I. Cellular membranes are fluid mosaics of lipids and proteinsII. Membrane structure results in selective permeabilityIII. Passive transport is diffusion of a substance across a membrane with no energy investmentChapter 7- Membrane Structure and Function – Lecture 7I. Cellular membranes are fluid mosaics of lipids and proteins1. Lipids and proteins are the staple ingredients of membranes. 2. Most lipids = phospholipids which are amphipathic, meaning it has both a hydrophilic and hydrophobic region 3. Most of the proteins within membranes have both hydrophobic and hydrophilic regions. 4. Fluid mosaic model- the membrane is a fluid structure with a “mosaic” of various proteins embedded in or attached to a double layer (bilayer) of phospholipids. 5. A membrane is held together primarily by hydrophobic interactions, which are much weaker than covalent bonds. 6. Most lipids and some of the proteins can shift about laterally lateral movement of phospholipids within the membrane is rapid proteins are much larger than lipids and move more slowly. 7. The temperature at which a membrane solidifies depends on the types of lipids it is made of. 8. The membrane remains fluid to a lower temperature if it is rich in phospholipids with unsaturated hydrocarbon tails. 9. Cholesterol can be thought of as a “fluidity buffer” for the membrane, resisting changes in membrane fluidity that can be caused by changes in temperature. 10. Membranes must be fluid to work properly not too fluid. 11. Two major populations of membrane proteins: - Integral proteins – penetrate the hydrophobic interior of the lipidbilayer. The majority are transmembrane proteins, which span the membrane. - Peripheral proteins- not embedded in the lipid bilayer at all; they are appendages loosely bound to the surface of the membrane, often to exposed parts of integral proteins. 12. 6 major functions performed by proteins of the plasma membrane: - Transport (active, passive) - +/- energy, many types: uniport (transports 1 thing), symport (transport 2 things in the same direction), and antiport (transport 2 things in opposite directions) - Enzymatic activity (individual, linked groups) - Signal transduction (receptor proteins, message relays) - Cell-cell recognition – a cell’s ability to distinguish one type of neighboring cell from another. (ID tag, often use sugars, glycoproteins, and glycolipids) - Intercellular joining (temporary or permanent) - Attachment to the cytoskeleton and extracellular matrix (ECM).13. Membrane carbohydrates are usually short, branched chains of fewer than 15 sugar units. Some are covalently bonded to lipids, forming moleculescalled glycolipids. Most are covalently bonded to proteins, which are called glycoproteins. 14. Membranes have distinct inside and outside faces.II. Membrane structure results in selective permeability 1. Biological membrane = supramolecular structure – many molecules ordered into a higher level of organization- with emergent properties beyond those ofthe individual molecules. 2. One of the most important of those properties: the ability to regulate transport across cellular boundaries, a function essential to the cell’s existence. 3. The lipid bilayer is only one aspect of the gatekeeper system responsible for the selective permeability of a cell. Proteins built into the membrane play keyroles in regulating transport. 4. Cell membranes are permeable to specific ions and a variety of polar molecules can avoid contact with lipid bilayer by passing through transport proteins that span the membrane. III. Passive transport is diffusion of a substance across a membrane with no energy investment 1. Diffusion- the movement of molecules of any substance so that they spread out evenly into the available space. 2. Any substance will diffuse down its concentration gradient- the region along which the density of a chemical substance increases or decreases. 3. 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. 4. Osmosis- the diffusion of free water across a selectively permeable membrane.5. Tonicity- the ability of a surrounding solution to cause a cell to gain or lose water. 6. Isotonic- same 7. Hypertonic – more (non-penetrating solute)8. Hypotonic –
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