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Passive transport moves substances across the cell membrane down their concentration gradient without requiring cellular energy (ATP). Simple diffusion allows small nonpolar molecules (O₂, CO₂) to pass directly through the bilayer. Facilitated diffusion uses channel or carrier proteins for ions and polar molecules. Osmosis is the diffusion of water through a selectively permeable membrane toward regions of lower water potential (higher solute concentration).
Work through quantitative osmosis problems using the concepts of isotonic, hypotonic, and hypertonic solutions. Predict what happens to a red blood cell or plant cell in each environment and explain using water potential reasoning.
You already know from studying the cell membrane that the phospholipid bilayer is a selective barrier — hydrophobic in its core, hydrophilic at its surfaces. This structure is what makes passive transport possible: certain substances can cross it without any energy input from the cell, driven purely by concentration gradients.
The simplest form is simple diffusion: small, nonpolar molecules like O₂ and CO₂ dissolve directly into the lipid bilayer and pass through. They move from regions of high concentration to low concentration — the same thermodynamic principle (entropy increasing, free energy decreasing) that causes a drop of food coloring to spread through water. The membrane just provides the medium. No proteins involved, no energy required.
Facilitated diffusion works by the same logic — still down the concentration gradient, still no ATP — but uses membrane proteins to help molecules that cannot dissolve in the lipid core. Channel proteins form permanent hydrophilic pores (aquaporins for water, ion channels for Na⁺, K⁺, Cl⁻). Carrier proteins bind a specific molecule, change shape, and release it on the other side. The key point that trips people up: the word "facilitated" means *assisted*, not *energized*. The protein lowers the energy barrier for crossing, but the gradient does the work.
Osmosis is a special case of diffusion: the movement of *water* across a selectively permeable membrane. Water moves toward the side with lower water potential, which means toward higher solute concentration. In an isotonic solution, solute concentrations are equal on both sides and there is no net water movement. In a hypotonic solution (lower solute outside), water enters the cell and it swells. In a hypertonic solution (higher solute outside), water leaves and the cell shrinks. Plant cells experience this as turgor pressure or plasmolysis; animal cells experience swelling or crenation. Building the habit of thinking in terms of *water potential* (not just solute concentration) will serve you well when these concepts appear in more advanced physiology.
The contrast between passive and active transport comes next: active transport will introduce what happens when the cell needs to move substances *against* their gradient, which requires energy (ATP) and protein pumps. Keep in mind that passive transport sets the baseline — any movement requiring ATP is doing extra thermodynamic work precisely because it fights the spontaneous passive direction.