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3.17: Chloroplasts

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    3995
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    A typical plant cell (e.g., in the palisade layer of a leaf) might contain as many as 50 chloroplasts.

    alt
    Figure 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.
    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.


    This page titled 3.17: Chloroplasts is shared under a CC BY 3.0 license and was authored, remixed, and/or curated by John W. Kimball via source content that was edited to the style and standards of the LibreTexts platform.

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