2.6 Facilitated Diffusion

Facilitated diffusion is passive transport of molecules across the plasma membrane through membrane proteins (channel or carrier proteins)—no energy (ATP) needed and movement is down a concentration (and often electrochemical) gradient. It’s used for charged ions (Na+, K+) and large polar molecules that can’t cross the hydrophobic lipid bilayer on their own. Channel proteins (ion channels, aquaporins) provide selective pores (some gated); carrier proteins bind the solute and change shape to shuttle it across. Regular (simple) diffusion is passive movement of small nonpolar or very small uncharged molecules (like O2, CO2) directly through the lipid portion of the membrane without proteins. Key CED links: LO 2.6.A and EK 2.6.A.1–A.3. For a quick review, see the Topic 2.6 study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-transport/study-guide/7yV52dtD7Vj7ZadTEd2P); for unit context use (https://library.fiveable.me/ap-biology/unit-2) and extra practice (https://library.fiveable.me/practice/ap-biology).

Charged ions like Na+ and K+ can't pass through the membrane’s hydrophobic lipid bilayer because the bilayer’s interior repels charged, polar particles. Moving an ion through that oily core would require a lot of energy, so ions instead move through specific transport proteins (ion channels or carrier proteins) that span the membrane. These proteins provide a water-filled, polar pathway and often a selectivity filter that only fits certain ions. Many ion channels are gated (open or closed by voltage, ligand, or mechanical signals), and ion movement is passive when it goes down an electrochemical gradient—but it still needs the protein route. This process can change membrane polarization because ion flow moves charge across the membrane (EK 2.6.A.1 & EK 2.6.A.2). For a concise AP-aligned review, see the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-transport/study-guide/7yV52dtD7Vj7ZadTEd2P) and more practice problems (https://library.fiveable.me/practice/ap-biology).

Aquaporins are special channel proteins in the plasma membrane that let water move through quickly and passively (no energy) down its concentration gradient. Think of them as tiny water pipelines: their pore is narrow and lined so only water molecules fit in the right orientation—that’s selectivity. Because water is polar and the membrane’s lipid core repels it, aquaporins give a fast route for large amounts of water (EK 2.6.A.3). They’re important in kidneys, plant roots, and any cell that needs rapid water balance. On the AP exam you might be asked to link aquaporin structure (channel protein, selectivity filter) to function (facilitated diffusion, osmosis)—use those CED keywords: channel protein, concentration gradient, passive transport. For a quick review see the Topic 2 membrane-transport study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-transport/study-guide/7yV52dtD7Vj7ZadTEd2P). Want more practice? Try AP-style problems at (https://library.fiveable.me/practice/ap-biology).

Main difference: facilitated diffusion is passive (no cellular energy) and moves specific molecules down their concentration (or electrochemical) gradient through channel or carrier proteins (e.g., ion channels, aquaporins). Active transport uses energy (usually ATP) to move substances against their concentration/electrochemical gradient via carrier proteins (pumps), which can change membrane polarization (EK 2.6.A.1–A.3). Key points to remember for AP Bio: - Facilitated diffusion = passive, down gradient, needs channel/carrier, for charged ions and large polar molecules (Na+, K+, glucose, water via aquaporins). - Active transport = energy input, can move ions/molecules up their gradient (e.g., Na+/K+ pump), creates/maintains electrochemical gradients. - Both require specific proteins, but only active transport consumes ATP. This maps to LO 2.6.A on the CED. If you want a quick refresher, check the Topic 2.6 study guide on Fiveable (https://library.fiveable.me/ap-biology/unit-2/membrane-transport/study-guide/7yV52dtD7Vj7ZadTEd2P) and practice questions at (https://library.fiveable.me/practice/ap-biology).

Whether a molecule can cross the plasma membrane by itself depends on its size, polarity, and charge. The membrane’s hydrophobic lipid bilayer lets small nonpolar molecules (O2, CO2) slip through easily because they mix with the fatty core. Small polar molecules (like H2O) sometimes cross slowly or use aquaporins for fast movement (EK 2.6.A.3). Large polar molecules and charged ions (Na+, K+) can’t cross the hydrophobic core—they need channel or carrier proteins and move down concentration/electrochemical gradients via facilitated diffusion (EK 2.6.A.1–A.2). Ion channels have selectivity filters and may be gated; carrier proteins change shape to shuttle specific solutes. On the AP exam you should use terms like concentration gradient, electrochemical gradient, channel protein, carrier protein, aquaporin, and membrane polarization. For a quick review, check the Topic 2.6 study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-transport/study-guide/7yV52dtD7Vj7ZadTEd2P), the Unit 2 overview (https://library.fiveable.me/ap-biology/unit-2), and practice problems (https://library.fiveable.me/practice/ap-biology).

