Chapter 10: Photosynthesis

Overview

• Photosynthesis is essential for the nourishment of organisms in the biosphere.
• Converts solar energy into chemical energy.
• Conducted by plants, algae, some prokaryotes, and specific protists.
• Autotrophs: produce energy without ingesting it, considered producers in ecology.
  • Convert CO2 and inorganic materials into organic molecules.
  • Photoautotrophs: use sunlight to produce organic molecules.
• Heterotrophs: derive energy from other organisms, either directly from plants or from plant-consuming organisms.

Photosynthesis Process

• Takes place primarily in leaves, in structures called chloroplasts.
• Chloroplasts evolved from photosynthetic bacteria.
• Chlorophyll: the pigment responsible for leaf color, found in thylakoid membranes.
• Stomata: microscopic openings allowing gas exchange (O2 out, CO2 in).
• Two main stages:
  1. Light Reactions: occur in thylakoids, split water, produce O2, ATP, and NADPH.
  2. Calvin Cycle: occurs in stroma, uses CO2, ATP, and NADPH to produce sugar.

Light Reactions

• Involve electron transfer via photosystems and electron transport chains.
• Photosystem II: absorbs light, splits water, generates ATP.
• Photosystem I: absorbs light, generates NADPH.
• Energy captured is used for photophosphorylation.

Calvin Cycle

• Takes place in stroma, uses CO2, ATP, and NADPH to produce G3P, a precursor to glucose.
• Three stages:
  1. Carbon Fixation: CO2 is fixed into an organic molecule via rubisco enzyme.
  2. Reduction: ATP and NADPH convert the fixed carbon into G3P.
  3. Regeneration of RuBP: ATP is used to regenerate starting molecules for the cycle.

Electromagnetic Spectrum and Light Absorption

• Light travels in waves; visible light can be seen by the naked eye.
• Wavelengths determine energy level; shorter wavelengths are more energetic.
• Pigments absorb specific wavelengths; unabsorbed wavelengths are reflected (e.g., green light in leaves).

Chemiosmosis in Chloroplasts vs. Mitochondria

• Both use chemiosmosis to produce ATP but differ in energy sources and processes.
• Chloroplasts use light energy, mitochondria use chemical energy from food.
• ATP generation in chloroplasts occurs in the stroma, whereas in mitochondria it is in the matrix.

Alternative Photosynthesis Pathways

• C4 Plants: spatial separation of steps to minimize photorespiration.
  • Use PEP carboxylase to fix CO2 into 4-carbon molecules.
  • Store CO2 in bundle sheath cells for later use.
• CAM Plants: temporal separation of steps.
  • Stomata open at night to fix CO2 into organic acids and release it during the day for the Calvin Cycle.

Importance of Photosynthesis

• Primary source of organic compounds and oxygen for the biosphere.
• Oxygen produced is essential for the survival of aerobic organisms.

Summary

Photosynthesis is a complex process converting solar energy into chemical energy, forming the foundation of life by providing food and oxygen. It involves intricate steps of light absorption, electron transport, and carbon fixation, with mechanisms adapted to various environmental conditions.