Chloroplasts harvest the energy of sunlight to generate ATP. Within the stroma of the chloroplasts are membrane-bound disclike structures, called thylakoids (figure 3.56). These structures contain chlorophyll, a pigment that emits electrons when stimulated by the energy of sunlight. These electrons are then transferred to the electron transport chain embedded in the membrane of the thylakoids. Meanwhile, chlorophyll returns to its original state when water is oxidized to produce oxygen.

ATP generated in the chloroplast is used to convert CO2 to organic compounds such as sugar and starch. These are the very compounds that most cells degrade as a source of energy.

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a complex three-dimensional internal membrane system of flattened sheets, sacs, and tubes (figure 3.57). The rough endoplasmic reticulum has a characteristic bumpy appearance due to the multitude of ribosomes coating it. It is the site where proteins not destined for the cytoplasm are synthesized. These include proteins destined for the lumen of an organelle or for secretion outside the cell. Membrane proteins such as receptors are also synthesized on the rough ER. The ribosomes making these proteins attach to the rough ER. During the process of synthesis, the protein is

3.13 Membrane-Bound Organelles

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