Outer chloroplast envelope
| Cell biology | |
|---|---|
| The chloroplast | |
Components of a typical chloroplast
1 Granum
4 Stromal thylakoid |
1 Granum
2 Chloroplast envelope ◄ You are here
4 Stromal thylakoid
5 Stroma
6 Nucleoid (DNA ring)
7 Ribosome
8 Plastoglobulus
9 Starch granule
Chloroplasts contain several important membranes, vital for their function. Like , chloroplasts have a double-membrane envelope, called the chloroplast envelope, but unlike , chloroplasts also have internal membrane structures called thylakoids. Furthermore, one or two additional membranes may enclose chloroplasts in organisms that underwent secondary endosymbiosis, such as the euglenids and chlorarachniophytes.
The chloroplasts come via endosymbiosis by engulfment of a photosynthetic cyanobacterium by the eukaryotic, already mitochondriate cell. Over millions of years the endosymbiotic cyanobacterium evolved structurally and functionally, retaining its own DNA and the ability to divide by binary fission (not mitotically) but giving up its autonomy by the transfer of some of its genes to the nuclear genome.
Each of the envelope membranes is a lipid bilayer that is between 6 and 8 nm thick. The lipid composition of the outer membrane has been found to be 48% phospholipids, 46% galactolipids and 6% sulfolipids, while the inner membrane has been found to contain 16% phospholipids, 79% galactolipids and 5% sulfolipids in spinach chloroplasts.
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