2.8 Mechanisms of Transport

Active transport is the movement of ions or molecules across a membrane using metabolic energy (usually from ATP) and membrane proteins, moving solutes against their concentration or electrochemical gradient. It’s required to establish and maintain electrochemical gradients and membrane potential (EK 2.8.A.1). Examples: the Na+/K+ ATPase (electrogenic P-type ATPase) that pumps 3 Na+ out / 2 K+ in using ATP, and H+ ATPases (proton pumps). How it differs from passive transport: - Energy: active = uses metabolic energy (ATP or an existing gradient); passive = no metabolic energy required (driven by diffusion, concentration or electrochemical gradients). - Direction: active can move solutes from low → high concentration; passive only moves high → low. - Proteins: both can use membrane proteins, but active transport always requires a protein carrier (pump). Secondary active transport (symport/antiport) uses an ion gradient created by primary active transport (e.g., Na+/glucose symporter uses Na+ gradient). For AP review, see Topic 2.8 in the CED and the unit 2 study guide (https://library.fiveable.me/ap-biology/unit-2/tonicity-osmoregulation/study-guide/CCENUydXNVb7K4P7ikp7). For more practice, check the unit overview (https://library.fiveable.me/ap-biology/unit-2) and practice problems (https://library.fiveable.me/practice/ap-biology).

The Na+/K+ pump (sodium-potassium ATPase) is a membrane P-type ATPase that uses ATP to move ions against their gradients—it exports 3 Na+ out of the cell and imports 2 K+ into the cell per ATP hydrolyzed. Steps: pump binds 3 intracellular Na+ → ATP phosphorylates the protein → pump changes shape and releases Na+ outside → binds 2 extracellular K+ → dephosphorylation returns pump to original shape and K+ is released inside. That makes it electrogenic (net + charge leaves), helping set the resting membrane potential and an electrochemical gradient. It’s primary active transport (direct use of ATP) and drives secondary active transport (e.g., Na+/glucose symporter uses the Na+ gradient). On the AP exam, you should be able to describe this active transport mechanism and its role in membrane potential (EK 2.8.A.1). Review Topic 2.8 study guide (https://library.fiveable.me/ap-biology/unit-2/tonicity-osmoregulation/study-guide/CCENUydXNVb7K4P7ikp7), the unit overview (https://library.fiveable.me/ap-biology/unit-2), and practice questions (https://library.fiveable.me/practice/ap-biology).

Active transport needs ATP because it moves ions or molecules against their concentration or electrochemical gradients—from low to high concentration or against membrane potential. That uphill movement isn’t spontaneous, so proteins like pumps (e.g., the Na+/K+ ATPase, a P-type ATPase and electrogenic pump) use energy from ATP hydrolysis to change shape and carry ions across the membrane (primary active transport). Secondary active transport uses the energy stored in an electrochemical gradient (set up by an ATP-powered pump) to drive movement of another molecule (symport/antiport), but the gradient itself was created using metabolic energy. Passive transport (diffusion, facilitated diffusion through ion channels or uniports) moves substances down their concentration or electrochemical gradients, which is energetically favorable and needs no ATP. This distinction (ATP required vs. not) appears directly in EK 2.8.A.1 and is tested on the AP for explaining membrane potential and pumps (see the Topic 2.8 study guide: https://library.fiveable.me/ap-biology/unit-2/tonicity-osmoregulation/study-guide/CCENUydXNVb7K4P7ikp7). For more review and practice, check Unit 2 (https://library.fiveable.me/ap-biology/unit-2) and practice questions (https://library.fiveable.me/practice/ap-biology).

