3.5 - Photosynthesis

Photosynthesis is a fundamental biological process that converts light energy into chemical energy, stored in sugars and other organic molecules. This process is crucial for nourishing almost all living organisms, either directly or indirectly.

• Historical Context:

  • First evolved around 3 billion years ago in prokaryotic cyanobacteria, which are considered precursors to chloroplasts in modern plants.

  • Cyanobacteria significantly contributed to the oxygenation of Earth's atmosphere, and today, over 80% of the atmospheric oxygen comes from these organisms in the ocean.

• General Equation of Photosynthesis:

  • Simplified Reaction: CO2 + H2O + Light → C6H12O6 + O2

Comparison of Light Reactions and the Calvin Cycle

Here’s a brief comparison of the two stages of photosynthesis, focusing on the primary processes and their locations within the chloroplast:

Detailed Photosynthetic Processes

• Photosystems in the Thylakoid Membrane:

  • Composed of a reaction center complex surrounded by multiple light-harvesting complexes that contain chlorophyll.

  • Chlorophyll molecules absorb light, causing an electron to become "excited."

  • The excited electrons energy is transferred among molecules in a process where each recipient molecule is reduced as it gains electrons.

• Sequence of Electron Flow:

  • Photosystem II:

    • Absorption of light excites an electron which is then captured by an electron acceptor.

    • Water is split to replace the lost electron, releasing hydrogen ions and oxygen.

  • Electron Transport Chain:

    • Electrons move from Photosystem II to Photosystem I, pumping hydrogen ions into the thylakoid and creating a proton gradient.

  • Photosystem I:

    • Light re-excites the electron before it is transferred to another acceptor.

    • The electron is used, along with an enzyme, to reduce NADP+ into NADPH (a high-energy molecule).

• ATP Synthesis:

  • The proton gradient across the thylakoid membrane drives ATP synthase to produce ATP.

  • ATP Synthase Function:

    • Mechanism: ATP synthase is a crucial enzyme that harnesses the energy from a proton gradient across the thylakoid membrane to synthesize ATP. As protons flow back across the membrane through the ATP synthase channel, the enzyme catalyzes the addition of a phosphate group to ADP, forming ATP.

    • Reaction: ADP + P → ATP

    • Role in Photosynthesis: This process provides the energy currency for many cellular processes, including the Calvin cycle in photosynthesis.

Calvin Cycle in Photosynthesis

• The Calvin Cycle:

  • Function: The Calvin cycle, also known as the light-independent reactions or dark reactions, uses the ATP and NADPH produced during the light reactions to convert carbon dioxide into organic carbohydrates.

  • Energy and Reducing Power:

    • ATP: Supplies the energy needed for the biochemical reactions within the Calvin cycle.

    • NADPH: Provides the reducing power (electrons) necessary to convert the fixed carbon dioxide into a more reduced (energy-rich) form as part of carbohydrate molecules.

  • Phases of the Calvin Cycle:

    • Carbon Fixation: CO2 molecules are attached to five-carbon sugars by the enzyme RuBisCO, creating unstable six-carbon compounds that immediately split into two three-carbon molecules.

    • Reduction Phase: ATP and NADPH are used to convert the three-carbon molecules into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar. Some G3P molecules exit the cycle to be used in other metabolic pathways, while others remain to be recycled.

    • Regeneration of the CO2 Acceptor: ATP is used to convert some of the G3P back into RuBP (ribulose-1,5-bisphosphate), the molecule that accepts CO2, allowing the cycle to continue.