Passive transport is a fundamental process governing how substances move across the cell’s plasma membrane. This cellular movement occurs without the direct expenditure of metabolic energy, such as adenosine triphosphate (ATP). Passive transport relies entirely on the natural kinetic energy of molecules. The driving force is the concentration gradient, which is the difference in the concentration of a substance between two regions. Molecules move “down” this gradient, shifting from an area of high concentration to an area of low concentration until dynamic equilibrium is achieved.
The Core Mechanism: Simple Diffusion
The most straightforward form of passive transport is simple diffusion, where molecules pass directly through the phospholipid bilayer of the cell membrane. This mechanism does not require the assistance of any membrane-spanning proteins. Only specific types of molecules can utilize this direct route because the membrane’s core is hydrophobic, or water-repelling.
Substances that are small, non-polar, or lipid-soluble readily dissolve in the lipid bilayer and diffuse across it. Prime examples include gases that cells constantly exchange, such as oxygen and carbon dioxide. Both follow their respective gradients: oxygen moves in where concentration is low, and carbon dioxide moves out where concentration is high, continuing until equilibrium is reached.
Assisted Passage: Facilitated Diffusion
For molecules that cannot cross the hydrophobic core of the membrane, such as large, polar molecules or charged ions, facilitated diffusion provides an assisted pathway. Although it requires the help of specific transmembrane proteins, it remains a passive process because the substances still move down their existing concentration gradient. These protein helpers create a shield, allowing polar or charged compounds to bypass the fatty interior of the membrane.
Carrier Proteins
One type of helper is the carrier protein, which functions by physically binding to the specific molecule it transports, such as glucose. Upon binding, the protein undergoes a conformational change, a shift in its three-dimensional shape, that exposes the molecule to the opposite side of the membrane. This shape change mechanism is generally slower, moving molecules at a rate of approximately a thousand per second.
Channel Proteins
The second type of helper is the channel protein, which forms a selective, water-filled pore or tunnel through the membrane. These channels are highly specific, often allowing only one type of ion, such as sodium or potassium, to pass through. Channel proteins, including specialized water channels called aquaporins, are significantly faster than carriers because they do not need to change shape with every molecule they transport. Some channels are “gated,” meaning they can open or close in response to a signal, allowing the cell to regulate the flow of ions.
What Controls the Speed of Diffusion
Several physical factors determine the rate at which molecules are transported across a membrane through diffusion. The primary factor is the steepness of the concentration gradient; a larger difference in concentration between the two sides results in a faster rate of movement. As the system approaches equilibrium, the rate of net diffusion naturally slows down.
Temperature also plays a role because higher temperatures increase the kinetic energy of the molecules. This causes them to move faster and collide more frequently with the membrane.
The physical characteristics of the membrane influence the rate. A larger surface area provides more space for molecules to cross, while a thicker membrane increases the distance the substance must travel, slowing the process. Lastly, the size and mass of the diffusing molecule are inversely related to speed, meaning smaller molecules diffuse more rapidly than heavier ones.