Membrane Permeability

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

1. Core Concepts of Membrane Permeability★☆☆☆☆⏱ 3 min

Membrane permeability is a foundational concept for Unit 2: Cell Structure and Function, which accounts for 10-13% of the total AP Biology exam score. Questions about permeability appear regularly in both multiple-choice (MCQ) and free-response (FRQ) sections, and underpin all topics related to cellular exchange with the environment.

📘 Definition

Membrane Permeability

Permeability coefficient $P$

The continuous, quantitative property describing how easily a given solute or solvent can cross a biological phospholipid bilayer. Selective permeability describes the cell membrane's ability to allow only certain substances to cross.

Example:

Oxygen has a much higher $P$ (permeability) than sodium ions in pure phospholipid bilayers.

2. Permeability Ranking by Molecule Class★★☆☆☆⏱ 4 min

The interior of the phospholipid bilayer is hydrophobic (nonpolar), so permeability depends primarily on two properties of the crossing molecule: (1) polarity and charge, (2) molecular size.

Small nonpolar molecules (O₂, CO₂, steroid hormones): Highest permeability, dissolve easily in the hydrophobic core
Small uncharged polar molecules (water, glycerol, urea): Moderate permeability, cross slowly through gaps between phospholipids
Large uncharged polar molecules (glucose, sucrose): Very low permeability in pure bilayers
Charged ions (Na⁺, K⁺, Cl⁻): Extremely low permeability in pure bilayers due to thick hydration shells

Whole cell membranes contain embedded channel and transport proteins that dramatically increase permeability for low-permeability molecules like glucose and ions.

📐 Worked Example

Rank the following molecules from highest membrane permeability to lowest in a pure phospholipid bilayer (no protein channels): glucose, O₂, Na⁺ ion, glycerol. Justify your ranking.

1
First, categorize each molecule by polarity, charge, and size: O₂ is small and nonpolar, glycerol is small and uncharged polar, glucose is large and uncharged polar, Na⁺ is a charged ion.
2
Highest permeability belongs to small nonpolar O₂: it dissolves easily in the hydrophobic bilayer interior, so it crosses rapidly.
3
Next is small uncharged polar glycerol: it is polar, so it crosses more slowly than O₂, but its small size allows it to pass through gaps between phospholipids at measurable rates.
4
Next is large uncharged polar glucose: its large size and polar nature make it very slow to cross the pure bilayer, so permeability is much lower than glycerol.
5
Lowest permeability is charged Na⁺: the charge creates a strong hydration shell that prevents it from dissolving in the hydrophobic core, so permeability is nearly zero.
6
Final ranking:
7
$\text{O}_2 > \text{glycerol} > \text{glucose} > \text{Na}^+$

Exam tip:

Always check if the question specifies "pure phospholipid bilayer" (no proteins) or "cell membrane" (includes proteins). If proteins are present, permeability for ions and glucose will be much higher due to channels and transporters.

3. Factors Altering Membrane Permeability★★★☆☆⏱ 4 min

Membrane permeability is not fixed; it changes based on bilayer structural composition and environmental conditions. Three key factors are regularly tested on the AP exam:

Temperature: Increasing temperature increases phospholipid kinetic energy, increasing space between tails and raising permeability. Only extreme high temperatures disrupt the entire bilayer structure.
Fatty acid saturation: Saturated fatty acids have straight tails that pack tightly, lowering permeability. Unsaturated fatty acids have kinked tails from double bonds that prevent tight packing, raising permeability.
Cholesterol: Acts as a permeability buffer: at high temperatures, it restricts phospholipid movement and lowers permeability; at low temperatures, it prevents tight packing and maintains permeability.

📐 Worked Example

Two pure phospholipid bilayers are prepared: Bilayer A is made of 100% saturated 16-carbon fatty acids, Bilayer B is made of 100% unsaturated 16-carbon fatty acids. Both are held at 25°C, and permeability to glycerol is measured. Which bilayer has higher permeability, and what is the effect of increasing temperature by 15°C on both bilayers?

1
Recall the effect of saturation on packing: saturated fatty acids have straight tails that pack tightly, while unsaturated fatty acids have kinked tails that cannot pack tightly.
2
Tighter packing reduces space between phospholipids, which reduces permeability. So Bilayer B (unsaturated) has looser packing and higher initial permeability to glycerol than Bilayer A.
3
Increasing temperature by 15°C (a moderate change that does not disrupt the bilayer) increases the kinetic energy of phospholipids in both bilayers, increasing the average distance between fatty acid tails.
4
Increased spacing reduces the barrier to glycerol crossing, so permeability to glycerol increases in both bilayers.
5
The relative difference in permeability remains: Bilayer B will still have higher permeability than Bilayer A at the higher temperature.

Exam tip:

If the question does not specify temperature, cholesterol acts to maintain stable membrane permeability (buffer it against temperature changes), which is the core function you will be expected to state on the exam.

4. Experimental Analysis of Permeability★★★★☆⏱ 5 min

Common AP Biology lab experiments test membrane permeability using dialysis tubing (a synthetic membrane with similar permeability properties to cell membranes) or plant tissue. The core principle is that net water movement follows water potential, which depends on the permeability of the solutes:

Permeable solutes can cross the membrane, so they equalize concentration at equilibrium
Impermeable solutes cannot cross, so they remain on their original side and create a permanent water potential difference

\Psi = \Psi_s + \Psi_p

Where $Ψ_{s}$ (solute potential) = $- i C R T$ , and pressure potential $Ψ_{p} = 0$ for an open beaker. Net water always moves from higher (less negative) water potential to lower (more negative) water potential.

