Skills you’ll gain in this topic:
- Explain how facilitated diffusion moves molecules using transport proteins.
- Describe examples of molecules that require facilitated diffusion.
- Predict the movement of molecules based on concentration gradients.
- Understand why facilitated diffusion doesn’t require energy.
- Differentiate between channel and carrier proteins in facilitated diffusion.
When and Why Does Facilitated Diffusion Occur?
When molecules cannot move easily enough through the plasma membrane, facilitated diffusion occurs. Molecules cannot pass through the phospholipid bilayer of the plasma membrane easily when particles are either charged or polar.
Facilitated diffusion is a form of passive transport which does NOT require energy. Passive transport occurs when molecules go down the concentration gradient.
A concentration gradient is when particles or solutes move from a highly concentrated area of particles to a less concentrated area of particles. This process is aided by proteins located on the plasma membrane (membrane proteins) such as transport proteins: Channel proteins and carrier proteins.
Channel Proteins
Channel proteins are laid throughout the membrane to provide a Hydrophilic passage through for the molecules to avoid the Hydrophobic core (Hydrophobic means to avoid polar substances such as water. Hence the terms hydro-water and Phobic think of phobia, fear of something. While Hydrophilic is the opposite and attracts water). An example of a channel protein is Aquaporins which allow water (polar H20) to diffuse through the membrane. Aquaporins are essential for plant cells, red blood cells, etc.
Nerve and muscle cells have gated ion channel proteins to enable the flow of charged ions such as sodium and potassium present in the sodium potassium pump of action potentials. These charged ions, like Na+ and K-, need channel proteins in order to move through the membrane. This creates a polarized membrane, and it helps with the action potential (you'll learn more about this later. If you're confused, don't worry too much. Just remember this concept is used in nerve cells). If a signal such as an electrical signal is activated these channels open their gate to transmit these signals through cells.
Carrier Proteins
Carrier proteins alter their shape to allow the flow of molecules through the concentration gradient of the membrane similarly to an enzyme substrate complex. Their rate of transport is slower than that of channel proteins. Carrier proteins provide an easy way for hydrophilic molecules to pass through the concentration gradient.
Active Transport
Active transport is used when the substance is moving in the opposite direction than its natural tendency would take it. The substance is transported to a region of higher concentration from lower concentration. Because it's against nature (like riding a bike uphill), it requires the use of ATP.
This is well demonstrated with the sodium-potassium pump. It takes out three sodium ions into the cell and brings in two potassium ions. Because both ions are moving against nature, ATP is needed. This is primary active transport.
But there is something called secondary active transport. In this transport system, something is actively transported by using energy from another substance going through the membrane through simple diffusion. Think of it as a mom with a stroller going through a door. Let's say the baby going through the door is active transport. The baby alone going through the door would need a lot of energy, it can't go on its own. But the mom, on the other hand, was already going through the door through simple diffusion. So the mom's "momentum" is used to push the baby through the door as well.
Facilitated diffusion and active transport are essential processes that allow cells to move molecules across their membranes. Think of facilitated diffusion as using a doorway (channel protein) or an escort (carrier protein) to help molecules that can't cross the membrane on their own—like water through aquaporins or ions through channel proteins—but still following the natural flow from high to low concentration. Active transport, on the other hand, is like moving against a crowd—it requires energy (ATP) to push molecules against their concentration gradient, as seen in the sodium-potassium pump that maintains nerve cell function. Both processes are critical for cells to maintain proper balance, get nutrients, remove wastes, and keep functioning properly. Without these transport mechanisms, our cells couldn't maintain the internal environment needed for survival.
Vocabulary
The following words are mentioned explicitly in the College Board Course and Exam Description for this topic.
