3.17: Chloroplasts

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A typical plant cell (e.g., in the palisade layer of a leaf) might contain as many as 50 chloroplasts.

The chloroplast is made up of 3 types of membrane:

A smooth outer membrane which is freely permeable to molecules.
A smooth inner membrane which contains many transporters: integral membrane proteins that regulate the passage in an out of the chloroplast of
- small molecules like sugars
- proteins synthesized in the cytoplasm of the cell but used within the chloroplast
A system of thylakoid membranes

Thylakoids

The thylakoid membranes enclose a lumen: a system of vesicles (that may all be interconnected). At various places within the chloroplast these are stacked in arrays called grana (resembling a stack of coins). Four types of protein assemblies are embedded in the thylakoid membranes: These carry out the so-called light reactions of photosynthesis including:

Photosystem I which includes chlorophyll and carotenoid molecules
Photosystem II which also contains chlorophyll and carotenoid molecules
Cytochromes b and f
ATP synthase

The thylakoid membranes are surrounded by a fluid stroma, which contains all the enzymes, e.g., RUBISCO, needed to carry out the "dark" reactions of photosynthesis; that is, the conversion of CO₂ into organic molecules like glucose. A number of identical molecules of DNA, each of which carries the complete chloroplast genome. The genes encode some — but not all of the molecules needed for chloroplast function. The others are

transcribed from genes in the nucleus of the cell
translated in the cytoplasm and
transported into the chloroplast.

alt — Figure 3.17.2 Chloroplast from a corn cell courtesy of Dr. L. K. Shumway

alt — Figure 3.17.3 Inner surface of thylakoid courtesy of Kenneth R. Miller

The electron micrograph in Figure 3.17.3 shows the inner surface of a thylakoid membrane. Each particle may represent one photosystem II complex. In the functioning chloroplast, these particles may not be as highly ordered as seen here.