By Kimberly Yavorski | Updated Mar 24, 2022
Photosynthesis is the remarkable process by which plants turn carbon dioxide, water, and sunlight into food. Carbon dioxide enters through tiny leaf pores called stomata, while water travels up the plant’s vascular system to reach the leaves.
During photosynthesis, sunlight’s energy is harnessed to synthesize glucose from CO₂ and H₂O. This glucose fuels plant growth, and the process also releases oxygen—an essential component for the survival of virtually all ecosystems.
Note: While algae and certain bacteria perform photosynthesis, this article focuses on the plant‑based pathway.
In plants, photosynthesis takes place within chloroplasts located in leaf mesophyll cells and green stems. A single leaf contains tens of thousands of cells, each housing 40–50 chloroplasts.
Each chloroplast contains stacks of disc‑shaped thylakoids, called grana, suspended in a fluid called stroma. Light‑dependent reactions occur in the grana, whereas light‑independent reactions take place in the stroma.
Although the entire process can complete in under a minute, it comprises intricate steps divided into two primary stages: the light reactions and the dark reactions (Calvin Cycle).
The first stage captures solar energy to generate the energy carriers ATP and NADPH. Pigment molecules in the thylakoid membrane act as antennae, absorbing light and transferring energy to chlorophyll reaction centers.
Excited electrons travel through the electron transport chain, splitting water molecules into oxygen, protons, and electrons. Oxygen diffuses out of the chloroplast via leaf pores, while ATP and NADPH accumulate for the next stage.
In the second stage, the ATP and NADPH produced earlier power the fixation of atmospheric CO₂ into sugars. The enzyme ribulose‑1,5‑bisphosphate carboxylase (RuBisCO) catalyzes the first step, ultimately producing glucose and other carbohydrates that serve as the plant’s energy reserve.
