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- The Cytoplasm of eucaryotic cells contaiins various organelles made largely of membranes. Such membranes include the plasma membrane lining the outside of the cell, as well as a transport system which includes the endoplasmic reticulum, Golgi apparatus, vacuoles, lysosomes, and vesicles. In all of these the membrane is composed of phospholipids and proteins.
- Phospholipids are composed of a phosphorrous "head" that is polar to attract molecules to the cell membrane and nonpolar lipid "tails" that form a barrier to keep molecules from passing through the cell membrane.
- The Lipid bilayer floats on the cell's ssurface like oil on water, but the phosphorous "heads" hold the membrane together with their polar attraction.
- - Cell surface proteins embedded in the lipid bilayer perform three main functions:
- ) Channel proteins serve as passageways allowing only certain molecules in and out of the cell.
- ) Receptor Proteins transmit chemical signals (hormones) into the cell so that cells can communicate with each other.
- ) Marker proteins identify the cells as belonging to a community to prevent the immune system from attacking them.
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