Biology · Introductory biology · Concept

Membrane transport: diffusion, osmosis, pumps

A cell membrane is a phospholipid bilayer that lets some molecules through and holds others back. Small nonpolar molecules such as O₂ diffuse straight across it; ions, and polar solutes such as glucose, need a channel or a carrier protein. Passive transport runs down a concentration gradient and costs the cell no energy. Active transport moves a solute up its gradient and needs energy, usually from ATP, as in the sodium-potassium pump. Water crosses by osmosis, toward the side with more dissolved solute.

A selectively permeable bilayer

The bilayer has a hydrophobic core. Small nonpolar molecules such as O₂ and CO₂ dissolve in it and cross freely. Ions and larger polar molecules such as glucose barely cross on their own, so membrane proteins control their traffic.

Passive transport

Diffusion is the net movement of a substance from higher to lower concentration, down its concentration gradient, until it is evenly spread; the molecules keep moving both ways, but the net movement stops. Facilitated diffusion is diffusion through a protein: a channel protein forms a pore for particular ions or for water, and a carrier protein binds its solute and changes shape to release it on the other side. Neither costs energy, and neither can move a solute up its gradient.

Osmosis and tonicity

Osmosis is the diffusion of water across a selectively permeable membrane, from the side with less dissolved solute to the side with more. Tonicity compares a solution with the cell: in a hypotonic solution, with less solute, water moves in and an animal cell swells and may burst; in a hypertonic solution, with more, water moves out and the cell shrivels; an isotonic solution gives no net movement. A plant cell’s wall keeps it from bursting, so in fresh water it becomes firm, or turgid.

A cell membrane in a hypertonic solution: water leaves the cell

Solution outside	Net water movement	Animal cell	Plant cell
Hypotonic	Into the cell	Swells and may burst	Turgid (firm)
Isotonic	None	Normal	Flaccid (limp)
Hypertonic	Out of the cell	Shrivels	Plasmolyzed: the membrane pulls away from the wall

Active transport

Active transport moves a solute against its gradient and needs energy. The sodium-potassium pump uses one ATP to move 3 Na⁺ out of the cell and 2 K⁺ in, keeping Na⁺ high outside and K⁺ high inside. In secondary active transport, one gradient powers another: a sodium–glucose cotransporter lets Na⁺ flow back in down its gradient and carries glucose in with it, up glucose’s gradient.

Bulk transport

Large molecules and particles cross in vesicles. In endocytosis the membrane folds in around material and pinches off a vesicle: phagocytosis engulfs particles, and receptor-mediated endocytosis takes in particular molecules. In exocytosis, vesicles fuse with the membrane and release their contents, as cells secrete hormones and enzymes.

Common mistakes

Saying water moves toward the lower solute concentration: water moves toward the side with more dissolved solute.
Calling facilitated diffusion active because it uses a protein: it needs no energy and runs only down the gradient.
Mixing up hypotonic and hypertonic: both describe the solution compared with the cell, so a cell in a hypotonic solution gains water.
Thinking molecules stop moving at equilibrium: they keep crossing both ways; only the net movement stops.

Key terms

Plasma membrane: The phospholipid bilayer, studded with proteins, that surrounds every cell and controls what enters and leaves it.
Phospholipid bilayer: A lipid with a hydrophilic phosphate head and two hydrophobic fatty-acid tails. In water, phospholipids form a bilayer, heads out and tails in: the basis of every cell membrane.
Selective permeability: A membrane lets some substances through and not others. Small nonpolar molecules such as O₂ and CO₂ cross the bilayer directly; ions and large polar molecules need transport proteins.
Concentration gradient: A difference in a substance’s concentration between two regions. Diffusion runs down the gradient, from higher to lower concentration; moving a substance up its gradient takes energy.
Diffusion: The net movement of particles from where they are more concentrated to where they are less, down their concentration gradient. It needs no energy from the cell.
Facilitated diffusion: Diffusion through a channel or carrier protein, for ions and polar molecules that cannot cross the bilayer. It still runs down the gradient with no energy input: it is passive.
Osmosis: The diffusion of water across a selectively permeable membrane, toward the side with the higher concentration of solutes that cannot cross.
Tonicity: How a solution would change a cell’s volume. In a hypertonic solution a cell loses water and shrinks; in a hypotonic one it gains water and swells; in an isotonic one it stays the same.
Active transport: Moving a substance against its concentration gradient, from lower to higher concentration, using energy, usually from ATP. Pumps such as the sodium–potassium pump do this.
Membrane pump: A transport protein that uses energy, usually from ATP, to move ions against their gradient. The sodium–potassium pump moves 3 Na⁺ out and 2 K⁺ in for each ATP.
Electrochemical gradient: For an ion, the concentration difference and the voltage across the membrane together. Na⁺ flows into a resting neuron because both push it inward.
Bulk membrane transport: Transport using membrane-bound vesicles: endocytosis brings material into a cell, while exocytosis releases vesicle contents outside through membrane fusion.
Adenosine triphosphate: Adenosine triphosphate, the cell’s energy currency. Splitting off its last phosphate (ATP → ADP + Pᵢ) releases energy that pumps and many other processes use.

Work through an example

A red blood cell’s cytoplasm holds about 0.3 mol of dissolved particles per liter. The cell is placed in (a) pure water, (b) 0.15 M NaCl and (c) 0.5 M NaCl. NaCl splits into two ions, and neither crosses the membrane. Which way does water move in each, and what happens to the cell?

Predict osmosis in three solutions →

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