Overview

It is the most important chemical reaction on Earth. Without it, there would be no food and no oxygen. Plants take sunlight (which is intangible energy), water, and carbon dioxide (air), and turn them into sugar (solid food). It is basically magic: making matter out of light.

Core Idea

The core idea is Energy Conversion. The universe runs on energy. But you can’t eat sunlight. Photosynthesis is the bridge that converts solar energy (photons) into chemical bond energy (glucose). Animals then eat the plants to get that energy. All life is solar-powered.

Formal Definition

The process by which green plants and some other organisms use sunlight to synthesize foods from carbon dioxide and water. Formula: 6CO2 + 6H2O + Light Energy → C6H12O6 (Sugar) + 6O2 (Oxygen).

Intuition

Think of a solar panel charging a battery.

  • Chloroplast: The solar panel.
  • Chlorophyll: The green pigment that catches the sun.
  • ATP/NADPH: The electricity flowing through the wire.
  • Glucose: The charged battery (stored energy).

Examples

  • The Great Oxygenation Event: 2.4 billion years ago, cyanobacteria “invented” photosynthesis. They pumped so much oxygen into the air that it poisoned almost everything else on Earth (which was anaerobic) and created the ozone layer, allowing complex life to evolve.
  • Autumn Leaves: Why do leaves turn red/yellow? The green chlorophyll breaks down in the cold, revealing the other pigments (carotenoids) that were hiding underneath all along.
  • Sea Slugs: The Elysia chlorotica slug eats algae and steals its chloroplasts. It can then survive on sunlight alone for months. It is a solar-powered animal.

Common Misconceptions

  • Plants breathe CO2: They do take in CO2, but they also need Oxygen for respiration (just like us) at night when there is no sun.
  • Only plants do it: Half the oxygen you breathe comes from phytoplankton (algae/bacteria) in the ocean, not trees.
  • Light-Dependent Reactions: The “Day Shift.” Occurs in the thylakoid. Splits water, releases oxygen, makes ATP.
  • Calvin Cycle (Light-Independent): The “Night Shift.” Occurs in the stroma. Uses the ATP to build sugar out of CO2.
  • Carbon Sequestration: Plants suck carbon out of the air and lock it in their wood. This is our best defense against Climate Change.

Applications

  • Biofuels: Turning corn or algae into ethanol is basically harvesting stored photosynthesis to run cars.
  • Artificial Photosynthesis: Scientists are trying to build machines that do what leaves do—split water into hydrogen fuel using just sunlight.

Criticism / Limitations

  • Inefficiency: Photosynthesis is actually very inefficient (only about 1-2% of sunlight is converted to biomass). Evolution settled for “good enough,” not “perfect.”

Further Reading

  • Morton, Oliver. Eating the Sun: How Plants Power the Planet. 2007.
  • Blankenship, Robert. Molecular Mechanisms of Photosynthesis. 2002.