Types of Cell Transport

Why This Matters

Cell transport isn't just a list of vocabulary terms—it's the foundation for understanding how cells survive, communicate, and maintain the precise internal conditions that make life possible. Every process you study in biology, from nerve impulses to muscle contraction to immune responses, depends on cells moving the right substances across their membranes at the right time. On the AP exam, you're being tested on your ability to explain why a cell uses one transport method over another and how energy requirements connect to concentration gradients.

The key concepts here are thermodynamics (does this process require energy input?), membrane selectivity (what can pass through, and how?), and homeostasis (how do cells maintain internal balance?). Don't just memorize that the sodium-potassium pump uses ATP—understand that it must use ATP because it's fighting against the natural tendency of molecules to spread out. When you can explain the "why" behind each transport type, you'll crush both multiple choice and FRQ questions.

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Passive Transport: Going with the Flow

Passive transport processes share one critical feature: they require no energy input because substances move down their concentration gradient. Think of it like a ball rolling downhill—the movement happens spontaneously because it's thermodynamically favorable.

Simple Diffusion

• Small, nonpolar molecules pass directly through the lipid bilayer—this includes $O_2$, $CO_2$, and steroid hormones that dissolve easily in the hydrophobic membrane core
• No proteins required—the molecule's size and polarity allow it to slip between phospholipids without assistance
• Continues until equilibrium—movement doesn't stop, but net movement reaches zero when concentrations equalize on both sides

Facilitated Diffusion

• Polar or large molecules require protein assistance—glucose, amino acids, and ions can't cross the hydrophobic core alone
• Channel proteins vs. carrier proteins—channels form open tunnels (often for ions), while carriers bind and change shape to shuttle molecules across
• Still passive, still down the gradient—the protein provides a pathway, not energy; this distinction is heavily tested

Osmosis

• Water diffuses across selectively permeable membranes—water moves toward regions of higher solute concentration (lower water concentration)
• Tonicity determines cell fate—hypertonic solutions cause cells to shrink (crenation in animal cells, plasmolysis in plant cells), while hypotonic solutions cause swelling or lysis
• Aquaporins speed the process—these channel proteins dramatically increase water transport rates, making osmosis a form of facilitated diffusion

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Active Transport: Fighting the Gradient

Active transport moves substances against their concentration gradient—from low to high concentration. This is thermodynamically unfavorable, so it requires energy input, typically from ATP hydrolysis or an existing electrochemical gradient.

Primary Active Transport

• ATP hydrolysis directly powers the transport protein—the protein uses energy released from breaking ATP's phosphate bond to change shape and move molecules
• Sodium-potassium pump ($Na^+/K^+$-ATPase) is the classic example—pumps 3 $Na^+$ out and 2 $K^+$ in per ATP, establishing crucial ion gradients
• Creates electrochemical gradients—these gradients store potential energy used for nerve impulses, muscle contraction, and secondary active transport

Secondary Active Transport

• Uses gradients established by primary active transport—no direct ATP use, but indirectly depends on ATP that built the gradient
• Symport vs. antiport mechanisms—symporters move two substances in the same direction; antiporters move them in opposite directions
• Glucose absorption in intestines is a key example—$Na^+$ flowing down its gradient (established by the $Na^+/K^+$ pump) drives glucose uptake against glucose's gradient

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Bulk Transport: Moving the Big Stuff

Some materials are too large to cross the membrane through proteins. Bulk transport uses vesicles—membrane-bound compartments—to move large molecules, particles, or even whole cells. These processes require energy for membrane remodeling.

Endocytosis

• Membrane folds inward to engulf external material—creates an intracellular vesicle containing substances from outside the cell
• Three main types serve different functions—phagocytosis, pinocytosis, and receptor-mediated endocytosis (detailed below)
• Requires ATP for membrane restructuring—cytoskeletal proteins must reorganize to pull the membrane inward

Phagocytosis

• "Cell eating" engulfs large particles or whole cells—pseudopods extend around the target, forming a phagosome
• Critical for immune function—macrophages and neutrophils use phagocytosis to destroy pathogens and clear cellular debris
• Highly selective process—cells recognize specific surface markers on targets before engulfing them

Pinocytosis

• "Cell drinking" takes in extracellular fluid non-selectively—small vesicles capture dissolved solutes along with water
• Continuous sampling of the environment—allows cells to monitor and absorb nutrients from surrounding fluid
• Less selective than receptor-mediated endocytosis—captures whatever happens to be dissolved in the fluid droplet

Receptor-Mediated Endocytosis

• Highly selective uptake using specific receptors—target molecules (ligands) bind to receptors that cluster in coated pits lined with clathrin protein
• Efficient for low-concentration substances—cholesterol uptake via LDL receptors is the classic example
• Receptor recycling maintains efficiency—after vesicle contents are released, receptors return to the membrane for reuse

Exocytosis

• Vesicles fuse with the plasma membrane to release contents—the reverse of endocytosis; moves materials out of the cell
• Essential for secretion—neurons release neurotransmitters, endocrine cells release hormones, and goblet cells release mucus via exocytosis
• Adds membrane to the cell surface—balances membrane removed during endocytosis, maintaining cell size

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Quick Reference Table

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Self-Check Questions

1. Which two transport types both require membrane proteins but differ in their energy requirements? Explain what accounts for this difference.

2. A cell is placed in a hypertonic solution. Using your understanding of osmosis, predict what will happen to the cell and explain why water moves in that direction.

3. Compare and contrast primary and secondary active transport. Why is secondary transport sometimes called "co-transport," and how does it ultimately depend on ATP?

4. An FRQ asks you to explain how a neuron maintains its resting membrane potential. Which transport mechanism is most relevant, and what specific example would you use?

5. A mutation destroys a cell's clathrin proteins. Which transport process would be most affected, and what cellular functions might be impaired as a result?