3.3 Membrane Transport

The Membrane Is Selectively Permeable

The phospholipid bilayer freely admits small, nonpolar molecules (O2, CO2, small uncharged molecules like urea). Polar and charged molecules — water, ions, glucose, amino acids — need help getting across. That selectivity is the entire reason the cell needs the transport machinery covered in this section.

Praxis questions almost always sort transport into three buckets. Make those buckets the first thing you check:

1. Passive — no ATP, follows the gradient.
2. Active — ATP required (directly or indirectly), moves against the gradient.
3. Bulk — vesicles, used for material too large for channels.

Passive Transport

Passive transport always moves solutes down their concentration gradient until equilibrium is reached. No metabolic energy is spent; thermal motion does the work.

• Simple diffusion — small, nonpolar solutes (O2, CO2, steroid hormones) slip directly through the bilayer.
• Facilitated diffusion — polar or charged solutes pass through protein channels or carriers. Channels are open or gated tunnels (e.g., aquaporins for water, voltage-gated Na+ channels). Carriers bind a solute and change shape (e.g., GLUT transporters for glucose).
• Osmosis — diffusion of water across a selectively permeable membrane, driven by the concentration of solutes that water cannot easily cross with.

Tonicity Vocabulary

Tonicity describes the effective solute concentration of a solution relative to the cell. Memorize these three terms; nearly every Praxis osmosis item depends on them.

Water always moves from where it is more concentrated (low solute) to where it is less concentrated (high solute), so the technical phrase is "water follows the solute."

Active Transport

Active transport pushes solutes against their gradient, which requires energy.

• Primary active transport uses ATP directly. The flagship example is the Na+/K+ pump (Na+/K+-ATPase), which exchanges 3 Na+ out and 2 K+ in per ATP hydrolyzed. Because more positive charge leaves than enters, the pump also makes the cell interior slightly negative, which is essential for nerve and muscle excitability.
• Secondary active transport uses the potential energy stored in an ion gradient that a primary pump created. The Na+/glucose symporter in the intestinal lining drags glucose into the cell on the back of Na+ flowing down its gradient. No ATP is hydrolyzed at the symporter itself — but ATP was spent earlier by the Na+/K+ pump to build the gradient.

Bulk Transport

Large molecules and bulk fluid cannot pass through channels and must be packaged in vesicles. Both endocytosis and exocytosis consume ATP because vesicle formation and fusion remodel the cytoskeleton and membranes.

• Endocytosis — material enters the cell in a vesicle.
  • Phagocytosis — "cell eating"; pseudopodia engulf large particles (e.g., macrophages swallowing bacteria).
  • Pinocytosis — "cell drinking"; nonspecific uptake of extracellular fluid in small vesicles.
  • Receptor-mediated endocytosis — specific ligands bind receptors clustered in clathrin-coated pits, then the membrane invaginates and pinches off (LDL cholesterol uptake is the textbook example).
• Exocytosis — secretory vesicles fuse with the plasma membrane and release contents to the outside (neurotransmitter release, hormone secretion, mucus discharge).

Concentration vs. Electrochemical Gradients

For uncharged solutes, the concentration gradient alone determines direction. For ions, the cell has to consider both the chemical gradient (where the ion is more concentrated) and the electrical gradient (the charge of the destination). Together they form the electrochemical gradient.

A resting neuron, for example, has a high external Na+ concentration and a negative interior; both gradients push Na+ inward when channels open. The combination of those two forces is what voltage-gated channels release during an action potential. When the Praxis asks why a charged solute moves "against" a chemical gradient, the answer is almost always that the electrical component dominates.