Cell Membrane Transport Mechanisms

Why This Matters

The cell membrane isn't just a passive barrier—it's the gatekeeper that determines what enters and exits every living cell. On the AP Biology exam, you're being tested on your understanding of selective permeability, concentration gradients, electrochemical gradients, and how cells use energy to maintain internal conditions different from their environment. These concepts connect directly to Unit 2 (Cell Structure) and cascade into Unit 4 (Cell Communication), where membrane transport underlies everything from nerve impulses to hormone signaling.

Think of transport mechanisms as existing on a spectrum from "no energy required" to "significant ATP investment." The exam loves to test whether you can identify which mechanism applies to a given scenario and why that mechanism is necessary. Don't just memorize that the sodium-potassium pump uses ATP—know that it moves ions against their concentration gradient, which is thermodynamically unfavorable without energy input. Master the underlying principles, and you'll be ready for any FRQ that throws a novel transport scenario your way.

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Passive Transport: Moving Down the Gradient

Passive transport mechanisms share one fundamental principle: molecules move spontaneously from regions of high concentration to low concentration, driven by the second law of thermodynamics. No ATP required—the concentration gradient itself provides the driving force.

Simple Diffusion

• Small nonpolar molecules pass directly through the phospholipid bilayer—this includes $O_2$, $CO_2$, and $N_2$, which dissolve in the hydrophobic core
• No transport proteins needed—the hydrophobic interior of the membrane is permeable to these molecules because they lack charge
• Rate depends on gradient steepness, temperature, and molecular size—steeper gradients and higher temperatures increase kinetic energy and diffusion rate

Facilitated Diffusion

• Transport proteins assist polar or charged molecules—glucose, amino acids, and ions cannot cross the hydrophobic core alone
• Channel proteins vs. carrier proteins—channels form hydrophilic tunnels; carriers bind solutes and undergo conformational changes
• Saturation kinetics limit transport rate—when all proteins are occupied, adding more solute won't increase transport speed (this is a common FRQ concept)

Osmosis

• Water diffuses across membranes through aquaporins—these channel proteins dramatically increase water permeability beyond simple diffusion
• Water moves toward higher solute concentration—this dilutes the solute, equalizing concentrations on both sides
• Turgor pressure and cell survival depend on osmotic balance—plant cells need turgor for structure; animal cells can lyse in hypotonic solutions

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Active Transport: Working Against the Gradient

Active transport mechanisms share the requirement for energy input because they move substances against their concentration or electrochemical gradient. This is thermodynamically unfavorable—like pushing a boulder uphill—and requires ATP hydrolysis or coupling to another gradient.

Primary Active Transport (Sodium-Potassium Pump)

• Moves 3 $Na^+$ out and 2 $K^+$ in per ATP hydrolyzed—this specific ratio creates both concentration and electrical gradients
• Establishes the electrochemical gradient essential for neurons—resting membrane potential depends on this pump's continuous operation
• Consumes ~25% of cellular ATP in neurons—demonstrates the enormous energy investment cells make to maintain ion gradients

Ion Channels

• Gated channels open/close in response to specific signals—voltage-gated, ligand-gated, and mechanically-gated varieties exist
• Allow rapid, selective ion flow down electrochemical gradients—$Na^+$, $K^+$, $Ca^{2+}$, and $Cl^-$ each have dedicated channels
• Critical for action potentials and cellular signaling—neurons and muscle cells depend on coordinated channel opening for function

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Bulk Transport: Moving Large Cargo

When molecules are too large to pass through channels or carriers, cells use membrane-bound vesicles to transport them. These processes require ATP for membrane remodeling and vesicle movement along the cytoskeleton.

Endocytosis

• Plasma membrane invaginates to engulf external material—forms intracellular vesicles containing captured substances
• Phagocytosis ("cell eating") captures large particles—immune cells use this to engulf pathogens; pinocytosis captures dissolved solutes
• Receptor-mediated endocytosis provides specificity—clathrin-coated pits concentrate specific ligands before internalization

Exocytosis

• Vesicles fuse with plasma membrane to release contents—SNARE proteins mediate the membrane fusion event
• Essential for secretion of hormones and neurotransmitters—connects directly to cell communication topics in Unit 4
• Adds membrane material to the cell surface—balances membrane removal during endocytosis

Vesicular Transport

• Moves materials between organelles and plasma membrane—the endomembrane system relies on continuous vesicle trafficking
• COPI, COPII, and clathrin coat proteins direct vesicle formation—each type targets vesicles to specific destinations
• Motor proteins (dynein, kinesin) move vesicles along cytoskeleton—requires ATP hydrolysis for directional transport

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Quick Reference Table

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Self-Check Questions

1. Both facilitated diffusion and the sodium-potassium pump use membrane proteins. What is the fundamental difference that determines whether ATP is required?

2. A cell is placed in a hypertonic solution. Using your knowledge of osmosis, predict what will happen to a plant cell versus an animal cell—and explain why their fates differ.

3. Compare and contrast ion channels and carrier proteins: How do their mechanisms differ, and why might a cell use one over the other?

4. If an FRQ describes a neuron that cannot maintain its resting membrane potential, which transport mechanism has most likely failed? Explain how this failure would affect the cell's ability to generate action potentials.

5. Identify two transport mechanisms that exhibit saturation kinetics. What molecular feature do they share that explains this limitation?