Chloroplasts

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

1. A smooth outer membrane which is freely permeable to molecules.
2. 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
3. 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:
  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
  These carry out the so-called light reactions of photosynthesis.

  Links to (1) a discussion of the "light" reactions and (2) a graphic showing the 4 complexes in the thylakoid membrane.

• The thylakoid membranes are surrounded by a fluid stroma.
  The stroma 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.

    Link to a discussion of the "dark" reactions.

  • 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.

    Illustrated discussion of the chloroplast genome

The electron micrograph above on the right (courtesy of Dr. L. K. Shumway) shows the chloroplast from the cell of a corn leaf.

The electron micrograph on the left (courtesy of Kenneth R. Miller) 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.