13.1 Diffusion/Osmosis, Facilitated & Active Transport, Pinocytosis/Phagocytosis

Crossing the Membrane: Passive and Active Transport

Every cell survives by moving substances across its plasma membrane — a selectively permeable phospholipid bilayer studded with proteins. The PA-CAT Bulletin of Information, rev. 20240815 lists Cellular Transport among the General Biology topics (Table 6), and questions test whether you can distinguish mechanisms by energy requirement, direction, and substrate. Master the four transport categories below and you can answer any transport item confidently.

Passive Transport: Down the Gradient, No ATP

Passive transport moves solutes from higher concentration to lower concentration — down their electrochemical gradient — without spending ATP. Three variants appear on the exam:

• Simple diffusion: small, nonpolar molecules (O2, CO2, steroid hormones) slip directly through the phospholipid core. No protein is involved.
• Facilitated diffusion: polar or charged solutes (glucose, ions) that cannot cross the hydrophobic interior use membrane proteins — either channel proteins (open pores, gated by voltage or ligand) or carrier proteins (bind the solute and undergo a conformational change). It is still passive: net movement is down the gradient.
• Osmosis: water moves across a selectively permeable membrane from lower solute concentration to higher solute concentration. Water crosses the lipid bilayer slowly, but most osmosis occurs through aquaporins.

Tonicity: What Happens to a Cell

Tonicity describes the solute concentration of the extracellular fluid relative to the cytosol and predicts cell behavior — a frequent PA-CAT application.

A red blood cell in distilled water (hypotonic) swells and bursts; the same cell in 5% NaCl (hypertonic) shrivels. IV fluids are formulated isotonic (0.9% NaCl, normal saline) precisely to avoid lysing patient cells. Plant cells rely on hypotonic surroundings for turgor pressure, which keeps non-woody stems erect; a wilted plant has lost turgor because its extracellular fluid has become hypertonic through dehydration.

Active Transport: Against the Gradient, ATP Required

Active transport moves solutes from lower to higher concentration — against their gradient — and therefore requires energy input.

• Primary (direct) active transport: ATP is consumed directly by a pump protein. The classic example is the Na+/K+-ATPase (sodium-potassium pump), which hydrolyzes one ATP to export 3 Na+ and import 2 K+ each cycle, maintaining the resting membrane potential. Other primary pumps include Ca2+-ATPase (muscle relaxation) and H+-ATPase (stomach acidification).
• Secondary (indirect) active transport: a symporter or antiporter harnesses the gradient built by primary transport. The Na+/glucose symporter in intestinal epithelium rides the Na+ gradient back into the cell, dragging glucose up its own gradient. The Na+/Ca2+ exchanger in cardiac muscle is an antiporter.

Bulk Transport: Moving Big Cargo

Macromolecules and particles too large for channels or carriers enter or exit by membrane vesicle traffic.

• Endocytosis — the membrane invaginates and pinches off a vesicle inward. Phagocytosis (cell eating) engulfs large particles such as bacteria; macrophages and neutrophils are professional phagocytes. Pinocytosis (cell drinking) takes in small droplets of extracellular fluid and dissolved solutes — nonspecific bulk uptake. Receptor-mediated endocytosis uses clathrin-coated pits with specific receptors (e.g., LDL cholesterol uptake), the most selective form.
• Exocytosis — vesicles fuse with the plasma membrane to release contents outside the cell. Secretory cells release insulin, neurotransmitters, and digestive enzymes this way.

Factors That Affect Diffusion Rate

The rate of simple diffusion depends on four variables the PA-CAT may frame quantitatively: (1) concentration gradient — a steeper gradient increases net flux; (2) surface area — more membrane means more crossing sites, which is why intestinal villi and lung alveoli amplify surface area; (3) membrane thickness — thinner membranes diffuse faster; (4) temperature and molecular size — warmer, smaller molecules move faster. Fick's law summarizes this: rate is proportional to (surface area x concentration difference) divided by membrane thickness. Facilitated diffusion saturates: at high solute concentration all carrier proteins are occupied and the rate plateaus at a transport maximum (Tm), which is why glucose uptake shows saturation kinetics. Simple diffusion is not saturable because no protein is involved. Recognizing saturation kinetics on a graph is a reliable way to distinguish facilitated from simple diffusion on the exam.

Water Flux and Aquaporins

Although water is small and polar, it crosses the bare lipid bilayer slowly. Most osmosis occurs through aquaporins, channel proteins that pass billions of water molecules per second while excluding ions and protons. Aquaporin-2 in kidney collecting ducts is regulated by antidiuretic hormone (ADH): ADH triggers insertion of aquaporin-2 channels into the apical membrane, increasing water reabsorption and concentrating urine. Defects in aquaporin-2 cause nephrogenic diabetes insipidus, linking membrane transport directly to renal physiology.

Discriminating the Mechanisms — Exam Strategy

When a PA-CAT item describes transport, classify it with three questions: (1) Does it require ATP? Yes = active or bulk; No = passive. (2) Does it move down or against the gradient? Down = passive; against = active. (3) Is the cargo a molecule/ion or a particle/bulk fluid? Particle/bulk = endocytosis/exocytosis. Oxygen entering a lung capillary is simple diffusion; glucose uptake by a red blood cell is facilitated diffusion; iodide accumulation in the thyroid is secondary active transport; a macrophage ingesting a bacterium is phagocytosis. Memorize one canonical example for each mechanism and the classification becomes automatic.