2.2 Diffusion, Osmosis & Membrane Transport

2.2 Diffusion, Osmosis & Membrane Transport

Quick Answer: Substances cross the plasma membrane by passive transport (down a gradient, no ATP: simple diffusion, facilitated diffusion, osmosis) or active transport (against a gradient, needs ATP), plus endocytosis/exocytosis for large cargo. Osmosis is water movement toward higher solute concentration. Isotonic solutions keep cell volume stable; hypotonic causes swelling/lysis; hypertonic causes crenation—critical for IV fluid decisions and NEX Science items.

The plasma membrane does not simply wall the cell off from the world. It is a selectively permeable barrier that must import nutrients, export wastes, maintain ion gradients for nerve and muscle function, and keep the cell’s volume compatible with life. Understanding how materials cross that barrier is one of the highest-yield cell biology topics on the NEX and a daily concept in nursing (IV fluids, edema, dehydration, dialysis).

Selective Permeability and Concentration Gradients

A concentration gradient exists when a substance is more concentrated on one side of a membrane than the other. Net movement of that substance tends to run from high to low concentration until equilibrium—unless energy is used to reverse the flow.

What crosses easily by simple diffusion?

• Small nonpolar molecules: O₂, CO₂, N₂
• Small uncharged polar molecules to a limited extent: water (also uses aquaporins), urea somewhat freely

What does not cross the bilayer freely?

• Ions (Na⁺, K⁺, Cl⁻, Ca²⁺) — charge blocks passage
• Large polar molecules (glucose without a transporter)
• Macromolecules (proteins, polysaccharides)

These restricted substances need protein channels, carriers, or vesicular transport.

Passive Transport (No Cellular ATP Required)

Passive processes use the energy stored in the concentration (or electrochemical) gradient itself—not ATP hydrolyzed by the cell for that specific move.

Simple Diffusion

Simple diffusion is net movement of a substance from high to low concentration directly through the lipid bilayer (or through a pore) without a carrier protein. Example: oxygen diffusing from alveolar air into blood, then into tissues where O₂ is lower.

Rate increases with steeper gradient, higher temperature, greater surface area, and higher membrane permeability for that solute.

Facilitated Diffusion

Facilitated diffusion still moves solutes down their gradient without ATP, but requires membrane proteins:

• Channel proteins — open pores (often gated) for ions or water (aquaporins)
• Carrier proteins — bind the solute, change shape, release it on the other side (e.g., GLUT transporters for glucose)

Glucose entry into many cells is a classic facilitated-diffusion example. When insulin promotes GLUT4 insertion into muscle and fat cell membranes, more glucose can enter down its gradient—linking membrane transport to endocrine physiology you will meet later.

Osmosis

Osmosis is the net diffusion of water across a selectively permeable membrane toward the side with higher total solute concentration (lower water concentration). Water moves to dilute the more concentrated side.

• Osmolarity / osmolality describe solute particle concentration.
• The osmotic pressure of a solution is the pressure that would be needed to stop osmotic water influx into that solution.

In the body, water moves between intracellular fluid (ICF) and extracellular fluid (ECF) based largely on relative osmolarity of those compartments.

Tonicity: Hypotonic, Isotonic, and Hypertonic Solutions

Tonicity describes how a solution affects cell volume by driving osmosis. It depends on solutes that cannot freely cross the membrane (effective osmoles).

Clinical and exam anchors:

• 0.9% NaCl (normal saline) is approximately isotonic to plasma—standard for many IV infusions when you want volume without major cell-volume shifts.
• Pure water or very dilute solutions are hypotonic to cells—dangerous IV use; RBCs can hemolyze.
• Concentrated saline or mannitol solutions can be hypertonic—water is pulled out of cells/tissues (used carefully, e.g., reducing cerebral edema with hypertonic agents under protocol).

Red blood cells are the classic model: isotonic → biconcave disc; hypotonic → swollen sphere → hemolysis; hypertonic → crenated (spiky, shrunken).

Plant cells behave differently because of the rigid cell wall: hypotonic solutions create turgor pressure (firmness) rather than lysis; hypertonic solutions cause plasmolysis (membrane pulls away from wall). NEX items about human cells focus on animal-cell outcomes (lyse/crenate).

Active Transport (Requires Energy)

Active transport moves substances against their concentration or electrochemical gradient and requires energy, usually from ATP.

Primary Active Transport

The textbook example is the sodium-potassium pump (Na⁺/K⁺-ATPase):

• Pumps 3 Na⁺ out of the cell and 2 K⁺ in per ATP hydrolyzed
• Maintains low intracellular Na⁺ and high intracellular K⁺
• Creates the foundation for resting membrane potential in nerves and muscle and drives secondary transport

Without this pump, gradients collapse, cells swell (Na⁺ accumulates inside, water follows), and excitable tissues fail.

Secondary Active Transport (Cotransport)

Here, the energy stored in one ion’s gradient (often Na⁺ moving into the cell down its gradient) drives another solute against its gradient. Example: sodium-glucose linked transporters (SGLT) in intestinal and kidney epithelium—glucose is absorbed against its gradient by “riding” Na⁺ influx. The Na⁺ gradient was originally paid for by the Na⁺/K⁺ pump (primary active transport).

Bulk Transport: Endocytosis and Exocytosis

Large particles and bulk fluid cannot squeeze through channels; the membrane uses vesicles.

Exocytosis is how neurons release neurotransmitters and how endocrine cells secrete hormones. Phagocytosis is central to innate immunity—neutrophils and macrophages engulf pathogens, then fuse vesicles with lysosomes for destruction (linking back to Section 2.1).

Transport Types at a Glance

Nursing and NEX Connections

• IV fluids: Choosing isotonic vs hypertonic vs hypotonic fluids is applied osmosis. Wrong tonicity harms RBCs and shifts fluid between ICF and ECF.
• Edema and dehydration: Reflect fluid shifts driven by hydrostatic and osmotic (oncotic) forces—built on the same gradient logic.
• Medications: Some drugs block ion channels or pumps (e.g., digoxin affects Na⁺/K⁺-ATPase indirectly via Na⁺/Ca²⁺ exchange context; many antiarrhythmics and local anesthetics alter Na⁺ channels).
• Kidneys and GI tract: Reabsorption of glucose, amino acids, and electrolytes depends on facilitated and active transport—failure shows as glycosuria when transporters are saturated (diabetes).

Exam Traps

• Osmosis direction: Water moves toward the hypertonic (higher solute) side—not “toward water.”
• Passive ≠ no protein: Facilitated diffusion is passive but requires a protein; simple diffusion does not.
• Active ≠ always into the cell: Active transport can move substances in or out; the defining feature is against the gradient + energy.
• Isotonic ≠ iso-osmolar always in complex solutions: For NEX-level questions, treat isotonic as “no net water shift / stable cell volume.”
• Hypotonic IV free water: Causes cells to swell, not shrink.
• Crenation vs lysis: Hypertonic → crenation; hypotonic → lysis.

Memorize the decision tree: Is ATP required? Is movement with or against the gradient? Is the cargo a small solute or bulk material? Those three answers place almost every NEX membrane-transport item correctly.