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The Structure of Cells

Author(s): Center for Educational Outreach


Chloroplasts are enclosed in a double membrane system. They consist of flattened sacs, called thylakoids, which can be stacked into granum. Thylakoids are surrounded by fluid called stroma. Within the stroma are DNA, ribosomes and enzymes.

Grana are stacks of thylakoids (organelles) that join together to form one functional unit. This is the site of light-dependent photosynthetic reactions with photosynthetic pigment in the membrane.

Image notes: On the left is a light-microscopic image of chloroplasts in the cells of Plagiomnium affine, the many-fruited thyme moss. On the right is a transmission electronic image of a single chloroplast within Coleus blumei leaf. The three large round areas are starch granules. The dark “lines” are grana.