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Chapter 7 Membrane Structure and Function I. Cellular Membranes - The plasma membrane is the “edge of life”, boundary that separates the living cell from its nonliving surroundings. - The plasma membrane exhibits selective permeability allowing some substances to cross it more easily than others. - The fluid mosaic model states that cellular membranes are fluid bilayer of lipids with a “mosaic” of various proteins embedded in it. - Phospholipids are the most abundant lipid in the plasma membrane and contain both hydrophobic and hydrophilic regions. A. Membrane Models: Scientific Inquiry - Membranes have been chemically analyzed and found to be composed of proteins and lipids. - Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer. - The Davson-Danielli sandwich model (1960’s) of membrane structure stated that the membrane was made up of a phospholipid bilayer sandwiched between two protein layers. - Was supported by electron microscope pictures of membranes. - In 1972, Singer and Nicolson proposed that membrane proteins are dispersed and individually inserted into the phospholipid bilayer. - Freeze-fracture studies of the plasma membrane supported the fluid mosaic model of membrane structure. B. The Fluidity of Membranes - Phospholipids in the plasma membrane can move within the bilayer. - Proteins in the plasma membrane can drift within the bilayer. - The type of hydrocarbon tails in phospholipids affects the fluidity of the plasma membrane. - The steroid cholesterol has different effects on membrane fluidity at different temperatures. C. Membrane Proteins and Their Functions - A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer. - Phospholipids form the main fabric of the membrane, but various proteins determine most of the membranes specific function. - Integral proteins penetrate the hydrophobic core of the lipid bilayer. - Integral proteins are often transmembrane proteins, completely spanning the membrane. - Peripheral proteins are appendages loosely bound to the surface of the membrane. D. The Role of Membrane Carbohydrates in Cell-Cell Recognition - Cell-cell recognition is a cell’s ability to distinguish one type of neighboring cell from another. - Membrane carbohydrates interact with the surface molecules of other cells, facilitating cell-cell recognition. E. Synthesis and Sidedness of Membranes - Membranes have distinct inside and outside faces. - This affects the movement of proteins synthesized in the endomembrane system. - Membrane proteins and lipids are synthesized in the ER and Golgi apparatus. II. Selective Permeability of Membranes - The biological membrane is an exquisite example of a supramolecular structure with emergent properties beyond those of the individual molecules. - A cell must exchange materials (sugars, amino acids, O2, CO2, ions, etc.) with its surroundings, a process controlled by the plasma membrane. A. The Permeability of the Lipid Bilayer - Hydrophobic molecules (hydrocarbons, O2, CO2, etc.) are lipid soluble and can pass through the membrane rapidly. - Polar molecules (glucose, H2O, etc.) and ions do not cross the membrane rapidly. B. Transport Proteins - Transport proteins allow passage of specific hydrophilic substances across the membrane. III. Passive Transport of Substances across Membranes - Diffusion is the tendency for molecules of any substance to spread out evenly into the available space. - Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another. - Passive transport is diffusion of a substance across a membrane with no energy investment. A. Osmosis - the movement of water across a selectively permeable membrane - affected by the concentration gradient of dissolved substances 1. Water Balance of Cells without Walls - Tonicity, the ability of a solution to cause a cell to gain or lose water, has a great impact on cells without walls. - If a solution is isotonic, the concentration of solutes is the same as it is inside the cell and there will be no net movement of water. - If a solution is hypertonic, the concentration of solutes is greater than it is inside the cell and the cell will lose water. - If a solution is hypotonic, the concentration of solutes is less than it is inside the cell and the cell will gain water. - Animals and other organisms without rigid cell walls living in hypertonic or hypotonic environments must have special adaptations for osmoregulation. 2. Water Balance of Cells with Walls - Cell walls help plants maintain water balance. - If a plant cell is turgid, it is in a hypotonic environment and is very firm, a healthy state in most plants. - If a plant cell is flaccid, it is in an isotonic or hypertonic environment. B. Facilitated Diffusion - In facilitated diffusion, transport proteins speed the movement of molecules (polar molecules, ions) across the plasma membrane (passive transport). - Two types of transport proteins: - Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane. - Gated channels allow specific ions to pass through when triggered by a stimulus. - Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane. IV. Active Transport A. The Need for Energy in Active Transport - Active transport moves substances against their concentration gradient and requires energy, usually in the form of ATP. - The sodium-potassium pump is one type of active transport system. B. Maintenance of Membrane Potential by Ion Pumps - Membrane potential is the voltage (electrical potential energy) difference across a membrane (acts as a battery). - An electrochemical gradient is caused by the concentration gradient of ions across a membrane. - An electrogenic pump is a transport protein that generates the voltage across a membrane. C. Cotransport: Coupled Transport by a Membrane Protein - Cotransport: active transport driven by a concentration gradient. - Cotransport occurs when active transport of a specific solute indirectly drives the active transport of another solute. V. Bulk Transport across the Plasma Membrane - Large proteins and polysaccharides cross the membrane by different mechanisms. A. Exocytosis - Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents. B. Endocytosis - The cell takes in macromolecules by forming new vesicles from the plasma membrane. - Three types of endocytosis: 1. Phagocytosis 2. Pinocytosis 3. Receptor-Mediated Endocytosis