Chapter 7 - Overview Life at the Edge The plasma membrane...

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Overview: Life at the Edge The plasma membrane is the boundary that separates the living cell from its surroundings The plasma membrane exhibits selective permeability , allowing some substances to cross it more easily than others Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins Phospholipids are the most abundant lipid in the plasma membrane Phospholipids are amphipathic molecules , containing hydrophobic and hydrophilic regions The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it Membrane Models: Scientific Inquiry Membranes have been chemically analyzed and found to be made of proteins and lipids Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins Later studies found problems with this model, particularly the placement of membrane proteins, which have hydrophilic and hydrophobic regions In 1972, J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water Freeze-fracture studies of the plasma membrane supported the fluid mosaic model Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer
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The Fluidity of Membranes Phospholipids in the plasma membrane can move within the bilayer Most of the lipids, and some proteins, drift laterally Rarely does a molecule flip-flop transversely across the membrane (a) Lipids move laterally in a membrane, but flip-flopping across the membrane is quite rare (b) Unsaturated hydrocarbons tails of phospholipids have kinks that keep the molecules from packing together, enhancing membrane fluidity (c) Cholesterol reduces membrane fluidity at moderate temperatures by reducing phospholipids movement, but at low temperatures it hinders solidification by disrupting the regular packing of phospholipids. David Frye and Michael Edidin, at Johns Hopkins University, labeled the plasma membrane proteins of a mouse cell and human cell with two different markers on the hybrid cells. The mixing of the mouse and human membrane proteins indicates that at least some membrane proteins move sideways within the plane of the plasma membrane. As temperatures cool, membranes switch from a fluid state to a solid state The temperature at which a membrane solidifies depends on the types of lipids Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids Membranes must be fluid to work properly; they are usually about as fluid as salad oil The steroid cholesterol has different effects on membrane fluidity at different temperatures At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids
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