Photosynthesis is nature's way of turning sunlight into food. Plants use light energy to make glucose from carbon dioxide and water, releasing oxygen as a byproduct. This process is crucial for life on Earth, providing energy for nearly all ecosystems.

The process happens in two stages: light-dependent reactions in thylakoid membranes and light-independent reactions in the stroma. Key players include chlorophyll, photosystems, and enzymes like RuBisCO. Together, they create the energy-rich molecules ATP and NADPH.

Photosynthesis: Energy Conversion

Overview and Importance

• Photosynthesis converts light energy into chemical energy stored in glucose or other sugars
• Occurs in the chloroplasts of plant cells, specifically in the thylakoid membranes and stroma
• Overall reaction: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2
  • Divided into light-dependent and light-independent reactions
• Crucial for life on Earth
  • Provides energy for nearly all ecosystems by converting light energy into usable chemical energy for other organisms

Process and Components

• Light-dependent reactions capture light energy to produce ATP and NADPH
• Light-independent reactions (Calvin cycle) use ATP and NADPH to fix carbon dioxide into organic compounds
• Occurs in two stages:
  1. Light-dependent reactions in the thylakoid membranes
  2. Light-independent reactions (Calvin cycle) in the stroma
• Key components:
  • Chlorophyll pigments (absorb light energy)
  • Photosystems I and II (protein complexes that initiate electron transport)
  • Electron transport chain (series of redox reactions)
  • ATP synthase (generates ATP using proton gradient)
  • Enzymes (RuBisCO, catalyzes carbon fixation)

Photosystems in Light-Dependent Reactions

Photosystem II (PSII)

• Protein complex in the thylakoid membrane that absorbs light energy at 680 nm
• Excites electrons that are passed to the electron transport chain
• Electron vacancies in PSII are filled by splitting water molecules (photolysis)
  • Releases oxygen as a byproduct
• Electrons from PSII are transported through a series of redox reactions, including the cytochrome b6f complex, to photosystem I

Photosystem I (PSI)

• Protein complex that absorbs light energy at 700 nm, further exciting electrons
• High-energy electrons from PSI are used to reduce NADP+ to NADPH
• Electron transport between PSII and PSI creates a proton gradient across the thylakoid membrane
  • Proton gradient is used by ATP synthase to generate ATP from ADP and inorganic phosphate (Pi)
• PSI and PSII work in tandem to capture light energy and convert it into chemical energy (ATP and NADPH)

Calvin Cycle: Carbon Fixation and Glucose

Carbon Fixation

• Uses ATP and NADPH generated during light-dependent reactions to fix CO2 into organic compounds
• Enzyme RuBisCO catalyzes the first major step by fixing CO2 to ribulose bisphosphate (RuBP), a 5-carbon sugar
  • Forms a 6-carbon compound that splits into two 3-carbon molecules
• 3-carbon molecules are reduced using ATP and NADPH to form 3-phosphoglycerate (3-PGA)

Glucose Synthesis

• 3-PGA is converted into glyceraldehyde 3-phosphate (G3P), a simple sugar
• Some G3P molecules are used to regenerate RuBP to continue the cycle
• Other G3P molecules are used to synthesize glucose and other organic compounds
• Calvin cycle occurs in three stages: carbon fixation, reduction, and regeneration
  • Repeated to produce a net gain of one G3P molecule for every three CO2 molecules fixed

Light-Independent Nature

• Calvin cycle is termed "light-independent" because it does not directly require light energy
• However, it is dependent on the products (ATP and NADPH) generated during light-dependent reactions
• Light-independent reactions occur in the stroma of chloroplasts

Photosynthesis vs Cellular Respiration

Similarities

• Both are essential processes in energy transformations of living organisms
• Involve electron transport chains and creation of proton gradients across membranes
  • Thylakoid membrane in photosynthesis
  • Inner mitochondrial membrane in cellular respiration
• Generate ATP through chemiosmosis
• Interconnected processes in ecosystems
  • Products of one process serve as reactants for the other, creating a continuous cycle of energy flow and matter exchange

Differences

• Photosynthesis is an anabolic process that converts light energy into chemical energy stored in glucose
  • Cellular respiration is a catabolic process that breaks down glucose to release energy as ATP
• In photosynthesis, light energy splits water molecules, releasing oxygen
  • In cellular respiration, oxygen is consumed as the final electron acceptor
• Photosynthesis occurs in chloroplasts of plant cells
  • Cellular respiration occurs in mitochondria of both plant and animal cells
• Overall equations are the reverse of each other:
  • Photosynthesis: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2
  • Cellular respiration: C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP