Biology · Unit 2: Cell Structure and Function · 14 min read · Updated 2026-05-10
AP Biology Plasma Membranes — AP Biology
AP Biology · Unit 2: Cell Structure and Function · 14 min read
1. The Fluid Mosaic Model★★☆☆☆⏱ 4 min
Proposed by Singer and Nicolson in 1972, the fluid mosaic model is the currently accepted model of plasma membrane structure. It describes the membrane as a dynamic structure with diverse embedded components suspended in a phospholipid bilayer.
**Phospholipids**: Amphipathic molecules with hydrophilic phosphate heads and hydrophobic fatty acid tails that spontaneously form bilayers in aqueous environments.
**Fluidity**: Phospholipids move laterally within their layer; fluidity is regulated by temperature, fatty acid saturation, and (in animal cells) cholesterol.
**Proteins**: Integral proteins embed in the bilayer; peripheral proteins bind to the membrane surface.
**Carbohydrates**: Bound to lipids/glycolipids or proteins/glycoproteins for cell-cell recognition.
2. Selective Permeability and Membrane Transport★★☆☆☆⏱ 3 min
The amphipathic structure of the phospholipid bilayer creates selective permeability, meaning only specific molecules can cross freely, while others require specialized proteins or energy. Permeability depends on solute size and polarity/charge:
**High permeability (no protein needed):** Small nonpolar molecules (e.g., $O_2$, $CO_2$, steroid hormones) diffuse rapidly.
**Low permeability (very slow diffusion):** Small polar uncharged molecules (e.g., water, urea) diffuse slowly, most movement via aquaporins.
**No permeability (requires protein):** Large polar molecules (e.g., glucose, sucrose) and all charged ions cannot cross the hydrophobic core.
Transport is divided into two broad categories: passive transport (no energy, moves down concentration gradients, includes simple and facilitated diffusion) and active transport (requires energy input, moves against gradients, includes bulk transport via vesicles).
3. Water Potential and Osmosis★★★☆☆⏱ 4 min
Osmosis is the net diffusion of free water across a selectively permeable membrane. Water potential (symbol $\Psi$, psi) measures the potential energy of water to move. Water always moves from an area of higher (less negative) water potential to an area of lower (more negative) water potential.
Ψ = Ψ_s + Ψ_p
Where $Ψ_s$ = solute potential and $Ψ_p$ = pressure potential. Solute potential is calculated as:
Ψ_s = -iCRT
$i$ = ionization constant (number of ions solute dissociates into, 1 for covalent solutes)
$C$ = molar concentration (mol/L)
$R$ = pressure constant ($0.0831 \frac{L·bar}{mol·K}$ for AP Biology)
$T$ = temperature in Kelvin ($^\circ C + 273$)
Solute potential is always negative (adding solute reduces free water). Pressure potential is 0 for open systems, positive in turgid plant cells, and negative in plant xylem.
4. Experimental Application of Membrane Concepts★★★★☆⏱ 3 min
Plasma membrane concepts frequently appear in AP Biology FRQs that require calculation and prediction for experimental scenarios, like testing salt tolerance in crop plants.
Common Pitfalls
Why: Students memorize cholesterol's role in animal cells and incorrectly generalize it to all cell types
Why: Students remember water moves from low solute to high solute, but mix up direction when working with negative water potential values
Why: Students overgeneralize the hydrophobic core rule to all polar molecules, regardless of size
Why: Students forget that ionic solutes dissociate into multiple ions, which changes the total number of solute particles
Why: Students forget that secondary active transport uses the gradient of one molecule to power movement of another against its gradient
Why: Students overgeneralize common diagrams that show all integral proteins spanning the bilayer