Rucete ✏ Campbell Biology In a Nutshell
Unit 2 THE CELL — Concept 10.2 Photosynthesis Converts Light Energy to the Chemical Energy of Food
Photosynthesis uses light energy to produce sugars through a process that occurs in chloroplasts. It consists of two major stages: the light reactions and the Calvin cycle.
The Site of Photosynthesis
- Photosynthesis occurs in chloroplasts, mostly within leaf mesophyll cells
- CO₂ enters through stomata; water is delivered via veins
- Chloroplast structure: outer and inner membranes, stroma (fluid), and thylakoids (membrane sacs)
- Thylakoids contain chlorophyll, which captures light energy
- Thylakoids may be stacked into grana
Photosynthesis as a Redox Process
- Photosynthesis reverses the direction of electron flow seen in cellular respiration
- Water is split, providing electrons and protons; oxygen is released as a by-product
- Electrons are transferred to carbon dioxide to form sugar
- Overall reaction (simplified):
6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂
Evidence for Water Splitting
- Early hypothesis: O₂ came from CO₂
- Van Niel proposed O₂ came from H₂O, based on sulfur bacteria experiments
- Later confirmed with ¹⁸O isotope tracing: O₂ originates from H₂O, not CO₂
Two Stages of Photosynthesis
- Light reactions (in thylakoids):
- Split water, releasing O₂
- Generate ATP and NADPH using light energy
- Calvin cycle (in stroma):
- Fixes CO₂ into organic molecules
- Uses ATP and NADPH to produce sugar
- Does not require light directly, but depends on light reactions for energy inputs
Light and Pigments
- Light behaves as both waves and particles (photons)
- Energy of photons is inversely related to wavelength (shorter = more energy)
- Visible light (380–740 nm) drives photosynthesis
- Pigments like chlorophyll absorb specific wavelengths; green is reflected
In a Nutshell
Photosynthesis occurs in chloroplasts and consists of light reactions and the Calvin cycle. Light energy is converted into ATP and NADPH, which are then used to fix CO₂ into sugars. The oxygen released comes from water, not carbon dioxide.