Mitochondria and chloroplasts are not part of the endomembrane system.In contrast to organelles of the endomembrane system, each mitochondrion or chloroplast has two membranes separating the innermost space from the cytosol.Their membrane proteins are not made by the ER, but rather by free ribosomes in the cytosol and by ribosomes within the organelles themselves.Both organelles have small quantities of DNA that direct the synthesis of the polypeptides produced by these internal ribosomes.Mitochondria and chloroplasts grow and reproduce as semiautonomous organelles.Almost all eukaryotic cells have mitochondria.There may be one very large mitochondrion or hundreds to thousands of individual mitochondria.The number of mitochondria is correlated with aerobic metabolic activity.A typical mitochondrion is 1–10 microns long.Mitochondria are quite dynamic: moving, changing shape, and dividing.Mitochondria have a smooth outer membrane and a convoluted inner membrane with infoldings called cristae.The inner membrane divides the mitochondrion into two internal compartments.The first is the intermembrane space, a narrow region between the inner and outer membranes.The inner membrane encloses the mitochondrial matrix,a fluid-filled space with DNA, ribosomes, and enzymes.Some of the metabolic steps of cellular respiration are catalyzed by enzymes in the matrix.The cristae present a large surface area for the enzymes that synthesize ATP.The chloroplast is one of several members of a generalized class of plant structures called plastids.Amyloplasts are colorless plastids that store starch in roots and tubers.Chromoplasts store pigments for fruits and flowers.Chloroplasts contain the green pigment chlorophyll as well as enzymes and other molecules that function in the photosynthetic production of sugar.Chloroplasts measure about 2 microns × 5 microns and are found in leaves and other green organs of plants and algae.The contents of the chloroplast are separated from the cytosol by an envelope consisting of two membranes separated by a narrow intermembrane space.Inside the innermost membrane is a fluid-filled space, the stroma,in which float membranous sacs, the thylakoids.The stroma contains DNA, ribosomes, and enzymes.
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The thylakoids are flattened sacs that play a critical role in converting light to chemical energy. In some regions, thylakoids are stacked like poker chips into grana.The membranes of the chloroplast divide the chloroplast into three compartments: the intermembrane space, the stroma, and the thylakoid space.Like mitochondria, chloroplasts are dynamic structures.Their shape is plastic, and they can reproduce themselves by pinching in two.
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