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In some plants such as cacti, chloroplasts are found in the stems, [183] though in most plants, chloroplasts are concentrated in the leaves. One square millimeter of leaf tissue can contain half a million chloroplasts. [11] Within a leaf, chloroplasts are mainly found in the mesophyll layers of a leaf, and the guard cells of stomata.
The several plastids including chloroplasts are distributed among some but not all eukaryotes. There are also occasional exceptions to the number of membranes surrounding organelles, listed in the tables below (e.g., some that are listed as double-membrane are sometimes found with single or triple membranes).
Chloroplasts have their own genome, which encodes a number of thylakoid proteins. However, during the course of plastid evolution from their cyanobacterial endosymbiotic ancestors, extensive gene transfer from the chloroplast genome to the cell nucleus took place. This results in the four major thylakoid protein complexes being encoded in part ...
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 7% sulfolipids, while the inner membrane has been found to contain 16% phospholipids, 79% galactolipids and 5% sulfolipids in spinach chloroplasts.
Chloroplasts can only be found in plants and algae, and they capture the sun's energy to make carbohydrates through photosynthesis. Diagram of the endomembrane system Endoplasmic reticulum : The endoplasmic reticulum (ER) is a transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules ...
For example, chloroplasts in plants and green algae have lost all phycobilisomes, the light harvesting complexes found in cyanobacteria, red algae and glaucophytes, but instead contain stroma and grana thylakoids. The glaucocystophycean plastid—in contrast to chloroplasts and rhodoplasts—is still surrounded by the remains of the ...
Euglena chloroplasts contain pyrenoids, used in the synthesis of paramylon, a form of starch energy storage enabling Euglena to survive periods of light deprivation. The presence of pyrenoids is used as an identifying feature of the genus, separating it from other euglenoids, such as Lepocinclis and Phacus. [14]
Organelles (literally "little organs") are usually membrane-bound structures inside the cell that have specific functions. Some major organelles that are suspended in the cytosol are the mitochondria, the endoplasmic reticulum, the Golgi apparatus, vacuoles, lysosomes, and in plant cells, chloroplasts.