2.4 Membrane Permeability

Membrane permeability is how easily substances cross the plasma membrane—and it’s selective because the membrane has a phospholipid bilayer with a hydrophobic (nonpolar) core. Small nonpolar molecules (O2, CO2, N2) cross freely by simple diffusion, small uncharged polar molecules (like some H2O or NH3) pass a little, but ions and large polar molecules can’t cross the hydrophobic core without membrane proteins. Cells use integral proteins (channels, carriers, aquaporins) for facilitated diffusion or active transport to move those substances. This selective permeability keeps the internal environment different from the outside (homeostasis), controls nutrient uptake, waste removal, signal transduction, and prevents osmotic lysis (cell wall helps too in plants/bacteria). For AP Bio, this links to LO 2.4.A/B—know the roles of phospholipids, transport proteins, aquaporins, channels, and how cell walls affect osmotic stability. Review the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/plasma-membranes/study-guide/1aW0ZDGzS56ism3BJwTi), the Unit 2 overview (https://library.fiveable.me/ap-biology/unit-2), and practice questions (https://library.fiveable.me/practice/ap-biology) to prep for MC and FRQ items on membranes.

The phospholipid bilayer is what makes the membrane selectively permeable because of its polar heads and nonpolar tails. Phospholipid heads face the watery inside and outside of the cell, while the hydrophobic hydrocarbon tails form a nonpolar interior (the hydrophobic core). That hydrophobic interior blocks ions and large polar molecules from crossing directly (EK 2.4.A.1, EK 2.4.A.3). Small nonpolar molecules (N2, O2, CO2) diffuse freely through the core, and small uncharged polar molecules (like H2O or NH3) cross a bit but slowly (EK 2.4.A.2–A.3). Large polar molecules and ions need embedded proteins—channels, carrier proteins, aquaporins, or porins—for facilitated diffusion or active transport. Cholesterol in the membrane also affects fluidity and permeability by packing between phospholipids. For quick review tied to the CED, see the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/plasma-membranes/study-guide/1aW0ZDGzS56ism3BJwTi), the Unit 2 overview (https://library.fiveable.me/ap-biology/unit-2), and extra practice problems (https://library.fiveable.me/practice/ap-biology).

Oxygen (O2) and carbon dioxide (CO2) cross membranes easily because they’re small, nonpolar molecules. The phospholipid bilayer has a hydrophobic core (nonpolar fatty-acid tails) that lets other nonpolar molecules dissolve and diffuse straight through by simple diffusion (EK 2.4.A.1–A.2). Glucose is large and polar, so it can’t pass the hydrophobic interior—the bilayer repels polar/charged stuff (EK 2.4.A.3). That’s why glucose needs membrane proteins (carrier proteins or facilitated diffusion; sometimes active transport) to get across. Small uncharged polar molecules (like H2O) can cross a bit or use aquaporins, but big polar sugars can’t without transporters. This is exactly what LO 2.4.A expects you to explain on the AP (use terms: phospholipid bilayer, hydrophobic core, transport proteins, facilitated diffusion). For a quick CED-aligned refresher, see the Topic 2.4 study guide (https://library.fiveable.me/ap-biology/unit-2/plasma-membranes/study-guide/1aW0ZDGzS56ism3BJwTi) and try practice questions at (https://library.fiveable.me/practice/ap-biology).

Short answer: hydrophobic (nonpolar) stuff crosses the membrane much more easily than hydrophilic (polar or charged) stuff because the membrane’s interior is hydrophobic—the phospholipid tails form a nonpolar core that repels polar molecules and ions (CED EK 2.4.A.1–3). Details you should know for AP Bio: - Small nonpolar molecules (N2, O2, CO2) diffuse freely through the bilayer (simple diffusion). - Small polar, uncharged molecules (H2O, NH3) can cross a little but much slower; water also uses aquaporins for faster movement. - Ions and large polar molecules can’t get through the hydrophobic core—they need integral proteins (ion channels, carrier proteins) or facilitated diffusion / active transport. - This selective permeability is why membranes control cell internal conditions (EK 2.4.A.2–3). Want practice? Review the Topic 2.4 study guide (https://library.fiveable.me/ap-biology/unit-2/plasma-membranes/study-guide/1aW0ZDGzS56ism3BJwTi) and try problems at (https://library.fiveable.me/practice/ap-biology).

Good question—water’s polarity does seem like it should stop it from crossing the hydrophobic membrane, but there are two ways it gets through. 1) Simple diffusion/osmosis: Water is small and uncharged, so a little H2O can squeeze across the phospholipid bilayer’s hydrophobic core by simple diffusion (EK 2.4.A.3). This is slow compared to nonpolar gases. 2) Facilitated diffusion via aquaporins: Most cells speed water movement with channel proteins called aquaporins (integral membrane proteins). Those channels let lots of water pass quickly without letting ions through (EK 2.4.A.2, keywords: aquaporins, facilitated diffusion, hydrophobic core). On the AP exam, you should be able to explain selective permeability by referencing the bilayer’s hydrophobic interior and transport proteins (LO 2.4.A). For a concise review, check the Topic 2.4 study guide (https://library.fiveable.me/ap-biology/unit-2/plasma-membranes/study-guide/1aW0ZDGzS56ism3BJwTi) and try practice questions (https://library.fiveable.me/practice/ap-biology).