Channel proteins are membrane proteins that form hydrophilic pores through the lipid bilayer so charged ions or water can flow down their electrochemical/concentration gradients. Examples: ion channels (Na+, K+) with selectivity filters and gated channels, and aquaporins that move lots of water quickly (EK 2.6.A.1, 2.6.A.3). Transport (carrier) proteins are a broader class that includes channels but usually refers to proteins that bind specific solutes (like glucose or amino acids) and change shape to shuttle them across the membrane. Carriers move larger polar molecules or ions via facilitated diffusion (no energy) and are typically slower than open channels; they can function as uniporters, symporters, or antiporters. Key difference: channels provide an open pore for rapid flow; carriers bind then undergo conformational change for transport (selectivity + slower rate). These distinctions show up on the AP exam under LO 2.6.A—review the Topic 2 study guide for membrane transport (https://library.fiveable.me/ap-biology/unit-2/membrane-transport/study-guide/7yV52dtD7Vj7ZadTEd2P) and practice questions (https://library.fiveable.me/practice/ap-biology).

Big polar molecules can’t just slip through the membrane because of both size and chemistry. The plasma membrane’s interior is hydrophobic (fatty acid tails), so small nonpolar molecules cross easily, but polar or charged molecules are repelled by that hydrophobic core. Large size makes it even harder—they can’t fit between lipid molecules. Charged ions (Na+, K+) also face an electrochemical barrier and need ion channels (EK 2.6.A.1). So cells use transport proteins: channel proteins or carrier proteins let large polar molecules move down their concentration gradients without energy (facilitated diffusion, EK 2.6.A.2). Water moves fast through aquaporins (EK 2.6.A.3). On the exam know the terms: channel vs carrier, selectivity filter, gated channel, passive transport—all tied to LO 2.6.A. Review the Topic 2 study guide for quick examples (https://library.fiveable.me/ap-biology/unit-2/membrane-transport/study-guide/7yV52dtD7Vj7ZadTEd2P) and practice questions (https://library.fiveable.me/practice/ap-biology).

Facilitated diffusion does NOT require cellular energy. It’s a type of passive transport (LO 2.6.A; EK 2.6.A.2) where molecules move down their concentration (or electrochemical) gradient through membrane proteins. Charged ions (Na+, K+) and large polar molecules can’t cross the lipid bilayer directly, so they use channel or carrier proteins (EK 2.6.A.1). Channels (including gated ion channels and aquaporins for water) provide a selective pore; carrier proteins change shape to shuttle substrates—still without ATP input. Remember: “facilitated” = helps movement, but “diffusion” = down the gradient and energy-free. For AP study, review EK 2.6.A.1–A.3 in the Unit 2 study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-transport/study-guide/7yV52dtD7Vj7ZadTEd2P) and try practice questions (https://library.fiveable.me/practice/ap-biology) to see this tested.

When a membrane becomes polarized by ion movement it means there’s an uneven charge across the plasma membrane—one side is more negative and the other more positive. Facilitated diffusion of ions (like Na+ and K+) through ion channels moves charge as well as concentration, so if more positive ions exit or more negative ions enter, the inside becomes relatively negative. That separation of charge creates a membrane potential (an electrochemical gradient) that can influence further passive ion flow, open gated channels, and power processes like nerve signaling. This idea ties directly to LO 2.6.A and EK 2.6.A.1: charged ions need channels to cross, and their movement can polarize the membrane. Want to practice? Review Topic 2.6 in the Fiveable study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-transport/study-guide/7yV52dtD7Vj7ZadTEd2P), skim Unit 2 overview (https://library.fiveable.me/ap-biology/unit-2), and try related practice problems (https://library.fiveable.me/practice/ap-biology).

Aquaporins are channel proteins that let water cross fast without energy input—they do facilitated diffusion down a concentration gradient (EK 2.6.A.2, A.3). Their pore is narrow and lined with specific hydrophilic amino acids and a “selectivity filter” that fits single-file water molecules; this excludes ions and other solutes (keywords: channel protein, selectivity filter, passive transport). A key structural feature orients water so protons can’t hop through, preserving membrane polarization. Because each channel offers an open, low-resistance path, many water molecules pass per second (aquaporins can move on the order of 10^9 water molecules/sec per channel), so tissues with lots of aquaporins (kidney, plant roots) get huge net fluxes. For AP review, focus on how structure (narrow pore, lining residues) determines permeability and selectivity (see the Topic 2.6 study guide for membrane transport: https://library.fiveable.me/ap-biology/unit-2/membrane-transport/study-guide/7yV52dtD7Vj7ZadTEd2P). For broader unit review and practice, check Unit 2 (https://library.fiveable.me/ap-biology/unit-2) and practice questions (https://library.fiveable.me/practice/ap-biology).