Facilitated diffusion and active transport both move molecules across membranes using membrane proteins, but they differ in energy use and direction relative to gradients. Facilitated diffusion: passive—no ATP—molecules move down their concentration (or electrochemical) gradient through channels or carrier proteins (e.g., ion channels, GLUT transporters). Active transport: requires metabolic energy (usually ATP) and membrane proteins to move solutes against their gradient, creating or maintaining electrochemical gradients and the resting membrane potential (examples: Na+/K+ ATPase, H+ ATPase). Active transport can be primary (direct ATP hydrolysis by a pump) or secondary (uses energy stored in an ion gradient for symport/antiport, like the Na+/glucose symporter). On the AP exam, you should name pumps/channels and mention ATP and electrochemical gradients (EK 2.8.A.1). For a quick review, see the Topic 2 study guide on tonicity/osmoregulation (https://library.fiveable.me/ap-biology/unit-2/tonicity-osmoregulation/study-guide/CCENUydXNVb7K4P7ikp7) and more unit resources (https://library.fiveable.me/ap-biology/unit-2). Practice questions are at (https://library.fiveable.me/practice/ap-biology).

Moving a molecule "uphill" (against its concentration or electrochemical gradient) requires energy and membrane proteins—that’s active transport (LO 2.8.A, EK 2.8.A.1). In primary active transport a pump uses ATP directly: e.g., the Na+/K+ ATPase hydrolyzes 1 ATP to move 3 Na+ out and 2 K+ in, making the pump electrogenic and helping maintain membrane potential. In secondary active transport, the cell uses the energy stored in an existing electrochemical gradient (set up by a pump) to move a second solute: a symporter brings two molecules the same way (like the Na+/glucose symporter), an antiporter moves them opposite ways. Pumps are membrane proteins (P-type ATPases, H+ ATPases) and ATP hydrolysis is the metabolic energy source. For AP prep, know definitions, examples (Na+/K+ pump, proton pump), and how gradients store work (these are testable concepts). Review the Unit 2 study guide on tonicity/osmoregulation for practice (https://library.fiveable.me/ap-biology/unit-2/tonicity-osmoregulation/study-guide/CCENUydXNVb7K4P7ikp7) and try problems at (https://library.fiveable.me/practice/ap-biology).

Membrane proteins are proteins embedded in the lipid bilayer that create pathways or do work to move specific ions and molecules across the membrane. For active transport you need membrane proteins because small nonpolar stuff can pass by diffusion, but charged ions and many polar molecules can’t—so pumps and transporters (like the Na⁺/K⁺ ATPase, H⁺ ATPase, uniports, symports, and antiports) bind those solutes and use energy to move them. In primary active transport a pump hydrolyzes ATP (EK 2.8.A.1) to move ions against their concentration or electrochemical gradients; electrogenic pumps (e.g., Na⁺/K⁺) also help maintain membrane potential. Secondary active transport uses the gradient established by a pump (no direct ATP hydrolysis) to drive cotransport (symport/antiport). This is exactly what the CED expects you to know for LO 2.8.A. For a quick topic review, check the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/tonicity-osmoregulation/study-guide/CCENUydXNVb7K4P7ikp7) and more practice at (https://library.fiveable.me/practice/ap-biology).

The Na+/K+ pump (sodium-potassium ATPase) is a primary active transport protein that uses ATP to move ions against their gradients: it pumps 3 Na+ out of the cell and 2 K+ into the cell per ATP hydrolyzed. Because more positive charge leaves than enters, the pump is electrogenic and helps create a small net negative charge inside the cell—a key contributor to the resting membrane potential (around −70 mV in many animal cells). It also maintains the steep Na+ and K+ concentration gradients cells need for secondary active transport (like the Na+/glucose symporter) and for normal excitability. Note: the pump helps establish and maintain membrane potential but works with K+ leak channels and other ion fluxes to set the final resting value (EK 2.8.A.1; Na+/K+ ATPase). For review, check the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/tonicity-osmoregulation/study-guide/CCENUydXNVb7K4P7ikp7), the unit overview (https://library.fiveable.me/ap-biology/unit-2), and practice questions (https://library.fiveable.me/practice/ap-biology).

