10.2: The structure of the chloroplast

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In plants, photosynthesis takes place primarily in leaves, which consist of many layers of cells and have differentiated top and bottom sides. The process of photosynthesis occurs not on the surface layers of the leaf, but rather in a middle layer called the mesophyll (Figure \(\PageIndex{1}\)).

The fine scale structure of a leaf featuring the major tissues; the upper and lower epithelia (and associated cuticles), the palisade and spongy mesophyll and the guard cells of the stoma. Key plant cell organelles (the cell wall, nucleus, chloroplasts, vacuole and cytoplasm) are also shown. — **Figure \(\PageIndex{1}\):** Not all cells of a leaf carry out photosynthesis. Cells within the middle layer of a leaf have chloroplasts, which contain the photosynthetic apparatus. (credit Zephyris; wikimedia)

The gas exchange of carbon dioxide and oxygen occurs through small, regulated openings called stomata.

Colorized electron microscope image of a stomate on the leaf of a tomato plant. — **Figure \(\PageIndex{2}\):** Tomato leaf stomate (singular of stomata). Photo credit: Photohound; Wikimedia; Public Domain.

In eukaryotes, photosynthesis takes place inside an organelle called a chloroplast. Some prokaryotes can perform photosynthesis, but they do not contain chloroplasts (or other membrane-bound organelles). In plants, chloroplast-containing cells exist in the mesophyll. Chloroplasts are surrounded by a double membrane similar to the double membrane found within a mitochondrion. Within the chloroplast is a third membrane that forms stacked, disc-shaped structures called thylakoids. Embedded in the thylakoid membrane are molecules of chlorophyll, a pigment (a molecule that absorbs light) through which the entire process of photosynthesis begins. Chlorophyll is responsible for the green color of plants. The thylakoid membrane encloses an internal space called the thylakoid lumen or space. Other types of pigments are also involved in photosynthesis, but chlorophyll is by far the most important. As shown in Figure \(\PageIndex{3}\), a stack of thylakoids is called a granum, and the space surrounding the granum is called stroma (not to be confused with stomata, the openings on the leaves).

This illustration shows a chloroplast, which has an outer membrane and an inner membrane. The space between the outer and inner membranes is called the intermembrane space. Inside the inner membrane are flat, pancake-like structures called thylakoids. The thylakoids form stacks called grana. The liquid inside the inner membrane is called the stroma, and the space inside the thylakoid is called the thylakoid lumen. — **Figure \(\PageIndex{3}\):** Structure of the chloroplast. Note that the chloroplast is surrounded by a double membrane, but also contains a third set of membranes, which enclose the thylakoids.

Just like the structure of the mitochondria was important to its ability to perform aerobic cellular respiration, the structure of the chloroplast allows the process of photosynthesis to take place. Both the light-dependent reactions and the Calvin cycle take place inside of the chloroplast.

References

Unless otherwise noted, images on this page are licensed under CC-BY 4.0 by OpenStax.

Text adapted from: OpenStax, Concepts of Biology. OpenStax CNX. May 18, 2016 http://cnx.org/contents/b3c1e1d2-839...9a8aafbdd@9.10