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|Chapter Three: Cell Structures|
Plastids are organelles which are found only in autotrophic cells. In many respects, plastids are similar to entire prokaryotic organisms; they contain their own DNA and can replicate themselves. Like the mitochondria, plastids have an outer membrane which separates them from the cytoplasm and a highly folded inner membrane. Plastids come in three different types: leucoplasts, chloroplasts, and chromoplasts.
Leucoplasts are colorless plastids which store starch (a carbohydrate), proteins, and lipids. These materials are released from the leucoplast when the cell requires them.
Chloroplasts are the organelles in which photosynthesis (see Chapter Four) takes place. Photosynthesis is an important process by which autotrophic cells manufacture their own food. Chloroplasts contain the green pigment chlorophyll (this is why plant leaves are green) which absorbs light to provide the energy necessary to complete photosynthesis.
Chloroplasts have two membranes: an outer membrane and an inner membrane. A solution called the stroma fills the part of the chloroplast inside of the inner membrane. In this area, there are stacks of flattened vesicles. The stacks themselves are known as grana, and the vesicles are called thylakoids. The thylakoids are where photosynthesis actually occurs.
Chromoplasts are very similar to chloroplasts, but they do not contain the green pigment chlorophyll. Instead, they contain other pigments which give color to flowers and to leaves during the fall. These other pigments absorb colors of light than chlorophyll.