Chapter 13: The Plasma Membrane
Chapter Summary
STRUCTURE OF THE PLASMA MEMBRANE
The Phospholipid Bilayer: The fundamental structure of the plasma membrane is a phospholipid bilayer, which also contains glycolipids and cholesterol.
Membrane Proteins: Associated proteins are responsible for carrying out specific membrane functions. Membranes are viewed as fluid mosaics in which proteins are inserted into phospholipid bilayers.
Mobility of Membrane Proteins: Proteins are free to diffuse laterally through the phospholipid bilayer. However, the mobility of some proteins is restricted by their associations with other proteins or specific lipids. In addition, tight junctions prevent proteins from moving between distinct plasma membrane domains of epithelial cells.
The Glycocalyx: The cell surface is covered by a carbohydrate coat called the glycocalyx. Cell surface carbohydrates serve as markers for cell-cell recognition.
TRANSPORT OF SMALL MOLECULES
Passive Diffusion: Small hydrophobic molecules are able to cross the plasma membrane by diffusing through the phospholipid bilayer.
Facilitated Diffusion and Carrier Proteins: The passage of most biological molecules is mediated by carrier or channel proteins that allow polar and charged molecules to cross the plasma membrane without interacting with its hydrophobic interior.
Ion Channels: Ion channels mediate the rapid passage of selected ions across the plasma membrane. They are particularly well characterized in nerve and muscle cells, where they are responsible for the transmission of electric signals.
Active Transport Driven by ATP Hydrolysis: Energy derived from ATP hydrolysis can drive the transport of molecules against their electrochemical gradients.
Active Transport Driven by Ion Gradients: Ion gradients are frequently used as a source of energy to drive the active transport of other molecules.
ENDOCYTOSIS
Phagocytosis: Cells ingest large particles, such as bacteria and cell debris, by phagocytosis.
Receptor-Mediated Endocytosis: The best-characterized form of endocytosis is receptor-mediated endocytosis, which provides a mechanism for the selective uptake of specific macromolecules.
Protein Trafficking in Endocytosis: Molecules taken up by endocytosis are transported to endosomes, where they are sorted for recycling to the plasma membrane or degradation in lysosomes.
