Key ideas
Membranes control what enters and leaves a cell. Substances cross by passive processes that need no energy or by active transport that does, and water moves by osmosis down a water-potential gradient.
- The membrane is a phospholipid bilayer with proteins (fluid mosaic model). Small nonpolar molecules diffuse through it, while ions and polar molecules need channels or carriers.
- Diffusion and facilitated diffusion go down a concentration gradient and need no ATP. Active transport goes against the gradient and uses ATP.
- Osmosis is the movement of water from higher to lower water potential. A cell swells in a hypotonic solution and shrinks in a hypertonic one.
- Water potential is Ψ = Ψs + Ψp. Adding solute lowers Ψs, and pressure raises Ψp. The diffusion rate increases with surface area and gradient and falls with distance.
Key equations
Ψ = Ψs + ΨpΨs = −iCRTrate ∝ surface area × gradient ÷ distance
Worked example
A plant cell has Ψs = −0.7 MPa and Ψp = +0.3 MPa, so Ψ = −0.4 MPa. It is placed in a solution with Ψ = −0.2 MPa. Water moves from higher to lower water potential, from the solution (−0.2) into the cell (−0.4), so the cell gains water and becomes turgid.
Common mistakes
- Saying water moves toward the lower solute concentration. It moves toward the lower water potential, which is the higher solute concentration.
- Confusing the tonicity of the cell with that of the solution.
- Thinking facilitated diffusion uses ATP. It does not; it only uses a protein.
Practice
Test yourself with practice questions on this topic: Membranes and transport