What Is Photosynthesis

Overview

Photosynthesis is arguably the most important chemical process on Earth. It provides the oxygen we breathe, the food we eat (directly or indirectly), and the fossil fuels that power modern civilization (which are ancient photosynthetic organisms compressed over millions of years). Without photosynthesis, life as we know it would not exist.

The Two Stages

Stage 1 — Light-Dependent Reactions: These occur in the thylakoid membranes inside chloroplasts. Chlorophyll absorbs sunlight (primarily blue and red wavelengths, reflecting green — which is why plants look green). This light energy splits water molecules (H₂O) into oxygen, hydrogen ions, and electrons. The oxygen is released as a byproduct. The energy is stored as ATP and NADPH.

Stage 2 — The Calvin Cycle: This takes place in the stroma of the chloroplast and does not require light directly. Using the ATP and NADPH from Stage 1, the Calvin cycle takes carbon dioxide (CO₂) from the air and converts it into glucose (C₆H₁₂O₆) through a series of chemical reactions. This process is also called carbon fixation.

Types of Photosynthesis

• C3 photosynthesis: The most common type, used by about 85% of plant species including rice and wheat
• C4 photosynthesis: More efficient in hot, dry conditions — used by corn, sugarcane, and many tropical grasses
• CAM photosynthesis: Used by desert plants like cacti — they open their stomata at night to conserve water

Why It Matters

Photosynthesis is central to the global carbon cycle. Plants absorb about 25% of human CO₂ emissions, making forests crucial carbon sinks. Deforestation reduces this capacity, accelerating climate change. Research into artificial photosynthesis aims to replicate this process to produce clean energy.