Passive Transport: Diffusion, Osmosis, and Facilitated Diffusion

Core Concept

Passive transport is the movement of substances across a membrane down their concentration gradient (from high to low concentration) without the cell spending metabolic energy. It includes simple diffusion, osmosis (the diffusion of water), and facilitated diffusion (movement of solutes through channel or carrier proteins). In every case, the driving force is the random thermal motion of molecules and the tendency of a system to spread out toward equilibrium.

Key Concepts

Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, driven by random molecular motion. Individual molecules move in all directions, but because there are more particles on the high-concentration side, there is a net flow down the gradient until concentrations equalize (equilibrium). Diffusion is faster with steeper gradients, higher temperature, smaller molecules, and a shorter distance.

Simple vs. Facilitated Diffusion

Simple diffusion occurs when a molecule crosses the lipid bilayer directly — possible mainly for small, nonpolar molecules (O₂, CO₂) and small polar molecules like water. Facilitated diffusion moves solutes that cannot cross the lipid bilayer on their own (ions, glucose, amino acids) through transport proteins, still down their gradient and still without energy.

Channels and Carriers

Channel proteins form hydrophilic pores through the membrane; many are gated (opened by ligands, voltage, or mechanical force). They are fast and typically ion-selective. Carrier proteins (also called transporters) bind their solute and undergo a conformational change to release it on the other side. Carriers can be saturated because they must physically bind and change shape, so they reach a maximum transport rate (Vmax).

Aquaporins

Water crosses membranes slowly by simple diffusion but rapidly through aquaporins, specialized channel proteins that form water-selective pores. Aquaporins greatly speed up osmosis in cells that must move large volumes of water quickly, such as kidney tubule cells and plant root cells.

Osmosis and Tonicity

Osmosis is the diffusion of water across a selectively permeable membrane, from the side with lower solute concentration (more free water) to the side with higher solute concentration (less free water). Tonicity describes the ability of a surrounding solution to make a cell gain or lose water:

• Isotonic — equal solute concentration to the cell; no net water movement.
• Hypertonic — higher solute (lower free water) than the cell; water leaves, cell shrinks.
• Hypotonic — lower solute (higher free water) than the cell; water enters, cell swells.

Effects on Animal vs. Plant Cells

In a hypertonic solution, an animal cell shrivels (crenates) and a plant cell undergoes plasmolysis (the plasma membrane pulls away from the cell wall as the vacuole loses water). In a hypotonic solution, an animal cell — lacking a cell wall — swells and may burst (lyse), while a plant cell becomes turgid (firm) because the cell wall resists expansion; turgor pressure is normal and healthy for plants. In an isotonic solution, an animal cell is normal ("flaccid" is the term for a plant cell in isotonic conditions — limp but intact).

How It Works

The mechanism is statistical, not directional. (1) Every molecule vibrates and moves randomly. (2) In a concentration gradient, more molecules are available to move away from the crowded region than toward it, so the net movement is downhill. (3) For solutes that cannot cross the lipid bilayer, channel proteins provide water-filled pores, or carrier proteins bind the solute and change shape to shuttle it across — still driven only by the gradient. (4) Water follows the solutes: because solutes reduce the concentration of free water, water moves toward the compartment with more solute. No step requires ATP; the cell simply provides a permeable surface and, where needed, the appropriate protein.

How it works

The mechanism is statistical, not directional. (1) Every molecule vibrates and moves randomly. (2) In a concentration gradient, more molecules are available to move away from the crowded region than toward it, so the net movement is downhill. (3) For solutes that cannot cross the lipid bilayer, channel proteins provide water-filled pores, or carrier proteins bind the solute and change shape to shuttle it across — still driven only by the gradient. (4) Water follows the solutes: because solutes reduce the concentration of free water, water moves toward the compartment with more solute. No step requires ATP; the cell simply provides a permeable surface and, where needed, the appropriate protein.

Common confusions

• "Passive transport uses a little energy." Wrong — passive transport requires no cellular energy at all; the gradient provides the driving force.
• "Facilitated diffusion is active transport." Wrong — facilitated diffusion uses proteins but still moves solutes down the gradient without ATP.
• "Osmosis is the movement of solute." Wrong — osmosis is specifically the movement of water (solvent), not solute.
• "Hypertonic means more water." Wrong — hypertonic means more solute (therefore less free water); water leaves the cell placed in it.
• "Plant cells burst in hypotonic solution." Wrong — the cell wall prevents lysis; instead the plant cell becomes turgid, which is normal.
• "Water can't cross the lipid bilayer." Wrong — water crosses slowly by simple diffusion and rapidly through aquaporins.

Quick review

• Passive transport = down-gradient, no energy: diffusion, osmosis, facilitated diffusion.
• Simple diffusion handles small nonpolar molecules directly through the bilayer.
• Channels = pores; carriers = conformational-change shuttles (saturable, Vmax).
• Aquaporins accelerate water movement.
• Tonicity drives osmosis: hypertonic shrinks, hypotonic swells, isotonic = no change.
• Animal cell: crenates (hypertonic) / lyses (hypotonic). Plant cell: plasmolyzes / turgid.