Chapter 7: Membrane Structure and Function

• What are the main components of membranes?

  Lipids, proteins, and carbohydrates
• What is the most abundant lipid in most membranes?

  Phospholipids
• What does it mean for a phospholipid to be amphipathic?

  It has both a hydrophilic region and a hydrophobic region
• Why can a phospholipid bilayer exist as a stable boundary between two aqueous compartments?

  Because it shelters the hydrophobic tails from water while exposing the hydrophilic heads to water
• How are membranes held together?

  Mainly by hydrophobic interactions
• What happens to a membrane as temperature decreases?

  It remains fluid until the phospholipids settle into a closely packed arrangement
• What factors influence the temperature at which a membrane solidifies?

  The types of lipids it is made of
• How do unsaturated hydrocarbon tails affect membrane fluidity?

  They prevent close packing, making the membrane more fluid
• What are integral membrane proteins?

  Proteins that are integrated into the membrane
• What distinguishes transmembrane proteins from other integral proteins?

  They stretch from one side of the membrane to the other
• What is the nature of the portions of an integral membrane protein found inside the membrane?

  They are hydrophobic
• What are peripheral membrane proteins?

  Proteins found on the outside and inside surfaces of membranes
• What do carbohydrates form when bound to proteins and lipids in membranes?


  Glycoproteins and glycolipids
• What role do carbohydrates play in the immune system?


  They help cells recognize each other
• What are the main components of the plasma membrane and their locations?

  • Phospholipids: Main fabric of the membrane
  • Cholesterol: Between hydrophobic tails of phospholipids
  • Integral proteins: Embedded in the bilayer
  • Peripheral proteins: On inner or outer surface
  • Carbohydrates: Attached to proteins or lipids on the extracellular side
• How does the cell membrane differ from the phospholipid bilayer?

  The cell membrane contains other macromolecules like proteins and carbohydrates
• What does selective permeability mean in the context of membranes?


  It allows some substances to cross more easily than others
• Which types of molecules can easily cross the lipid bilayer?


  Nonpolar molecules, such as hydrocarbons
• Why do polar molecules pass slowly through a lipid bilayer?

  Because the hydrophobic interior impedes their passage
• How do transport proteins assist in selective permeability?

  They allow specific ions and polar molecules to cross the membrane
• What are channel proteins?

  Transport proteins with a hydrophilic channel for molecules or ions
• What is the function of aquaporins?

  They facilitate the passage of water molecules through the membrane
• How do carrier proteins function in membrane transport?

  They hold onto their passengers and change shape to shuttle them across
• What determines the specificity of a transport protein?

  It is specific for the substance it translocates
• What factors contribute to the selective permeability of a membrane?


  • The discriminating barrier of the lipid bilayer
  • Specific transport proteins built into the membrane
• What type of energy do molecules have due to their constant motion?

  Thermal energy
• What is diffusion?


  The movement of particles of any substance so that they spread out into the available space
• How does the motion of individual molecules relate to the overall process of diffusion?


  Each molecule moves randomly, yet diffusion of a population of molecules may be directional
• In which direction does a substance diffuse in the absence of other forces?


  From where it is more concentrated to where it is less concentrated
• What is the term for the region along which the density of a chemical substance increases or decreases?

  Concentration gradient
• Is diffusion a spontaneous process, and does it require energy input?


  Yes, diffusion is a spontaneous process that needs no input of energy
• What is passive transport?

  The diffusion of a substance across a biological membrane that requires no energy
• What drives diffusion across cell membranes?

  The concentration gradient itself represents potential energy and drives diffusion
• How do selectively permeable membranes affect diffusion rates of various molecules?


  They have different effects on the rates of diffusion of various molecules
• How do aquaporins affect water diffusion across cell membranes?


  Water can diffuse very rapidly across the membranes of cells with aquaporins
• What is osmosis?


  The diffusion of free water across a selectively permeable membrane
• What is the significance of water movement across cell membranes?


  The balance of water between the cell and its environment is crucial to organisms
• What factors must be considered to explain a cell's behavior in a solution?

  Both solute concentration and membrane permeability
• What does tonicity refer to?


  The ability of a surrounding solution to cause a cell to gain or lose water
• How does the concentration of nonpenetrating solutes affect water movement?


  If there is a higher concentration of nonpenetrating solutes in the surrounding solution, water will tend to leave the cell