Membrane Structure and Function Notes

Chapter 7: Membrane Structure and Function 7.1 “Cellular membranes are fluid mosaics of lipids and proteins”  Lipids and proteins are the staple ingredients of membranes, although carbohydrates are also important. The most abundant lipids in most membranes are phospholipids. The ability of phospholipids to form membranes is inherent in their molecular structure. A phospholipid is an amphipathic molecule The Fluidity of Membranes  A membrane is held together primarily by hydrophobic interactions, which are much weaker than covalent bonds  The lateral movement of phospholipids within the membrane is rapid  Adjacent phospholipid switch positions about 107 per second, which means that a phospholipid can travel about um—the length of a typical bacterial cell—in 1 second. Proteins are larger than lipids and move more slowly, but some do drift The Role of Membrane Carbohydrates in Cell-Cell Recognition  Cell-cell recognition, a cell’s ability to distinguish one type of neighboring cell from another, is crucial to the functioning of an organism  The way cells recognize other cells is by binding to surface molecules, often carbohydrates, on the plasma membrane 7.2 “Membrane structure results in selective permeability” The Permeability of the Lipid Bilayer  Hydrophobic (nonpolar) molecules, such as hydrocarbons, carbon dioxide, and oxygen, can dissolve in the lipid bilayer of the membrane and cross it with ease, without the aid of membrane proteins  The hydrophobic core of the membrane impedes the direct passage of ions and polar molecules, which are hydrophilic, through the membrane Transport Proteins  Cell membranes are permeable to specific ions and a variety of polar molecules  These hydrophilic substances can avoid contact with the lipid bilayer by through transport proteins that span the membrane 7.3 “Passive transport is diffusion of a substance across a membrane with no energy investment”  Molecules have a type of energy called thermal motion (heat). One result of thermal motion is diffusion  Any substance will diffuse down its concentration gradient, no work must be done to make this happen  The diffusion of a substance across a biological membrane is called passive transport because the cell does not have to expend energy to make it happen Effects of Osmosis on Water Balance  The movement of water across cell membranes and the balance of water between the cell and its environment is crucial to organisms Water Balance of Cells Without Walls  When considering the behavior of a cell in a solution both solute concentration and membrane permeability must be considered  Both factors are taken into account in the concept of tonicity Water Balance of Cell with Walls  When a cell with a wall is immersed in a hypotonic solution, the wall helps maintain the water balance Facilitated Diffusion: Passive Transport Aided by Proteins  Two types of transport proteins are channel proteins and carrier proteins  Channel proteins simply provide corridors that allow a specific molecule or ion to cross the membrane  Carrier proteins seem to undergo a subtle change in shape that somehow that translocates the solute-binding site across the membrane 7.4 “Active transport uses energy to move solutes against their gradients” The Need for Energy in Active Transport  To pump a molecule across a membrane against the gradient require work, the cell must expend energy  The transport proteins that move solutes against a concentration gradient are all carrier proteins  Active transport enables a cell to maintain internal concentrations of small molecules that differ from concentration in its environment  ATP supplies the energy for most active transport Maintenance of Membrane Potential by Ion Pumps  All cells have voltages across their plasma membranes  The cytoplasm of a cell is negative in charge compared to the extracellular fluid because of an equal distribution of anions and cations on opposite sides of the membranes  The membrane potential acts like a battery, an energy source that affects the traffic of all charged substances across the membrane Cotransport: Coupled Transport by a Membrane Protein  A substance that has been pumped across a membrane can do work as it moves back across the membrane by diffusion, analogous to water that has been pumped uphill and performs work as it flows back down  Another specialized transport protein, a cotransporter separate from the pump, can couple the “downhill” diffusion of this substance to the “uphill” transportation of a second substance against its own concentration gradient 7.5 “Bulk transport across the plasma membrane occurs by exocytosis and endocytosis” Exocytosis  Many secretory cells use exocytosis to export their products  When plant cells are making walls, exocytosis delivers proteins and certain carbohydrates from Golgi vesicles to outside of the cell Endocytosis  A small area of the plasma membrane sinks inward to form a pocket. As the pocket deepens, it pinches in, forming a vesicle containing material that had been outside the cell  There are three types of endocytosis: phagocytosis, pinocytosis, and receptormediated endocytosis