| Term | Definition |
|---|---|
| central vacuole | A large organelle in plant cells that stores water and solutes, playing a role in maintaining turgor pressure and osmoregulation. |
| concentration gradient | A difference in the concentration of a substance across a membrane, with higher concentration on one side and lower concentration on the other. |
| contractile vacuole | An organelle in protists that collects and expels excess water to maintain osmotic balance. |
| homeostasis | The maintenance of stable internal environmental conditions in an organism despite external and internal changes. |
| hypertonic | A solution with a higher solute concentration relative to another solution, causing water to move out of the cell. |
| hypotonic | A solution with a lower solute concentration relative to another solution, causing water to move into the cell. |
| isotonic | A solution with the same solute concentration as another solution, resulting in no net movement of water across the membrane. |
| osmolarity | The concentration of solutes in a solution, which determines the direction of water movement across membranes. |
| osmoregulation | The process by which organisms maintain water balance and control their internal solute composition and water potential. |
| osmoregulatory mechanism | Physiological processes that organisms use to maintain water balance and regulate internal solute composition. |
| osmosis | The movement of water across a semipermeable membrane from regions of high water potential to regions of low water potential. |
| pressure potential | The component of water potential representing the physical pressure exerted on water in a cell, often due to cell wall rigidity. |
| solute concentration | The amount of dissolved solutes per unit volume of solution, which affects water movement across membranes. |
| solute potential | The component of water potential that represents the effect of dissolved solutes in lowering the potential energy of water. |
| water balance | The regulation of water movement into and out of cells to maintain proper cellular function and organism homeostasis. |
| water potential | The potential energy of water in a system, determined by pressure potential and solute potential, that drives water movement. |
Frequently Asked Questions
What is tonicity and how does it affect cells?
Tonicity describes how the solute concentration outside a cell compares to the inside (hypotonic, hypertonic, isotonic) and determines water movement by osmosis—water flows from hypotonic (high water potential) to hypertonic (low water potential). Use water-potential ideas (ψ = ψp + ψs; ψs = −iCRT) to predict direction quantitatively. Effects on cells: in a hypotonic solution animal cells may swell and lyse; plant cells gain turgor pressure (central vacuole fills) and stay firm. In hypertonic solutions animal cells shrink (crenate) and plant cells plasmolyze (loss of turgor). Cells control this with aquaporins, contractile vacuoles in protists, vacuoles in plants, and organismal osmoregulation (ADH, kidneys—loop of Henle) to maintain homeostasis (LO 2.7.A/B). For AP prep, review examples and practice applying ψs = −iCRT on free-response and multiple-choice (see sample Q10 in the CED). For a focused review, check the Topic 2 study guide (facilitated diffusion/tonicity) on Fiveable (https://library.fiveable.me/ap-biology/unit-2/facilitated-diffusion/study-guide/i3qUckt9PGfT4pQlHq5B) and try practice problems (https://library.fiveable.me/practice/ap-biology).
Why does water move from hypotonic to hypertonic solutions?
Water moves from a hypotonic to a hypertonic solution because of osmosis—net diffusion of water down its water potential gradient. A hypotonic solution has higher water potential (less solute) and a hypertonic solution has lower water potential (more solute). Water potential ψ = ψp + ψs, and adding solute makes ψs more negative, lowering ψ. So water flows from the region of higher ψ (hypotonic) into the region of lower ψ (hypertonic) until potentials equilibrate or pressure stops flow. Across cell membranes this happens through aquaporins or the lipid bilayer and explains turgor in plant cells (hypotonic outside → water into central vacuole) and plasmolysis in hypertonic conditions (water leaves cell). This is exactly what LO 2.7.A/EK 2.7.A.1 and EK 2.7.B.2 cover. For a quick refresher, check the Topic 2.7 study guide (https://library.fiveable.me/ap-biology/unit-2/facilitated-diffusion/study-guide/i3qUckt9PGfT4pQlHq5B) and more unit review (https://library.fiveable.me/ap-biology/unit-2). Practice problems are at (https://library.fiveable.me/practice/ap-biology).
What's the difference between hypotonic, hypertonic, and isotonic solutions?
Hypotonic, hypertonic, and isotonic describe how solute concentrations outside a cell compare to inside—and they predict water movement by osmosis (water goes from high water potential → low water potential; from low solute concentration → high solute concentration). - Hypotonic: external solution has lower solute concentration than the cell. Water enters the cell. Plant cells become turgid (good—turgor pressure supports the cell wall); animal cells may swell and lyse. - Hypertonic: external solution has higher solute concentration. Water leaves the cell. Plant cells plasmolyze (central vacuole shrinks, membrane pulls away); animal cells crenate (shrink). - Isotonic: equal solute concentration; no net water movement. Cells stay same volume (red blood cells prefer this). Connect to AP CED: use ψ = ψp + ψs and ψs = −iCRT to predict direction of water flow (LO 2.7.A/B). For quick review, see the Topic 2 study guide on facilitated diffusion (https://library.fiveable.me/ap-biology/unit-2/facilitated-diffusion/study-guide/i3qUckt9PGfT4pQlHq5B) and more unit resources (https://library.fiveable.me/ap-biology/unit-2). Practice problems: https://library.fiveable.me/practice/ap-biology.