An electrochemical gradient is the combined difference in solute concentration (chemical gradient) and electrical charge (voltage) across a membrane. Cells create it using membrane proteins that move ions with metabolic energy (e.g., Na+/K+ ATPase, H+ ATPase—electrogenic pumps) (EK 2.8.A.1). The gradient stores potential energy and sets the membrane potential (resting potential). It’s important because cells use that stored energy for work: driving secondary active transport (Na+/glucose symporter = symport), powering ATP synthesis in mitochondria/chloroplasts (proton gradient → ATP synthase), and enabling nerve and muscle signaling. On the AP exam you should be able to name pumps (Na+/K+ pump), explain how ATP is used, and link gradients to transport types (primary vs. secondary active transport). Review this topic in the Topic 2.8 study guide (https://library.fiveable.me/ap-biology/unit-2/tonicity-osmoregulation/study-guide/CCENUydXNVb7K4P7ikp7), Unit 2 overview (https://library.fiveable.me/ap-biology/unit-2), and practice more at (https://library.fiveable.me/practice/ap-biology).

ATPase is a membrane protein that uses energy from ATP to move ions against their concentration gradients. In simple terms: it “burns” ATP (ATP hydrolysis) to change shape and pump specific ions (like the Na+/K+ ATPase moving 3 Na+ out and 2 K+ in). That pumping is primary active transport, is electrogenic (creates charge difference), and helps set the resting membrane potential and electrochemical gradients (EK 2.8.A.1). Those gradients power secondary active transport (e.g., the Na+/glucose symporter) and affect things like nerve signals and osmoregulation. Other ATPases include H+ (proton) pumps and P-type ATPases with similar mechanisms. For AP exam relevance, know that ATPases are membrane proteins required for active transport and for establishing membrane potential (LO 2.8.A). For a quick Topic 2 review, check the Fiveable Unit 2 study guide (https://library.fiveable.me/ap-biology/unit-2/tonicity-osmoregulation/study-guide/CCENUydXNVb7K4P7ikp7) and more practice problems (https://library.fiveable.me/practice/ap-biology).

Short answer: Some molecules move down their concentration or electrochemical gradients so they don’t need cellular energy (passive transport: simple diffusion or facilitated diffusion through channels/carriers). But when a cell must move solutes against a gradient (from low to high concentration or against an electrical gradient), it has to spend metabolic energy—usually ATP—because that’s thermodynamically unfavorable. AP terms: active transport (primary: ATP-powered pumps like the Na+/K+ ATPase; secondary: using energy stored in an ion gradient to drive transport—symport/antiport like the Na+/glucose symporter). Pumps can be electrogenic (create membrane potential). EK 2.8.A says membrane proteins + ATP are required to establish/maintain electrochemical gradients and resting membrane potential. For extra review on tonicity/osmoregulation and practice problems, check the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/tonicity-osmoregulation/study-guide/CCENUydXNVb7K4P7ikp7) and lots of practice at (https://library.fiveable.me/practice/ap-biology).

If the Na⁺/K⁺ ATPase (sodium-potassium pump) stops working, a cascade of problems follows. Normally the pump uses ATP to move 3 Na⁺ out and 2 K⁺ in, creating and maintaining an electrochemical gradient and the resting membrane potential (EK 2.8.A.1). Without it, intracellular Na⁺ rises and K⁺ drops, so the membrane potential depolarizes—neurons and muscle cells can’t fire properly. Osmotic balance is upset (water follows Na⁺), so cells may swell and even lyse. Secondary active transport that depends on the Na⁺ gradient (e.g., Na⁺/glucose symporter) fails, disrupting nutrient uptake. Because the pump is electrogenic, its loss reduces the cell’s ability to restore ion gradients after activity and quickly impairs cell function and viability. For AP review, connect this to primary active transport, electrochemical gradients, and membrane potential (see the Topic 2.8 study guide: https://library.fiveable.me/ap-biology/unit-2/tonicity-osmoregulation/study-guide/CCENUydXNVb7K4P7ikp7; unit overview: https://library.fiveable.me/ap-biology/unit-2). Practice problems: https://library.fiveable.me/practice/ap-biology.

