Passive Transport: Diffusion, Osmosis, and Water Potential
Passive transport is the movement of substances across a membrane without the cell using energy, and it includes diffusion, facilitated diffusion and osmosis. In brief: diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down the concentration gradient. Osmosis is the diffusion of water across a partially permeable membrane, from a region of higher water potential to one of lower water potential.
Passive transport moves oxygen into cells, carbon dioxide out and water in and out, and it explains why plant cells swell and why salt can preserve food. This guide explains diffusion and the factors that affect it, introduces water potential, describes osmosis in animal and plant cells, shows how to calculate a percentage change in a potato experiment and corrects common mistakes.
What is diffusion?
Particles in liquids and gases are in constant random motion. Because of this motion, particles spread out from where they are more concentrated to where they are less concentrated until they are evenly distributed. The difference in concentration between two places is the concentration gradient, and diffusion occurs “down” the gradient. No energy from the cell is needed, because the energy comes from the motion of the particles themselves.
In living things, diffusion moves oxygen from the alveoli into the blood, carbon dioxide from cells into the blood, and digested food molecules from the intestine into the villi. It works well over short distances, but it is too slow over long distances.
What affects the rate of diffusion?
- Concentration gradient: the steeper the gradient, the faster the diffusion.
- Temperature: higher temperature gives the particles more kinetic energy, and the rate increases.
- Surface area: a larger area of membrane allows more particles to cross.
- Distance: a thinner exchange surface or a shorter distance means faster diffusion.
- Particle size and type: small and nonpolar molecules cross lipid membranes more easily.
Fick’s law summarizes the first three factors: the rate of diffusion is proportional to the surface area multiplied by the concentration difference, divided by the thickness of the membrane.
What is facilitated diffusion?
Many molecules, including glucose, amino acids and ions, cannot cross the lipid bilayer by simple diffusion. In facilitated diffusion, channel proteins or carrier proteins in the membrane help them to cross. The movement is still passive and goes down the concentration gradient, and no cell energy is used. The channels and carriers are specific, so each type only passes certain particles, and the rate reaches a maximum when all the carriers are in use.
What is osmosis?
Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential. A partially permeable membrane lets water pass but not many of the dissolved particles (solutes). Water moves to the side with more solute, which has a lower water potential, and this dilutes it.
A common simple description says that water moves from a dilute solution to a more concentrated solution. This is correct when the membrane blocks the solute, but using water potential is more accurate and is the standard in advanced courses.
What is water potential?
Water potential, symbol Ψ (psi), is a measure of the tendency of water to move from one place to another. It is measured in kilopascals (kPa). Pure water at standard pressure has the highest water potential, defined as zero. Adding a solute lowers the water potential, so solutions have negative values. The more concentrated the solution, the more negative the value.
Water always moves toward the lower (more negative) water potential. In plants, the water potential is the sum of the solute potential (always negative) and the pressure potential (positive in a swollen cell). In a swollen plant cell, the pressure of the wall pushing back can balance the tendency of water to enter, and net movement stops.
What happens to cells in different solutions?
| Solution outside | Water movement | Animal cell (red blood cell) | Plant cell |
|---|---|---|---|
| Hypotonic (more dilute) | Into the cell | Swells and may burst | Becomes turgid (firm) |
| Isotonic (same) | No net movement | Normal shape | Flaccid (soft) |
| Hypertonic (more concentrated) | Out of the cell | Shrinks, becomes crenated | Plasmolysis (membrane pulls from wall) |
Animal cells have no wall, so they burst in very dilute solutions, which is why intravenous fluids are made isotonic to blood. Plant cells are protected by the cell wall, so they become turgid, and turgor pressure keeps leaves and stems firm. Wilting occurs when cells lose water and become flaccid.
Passive transport compared
| Feature | Simple diffusion | Facilitated diffusion | Osmosis |
|---|---|---|---|
| Substance moved | Small, nonpolar molecules (O2, CO2) | Ions, glucose, amino acids | Water |
| Direction | Down the concentration gradient | Down the concentration gradient | Toward lower water potential |
| Membrane proteins needed | No | Yes (channels or carriers) | Sometimes (aquaporins) |
| Cell energy (ATP) used | No | No | No |
| Rate limited by | Gradient, area, distance | Number of carriers (saturation) | Water potential difference |
Worked example: a potato osmosis experiment
Potato cylinders of equal size are weighed, placed in sucrose solutions of different concentrations for 30 minutes, dried and weighed again. The results are shown below.
| Sucrose (mol/L) | Start mass (g) | End mass (g) | Change (g) | Change (%) |
|---|---|---|---|---|
| 0.0 | 2.00 | 2.30 | +0.30 | +15.0 |
| 0.2 | 2.00 | 2.10 | +0.10 | +5.0 |
| 0.4 | 2.00 | 1.90 | −0.10 | −5.0 |
| 0.6 | 2.00 | 1.70 | −0.30 | −15.0 |
- Percentage change: change ÷ start mass × 100. For 0.0 mol/L: 0.30 ÷ 2.00 × 100 = +15.0 percent. For 0.6 mol/L: −0.30 ÷ 2.00 × 100 = −15.0 percent.
- Interpretation: in pure water, the water potential outside is higher, so water enters the potato by osmosis and the mass increases. In 0.6 mol/L the potato loses water to the stronger solution.
- Isotonic point: the percentage change goes from +5 at 0.2 to −5 at 0.4. A straight line between these points crosses zero at 0.3 mol/L. This is the estimated concentration of the potato cell sap, at which there is no net water movement.
Variables to control are the size and surface area of the cylinders, the volume of solution, the temperature and the time. Blotting the potatoes dry in the same way each time reduces error. Repeating each concentration at least three times would make the estimate more reliable.
How were these processes discovered?
Jean-Antoine Nollet observed osmosis in 1748 when he found that water passed through an animal bladder into alcohol. Thomas Graham studied diffusion of gases and liquids in the nineteenth century and formulated Graham’s law. Adolf Fick published his laws of diffusion in 1855. Wilhelm Pfeffer and Jacobus van ‘t Hoff measured osmotic pressure in the 1880s, and van ‘t Hoff received the first Nobel Prize in Chemistry in 1901.
Which mistakes are common?
Mistake 1: Saying that water moves “to where there is less water”
Use water potential: water moves from higher to lower water potential. Where there is more solute, the water potential is lower.
Mistake 2: Mixing up diffusion and osmosis
Osmosis is the special case of diffusion for water across a partially permeable membrane.
Mistake 3: Thinking that equilibrium means movement stops
At equilibrium, particles still move, but the net movement is zero.
Mistake 4: Saying that facilitated diffusion uses energy
It uses proteins but no cell energy, because particles move down the gradient.