I'm confused about water potential - can someone explain it in simple terms?
Think of water potential (ψ) as how badly water “wants” to move—higher ψ means water is freer to move. Water always moves from regions of higher ψ to lower ψ. Two things set ψ: pressure potential (ψp, physical pressure) and solute potential (ψs, how much solute lowers free water). So ψ = ψp + ψs. Solutes make ψs more negative; adding salt lowers ψ, pulling water in. You can calculate ψs with ψs = −iCRT (i = ionization constant, C = molarity, R = 0.0831 L·bar/mol·K, T in K). How this matters: water moves from a hypotonic (higher ψ) outside into a hypertonic (lower ψ) inside, causing turgor in plants or plasmolysis if water leaves. Protists use contractile vacuoles; plants use central vacuoles to regulate ψ and survive. This is core for LO 2.7.A/B on the AP exam—be ready to explain movement using ψ and to do simple ψs calculations. For a quick review, see the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/facilitated-diffusion/study-guide/i3qUckt9PGfT4pQlHq5B) and practice problems (https://library.fiveable.me/practice/ap-biology).
How do you calculate solute potential using that formula with i, C, R, and T?
Use ψs = −iCRT. Plug in: - i = ionization constant (how many particles one solute gives: glucose = 1, NaCl ≈ 2, CaCl2 ≈ 3), - C = molarity (mol·L⁻¹), - R = 0.0831 L·bar·mol⁻¹·K⁻¹, - T = temperature in Kelvin (°C + 273). Work step-by-step and keep units: ψs (bars) = −(i)(C in mol·L⁻¹)(0.0831)(T in K). Example: 0.10 M NaCl at 25°C: i = 2, C = 0.10, T = 25 + 273 = 298 K ψs = −2 × 0.10 × 0.0831 × 298 ≈ −4.95 bars On the AP, show substitutions, units, and reasonable sig figs. For more practice on tonicity/osmoregulation, check the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/facilitated-diffusion/study-guide/i3qUckt9PGfT4pQlHq5B) and the unit page (https://library.fiveable.me/ap-biology/unit-2).
What happens to plant cells when they're placed in different tonic solutions?
If you put a plant cell in a hypotonic solution (lower solute outside), water enters by osmosis—the central vacuole fills, pressure potential (ψp) rises, and the cell becomes turgid; turgor pressure helps support the plant (good for growth/homeostasis; LO 2.7.A, EK 2.7.A.1). In an isotonic solution, there’s no net water movement so the cell is flaccid (no extra support). In a hypertonic solution (higher solute outside), water leaves the central vacuole, ψs becomes more negative, the plasma membrane pulls away from the cell wall (plasmolysis) and the cell wilts—damaging if prolonged. You can connect this to the water-potential equation ψ = ψp + ψs and ψs = −iCRT to predict direction quantitatively. For a quick AP-focused review and practice on these ideas, see the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/facilitated-diffusion/study-guide/i3qUckt9PGfT4pQlHq5B), the unit overview (https://library.fiveable.me/ap-biology/unit-2), and practice problems (https://library.fiveable.me/practice/ap-biology).
Why do contractile vacuoles in protists keep pumping out water?
Protists live in freshwater that’s usually hypotonic to their cytoplasm, so water constantly flows into the cell by osmosis (water moves from high water potential to low). If they didn’t remove that water, the cell would swell and could burst. Contractile vacuoles collect excess water (often after ions are actively pumped into the vacuole to lower its solute potential) and periodically contract to pump it out—an energy-consuming osmoregulatory process that maintains internal osmolarity and prevents lysis. This is the classic AP illustrative example for osmoregulation and tonicity in Topic 2.7 (LO 2.7.A and LO 2.7.B). For a quick review of membrane transport and related examples, check the Topic 2 study guide on Fiveable (https://library.fiveable.me/ap-biology/unit-2/facilitated-diffusion/study-guide/i3qUckt9PGfT4pQlHq5B) and try practice problems at (https://library.fiveable.me/practice/ap-biology).
How does osmoregulation help organisms survive in different environments?