Ions need transport proteins because the phospholipid bilayer has a hydrophobic (nonpolar) core that repels charged, polar particles. Charged ions (Na+, K+, Cl–, etc.) carry a hydration shell and can’t pass through the nonpolar fatty-acid tails—so simple diffusion won’t work (EK 2.4.A.1–A.3 / LO 2.4.A). Transport proteins (ion channels and carrier proteins) provide a polar pathway across the membrane: ion channels form selective pores for fast, passive movement down electrochemical gradients; carrier proteins (including pumps) bind and change shape to move solutes, sometimes using ATP for active transport against gradients. Size, charge, and hydration explain why ions need proteins; small uncharged gases (O2, CO2) cross freely. This is a core AP idea you should know for multiple-choice and free-response questions on membrane permeability (see the Topic 2.4 study guide for review: https://library.fiveable.me/ap-biology/unit-2/plasma-membranes/study-guide/1aW0ZDGzS56ism3BJwTi). For extra practice, try problems at (https://library.fiveable.me/practice/ap-biology).

The interior of cell membranes is hydrophobic because the membrane is a phospholipid bilayer whose phospholipids have polar (hydrophilic) heads facing the water on each side and nonpolar hydrocarbon tails pointing inward. Those fatty acid tails are nonpolar and repel water, creating a hydrophobic core (EK 2.4.A.1, EK 2.4.A.3). That nonpolar region prevents ions and large polar molecules from crossing directly, so those substances need integral membrane proteins (channels, carriers, aquaporins) to get through. Cholesterol also sits among the tails and affects membrane fluidity and packing, which can strengthen the hydrophobic barrier. On the AP exam, be ready to connect “hydrophobic interior” to selective permeability: small nonpolar gases (N2, O2, CO2) diffuse easily, small uncharged polar molecules (like some H2O) pass slowly, and ions/polar solutes require transport proteins. For a concise review, see the Topic 2.4 study guide (https://library.fiveable.me/ap-biology/unit-2/plasma-membranes/study-guide/1aW0ZDGzS56ism3BJwTi); more practice is at (https://library.fiveable.me/practice/ap-biology).

Small nonpolar gases (N2, O2, CO2) cross the plasma membrane by simple diffusion: they dissolve in the membrane’s hydrophobic core and move down their concentration gradient without needing proteins. The phospholipid bilayer has nonpolar hydrocarbon tails that make a hydrophobic interior (EK 2.4.A.1–2), so small nonpolar molecules “freely pass” across the membrane (EK 2.4.A.2). Rate of crossing depends on concentration difference, membrane fluidity (temperature, cholesterol), molecule size, and membrane thickness. By contrast, ions and large polar molecules can’t cross the hydrophobic core and need channels or carriers (facilitated diffusion). (For AP-style answers, emphasize membrane structure → selective permeability → simple diffusion.) Review this in the Topic 2.4 study guide (https://library.fiveable.me/ap-biology/unit-2/plasma-membranes/study-guide/1aW0ZDGzS56ism3BJwTi) and practice related questions at (https://library.fiveable.me/practice/ap-biology).

Plant cells have rigid cell walls because of what they need to do that animal cells don’t. The cell wall (made mainly of cellulose in plants) provides a strong structural boundary that maintains shape, resists gravity, and protects cells from bursting when water enters by osmosis (prevents osmotic lysis)—all points aligned with EK 2.4.B. Plant cells sit in hypotonic environments and use a rigid wall plus a large central vacuole to create turgor pressure for support. Animal cells lack walls so they can be more flexible, move, and engage in processes like endocytosis/exocytosis; instead they rely on an extracellular matrix for structural support. This is tested in the CED under LO 2.4.B (cell wall role) and connects to membrane permeability ideas (hydrophobic core, transport proteins) in LO 2.4.A. For more review, see the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/plasma-membranes/study-guide/1aW0ZDGzS56ism3BJwTi) or the unit overview (https://library.fiveable.me/ap-biology/unit-2). Practice problems are at (https://library.fiveable.me/practice/ap-biology).

Short answer: the plasma membrane is the selective barrier that controls what actually gets into the cytoplasm; the cell wall is a mostly structural, porous layer outside the membrane that doesn’t do the same level of molecular selection. Why: the phospholipid bilayer has a hydrophobic core, so small nonpolar molecules (N2, O2, CO2) diffuse freely, small uncharged polar molecules (like H2O, NH3) cross slowly or through aquaporins, and ions/large polar molecules need channels or carrier proteins (LO 2.4.A, EKs 2.4.A.1–A.3). By contrast, cell walls (cellulose in plants, peptidoglycan in bacteria, chitin in fungi) provide shape and protect against osmotic lysis but are porous—many solutes and water pass through the wall to reach the membrane; some microbes have porins in outer layers, but selective transport is still done by the membrane (LO 2.4.B, EK 2.4.B.1). For a focused review on membranes and permeability for the AP exam, see the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/plasma-membranes/study-guide/1aW0ZDGzS56ism3BJwTi). You can practice related questions at https://library.fiveable.me/practice/ap-biology.

