounds are essential for structure, function, and energy storage within the cell.

Chloroplast Structure and Function

• Chloroplasts are responsible for converting light energy to chemical energy during photosynthesis.
• Chloroplasts have a double membrane structure.
• They contain thylakoids that contain photosystems.
• Thylakoids are stacked into grana.
• The stroma is the fluid surrounding the thylakoids where the Calvin cycle occurs.