Transport Across Cell Membranes

Selective permeability determines what enters and leaves

Cells constantly exchange substances with their surroundings, but they cannot allow everything to move at the same rate or in either direction. The plasma membrane combines a lipid barrier with specific transport proteins. Whether a molecule crosses depends on its size, charge, lipid solubility, the presence of transport proteins and the gradients across the membrane. A gradient is a difference in a quantity such as concentration or electrical potential across space. Gradients allow transport to be directional without a continuous outside push.

Some uncharged, relatively lipid-soluble molecules cross directly through the phospholipid layer. In simple diffusion, net movement occurs from higher concentration to lower concentration because random molecular motion creates a larger average flux from the side with more molecules. Diffusion does not require the cell to spend ATP to drive the movement down the gradient. At equilibrium molecules continue moving in both directions, but the net flux can become zero.

Facilitated diffusion provides a selective route

Many polar solutes and ions cross the membrane with help from a protein channel or carrier. The transport protein provides a pathway compatible with the molecule's chemical properties. Facilitated diffusion still moves substances down their relevant gradient and does not directly use ATP to force them uphill. Channels provide passages for selected ions or water, whereas carriers bind a substrate and undergo conformational changes to expose it to the other side.

A protein can therefore speed transport without making it active. The important distinction is whether movement is down or against the relevant gradient and whether energy is coupled to the process. In the case of ions, electrical charge influences movement as well as concentration; electrochemical gradients give the more complete driving force. Introductory examples often focus on concentrations because they are easier to visualise.

Osmosis is about water

Water moves across selectively permeable membranes in response to differences in water potential. A useful classroom approximation is to say that water tends to move from a more dilute solution towards a more concentrated solution when solutes cannot cross freely. The underlying mechanism reflects the chemical potential of water, not an ability of water to sense solute particles. Water can pass through the lipid bilayer and, in many cells, through aquaporin channels.

If a plant cell is placed in a solution where water tends to enter, the expanding vacuole presses the membrane against the wall and produces turgor pressure. In a sufficiently concentrated outside solution, water may leave and the living contents can shrink away from the wall, called plasmolysis. These changes show that walls provide mechanical support while the membrane remains the selective boundary. Animal cells lack a rigid wall and respond differently to strongly hypotonic or hypertonic surroundings.

Active transport maintains gradients

When a cell must accumulate a solute at a higher concentration than outside, it usually needs an energy-coupled mechanism. In primary active transport, a membrane protein directly uses energy from ATP hydrolysis to move a selected ion or molecule against its electrochemical gradient. The sodium–potassium pump of animal cells typically exports three Na⁺ ions and imports two K⁺ ions for each ATP hydrolysed during one transport cycle, helping maintain ionic gradients important for many cell activities.

Not all uphill transporters consume ATP themselves; some use energy stored in the downhill movement of another substance, a process called secondary active transport. This distinction helps reconcile the general statement that active transport requires energy with the observation that individual proteins can be powered in different ways. The driving energy is never created by the transport process; it comes from ATP or another pre-existing gradient.

Solve transport questions systematically

Start by identifying the substance and whether the membrane permits its passage. Next determine concentrations and, for ions, the membrane potential. Then decide whether a transport protein is present and whether ATP or another source of energy is coupled. Only after those steps should you label the process. Remember that a nonpolar solute diffusing through lipid and glucose crossing via a carrier can both be passive even though they follow different physical routes.

Quick Quiz

Model Answers

1. Simple diffusion traverses the lipid barrier down a gradient; facilitated diffusion uses a channel/carrier down a gradient; active transport moves against an electrochemical gradient with an energy source.

2. The solute's existing concentration or electrochemical gradient provides the driving force. The protein lowers the barrier but does not need to force net uphill movement.

3. Water tends to move from higher to lower water potential across a selective membrane. In a dilute environment, plant vacuoles take up water and turgor can rise.

4. During a typical cycle, one ATP drives 3 Na⁺ out and 2 K⁺ in, maintaining concentration and electrical gradients.

5. Facilitated diffusion is likely if a specific channel or carrier provides the route down the concentration gradient.