Membranes160-page5 - Membrane Structure and Function 5...

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Membrane Structure and Function - 5 Moving Materials Through Membranes A significant part of membrane activity involves transporting materials through it in one direction or the other. Recall that the plasma membrane is selectively, or differentially, permeable. This means that: Some materials freely pass - the membrane is permeable to such molecules and whether they are inside or outside of the cell depends on other factors Some materials are excluded Some materials enter or leave the cell only by the using cell energy For example, small hydrophobic molecules, such as CO 2 , O 2 and small lipids, dissolve in the membrane and pass through readily. Tiny polar molecules, such as H 2 O and alcohol, can also slip between the phospholipid molecules. Ions and most nutrient molecules do not move freely through membranes, but are often carried by the transport protein channels, either with or without the use of energy. Most large
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