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Review of Membrane Structure

review 2

concepts 1

How Do Molecules Cross the Plasma Membrane?

  • The plasma membrane is selectively permeable; hydrophobic molecules and small polar molecules can diffuse through the lipid layer, but ions and large polar molecules cannot.

  • Integral membrane proteins enable ions and large polar molecules to pass through the membrane by passive or active transport.

    • Proteins which form channels may be utilized to enable the transport of water and other hydrophilic molecules; these channels are often gated to regulate transport rate.

    • In facilitated transport, hydrophilic molecules bind to a "carrier" protein; this is a form of passive transport.

    • In active transport, hydrophilic molecules also bind to a carrier protein, but energy is utilized to transport the molecules against their concentration gradient; in some cases, indirect energy sources are used.

    • Ions are often used in secondary transport systems because they generate an electrochemical gradient.

  • Bulk transport mechanisms enable large molecules and even larger objects to cross the plasma membrane.

    • The process of exocytosis expels large molecules from the cell and is used for cell secretion.

    • Receptor-mediated endocytosis allows specific large molecules to be taken in by the cell.

    • Other forms of endocytosis allow liquids to be taken in by a process called pinocytosis and large objects, such as debris or microorganisms, to be taken in by phagocytosis.

review 2

concepts 1