📐 Worked Example

A dialysis bag is permeable to water and monosaccharides, but impermeable to disaccharides. The bag is filled with 0.2 M sucrose (disaccharide) and 0.1 M glucose (monosaccharide), then placed in a beaker containing 0.1 M sucrose and 0.2 M glucose. What is the expected change in mass of the bag before equilibrium, and what are the solute concentrations at equilibrium (assume equal volumes inside and outside the bag)?

1
First categorize solutes by permeability: sucrose (disaccharide) is impermeable, glucose (monosaccharide) is permeable.
2
At equilibrium, permeable glucose will equalize its concentration across the membrane. Total glucose in the system is 0.1 M (inside) + 0.2 M (outside), split equally across two equal volumes, so glucose concentration becomes 0.15 M on both sides.
3
Impermeable sucrose remains on its original side: 0.2 M inside the bag, 0.1 M outside the bag.
4
Calculate total solute concentration at equilibrium: inside = 0.2 M sucrose + 0.15 M glucose = 0.35 M; outside = 0.1 M sucrose + 0.15 M glucose = 0.25 M.
5
Solute potential inside is $Ψ_{s (in)} = - 0.35 R T$ , which is more negative (lower water potential) than outside $Ψ_{s (o u t)} = - 0.25 R T$ . Net water moves into the bag, so the mass of the bag increases.
6
Final concentrations: 0.15 M glucose (both sides), 0.2 M sucrose (inside), 0.1 M sucrose (outside).

✓ Quick check

Test your understanding with this AP-style MCQ:

Aquaporins are channel proteins that increase the permeability of cell membranes specifically to water. Which of the following describes the effect of adding aquaporins to a pure phospholipid bilayer placed in a hypertonic solution?
- A) The rate of net water movement out of the bilayer increases, and the equilibrium mass of the bilayer does not change
- B) The rate of net water movement out of the bilayer increases, and the equilibrium mass of the bilayer is lower
- C) The rate of net water movement out of the bilayer does not change, and the equilibrium mass of the bilayer is lower
- D) The rate of net water movement out of the bilayer decreases, and the equilibrium mass of the bilayer does not change

Exam tip:

Always categorize solutes by permeability before calculating water potential. Do not add up initial total concentrations and assume they will equalize—impermeable solutes stay on their original side, which changes the final water potential difference.

5. Common Pitfalls

Wrong move:

Claiming cholesterol always decreases membrane permeability

Why:

Students memorize that cholesterol restricts phospholipid movement, but forget the effect depends on environmental temperature

Correct move:

Always specify the temperature context: cholesterol decreases permeability at high temperatures and increases permeability at low temperatures, acting as a permeability buffer

Wrong move:

Ranking glucose as lower permeability than Na⁺ in a whole cell membrane

Why:

Students confuse pure protein-free bilayers with whole cell membranes that contain transporters

Correct move:

Always adjust rankings based on the presence of proteins specified in the question; in most cell membranes, glucose permeability is much higher than resting Na⁺ permeability due to glucose transporters

Wrong move:

Calculating total initial solute concentration to predict mass change without separating permeable and impermeable solutes

Why:

Students assume all solutes will equalize, missing that impermeable solutes stay on their original side

Correct move:

First categorize each solute as permeable or impermeable, calculate equilibrium concentrations for permeable solutes, then sum total solute to find the water potential difference

Wrong move:

Claiming large nonpolar molecules have higher permeability than small polar molecules

Why:

Students only remember polarity and forget size as a determining factor

Correct move:

Always evaluate both polarity/charge and size: a large nonpolar molecule has lower permeability than small nonpolar O₂, but higher permeability than small charged ions

Wrong move:

Assuming any increase in temperature denatures membrane proteins and decreases permeability

Why:

Students confuse moderate temperature increases with extreme temperature increases

Correct move:

Only extreme high temperature (far above physiological range) denatures proteins and disrupts the bilayer; moderate temperature increases increase permeability by increasing phospholipid movement

6. Quick Reference Cheatsheet

Factor/Category	Effect on Permeability	Key AP Note
Small nonpolar molecules	Highest	Cross pure bilayers rapidly
Small uncharged polar	Moderate	Cross slowly through bilayer gaps
Large uncharged polar	Very low (pure bilayer)	Require transporters in cells
Charged ions	Nearly zero (pure bilayer)	Require channel proteins
Increased temperature	Increases	Only extreme temps disrupt bilayer
Increased unsaturated fatty acids	Increases	Kinked tails prevent tight packing
Cholesterol (high temp)	Decreases	Buffers excess fluidity
Cholesterol (low temp)	Increases	Prevents tight packing
Permeable solutes	Equalize at equilibrium	No permanent water potential gradient
Impermeable solutes	Stay on original side	Create permanent water potential gradient

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
Rank molecule permeability
2022 · FRQ
Factors affecting membrane permeability
2021 · MCQ
Dialysis tubing permeability experiment

Going deeper

study guideWater Potential Calculations for AP BiologyRequired prerequisite for experimental permeability problems

What's Next

Membrane permeability is a core concept that underpins all topics related to cellular transport, cell signaling, and osmoregulation in AP Biology. Understanding selective permeability prepares you to analyze passive and active transport across cell membranes, interpret results from common AP Biology labs, and answer FRQs about cellular adaptation to different environments. Mastering the rules of permeability and how solute permeability impacts water potential will help you earn full points on both MCQ and FRQ questions in Unit 2 and beyond.