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5: Structure and Function of Plasma Membranes
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5.1: Components and Structure - Components of Plasma Membranes
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The plasma membrane protects the cell from its external environment, mediates cellular transport, and transmits cellular signals.
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5.2: Components and Structure - Fluid Mosaic Model
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The fluid mosaic model describes the plasma membrane structure as a mosaic of phospholipids, cholesterol, proteins, and carbohydrates.
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5.3: Components and Structure - Membrane Fluidity
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The mosaic nature of the membrane, its phospholipid chemistry, and the presence of cholesterol contribute to membrane fluidity.
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5.4: Passive Transport - The Role of Passive Transport
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Passive transport, such as diffusion and osmosis, moves materials of small molecular weight across membranes.
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5.5: Passive Transport - Selective Permeability
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The hydrophobic and hydrophilic regions of plasma membranes aid the diffusion of some molecules and hinder the diffusion of others.
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5.6: Passive Transport - Diffusion
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Diffusion is a process of passive transport in which molecules move from an area of higher concentration to one of lower concentration.
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5.7: Passive Transport - Facilitated Transport
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Facilitated diffusion is a process by which molecules are transported across the plasma membrane with the help of membrane proteins.
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5.8: Passive Transport - Osmosis
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Osmosis is the movement of water through a semipermeable membrane according to the concentration gradient of water across the membrane, which is inversely proportional to the concentration of solutes. Semipermeable membranes, also termed selectively permeable membranes or partially permeable membranes, allow certain molecules or ions to pass through by diffusion.
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5.9: Passive Transport - Tonicity
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Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis. A solution’s tonicity often directly correlates with the osmolarity of the solution. Osmolarity describes the total solute concentration of the solution. A solution with low osmolarity has a greater number of water molecules relative to the number of solute particles; a solution with high osmolarity has fewer water molecules with respect to solute particles.
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5.10: Active Transport - Electrochemical Gradient
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To move substances against the membrane’s electrochemical gradient, the cell utilizes active transport, which requires energy from ATP.
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5.11: Active Transport - Primary Active Transport
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The sodium-potassium pump maintains the electrochemical gradient of living cells by moving sodium in and potassium out of the cell.
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5.12: Active Transport - Secondary Active Transport
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In secondary active transport, a molecule is moved down its electrochemical gradient as another is moved up its concentration gradient.
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5.13: Bulk Transport - Endocytosis
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Endocytosis is a type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. There are different variations of endocytosis, but all share a common characteristic: the plasma membrane of the cell invaginates, forming a pocket around the target particle. The pocket pinches off, resulting in the particle being contained in a newly-created intracellular vesicle formed from the plasma membrane.
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5.14: Bulk Transport - Exocytosis
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Exocytosis is the process by which cells release particles from within the cell into the extracellular space.