What the exam asks
- Multiple Choice (Section I): Stimulus-based sets often show a cross-section of a plasma membrane or a table of amino acid properties, asking you to identify where specific domains of a transmembrane protein sit or which substance crosses the fastest without a transport protein.
- Free Response (Section II): Particularly in Question 4 (Conceptual Analysis) and Question 5 (Analyze a Model or Visual Representation), you may be asked to describe how a structural change affects permeability, explain why an ion requires a channel, or predict the effect of removing a cell wall in a hypotonic environment.
Core ideas
Phospholipid orientation and the bilayer
Phospholipids are amphipathic molecules: each has a hydrophilic (polar/charged, water-attracting) phosphate head and two hydrophobic (nonpolar, water-repelling) fatty acid tails.
When placed in an aqueous environment—with water inside the cell (cytosol) and outside the cell (extracellular fluid)—phospholipids spontaneously assemble into a bilayer:
- Hydrophilic heads face outward toward the aqueous cytosol and extracellular fluid.
- Hydrophobic fatty acid tails point inward toward one another, creating a nonpolar interior core shielded from water.
The fluid mosaic model
The plasma membrane is described as a fluid mosaic: a flexible two-dimensional fluid of phospholipids studded with a mosaic of diverse proteins and other molecules that can drift laterally.
- Membrane proteins: Like phospholipids, embedded proteins fold according to polarity.
- Integral (transmembrane) proteins span the bilayer. Their interior-facing segments contain mostly nonpolar (hydrophobic) amino acid R-groups that interact with the fatty acid tails, while the segments exposed to the cytosol or extracellular fluid contain polar or charged (hydrophilic) R-groups.
- Peripheral proteins are not embedded in the hydrophobic core; they bind loosely to the hydrophilic heads or to exposed regions of integral proteins.
- Cholesterol (in animal cells): Steroid lipids wedged among the hydrophobic tails. Cholesterol acts as a fluidity buffer: at high temperatures it restrains phospholipid movement (preventing excessive fluidity), and at low temperatures it prevents fatty acid tails from packing tightly together (preventing solidification).
- Glycoproteins and glycolipids: Short carbohydrate chains attached to extracellular proteins or lipids. They face the outside of the cell and function in cell-to-cell recognition, adhesion, and signaling.