1.  Discuss the differences among diffusion, facilitated diffusion, active transport, and endocytosis.  Provide examples of substances that use each of the above mechanisms to cross the plasma membrane.

(50 points)

Answer (and grading guideline followed by TAs)

Simple diffusion: something that is nonpolar and small can cross the membrane along (with) (down) its concentration gradient (from the side of the membrane where it is in higher concentration to the side where it is in lower concentration). Examples of this would be O₂, CO₂ (accept other correct examples in all cases) (~ 13 points).  Facilitated diffusion: a substance is also moving across the membrane down its concentration gradient, but it needs to move through a channel or in a transporter because it is polar or a bit larger.  Examples: glucose, sucrose.  (another example can be water- although this works as an example for simple diffusion also). Both simple and facilitated diffusion are passive transport- they do not require energy to occur (~ 13 points).  Active transport moves a substance or ion against its concentration gradient: from the side of the membrane where it is in lower concentration to the side where it is in higher concentration so energy (ATP) is expended. An example is the Na+/K+ pump (~ 12 points).     Endocytosis: Endocytosis moves substances into the cell by the invagination (“pinching in”) of the plasma membrane. Therefore, the substance of interest enters the cell in a membrane-enclosed vesicle. (~ 12 points).   A protein is an example of something that would move into a cell via endocytosis.

2.   Describe the structure, function and relative fluidity of a phospholipid bilayer. Be sure to include what types of molecules are found in the bilayer, their ability to move, etc.

Answer (and grading guideline followed by TAs)

The phospholipid bilayer is a structure that is produced by the spontaneous arrangement of amphipathic phospholipids in an aqueous solution.  The phospholipids arrange themselves tail-to-tail in a self-enclosed structure that shields the hydrophobic tails from interaction with water. The hydrophilic heads of the phospholipids are in contact with water on both sides of the bilayer (~13 points). This bilayer is insoluble in water and, therefore, serves as a barrier that can separate one compartment from the outside environment (such as the inside of a cell from the surrounding extracellular fluid). The hydrophobic interior of the bilayer restricts passage of all substances that are highly polar and of a certain size (~12 points). The more saturated the fatty acid tails of the phospholipids, the less permeable the membrane. (~5 points). Cholesterol inclusion in membranes also reduces permeability because of its rigid ring structure (~5 points). Proteins play many roles in the lipid bilayer- one of which is allowing polar and large molecules to cross the lipid bilayer by serving as channels and transporters and another is allowing ion concentration gradients to be set up by serving as pumps (~10 points).  Phospholipids will (and proteins may, if not tethered) move laterally within the plane of the bilayer. (~5 points).