AP Biology: Membrane Transport – Passive (Diffusion, Osmosis, Facilitated) vs Active Transport (Pumps, Endo/Exocytosis)

Membrane Transport – Passive (Diffusion, Osmosis, Facilitated) vs Active Transport (Pumps, Endo/Exocytosis)

Concept Summary

• Passive transport: Movement of molecules across a membrane without energy input, driven by concentration gradients; essential for maintaining cellular homeostasis.
• Diffusion: Net movement of particles from high to low concentration until equilibrium; primary mechanism for small, nonpolar molecules (e.g., O?, CO?).
• Osmosis: Diffusion of water across a selectively permeable membrane; critical for cell volume regulation and turgor pressure in plants.
• Facilitated diffusion: Passive transport of polar/charged molecules (e.g., glucose, ions) via channel or carrier proteins; no ATP required but requires protein assistance.
• Active transport: Movement of molecules against their gradient using ATP or electrochemical gradients; includes pumps (e.g., Na?/K? ATPase) and bulk transport (endo/exocytosis).

Core Questions

WHAT (definitional)

Q: What is a concentration gradient? A: A difference in solute concentration between two regions, driving passive transport. Trap/Clarification: Gradients refer to solute concentration, not solvent (e.g., water in osmosis moves toward higher solute concentration).

Q: What distinguishes facilitated diffusion from simple diffusion? A: Facilitated diffusion requires membrane proteins (channels/carriers) for polar/charged molecules; simple diffusion does not. Trap/Clarification: Both are passive, but facilitated diffusion is saturable (rate limited by protein availability).

WHY (causal/explanatory)

Q: Why is osmosis critical for cell survival? A: It maintains water balance, preventing lysis (hypotonic) or crenation (hypertonic) in animal cells and turgor pressure in plant cells. Trap/Clarification: Osmosis is not about solute movement—water moves to dilute the higher solute concentration.

Q: Why do cells use active transport instead of passive transport? A: To accumulate molecules against gradients (e.g., nutrient uptake, ion gradients for nerve impulses) or maintain disequilibrium (e.g., Na?/K? pump). Trap/Clarification: Active transport requires energy (ATP or cotransport), but cotransport (e.g., symport/antiport) is secondary active transport (uses gradients, not direct ATP).

HOW (process/application)

Q: How does the Na?/K? pump work? A: 3 Na? out, 2 K? in per ATP hydrolyzed, creating electrochemical gradients; steps: (1) Na? binds, (2) ATP phosphorylates pump, (3) conformational change, (4) Na? released, (5) K? binds, (6) dephosphorylation resets pump. Trap/Clarification: The pump is electrogenic (creates voltage) but not the primary driver of resting membrane potential (leak channels matter more).

Q: How is water potential (?) calculated? A: ? = + , where = pressure potential (turgor in plants), = solute potential ( = -iCRT; i = ionization constant, C = molarity, R = gas constant, T = temp in Kelvin). Trap/Clarification: Pure water has-= 0; adding solutes lowers ? (more negative), so water moves toward lower (more negative) ?.

CAN (conditions/possibilities)

Q: Can facilitated diffusion occur without a concentration gradient? A: No—it’s passive, so it requires a gradient (though proteins lower activation energy). Trap/Clarification: Facilitated diffusion can saturate (max rate at high [solute]), unlike simple diffusion.

Q: Under what conditions does exocytosis occur? A: When cells secrete large molecules (e.g., hormones, neurotransmitters) or insert membrane proteins/lipids; triggered by Ca²? influx or signaling pathways. Trap/Clarification: Exocytosis adds membrane (vesicle fusion), while endocytosis removes membrane (vesicle formation).

Quick Facts & Traps

• Fact: Aquaporins are water-specific channel proteins that increase osmosis rate without changing direction.
• Trap: "Osmosis moves solutes"-Reality: Osmosis moves water toward higher solute concentration.
• Fact: Electrochemical gradients combine concentration + charge (e.g., Na?/K? pump creates both).
• Trap: "Active transport always uses ATP"-Reality: Secondary active transport (e.g., glucose symport) uses gradients from primary active transport.
• Fact: Tonicity (hypo/iso/hyper) describes solute concentration outside the cell relative to inside; osmolarity is total solute concentration.
• Trap: "Facilitated diffusion is active transport"-Reality: It’s passive (no energy), but requires proteins.

Rapid-Fire True/False

• Statement: Diffusion stops when equilibrium is reached. Answer: FALSE Why the common mistake happens: Students confuse net movement (stops) with individual molecule movement (continues randomly).

• Statement: The Na?/K? pump directly generates the resting membrane potential. Answer: FALSE Why the common mistake happens: The pump creates gradients, but leak channels (K? efflux) establish the resting potential.

• Statement: Endocytosis and exocytosis are forms of active transport. Answer: TRUE Why the common mistake happens: Students overlook that vesicle formation/fusion requires ATP, even though no direct molecule "pumping" occurs.