Types of Cellular Transport

Why This Matters

Cellular transport is the foundation of how cells stay alive—it's how they acquire nutrients, expel waste, communicate with each other, and maintain the precise internal conditions necessary for biochemical reactions. On the AP exam, you're being tested on your understanding of membrane structure, energy use, concentration gradients, and homeostasis. These concepts show up everywhere: from explaining how neurons fire to why your kidneys can concentrate urine to how plant cells stay rigid.

Don't just memorize a list of transport types. Instead, focus on why each mechanism exists and what determines which one a cell uses. Ask yourself: Does this process require energy? Does it need a protein? Is the substance moving with or against its gradient? Master these distinctions, and you'll be ready for any multiple-choice question or FRQ that tests transport concepts.

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Passive Transport: Going With the Flow

Passive transport requires no cellular energy because substances move down their concentration gradient—from high to low concentration. The driving force is the random thermal motion of molecules, which naturally disperses them until equilibrium is reached. The membrane's structure determines which molecules can pass freely and which need help.

Simple Diffusion

• Small, nonpolar molecules pass directly through the phospholipid bilayer—examples include $O_2$, $CO_2$, and steroid hormones
• No proteins or energy required—the hydrophobic core of the membrane is permeable to these molecules
• Continues until dynamic equilibrium—molecules still move, but net movement stops when concentrations equalize

Facilitated Diffusion

• Polar or large molecules require transport proteins—glucose, amino acids, and ions cannot cross the hydrophobic bilayer alone
• Channel proteins vs. carrier proteins—channels form hydrophilic tunnels; carriers bind and change shape to shuttle molecules
• Still passive (no ATP)—the concentration gradient provides all the energy needed for movement

Osmosis

• Water diffuses through aquaporins or directly through the membrane—moving toward regions of higher solute concentration (lower water concentration)
• Determines cell volume and turgor pressure—critical for plant rigidity and preventing animal cells from bursting or shriveling
• Tonicity matters for the exam—hypertonic, hypotonic, and isotonic solutions predict water movement direction

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Active Transport: Fighting the Gradient

When cells need to move substances against their concentration gradient—from low to high concentration—they must spend energy. This is thermodynamically unfavorable, so ATP hydrolysis or stored electrochemical gradients power the process. Active transport is how cells create and maintain the concentration differences essential for life.

Primary Active Transport

• ATP hydrolysis directly powers the transport protein—the protein changes shape when ATP is broken down
• Sodium-potassium pump ($Na^+/K^+$-ATPase) is the classic example—pumps 3 $Na^+$ out and 2 $K^+$ in per ATP, creating electrochemical gradients
• Essential for nerve impulses, muscle contraction, and cell volume regulation—uses up to 25% of a cell's ATP

Secondary Active Transport

• Uses the gradient created by primary active transport as an energy source—no direct ATP use, but depends on ATP indirectly
• Symport moves two substances in the same direction; antiport moves them opposite—glucose-sodium symporter in intestinal cells is a key example
• Couples "downhill" movement of one substance to "uphill" movement of another—the ion gradient stores potential energy like a battery

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Bulk Transport: Moving the Big Stuff

Some materials are too large for transport proteins—entire proteins, bacteria, or large quantities of fluid. Cells use vesicle-mediated transport, which requires membrane remodeling and significant energy investment. These processes involve the cytoskeleton and membrane fusion machinery.

Endocytosis (Bringing Materials In)

Phagocytosis

• "Cell eating"—engulfs large particles like bacteria or debris—the membrane extends pseudopods around the target
• Critical for immune function—macrophages and neutrophils use this to destroy pathogens
• Forms a phagosome that fuses with lysosomes—lysosomal enzymes digest the engulfed material

Pinocytosis

• "Cell drinking"—takes in extracellular fluid nonselectively—small vesicles form continuously at the membrane
• Allows cells to sample their environment—captures dissolved solutes along with the fluid
• Common in cells that absorb nutrients—intestinal and kidney cells rely heavily on this process

Receptor-Mediated Endocytosis

• Highly selective—only captures molecules that bind specific receptors—receptors cluster in coated pits lined with clathrin protein
• Efficient uptake of specific substances—cholesterol (via LDL receptors), iron (via transferrin), and some hormones enter this way
• Defects cause disease—familial hypercholesterolemia results from faulty LDL receptors

Exocytosis (Sending Materials Out)

Exocytosis

• Vesicles fuse with the plasma membrane to release contents—used for secretion of hormones, neurotransmitters, and digestive enzymes
• Adds membrane to the cell surface—balances membrane removed during endocytosis
• Regulated vs. constitutive pathways—some secretion requires a signal (like calcium for neurotransmitter release); some occurs continuously

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Quick Reference Table

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Self-Check Questions

1. Which two transport mechanisms both require membrane proteins but differ in their energy requirements? Explain what determines whether a cell uses one versus the other.

2. A cell needs to accumulate iodine ions against their concentration gradient. What type of transport must be involved, and what would happen if you blocked ATP production?

3. Compare and contrast phagocytosis and receptor-mediated endocytosis in terms of selectivity, typical cargo, and cellular functions.

4. If a patient has a genetic mutation affecting their $Na^+/K^+$-ATPase, which types of transport would be directly affected? Which would be indirectly affected? Explain your reasoning.

5. An FRQ asks you to explain how a cell absorbs glucose from the intestinal lumen where glucose concentration is lower than inside the cell. Which transport mechanism(s) would you describe, and how do they work together?