Osmoregulation keeps an organism’s internal water and solute balance so cells keep working across different environments. Water moves by osmosis from regions of high water potential to low (ψ = ψp + ψs; ψs = −iCRT), so organisms use pumps, membranes, and structures to control ψs and ψp. Examples: freshwater protists use contractile vacuoles to expel excess water in hypotonic environments; plant central vacuoles build turgor pressure to stay rigid and avoid plasmolysis in hypertonic soils; animals regulate blood osmolarity via the nephron and ADH to conserve or excrete water. These mechanisms maintain homeostasis, support growth, and let species live in fresh, marine, or terrestrial habitats. On the AP exam, you should be able to explain tonicity (hypo/hyper/isotonic), calculate solute potential, and connect structures to function (e.g., aquaporins, vacuoles, loop of Henle). For a quick Topic 2.7 review see the facilitated diffusion study guide (https://library.fiveable.me/ap-biology/unit-2/facilitated-diffusion/study-guide/i3qUckt9PGfT4pQlHq5B), unit overview (https://library.fiveable.me/ap-biology/unit-2), and extra practice (https://library.fiveable.me/practice/ap-biology).
What's the relationship between osmolarity and water movement across membranes?
Osmolarity (solute concentration) sets the direction of water movement across membranes: water flows from regions of low osmolarity (fewer solutes, more free water) toward regions of high osmolarity (more solutes, less free water). In AP terms, water moves by osmosis from hypotonic → hypertonic regions (LO 2.7.A/B). You can also think in water potential: ψ = ψp + ψs, where ψs = −iCRT, so adding solute lowers ψ and pulls water in. Membrane proteins like aquaporins speed osmotic flow but don’t change the direction. Biological consequences: plant cells in hypotonic solutions become turgid (central vacuole fills, turgor pressure rises); in hypertonic solutions they plasmolyze. Protists use contractile vacuoles and animals use osmoregulation (e.g., ADH, kidneys) to maintain internal osmolarity. For more AP-aligned review and practice on this topic, see the Topic 2 study guide (facilitated diffusion) (https://library.fiveable.me/ap-biology/unit-2/facilitated-diffusion/study-guide/i3qUckt9PGfT4pQlHq5B) and thousands of practice questions (https://library.fiveable.me/practice/ap-biology).
I don't understand how pressure potential and solute potential work together - help?
Think of water potential (ψ) as the “score” that tells water which way to move: ψ = ψp + ψs. Pressure potential (ψp) is the physical pressure (positive in a turgid plant cell because the cell wall pushes back). Solute potential (ψs) is always zero or negative—adding solute makes ψs more negative (ψs = −iCRT), which lowers overall ψ. Water moves from higher ψ to lower ψ. So if you add solute inside a cell, ψs drops (more negative) → whole ψ drops → water flows in until ψp rises enough to balance (turgor). In a plant in a hypotonic external solution, ψp increases and the cell becomes turgid; in a hypertonic external solution, ψs outside is lower (more negative) so cell ψ is higher and water leaves → plasmolysis. Protists use contractile vacuoles to raise ψp and expel water in very hypotonic environments. For AP: be ready to use ψ = ψp + ψs and ψs = −iCRT on free-response and to explain tonicity effects (LO 2.7.A/B). More practice and concise examples are in the Topic 2.7 study guide (https://library.fiveable.me/ap-biology/unit-2/facilitated-diffusion/study-guide/i3qUckt9PGfT4pQlHq5B) and the unit overview (https://library.fiveable.me/ap-biology/unit-2).
Why do red blood cells burst in distilled water but shrivel in salt water?
Red blood cells burst in distilled water because distilled water is hypotonic relative to the cell interior: it has a much lower solute concentration, so water moves into the cell by osmosis (water moves from high water potential → low water potential). The plasma membrane of an RBC is flexible but has limits; too much influx raises internal pressure potential (ψp) and the cell lyses (bursts). In salt water the external solution is hypertonic: it has higher solute concentration than the cytoplasm, so water leaves the cell, lowering ψp and causing the cell to shrivel (crenate). Aquaporins speed water movement but don’t change direction—tonicity does. This fits LO 2.7.A/B (osmotic gradients, water potential ψ = ψp + ψs) on the CED. For a quick refresher on these terms and AP-style practice, check the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/facilitated-diffusion/study-guide/i3qUckt9PGfT4pQlHq5B) and more practice problems at (https://library.fiveable.me/practice/ap-biology).
