Membrane Transport

AP Biology· AP Biology CED — Cell Structure and Function· 14 min read

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.

📘 Definition

Selective Permeability

The ability of a cell membrane to restrict movement of some molecules while allowing others to cross, a property that arises directly from the hydrophobic core of the phospholipid bilayer.

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.

📐 Worked Example

A researcher reconstitutes an artificial phospholipid bilayer (no integral proteins) and measures the relative rate of movement of three molecules: oxygen (O₂), glucose, and sodium ion (Na⁺). Predict the order of movement from fastest to slowest and justify your prediction.

1
Categorize each molecule by size and chemical properties:
2
- O₂: small nonpolar molecule
- Glucose: large polar molecule
- Na⁺: charged ion
3
The hydrophobic core of the bilayer only allows nonpolar molecules to dissolve and diffuse freely; it strongly repels charged molecules and blocks large polar molecules from crossing at appreciable rates.
4
Final order (fastest to slowest):
5
$O_2 > \text{glucose} > Na^+$

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 ( $Ψ$ , 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 $Ψ_{s}$ = solute potential (also called osmotic potential) and $Ψ_{p}$ = 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 ( $0.0831 L \cdot ba r / m o l \cdot K$ ), 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.

📐 Worked Example

Calculate the total water potential of a 0.20 M sucrose solution in an open beaker at 27°C. If a plant cell with $Ψ = - 0.8 ba r s$ is placed in this solution, what direction will water move?

1
Convert temperature to Kelvin:
2
$27^\circ C + 273 = 300\ K$
3
Sucrose does not ionize in water, so $i = 1$ .
4
Calculate solute potential:
5
$\Psi_s = -(1)(0.20\ mol/L)(0.0831\ L\cdot bar/mol\cdot K)(300\ K) = -5.0\ bars$
6
Open beakers have 0 pressure potential (atmospheric pressure), so total water potential of the solution is:
7
$\Psi = -5.0 + 0 = -5.0\ bars$
8
Water moves from higher water potential to lower water potential. The plant cell has $Ψ = - 0.8 ba r s$ , which is higher than the solution's $- 5.0 ba r s$ , so net water movement is out of the plant cell into the solution.

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.

📐 Worked Example

A researcher treats cultured mammalian cells with a toxin that inhibits ATP synthesis. After treatment, they observe that intracellular Na⁺ concentration increases and intracellular K⁺ concentration decreases, but facilitated diffusion of glucose across the membrane is unchanged. Explain this observation.

1
The Na⁺/K⁺ pump is an active transport protein that requires ATP to move Na⁺ out and K⁺ in against their concentration gradients.
2
Inhibiting ATP synthesis removes the energy source required for the pump to function, so it can no longer maintain the gradient. Na⁺ leaks down its gradient into the cell, and K⁺ leaks out, causing the observed concentration changes.
3
Facilitated diffusion of glucose is a passive process that moves glucose down its concentration gradient through a carrier protein, and does not require ATP.
4
Because no energy is needed for facilitated diffusion, it is unaffected by the loss of ATP, matching the researcher's observation.

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.

📐 Worked Example

The genetic condition familial hypercholesterolemia is caused by a mutation that eliminates functional LDL receptor proteins on the surface of human body cells. LDL carries cholesterol in the bloodstream, and LDL enters cells via receptor-mediated endocytosis. Predict the effect of this mutation on blood cholesterol levels and justify your prediction.

1
Receptor-mediated endocytosis requires specific receptor proteins to bind target molecules before endocytosis can occur.
2
Without functional LDL receptors, LDL cholesterol cannot bind to body cell surfaces and cannot be taken up into cells from the bloodstream.
3
Unabsorbed LDL remains in the bloodstream, leading to chronically elevated blood cholesterol levels, which causes the cardiovascular symptoms of familial hypercholesterolemia.

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.

