Transport across Membranes

• What is the fluid mosaic model of membranes?

  A model describing membranes as a phospholipid bilayer with embedded proteins.
• What does the term "fluid" refer to in the fluid mosaic model?

  It refers to the phospholipid bilayer's flexible shape, allowing individual phospholipids to move.
• What does the term "mosaic" refer to in the fluid mosaic model?

  It refers to the different sizes and shapes of extrinsic and intrinsic proteins embedded in the membrane.
• What is the role of cholesterol in membranes?

  Cholesterol connects phospholipids and reduces fluidity, making the bilayer more stable.
• What is the role of glycolipids in membranes?

  Glycolipids are involved in cell signaling and cell recognition.
• What are the functions of extrinsic proteins in membranes?

  Extrinsic proteins serve as binding sites, receptors, antigens, and are involved in cell signaling.
• What are the functions of intrinsic proteins in membranes?

  Intrinsic proteins function as electron carriers, channel proteins, and carrier proteins for transport.
• What are the functions of membranes within cells?

  Membranes provide an internal transport system, are selectively permeable, provide reaction surfaces, and isolate organelles.
• What are the functions of the cell-surface membrane?


  The cell-surface membrane isolates the cytoplasm, is selectively permeable, and is involved in cell signaling and recognition.
• Name three factors that affect membrane permeability.

  Temperature, pH, and the use of solvents.
• How does temperature affect membrane permeability?

  High temperature denatures membrane proteins and increases the kinetic energy of phospholipids, causing them to move apart.
• How does pH affect membrane permeability?

  pH changes the tertiary structure of membrane proteins, affecting their function.
• How can the use of a solvent affect membrane permeability?

  Solvents may dissolve the membrane, increasing permeability.
• How can colorimetry be used to investigate membrane permeability?

  1. Use plant tissue with soluble pigment; disrupt membranes to increase permeability.
  2. Select a colorimeter filter with a complementary color.
  3. Set colorimeter to 0 using distilled water; measure absorbance/% transmission.
  4. High absorbance/low transmission indicates more pigment in solution.
• Define osmosis.


  Osmosis is the diffusion of water across semi-permeable membranes from higher to lower water potential.
• What is water potential (ψ)?


  Water potential is the pressure created by water molecules, measured in kPa.
• What is the water potential of pure water at 25℃ and 100 kPa?

  The water potential of pure water at these conditions is 0.
• How does the presence of solute affect water potential?

  More solute results in a more negative water potential.
• How does osmosis affect plant cells?

  Osmosis into plant cells causes the protoplast to swell, making the cell turgid.
• How does osmosis affect animal cells?

  Osmosis into animal cells can lead to lysis, while osmosis out of cells can cause crenation.
• How can a student produce a desired concentration of solution from a stock solution?

  • Volume of stock solution = (required concentration x final volume needed) / concentration of stock solution.
  • Volume of distilled water = final volume needed - volume of stock solution.
• Define simple diffusion.


  Simple diffusion is a passive process where small, lipid-soluble molecules move from high to low concentration through the bilayer.
• Define facilitated diffusion.


  Facilitated diffusion is a passive process where specific proteins transport large or polar molecules down their concentration gradient.
• How do channel proteins work?

  Channel proteins create hydrophilic channels that bind to specific ions, allowing them to pass through the membrane.
• How do carrier proteins work?

  Carrier proteins bind to complementary molecules and change shape to release them on the other side of the membrane.
• Name five factors that affect the rate of diffusion.

  Temperature, diffusion distance, surface area, size of molecule, and concentration gradient.
• State Fick’s law.


  Fick's law states that the rate of diffusion is proportional to surface area x difference in concentration / diffusion distance.
• How are cells adapted to maximize the rate of transport across their membranes?

  Cells have many carrier/channel proteins and folded membranes to increase surface area.
• How does the graph of concentration against rate differ for simple vs facilitated diffusion?

  Simple diffusion shows a straight diagonal line, while facilitated diffusion levels off when proteins are saturated.
• Define active transport.


  Active transport is the process where ATP hydrolysis causes a carrier protein to transport molecules against their concentration gradient.
• Compare and contrast active transport and facilitated diffusion.

  Both may involve carrier proteins; active transport requires energy, while facilitated diffusion is passive.
• Define co-transport.


  Co-transport is the movement of a substance against its concentration gradient coupled with another substance moving down its gradient.
• How is co-transport involved in the absorption of glucose/amino acids in the small intestine?

  1. Na+ is actively transported out of epithelial cells into the bloodstream.
  2. Na+ concentration is lower in epithelial cells than in the gut lumen.
  3. Glucose/amino acids are transported from the lumen to epithelial cells coupled with Na+ diffusion down its gradient.