3.3 Photosynthesis and light-driven processes

Photosynthesis is nature's way of turning sunlight into food. This process, occurring in plants and some bacteria, captures light energy and converts it into chemical energy stored in sugars. It's the foundation of most life on Earth.

The light-dependent reactions split water and produce ATP and NADPH. These energy-rich molecules then power the Calvin cycle, which uses CO2 to make glucose. This two-stage process showcases how organisms efficiently transform and store energy.

Photosynthesis Principles and Stages

Overview of Photosynthesis

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• Photosynthesis converts light energy into chemical energy stored in sugars or other organic compounds in plants, algae, and some bacteria
• The overall equation for photosynthesis: 6CO2 + 6H2O + light energy → C6H12O6 (glucose) + 6O2
  • Involves the reduction of carbon dioxide to carbohydrates
  • Involves the oxidation of water to molecular oxygen

Stages of Photosynthesis

• Photosynthesis occurs in two main stages
  1. Light-dependent reactions (light reactions)
     • Occur in the thylakoid membranes of chloroplasts
     • Involve the absorption of light energy, splitting of water, and generation of ATP and NADPH
  2. Light-independent reactions (dark reactions or Calvin cycle)
     • Take place in the stroma of chloroplasts
     • Use ATP and NADPH produced in light reactions to reduce CO2 to form glucose and other organic compounds
• Photosynthetic pigments (chlorophylls and carotenoids) are essential for absorbing light energy
  • Chlorophyll a is the primary pigment involved in photosynthesis
  • Other pigments help extend the range of light absorption

Light-Harvesting Complexes and Reaction Centers

Light-Harvesting Complexes (LHCs)

• LHCs are protein-pigment complexes that capture light energy and transfer it to reaction centers
  • Contain various photosynthetic pigments (chlorophylls and carotenoids) which absorb light at different wavelengths
  • Arranged around reaction centers to maximize absorption and transfer of light energy
  • Arrangement and composition of LHCs vary among different photosynthetic organisms
    • Example: Green sulfur bacteria have chlorosomes, specialized light-harvesting structures

Reaction Centers

• Reaction centers are the sites where primary photochemical reactions of photosynthesis occur
• Two types of reaction centers
  1. Photosystem II (PSII)
     • Absorbs light energy to split water molecules (H2O) into electrons, protons (H+), and oxygen (O2)
     • Electrons are transferred to an electron transport chain
  2. Photosystem I (PSI)
     • Absorbs light energy to excite electrons
     • Excited electrons are transferred to NADP+ to form NADPH
• Reaction centers contain special pairs of chlorophyll a molecules responsible for primary charge separation events that initiate electron transport
  • P680 in PSII
  • P700 in PSI

Electron Transport and ATP Synthesis

Electron Transport Chain

• Electron transport involves the transfer of electrons from water (in PSII) to NADP+ (in PSI) through a series of redox reactions
  • Coupled with the generation of a proton gradient across the thylakoid membrane
• Electron transport chain consists of several protein complexes
  • Cytochrome b6f and plastocyanin shuttle electrons from PSII to PSI
  • As electrons move through the chain, protons (H+) are pumped from the stroma into the thylakoid lumen, creating a proton gradient (chemiosmosis)

ATP Synthesis

• ATP synthase, a large enzyme complex in the thylakoid membrane, uses the proton gradient to drive ATP synthesis
  • As protons flow back into the stroma through ATP synthase, released energy is used to phosphorylate ADP, forming ATP
• ATP and NADPH generated during light-dependent reactions are used in light-independent reactions (Calvin cycle) to
  • Reduce CO2
  • Form glucose and other organic compounds

Light-Dependent vs Light-Independent Reactions

Light-Dependent Reactions

• Occur in the thylakoid membranes of chloroplasts
• Require light energy to drive the reactions
• Involve
  • Absorption of light energy by photosynthetic pigments
  • Splitting of water
  • Generation of ATP and NADPH
• Do not directly involve the fixation of CO2

Light-Independent Reactions

• Occur in the stroma of chloroplasts
• Do not directly require light energy (hence "dark reactions")
  • Dependent on products of light reactions (ATP and NADPH)
• Involve
  • Fixation of CO2
  • Synthesis of glucose and other organic compounds using the Calvin cycle
• Indirectly dependent on light, as ATP and NADPH from light reactions are necessary for the Calvin cycle

Interconnection

• Both stages are essential for the overall process of photosynthesis
  • Light-dependent reactions provide energy and reducing power needed for light-independent reactions to fix CO2 and produce organic compounds

Photosynthesis: Light Energy to Chemical Energy

Importance of Photosynthesis

• Photosynthesis sustains life on Earth by converting light energy into chemical energy stored in organic compounds
• Chemical energy produced by photosynthesis is the primary energy source for most living organisms
  • Directly for plants and other photosynthetic organisms
  • Indirectly for animals and heterotrophs that depend on photosynthetic organisms for food
• Photosynthesis plays a vital role in the global carbon cycle
  • Removes CO2 from the atmosphere and incorporates it into organic compounds
  • Helps regulate atmospheric CO2 levels and mitigate effects of climate change

Implications and Applications

• The oxygen released as a byproduct of photosynthesis is essential for most life forms that depend on aerobic respiration
• Understanding photosynthesis is crucial for
  • Developing strategies to improve crop yields (agricultural optimization)
  • Optimizing biofuel production (renewable energy)
  • Addressing global environmental challenges (climate change and food security)