This unit is ≈12% of the Biology, across 7 lessons. Full syllabus
AP Biology · Lesson 4 of 7 · Unit 2
Membrane transport: passive, facilitated, active and bulk transport
3 quick checks ≈2% of the test9 min read · about 1 h 45 min with practicePreview
You’re reading a preview of this lesson. Already on Pro? Sign in to read it all.
Membrane transport sits at the heart of cellular homeostasis, connecting membrane structure (Topic 2.3) to cellular energetics (Unit 3) and cell signaling (Unit 4). On the AP Biology exam, you will rarely be asked to simply define a transport mechanism; instead, you will deduce the mechanism from experimental uptake graphs or predict what happens when an inhibitor blocks ATP synthesis or a specific membrane protein.
By the end you’ll be able to
Describe how selective permeability allows concentration gradients to form and how passive transport moves molecules down a gradient without direct input of metabolic energy.
Explain how channel and transport proteins, including aquaporins, enable facilitated diffusion of ions, large polar molecules and water.
Explain how active transport uses metabolic energy and membrane proteins to move molecules, sometimes against a gradient, and to maintain electrochemical gradients.
Preview
Upgrade to Pro to unlock the rest
You’ve read the first part of this lesson. The rest is part of Pro: about 6 more minutes of reading, plus the quick check.
Explain how the Na+/K+ ATPase pump contributes to the maintenance of the membrane potential.
Describe how endocytosis and exocytosis use energy to move large molecules or large amounts of material across the plasma membrane.
Predict the effect of inhibiting a transport protein or ATP production on the movement of a substance, and propose an investigation to test it.
What the exam asks
Identify transport mechanisms from data: Distinguish simple diffusion, facilitated diffusion, and active transport using rate-vs.-concentration graphs, tables showing intracellular vs. extracellular concentrations, or experimental results with metabolic inhibitors.
Relate solute chemistry to transport requirements: Explain why charged ions (Na+, K+, H+), large polar molecules (glucose, amino acids), and large quantities of water require specific transport proteins to cross the hydrophobic core of the phospholipid bilayer.
Analyze electrochemical gradients: Explain how electrogenic pumps such as the Na+/K+ ATPase establish and maintain membrane potential, and how cotransporters use that stored potential energy.
Predict the impact of disruptions: Predict and justify the consequences of mutating a transport protein, removing extracellular ions, or adding a chemical that stops ATP production (such as an electron transport chain inhibitor).
Core ideas
Selective permeability and passive transport
Because the interior of the phospholipid bilayer is hydrophobic, the plasma membrane is selectively permeable. This barrier allows cells to maintain internal concentrations of solutes that differ from the extracellular environment—a concentration gradient.
Passive transport is the net movement of a substance down its concentration (or electrochemical) gradient—from a region of higher concentration to a region of lower concentration—without the direct input of metabolic energy (ATP). It plays a primary role in the import of materials and the export of wastes.
Simple diffusion: Small, nonpolar molecules (such as O2, CO2, and N) dissolve readily in the hydrophobic lipid interior and diffuse directly across the membrane.
Active transport and electrochemical gradients
Active transport requires the direct or indirect input of metabolic energy (usually from ATP hydrolysis) and always uses specific membrane carrier proteins (pumps) to move substances against their concentration or electrochemical gradients (from low to high concentration).
Primary active transport and membrane potential: The Na+/K+ ATPase pump uses energy from ATP hydrolysis to pump 3 Na+ ions out of the cell for every ions pumped into the cell, moving both ions against their respective concentration gradients. Because it exports a net positive charge ( out vs. in) with each cycle, it contributes directly to the maintenance of the (voltage difference across the membrane, leaving the cytoplasm negatively charged relative to the extracellular fluid).
Facilitated diffusion: Charged ions and polar molecules cannot efficiently cross the hydrophobic interior on their own. They move down their gradient via hydrophilic pathways provided by transmembrane channel proteins (which form open or gated pores) or carrier/transport proteins (which bind the solute and undergo a conformational change). While single water molecules can slip between phospholipids slowly, rapid movement of large amounts of water across the membrane occurs via facilitated diffusion through channel proteins called aquaporins.
2 K+
+3
+2
membrane potential
Cotransport (secondary active transport): A cotransporter protein couples the passive diffusion of one solute (such as H+ or Na+) down its electrochemical gradient to the uphill transport of a second solute (such as sucrose or glucose) against its concentration gradient.