A stack of thylakoids (flattened sac-like structures) found inside chloroplasts in plant cells, where light reactions of photosynthesis occur.
From Latin granum meaning 'grain' or 'seed,' referring to the grain-like appearance of these stacked structures under early electron microscopes. The term became standard in plant biology in the mid-20th century.
Grana are basically tiny solar power plants stacked like coins—each thylakoid in the stack is a membrane loaded with pigments and proteins that capture light energy and convert it into chemical energy that powers all plant growth.
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