Membrane Transport — AP Biology
1. Selective Permeability and Passive Transport
Membrane transport describes the movement of ions, molecules, and other substances across biological cell membranes, a process required to maintain cellular homeostasis by controlling the cell's internal chemical environment. This topic makes up 3–4% of the total AP Biology exam score, appearing in both multiple-choice and free-response questions, often combined with other topics like cell size and osmoregulation.
Passive transport is any movement of molecules across the membrane that occurs down a concentration gradient (from higher to lower concentration) and does not require input of cellular energy (ATP). There are two subtypes: (1) simple diffusion, where molecules move directly across the bilayer without assistance, and (2) facilitated diffusion, where molecules move through integral membrane proteins (channels or carriers) because they cannot cross the hydrophobic core on their own.
Exam tip: On FRQ justifications for permeability, always link the molecule’s properties to the structure of the bilayer explicitly—AP readers require this connection to award full points, not just a ranking of rates.
2. Water Potential and Osmosis
Osmosis is the net diffusion of free water across a selectively permeable membrane, and its direction is predicted using water potential ($\Psi$, psi), a measure of the potential energy of water to move. Water always moves from a region of higher water potential to a region of lower water potential. The formula for total water potential is:
\Psi = \Psi_s + \Psi_p
where $\Psi_s$ = solute potential (also called osmotic potential) and $\Psi_p$ = pressure potential. Solute potential is always negative because adding solute reduces the number of free water molecules, lowering water potential; pure water has a solute potential of 0. The formula for solute potential is:
\Psi_s = -iCRT
where $i$ = ionization constant (number of particles a solute splits into when dissolved), $C$ = molar concentration of solute, $R$ = pressure constant ($0.0831\ L\cdot bar/mol\cdot K$), and $T$ = temperature in Kelvin ($^\circ C + 273$). Tonicity describes the effect of a solution on cell volume: hypotonic solutions have lower solute concentration than the cell, hypertonic have higher, and isotonic have equal solute concentration.
Exam tip: Double-check the ionization constant $i$ before solving: use $i=1$ for non-electrolytes (sucrose, glucose) and $i=2$ for NaCl (and other 1:1 salts that dissociate into two ions). AP questions often test this common mistake.
3. Active Transport and Electrochemical Gradients
Active transport is the movement of molecules across the membrane against their concentration gradient (from lower to higher concentration), which requires input of cellular energy (usually ATP) and is carried out by transmembrane protein pumps. The most well-studied example is the Na⁺/K⁺ ATPase pump, which pumps 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell, both against their gradients, using one ATP per cycle.
This pump establishes and maintains an electrochemical gradient (a combined gradient of charge and concentration) across the membrane, which is critical for nerve function, nutrient uptake, and muscle contraction. Cotransport is a secondary active transport process: a proton pump first uses ATP to establish a H⁺ gradient, then the downhill diffusion of H⁺ provides the energy to move a second molecule (e.g., sucrose) uphill against its gradient.
Exam tip: To earn full points when distinguishing active vs passive transport on FRQs, always explicitly state two key differences: (1) gradient direction (against vs down), and (2) energy requirement (ATP required vs no ATP required).
4. Bulk Transport
Bulk transport is the movement of large particles, macromolecules, or large volumes of extracellular fluid that cannot fit through transport proteins, and it requires energy for vesicle formation and movement, making it a form of active transport. It occurs via two main processes: endocytosis (movement into the cell) and exocytosis (movement out of the cell).
Endocytosis has three subtypes: phagocytosis ("cell eating"), where the cell engulfs large particles like bacteria into a food vacuole; pinocytosis ("cell drinking"), where the cell takes up small droplets of extracellular fluid with dissolved solutes; and receptor-mediated endocytosis, a highly specific process where receptors on the cell surface bind target molecules (e.g., LDL cholesterol) before the membrane folds in to bring them into the cell.
Exocytosis occurs when vesicles from the Golgi apparatus fuse with the cell membrane to release their contents outside the cell, for example, secretion of insulin from pancreatic cells or release of neurotransmitters from neurons.
Exam tip: Do not forget that bulk transport is a form of active transport—it requires energy, even though it does not use protein pumps. AP MCQs often test this common misconception.
Common Pitfalls
Why: Students generalize the $i=1$ rule used for sucrose to all solutes, forgetting that ionic salts dissociate into multiple particles in solution.
Why: Students mix up solute concentration and water potential, reversing the direction of movement.
Why: Students associate membrane proteins with active transport, forgetting that passive transport can also use proteins as long as movement is down the gradient.
Why: Students mix up the responses of animal and plant cells to tonicity, forgetting the rigid cell wall.
Why: Students overgeneralize the permeability rule to all polar molecules regardless of size.
Why: Students memorize the pump’s function but forget the unequal movement that creates the resting membrane potential.