Passive transport allows cells to move substances across their membranes without expending metabolic energy, such as adenosine triphosphate (ATP). This movement relies on the inherent kinetic energy of molecules. The driving force behind all passive transport is the concentration gradient, which is the difference in the concentration of a substance between two regions. Molecules naturally move from an area of high concentration to an area of low concentration, a movement often described as moving “down the gradient.” This downhill flow is energetically favorable and therefore requires no cellular energy input. The three primary mechanisms of passive transport are simple diffusion, facilitated diffusion, and osmosis.
Simple Diffusion Across the Membrane
Simple diffusion is the most straightforward type of passive transport, where molecules pass directly through the phospholipid bilayer of the cell membrane without any assistance. This direct passage is only possible for molecules that are small, nonpolar, or lipid-soluble enough to dissolve into the hydrophobic core of the membrane.
The rate of transport is determined by the steepness of the concentration gradient and the molecule’s ability to dissolve in lipids. Small, uncharged gases, such as oxygen (O2) and carbon dioxide (CO2), are classic examples of substances that cross membranes this way. This mechanism is important in gas exchange, such as in the lungs, where oxygen moves from the high concentration in the alveoli into the blood, while carbon dioxide moves out.
Facilitated Diffusion: Using Transport Proteins
Facilitated diffusion is the passive movement of molecules that are too large or too polar to cross the lipid bilayer on their own, even though a favorable concentration gradient exists. These molecules, such as glucose, ions like sodium and potassium, or amino acids, require the assistance of specific transmembrane proteins to shield them from the membrane’s hydrophobic interior. Two main classes of transport proteins mediate this process: channel proteins and carrier proteins.
Channel Proteins
Channel proteins form open, hydrophilic pores that extend through the membrane, acting like water-filled tunnels. They allow for the rapid passage of specific ions or small polar molecules across the membrane. These channels are highly selective, often allowing only one type of ion, such as sodium, to pass through.
Many channels can be regulated by being “gated” to open or close in response to specific cellular signals. The speed of transport through channels is exceptionally fast, allowing millions of ions to cross per second, which is necessary for processes like nerve signaling.
Carrier Proteins
Carrier proteins operate differently by physically binding to the molecule they transport. Once a specific substance, like glucose, binds to the carrier protein on one side of the membrane, the protein undergoes a subtle change in its three-dimensional shape.
This conformational change exposes the binding site to the opposite side of the membrane, releasing the molecule down its concentration gradient. While slower than channel proteins, carrier proteins are highly specific and are responsible for transporting essential nutrients like sugars and amino acids into the cell.
Osmosis: The Movement of Water
Osmosis is a specific type of passive transport defined as the net movement of water across a selectively permeable membrane. Water moves from a region of higher water concentration (which corresponds to a lower solute concentration) to a region of lower water concentration (a higher solute concentration). Although water is small enough to pass slowly through the lipid bilayer, its movement is greatly accelerated by specialized channel proteins called aquaporins.
The movement of water is driven by differences in tonicity, which describes how an external solution affects cell volume and shape. In a hypertonic solution, the external environment has a higher solute concentration than the cell’s interior, causing water to flow out and the cell to shrink, a process known as crenation in red blood cells.
Conversely, a hypotonic solution has a lower solute concentration, causing water to rush into the cell, potentially leading to swelling and bursting (cytolysis) if the cell lacks a protective wall. An isotonic solution has the same solute concentration as the cell, resulting in no net water movement, which is the ideal state for cells. Aquaporin channels allow for the rapid adjustment of water content, maintaining water balance.