Bio Chapter 7

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Like all biological membranes, the plasma membrane exhibits

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selectivepermeability

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selectivepermeability

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it allows some substances to cross it more easily than others.The ability of the cell to discriminate in its chemical exchanges with its environment

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..... and ..... are the staple ingredients of membranes

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Lipids and proteins

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The most abundantlipids in most membranes are

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phospholipids

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A phospholipid is an ...... molecule

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amphipathic

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amphipathic means

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it has both a hydrophilic region and ahydrophobic region

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most membrane proteins are

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amphipathic

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proteins can reside in the phospholipid bilayerwith their hydrophilic regions protruding. This molecularorientation maximizes

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contact of hydrophilicregions of proteins with water

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fluid mosaic model

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the membraneis a mosaic of protein molecules bobbing in afluid bilayer of phospholipids.

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The proteins are not randomly distributed in the membrane,however. Groups of proteins are often associated in long-lasting,......, where they carry out common functions.

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specialized patches

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A membrane is held together primarily by

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hydrophobicinteractions

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Most of the lipids and some of theproteins can shift about

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laterally

The lateral movement of phospholipids within the membraneis rapid

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Very rarely, also, a lipid may

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flip-flop acrossthe membrane

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Proteins are much larger than lipids andmove more slowly, but some membrane proteins do drift Some membrane proteins seem to move in a highly directed manner, perhaps driven along .......in the cellby ..... connected to the membrane proteins’cytoplasmic regions. However, many other membrane proteinsseem to be held immobile by their attachment to thecytoskeleton or to the extracellular matrix

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cytoskeletal fibers

motor proteins

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The temperature at which a membranesolidifies depends on

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the types of lipids it is made of.

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The membrane remains fluid to a lower temperature if it isrich in phospholipids with

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unsaturated hydrocarbon tails

Because of kinks in the tailswhere double bonds are located, unsaturated hydrocarbontails cannot pack together as closely as saturated hydrocarbontails, making the membrane more fluid

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At relatively high temperatures—at 37°C, the body temperature of humans, for example—cholesterol makes the membrane

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less fluid by restrainingphospholipid movement

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because cholesterol alsohinders the close packing of phospholipids, it

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lowers thetemperature required for the membrane to solidify

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cholesterol can be thought of as a

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“fluidity buffer”

resisting changes in membrane fluidity that canbe caused by changes in temperature

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the fluidityof a membrane affects both

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its permeability

and the abilityof membrane proteins to move to where their function isneeded

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When a membrane solidifies,..... membranes that are too fluid .......

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enzymatic proteins become inactive, cannot support proteinfunction either

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Therefore, extreme environments posea challenge for life, resulting in

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evolutionary adaptations like membrane lipid composition

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