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.