Think of the membrane like a water-filled fence: the phospholipid bilayer has a hydrophobic (nonpolar) core. Small nonpolar molecules (O2, CO2) are also nonpolar, so they dissolve in and slip through that core by simple diffusion. Ions (Na+, K+, Cl–), however, are charged and surrounded by water (hydration shells). Moving a charged, hydrated particle into the nonpolar core is energetically unfavorable, so ions can’t cross on their own. That’s why EK 2.6.A.1 says charged ions require channel proteins (ion channels) or carriers for facilitated diffusion. Ion channels have selectivity filters and often gated openings that let ions pass down electrochemical gradients without ATP. Water and some polar solutes use aquaporins or carrier proteins (EK 2.6.A.2–3). For more AP-aligned review, see the Topic 2 membrane-transport study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-transport/study-guide/7yV52dtD7Vj7ZadTEd2P) and Unit 2 overview (https://library.fiveable.me/ap-biology/unit-2). Practice questions: (https://library.fiveable.me/practice/ap-biology).

Step-by-step: 1. A molecule (charged ion or large polar solute) is stuck outside the lipid bilayer because the membrane’s hydrophobic core repels it. 2. Movement is driven only by diffusion—substances go down their concentration (and for ions, electrochemical) gradient; no ATP used. 3. If it’s an ion, it finds a selective ion channel (with a selectivity filter); if it’s a big polar molecule or sugar, it finds a carrier (transport) protein or an aquaporin for water. 4. For a gated channel, a signal opens the gate; for a carrier, the molecule binds to a specific site. 5. Channel: the molecule passes through the hydrophilic pore. Carrier: binding triggers a conformational change that shuttles the molecule across. 6. The molecule exits on the other side; net movement continues until equilibrium or electrochemical forces balance. 7. For ions, this movement can polarize the membrane (change membrane potential). Review this topic on the Fiveable study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-transport/study-guide/7yV52dtD7Vj7ZadTEd2P) and try practice problems (https://library.fiveable.me/practice/ap-biology).

Because Na+ and K+ are different sizes and interact differently with water, the membrane needs different channels that match each ion’s shape and chemistry. Ion channels have a selectivity filter—specific amino acids arranged so only the right ion (after shedding some of its hydration shell) fits and is stabilized. K+ is larger than Na+, so K+ channels have a pore and carbonyl oxygen arrangement that perfectly stabilizes K+ but not Na+. Na+ channels have a narrower filter that fits Na+ instead. That selectivity is essential for moving ions down electrochemical gradients without using ATP (facilitated diffusion) and for creating membrane polarization—different channels let cells control Na+ vs K+ flow precisely (e.g., during action potentials). Some channels are gated too, so cells open/close them in response to voltage or ligands. For AP review, see Topic 2.6 in the Fiveable study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-transport/study-guide/7yV52dtD7Vj7ZadTEd2P) and practice questions (https://library.fiveable.me/practice/ap-biology).

Both simple and facilitated diffusion move substances down their concentration gradient (from high to low) without using cell energy—that’s passive transport (EK 2.6.A.2). The key difference: simple diffusion works for small nonpolar molecules (O2, CO2) that can slip through the lipid bilayer, while facilitated diffusion needs channel or carrier proteins for ions and large polar molecules (Na+, K+, glucose) that can’t cross the hydrophobic core (EK 2.6.A.1). For ions, movement is actually driven by an electrochemical gradient (concentration + charge) and channels can be selective or gated. Carrier proteins can saturate—there’s a maximum transport rate—whereas simple diffusion rate just depends on the gradient and membrane permeability. Aquaporins are a special channel for quick water flow (EK 2.6.A.3). This is an AP topic (LO 2.6.A)—review the membrane-transport study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-transport/study-guide/7yV52dtD7Vj7ZadTEd2P) and practice questions (https://library.fiveable.me/practice/ap-biology) for examples.

Think of facilitated diffusion as molecules that can’t cross the lipid bilayer on their own using protein “doors.” Real-body examples you should know for LO 2.6.A: - Neurons: Voltage-gated Na+ and K+ channels let Na+ and K+ move down electrochemical gradients during action potentials, polarizing and repolarizing the membrane (gated ion channels, selectivity filters). - Glucose transport: GLUT family (e.g., GLUT4 in muscle/adipose moves glucose into cells after insulin causes transporters to the membrane; GLUT1 in red blood cells)—carrier proteins moving large polar glucose down its concentration gradient. - Water movement: Aquaporins in kidney collecting ducts and red blood cells permit rapid water osmosis (EK 2.6.A.3). - Chloride movement: Ligand- or voltage-gated Cl– channels (e.g., GABA receptor–linked Cl– channels) let Cl– move passively to change membrane potential. These match AP keywords: channel/carrier protein, gated channel, electrochemical gradient, passive transport. For a quick review, check the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-transport/study-guide/7yV52dtD7Vj7ZadTEd2P) and practice problems (https://library.fiveable.me/practice/ap-biology).