ch:10

Learning Outcomes

• Describe structures of chloroplasts.

• Explain light and pigment interactions.

• Describe light reactions producing O2, NADPH, and ATP.

• Explain Calvin Cycle utilization of CO2, NADPH, and ATP to produce sugar and H2O.

• Discuss C4 and CAM adaptations for dry conditions.

Overview of Photosynthesis

• Converts solar energy into chemical energy in chloroplasts.

• Involves autotrophs (e.g., plants, some prokaryotes) producing organic molecules.

• Heterotrophs depend on autotrophs.

Chloroplast Structure & Function

• Chloroplasts found in leaf mesophyll cells.

• CO2 enters and O2 exits via stomata.

• Contains chlorophyll, thylakoids, protein complexes for energy transfer.

Light Reactions

• Functions as a redox process; H2O is oxidized, CO2 is reduced to sugars.

• Light energy excites electrons:

  • Photosystems II (P680) and I (P700) generate ATP and NADPH.

  • Electrons flow through electron transport chains in thylakoid membranes.

Calvin Cycle

• Light-independent, anabolic process using ATP and NADPH.

• Steps:

  1. Carbon fixation (CO2 to organic molecule)

  2. Reduction (use of H from NADPH)

  3. Release G3P

  4. Regeneration of RuBP

• Produces one G3P for every three CO2.

Photorespiration

• Calvin Cycle efficiency drops when CO2 is low and O2 is high.

• Rubisco binds O2 instead of CO2, expending energy.

Adaptations to Reduce Photorespiration

• C4 plants fix CO2 into four-carbon compounds, segregating reactions into different cells.

• CAM plants open stomata at night for CO2 intake, reducing water loss in dry conditions.

Comparison with Cellular Respiration

• Both photosynthesis and respiration involve redox reactions but reverse electron flow.

• Photosynthesis uses: $ext{Energy + 6 H₂O + 6 CO₂} o ext{C₆H₁₂O₆ + 6 O₂}$.

• Cellular respiration uses: $ext{C₆H₁₂O₆ + 6 O₂} o ext{6 CO₂ + 6 H₂O + Energy}$.