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Where does photosynthesis occur in plant cells?

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Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — Photosynthesis Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

In plant cells, photosynthesis occurs entirely within specialized organelles called chloroplasts. These organelles contain the necessary photosynthetic apparatus, including pigments, enzymes, and electron-carrying chemicals.12

The process is divided into two interdependent stages that take place in different regions of the chloroplast:

Light Reactions: These occur exclusively within the thylakoids, which are closed, disk-like structures formed by internal membranes. In many plants, these thylakoids are tightly stacked into regions known as grana.32
Dark Reactions (Carbon-Assimilating Reactions): These occur in the stroma, the colorless, enzyme-rich matrix that surrounds the thylakoids.32

The structural integrity of these components is vital; for instance, the capacity for carbon fixation is lost if the chloroplast's outer membrane is damaged, causing the enzymes in the stroma to leak out. Similarly, intact thylakoids are required for the efficient conversion of light into chemical energy.3