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4g. Mitochondria and Chloroplasts
 | Mitochondria with inner and outer membranes shown 
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Any factory needs some sort of energy source. This energy must be in a usable form. Most factories have power plants in which generators burn fuel to produce heat. This heat energy is used to make steam, which is then used to make electricity.
Building proteins is the main function of cells. But for this to occur, a cell must have an energy source, and that energy must be in a form that the cell can use. The mitochondrion and the chloroplast are the two organelles responsible for energy transformation (neither organelle truly produces energy).
Like our factory's power plant, mitochondria and chloroplasts transform one form of energy to another. Remember that nearly all the energy used by living things on Earth comes from the Sun. This section discusses how energy is made available for cell processes.
The Chloroplast
 | The parts of the chloroplast
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Energy enters the food chain through the chloroplasts. Chloroplasts don't exist in animal cells; they are present only in plants and some protists. CHLOROPLASTS trap light energy and convert it into the chemical bond energy of sugar. Living things use sugars as their primary energy source. The chloroplast has an outer membrane and an inner membrane. Within the chloroplast is a cavity called the STROMA where a third set of membranes is located. These innermost membranes, called THYLAKOIDS, are arranged in stacks called GRANA. Most of photosynthesis occurs along the thylakoid membranes with the help of the pigment CHLOROPHYLL.
The Mitochondrion
 | Muscle cell mitochondria
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Once energy is trapped in the form of sugar by photosynthesis, it must be converted into a form that the cell can easily use. Sugars are rich in energy, but the energy in their chemical bonds can't be released easily enough to be used in cell processes. Remember that the energy source cells use is a chemical called adenosine triphosphate (ATP).
Respiration releases the energy that was trapped by photosynthesis and converts it into the easily accessible, high-energy phosphate bonds of ATP. All living things (including plants) must respire to produce ATP. Eukaryotic cells have a special organelle called the mitochondrion that makes this process more efficient. The MITOCHONDRION, like the chloroplast, is a double membrane structure. The outer membrane of the mitochondrion is smooth, and the inner membrane is ruffled. The essential energy-releasing reactions of aerobic respiration takes place along these membranes.
 | A chemical reaction involving ATP creates energy for the cell.
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Origins
The mitochondrion and the chloroplast both have a number of unusual features that distinguish them from other organelles: they contain their own DNA that loops around like that of bacteria, they manufacture many of their own proteins, and they both reproduce by binary fission. This is very similar to what bacteria do. The similarity has led many scientists to conclude that these organelles may have evolved from independent bacteria that took up residence in early eukaryotic cells billions of years ago. These bacteria eventually became so dependent on their hosts, and vice versa, that they have essentially become one organism.
  
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