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3.17: Chloroplasts

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

Fig 3.17.1 Typical Chloroplast

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:

  1. Photosystem I which includes chlorophyll and carotenoid molecules
  2. Photosystem II which also contains chlorophyll and carotenoid molecules
  3. Cytochromes b and f
  4. 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.

    Fig 3.17.2 Chloroplast from a corn cell courtesy of Dr. L. K. Shumway

       

Fig. 3.17.3 Inner surface of thylakoid courtesy of Kenneth R. Miller

The electron micrograph in Fig. 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.

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