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coli, have a dual membrane system in which the plasma membrane (or inner membrane) is surrounded by the cell wall and a distinct outer membrane (Figure 12.8). In contrast to the plasma membrane, the outer membrane is highly permeable to ions and small polar molecules (in the case of E. coli, with molecular weights up to 600).
Cell parts and their functions. Within cells, special structures are responsible for particular functions. For example, mitochondria help release energy that the cell can use for other tasks, and the nucleus contains the genetic information. The cell membrane forms the boundary that controls what enters and leaves the cell.
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Figure 3.5 Simple Diffusion across the Cell (Plasma) Membrane The structure of the lipid bilayer allows small, uncharged substances such as oxygen and carbon dioxide, and hydrophobic molecules such as lipids, to pass through the cell membrane, down their concentration gradient, by simple diffusion.
Hace 2 días · Cell, in biology, the basic membrane-bound unit that contains the fundamental molecules of life and of which all living things are composed. A single cell may be a complete organism in itself, such as a bacterium, or it may acquire a specialized function, becoming a building block of a multicellular organism.
Proteins are the other major constituent of cell membranes, constituting 25 to 75% of the mass of the various membranes of the cell. The current model of membrane structure, proposed by Jonathan Singer and Garth Nicolson in 1972, views membranes as a fluid mosaic in which proteins are inserted into a lipid bilayer (Figure 2.48).
The plasma membrane is made up primarily of a bilayer of phospholipids with embedded proteins, carbohydrates, glycolipids, and glycoproteins, and, in animal cells, cholesterol. The amount of cholesterol in animal plasma membranes regulates the fluidity of the membrane and changes based on the temperature of the cell’s environment.
4.1: Membrane Structure and Composition. Since most cells live in an aqueous environment and the contents of the cell are also mostly aqueous, it stands to reason that a membrane that separates one side from the other must be hydrophobic to form an effective barrier against accidental leakage of materials or water.