Short way to remember: if a molecule moves down its concentration (or electrochemical) gradient, it doesn’t need metabolic energy; if it moves up (against) a gradient, it does. So: - No ATP (passive): simple diffusion, osmosis, facilitated diffusion through ion channels or carrier proteins—movement down gradient. - Needs ATP (active): primary active transport uses ATP directly (e.g., Na⁺/K⁺ ATPase, H⁺ ATPase—electrogenic pumps that help set membrane potential). Secondary active transport uses the electrochemical gradient set up by pumps (symport/antiport like Na⁺/glucose)—ATP used indirectly to establish that gradient. Quick mnemonic: “Down = downhill (no ATP). Up = uphill (ATP).” Remember EK 2.8.A: active transport requires metabolic energy and membrane proteins and maintains electrochemical gradients (Na⁺/K⁺ pump role). For a deeper review see the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/tonicity-osmoregulation/study-guide/CCENUydXNVb7K4P7ikp7), the whole Unit 2 overview (https://library.fiveable.me/ap-biology/unit-2), and extra practice questions (https://library.fiveable.me/practice/ap-biology).

ATP provides the metabolic energy cells use to move ions against their concentration or electrical gradients (primary active transport). Membrane proteins like the Na+/K+ ATPase hydrolyze ATP → ADP + Pi to change shape and pump 3 Na+ out and 2 K+ in, making the pump electrogenic and helping maintain the resting membrane potential (EK 2.8.A.1). Proton (H+) ATPases and other P-type ATPases work the same way. Those ATP-powered pumps establish electrochemical gradients that can be used for secondary active transport: the downhill flow of one ion (e.g., Na+) drives uphill transport of another molecule via symport or antiport (e.g., Na+/glucose symporter). On the AP, you should be able to name pumps, explain ATP hydrolysis driving conformational change, and connect pumps to membrane potential and electrochemical gradients (see Topic 2.8 in the CED). For a quick review, check the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/tonicity-osmoregulation/study-guide/CCENUydXNVb7K4P7ikp7) and practice questions (https://library.fiveable.me/practice/ap-biology).

Cells keep different concentrations across membranes by combining selective permeability with energy-driven pumps and channels (LO 2.8.A). The lipid bilayer blocks many ions; membrane proteins let specific solutes pass. Primary active transport uses ATP to move ions against their gradients—e.g., the Na+/K+ ATPase pumps 3 Na+ out and 2 K+ in per ATP hydrolyzed, creating an electrochemical gradient and contributing to the resting membrane potential (an electrogenic pump). Secondary active transport then uses that stored gradient: symports and antiports couple downhill movement of one ion (usually Na+ or H+) to drive uphill transport of another molecule (like the Na+/glucose symporter). Ion channels allow passive flow when gated, letting cells change membrane potential quickly. Together, these mechanisms establish and maintain different concentrations and the electrochemical gradients AP Bio asks you to know (see Topic 2.8 and the Fiveable study guide: https://library.fiveable.me/ap-biology/unit-2/tonicity-osmoregulation/study-guide/CCENUydXNVb7K4P7ikp7). For more review and practice problems, check the unit overview (https://library.fiveable.me/ap-biology/unit-2) and thousands of practice questions (https://library.fiveable.me/practice/ap-biology).

Know these clear examples for the AP exam (LO 2.8.A / EK 2.8.A): - Sodium-potassium ATPase (Na+/K+ pump): primary active transport, uses ATP hydrolysis to move 3 Na+ out and 2 K+ in—electrogenic and helps maintain membrane potential. - Proton pump (H+ ATPase): moves H+ against its gradient (important in plants, fungi, lysosomes)—a P-type ATPase example. - Secondary active transport (uses an electrochemical gradient set up by a pump): Na+/glucose symporter (cotransport) brings glucose into cells using Na+ gradient; Na+/Ca2+ antiport exchanges Na+ and Ca2+. - General terms to know: primary vs. secondary active transport, symport/antiport/uniport (uniport = single solute carrier), electrogenic pump, electrochemical gradient. These map directly to CED keywords and what you’ll see on multiple-choice/free-response. Review the Topic 2 study guide (https://library.fiveable.me/ap-biology/unit-2/tonicity-osmoregulation/study-guide/CCENUydXNVb7K4P7ikp7) and practice problems (https://library.fiveable.me/practice/ap-biology).