How do plant central vacuoles help with water balance and turgor pressure?
The central vacuole helps a plant control water potential (ψ = ψp + ψs) and therefore turgor pressure. By storing solutes (sugars, ions), the vacuole lowers the cell’s solute potential (ψs), making the interior more negative. Water moves by osmosis from higher water potential (outside or apoplast) into the vacuole through aquaporins, increasing pressure potential (ψp). That pressure presses the plasma membrane against the rigid cell wall—turgor—which supports stems and leaves and helps cells grow. In a hypotonic external environment the vacuole fills and turgor rises; in a hypertonic environment water leaves the vacuole, ψp falls, and the cell can plasmolyze (membrane pulls away from wall). For AP-style explanations, connect solute and pressure potentials and use the water-potential equation. For extra review, see the Unit 2 study guide (https://library.fiveable.me/ap-biology/unit-2/facilitated-diffusion/study-guide/i3qUckt9PGfT4pQlHq5B) and more practice questions (https://library.fiveable.me/practice/ap-biology).
What does it mean when they say water moves from high to low water potential?
It means water flows from regions where the overall potential energy of water (water potential, ψ) is higher to where it's lower—think of it like downhill movement. Water potential equals pressure potential plus solute potential (ψ = ψp + ψs). Solutes lower ψ (ψs is negative), so adding solute or having higher osmolarity makes ψ more negative (lower). Thus water moves from a hypotonic region (higher ψ, less solute) into a hypertonic region (lower ψ, more solute). In plants that’s why water enters cells, raising turgor pressure (ψp) until equilibrium; in strong salt solutions cells plasmolyze as water leaves. In protists, contractile vacuoles pump out excess water to keep ψ balanced. This idea is tested on the AP exam under LO 2.7.A/B, so be comfortable using ψ = ψp + ψs and describing hypotonic/isotonic/hypertonic scenarios (see the Topic 2 study guide for practice: https://library.fiveable.me/ap-biology/unit-2/facilitated-diffusion/study-guide/i3qUckt9PGfT4pQlHq5B; unit review: https://library.fiveable.me/ap-biology/unit-2).
Can someone explain the ionization constant in the solute potential equation?
The ionization constant i in ψs = −iCRT (the van’t Hoff factor) tells you how many particles a solute produces in solution—that’s what matters for solute potential. For non-electrolytes (sucrose, glucose) i = 1 because they don’t dissociate. For NaCl, i ≈ 2 (Na+ and Cl−). For CaCl2, i ≈ 3 (Ca2+ + 2 Cl−). Multiply i by C to get the effective molar concentration of dissolved particles, so more particles → more negative ψs → water flows toward that side (osmosis). Note: i is idealized; real solutions can have slightly different effective i because of ion pairing, but for AP calculations you use the integer values above. Knowing i is important for LO 2.7.A/B problems where you calculate water potential and predict water movement (you may see these kinds of calculations on the free-response; practice doing them). For more review: see the Topic 2 study guide (facilitated diffusion/topic 2 link) and practice problems (https://library.fiveable.me/practice/ap-biology).
How do organisms control their internal solute composition to maintain homeostasis?
Organisms control internal solute composition through a mix of passive and active processes to maintain water potential and homeostasis (LO 2.7.A/B). Water moves by osmosis (high water potential → low water potential) described by ψ = ψp + ψs and ψs = −iCRT. Passive routes: aquaporins let water cross membranes; cells gain/lose water when environments are hypo-, hyper-, or isotonic (turgor pressure vs. plasmolysis). Active control: membrane pumps and channels move ions (Na+, K+, Cl−) to set osmolarity; plants use central vacuoles to store solutes and create turgor. Protists use contractile vacuoles to expel excess water. Animals regulate via organs/hormones—kidneys (nephron, loop of Henle) and ADH adjust water reabsorption and urine concentration. Behaviorally, organisms seek/avoid salty or fresh water. For AP exam prep, focus on applying water-potential math and naming mechanisms (aquaporins, vacuoles, ADH, nephron)—see the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/facilitated-diffusion/study-guide/i3qUckt9PGfT4pQlHq5B) and more unit review (https://library.fiveable.me/ap-biology/unit-2) or practice questions (https://library.fiveable.me/practice/ap-biology).