5. Common Pitfalls

Wrong move:

Calculating solute potential for NaCl and using an ionization constant $i = 1$

Why:

Students generalize the $i = 1$ rule used for sucrose to all solutes, forgetting that ionic salts dissociate into multiple particles in solution.

Correct move:

Always confirm the solute before calculating: use $i = 1$ for non-ionizing solutes (sucrose, glucose) and $i = 2$ for 1:1 salts like NaCl.

Wrong move:

Stating water moves from higher solute concentration to lower solute concentration during osmosis

Why:

Students mix up solute concentration and water potential, reversing the direction of movement.

Correct move:

Always describe osmosis direction using water potential: water moves from higher $Ψ$ to lower $Ψ$ , which is from lower solute concentration to higher solute concentration.

Wrong move:

Claiming facilitated diffusion is active transport because it uses proteins

Why:

Students associate membrane proteins with active transport, forgetting that passive transport can also use proteins as long as movement is down the gradient.

Correct move:

Classify transport based on energy requirement and gradient direction, not whether a protein is involved.

Wrong move:

Stating plant cells burst in hypotonic solutions

Why:

Students mix up the responses of animal and plant cells to tonicity, forgetting the rigid cell wall.

Correct move:

Remember animal cells burst in hypotonic solution (no cell wall to resist pressure); plant cells become turgid (healthy) in hypotonic solution, and plasmolyze in hypertonic solution.

Wrong move:

Assuming all polar molecules cannot cross the phospholipid bilayer

Why:

Students overgeneralize the permeability rule to all polar molecules regardless of size.

Correct move:

Small polar molecules (water, urea) can cross the bilayer slowly, while large polar molecules (glucose, starch) require transport proteins to cross at biologically relevant rates.

Wrong move:

Stating the Na⁺/K⁺ pump moves equal numbers of Na⁺ and K⁺ across the membrane

Why:

Students memorize the pump’s function but forget the unequal movement that creates the resting membrane potential.

Correct move:

Recall the 3 out, 2 in rule: 3 Na⁺ exit the cell, 2 K⁺ enter, for every 1 ATP hydrolyzed.

6. Quick Reference Cheatsheet

Category	Formula / Key Rule	Notes
Total Water Potential	$Ψ = Ψ_{s} + Ψ_{p}$	$Ψ_{p} = 0$ for open solutions, positive for turgid plant cells
Solute Potential	$Ψ_{s} = - i C R T$	$i = 1$ for non-electrolytes, $i = 2$ for 1:1 salts; $R = 0.0831 L \cdot ba r / m o l \cdot K$ ; $T =^{\circ} C + 273$
Simple Diffusion	Down concentration gradient, no energy	Small nonpolar molecules cross freely, no proteins needed
Facilitated Diffusion	Down concentration gradient, no energy	Uses channel/carrier proteins for large/charged/polar molecules
Active Transport	Against concentration gradient, requires ATP	Uses protein pumps, creates and maintains concentration gradients
Bulk Transport	Requires energy for vesicle formation	Classified as active transport; endocytosis = into cell, exocytosis = out of cell
Osmosis Direction	Water moves from higher $Ψ$ to lower $Ψ$	Higher $Ψ$ = lower solute concentration
Tonicity (Animal Cells)	Hypotonic = burst, Isotonic = normal, Hypertonic = crenate	No cell wall to resist osmotic pressure
Tonicity (Plant Cells)	Hypotonic = turgid (healthy), Isotonic = flaccid, Hypertonic = plasmolyzed	Rigid cell wall generates positive turgor pressure

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2023 · MCQ
Permeability ranking question
2022 · FRQ
Osmosis water potential calculation

What's Next

Membrane transport is a core foundational concept for many other AP Biology topics, and mastery of this content will help you with combined questions that appear frequently on the AP exam. Water potential calculations and transport classification are common MCQ topics, while permeability justifications and osmosis experiments are frequent FRQ prompts. This topic connects closely to cell homeostasis, cell signaling, neuron function, and osmoregulation in whole organisms. Review related topics below to build a connected understanding of Unit 2 content.