Cell transport & osmosis

How water, ions, and molecules cross a membrane.

The idea

The phospholipid bilayer lets small nonpolar molecules such as O₂ and CO₂ slip through by simple diffusion, down their concentration gradient, at no energy cost. Ions and polar molecules such as glucose need help: facilitated diffusion moves them through channel or carrier proteins, still passively and still down the gradient. Osmosis is the diffusion of water across a selectively permeable membrane from higher water potential (Ψ) to lower; pure water has Ψ = 0, dissolved solutes make it negative (the solute potential Ψs), and pressure from a cell wall pushes it back up (the pressure potential Ψp). Active transport moves substances against their gradient using ATP — the sodium–potassium pump, for example, exports 3 Na⁺ and imports 2 K⁺ for every ATP it splits — while endocytosis and exocytosis move bulk cargo in vesicles.

Work through an example

A plant cell has Ψs = −0.8 MPa and Ψp = +0.3 MPa, so its water potential is −0.8 + 0.3 = −0.5 MPa. Drop it into a sucrose solution with Ψ = −0.2 MPa: water moves from −0.2 (higher) to −0.5 (lower), so it enters the cell. The cell swells until the wall pushes Ψp up to +0.6 MPa, giving −0.8 + 0.6 = −0.2 MPa, and net flow stops. For reference, 0.1 M sucrose at 25 °C has Ψs = −iCRT = −1 × 0.1 × 0.00831 × 298 ≈ −0.25 MPa.

What to watch for

Water does not move "toward the higher concentration" — that phrase describes the solute. Water moves toward the more concentrated solution, which is the side with the lower (more negative) water potential. Remember that every solution has a negative Ψ, so a cell at −0.5 MPa loses water to a beaker at −0.9 MPa even though both numbers look small.