Cell walls protect cells from osmotic lysis by providing a rigid, external mesh that resists the inward pressure created when water moves into a cell by osmosis. When a cell is in a hypotonic environment, water enters across the plasma membrane (sometimes through aquaporins) and raises internal turgor pressure. The cell wall (made of peptidoglycan in bacteria, cellulose in plants, or chitin in fungi) physically counteracts that pressure so the plasma membrane doesn’t burst—this is exactly what EK 2.4.B.1 describes. If water loss occurs instead, the rigid wall also helps maintain shape (plant cells become flaccid but don’t lyse). For AP review, see Topic 2.4 in the Unit 2 study guide (https://library.fiveable.me/ap-biology/unit-2/plasma-membranes/study-guide/1aW0ZDGzS56ism3BJwTi) and try practice questions at (https://library.fiveable.me/practice/ap-biology) to reinforce these ideas.

Good question—NH3 is polar, but two things let some of it cross the membrane. First, NH3 is small and uncharged, so it can diffuse slowly through the hydrophobic core of the phospholipid bilayer (EK 2.4.A.3). Second, ammonia exists in equilibrium with its ion form, NH4+. The charged NH4+ can’t cross the membrane without a transporter, but the uncharged NH3 can—so membranes are permeable to “small polar, uncharged” molecules like H2O and NH3 in small amounts. The rate is much lower than for nonpolar gases (N2, O2, CO2) and far lower than facilitated transport for ions or large polar molecules (EK 2.4.A.1–A.3). For a quick review of how membrane structure controls selective permeability, check the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/plasma-membranes/study-guide/1aW0ZDGzS56ism3BJwTi) and practice problems (https://library.fiveable.me/practice/ap-biology).

If a cell membrane weren’t selectively permeable, the cell would lose control of its internal environment (homeostasis). The phospholipid bilayer’s hydrophobic core normally blocks ions and large polar molecules, so without selectivity ions and solutes would flow freely in and out. Consequences: huge disruption of ion gradients (no membrane potential → nerves and muscles fail), uncontrolled osmosis (water could rush in and cause osmotic lysis or rush out and plasmolysis), loss of ATP production tied to gradients, and inability to regulate concentrations of nutrients and wastes. Transport proteins (ion channels, carrier proteins, aquaporins) and processes like facilitated diffusion would be useless or uncontrolled. Cell walls can help some cells resist osmotic lysis (EK 2.4.B.1), but many animal cells would still die. This connects directly to LO 2.4.A and the EKs about hydrophobic interior and selective passage of N2, O2, CO2 versus ions/polar molecules. For a quick refresher, check the Topic 2.4 study guide on Fiveable (https://library.fiveable.me/ap-biology/unit-2/plasma-membranes/study-guide/1aW0ZDGzS56ism3BJwTi) and try practice questions (https://library.fiveable.me/practice/ap-biology).

The hydrocarbon tails of phospholipids form the hydrophobic core of the bilayer—a nonpolar, water-repelling region. Ions are charged and highly hydrophilic, so moving an ion from water into that nonpolar interior is energetically unfavorable (it would have to lose its stabilizing interactions with water). Because the tails can’t stabilize charge, ions can’t cross the membrane by simple diffusion. That’s why cells use integral membrane proteins (ion channels and carrier proteins) to let ions through in a controlled way (EK 2.4.A.1–3 in the CED). Small nonpolar molecules cross freely and small uncharged polar molecules (like some H2O) pass slowly, but ions require protein pathways (facilitated diffusion or active transport). For a focused review, see the Topic 2 plasma membranes study guide (https://library.fiveable.me/ap-biology/unit-2/plasma-membranes/study-guide/1aW0ZDGzS56ism3BJwTi) and try practice questions (https://library.fiveable.me/practice/ap-biology).

Because of their lifestyles and needs. Bacteria, fungi, and plants have rigid cell walls (peptidoglycan in bacteria, chitin in fungi, cellulose in plants) that provide a structural boundary, act as a permeability barrier for some substances, and protect cells from osmotic lysis (EK 2.4.B.1). Those walls let stationary or single-celled organisms resist swelling when water moves in and maintain shape. Animal cells don’t need walls because they rely on a flexible plasma membrane and extracellular matrix for support and communication. Flexibility lets animal cells change shape, move, form tissues, and perform processes like endocytosis, phagocytosis, and tight cell–cell signaling that rigid walls would block. In multicellular animals, the extracellular matrix replaces many mechanical roles of a wall while preserving membrane permeability (links to LO 2.4.B and EK 2.4.A). If you want a quick AP-style review of membrane roles and permeability, check the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/plasma-membranes/study-guide/1aW0ZDGzS56ism3BJwTi). For unit review and lots of practice questions, see the unit page (https://library.fiveable.me/ap-biology/unit-2) and practice problems (https://library.fiveable.me/practice/ap-biology).