What is stroma?

Chloroplasts are the sites of photosynthesis in plant cells. They have two main structures: grana and stroma.

• Grana: These are stacks of disc-shaped membranes called thylakoids. The thylakoid membranes contain chlorophyll and other pigments, which capture light energy for Photosystem I and Photosystem II. Grana are interconnected by stroma thylakoids (also called intergranal lamellae), forming a continuous network.
• Stroma: This is the jelly-like, colourless fluid that surrounds the grana inside the chloroplast. It contains enzymes that help in the dark reactions (light-independent reactions) of photosynthesis.

Inside the thylakoid lumen (the inner space of the thylakoid), the light-dependent reactions of photosynthesis take place. These reactions convert solar energy into chemical energy.

Types of thylakoids:

• Grana thylakoids: Found in stacks (grana), mainly responsible for capturing light.
• Stroma thylakoids: Connect grana and also trap light energy for photosynthesis.
• Stroma in botany: The clear fluid in the chloroplast that surrounds the grana.
• Granum (plural: grana): A single stack of thylakoid discs. A chloroplast may contain 10–100 grana, all connected by stroma thylakoids.
• Function of grana: They provide a large surface area for light absorption, which increases the efficiency of photosynthesis.

Note:

Photosynthesis takes place inside chloroplasts of photosynthetic eukaryotes. It is the process of converting light energy into chemical energy, producing oxygen and energy-rich organic compounds.

The endosymbiotic theory suggests that chloroplasts (and mitochondria) originated from free-living bacteria. Chloroplasts are closely related to photosynthetic cyanobacteria, which are considered their ancestors.