1. Definition
Passive transport refers to the movement of molecules across a cell membrane without the use of energy, driven by concentration gradients.
2. Classification
Passive transport can be classified into two main types: simple diffusion and facilitated diffusion. Each process moves substances in a direction from areas of high concentration to low concentration.
2.1. Simple diffusion
In simple diffusion, certain molecules can pass through the membrane without external assistance. This process is limited to very small, nonpolar, and lipid-soluble molecules (e.g., water or urea) as well as gases (e.g., oxygen or carbon dioxide).
Simple diffusion does not require additional transport proteins and can, therefore, be considered the simplest form of passive transport.
2.2. Facilitated diffusion
In facilitated diffusion, certain molecules traverse the membrane with the assistance of transport or channel proteins. This assistance is necessary because these molecules are larger and polar, preventing them from passing through the membrane unaided. As in simple diffusion, the direction of transport follows the concentration gradient, moving toward the area of lower concentration.
Depending on whether transport proteins (carrier proteins) or channel proteins are involved, facilitated diffusion can be categorized as carrier-mediated or canal-mediated diffusion.
2.2.1 Carrier-mediated facilitated diffusion
This form of facilitated diffusion involves the uptake of a substrate by a carrier protein, which provides a binding site for the substrate. Due to the relatively slow rate of simple diffusion for the substrate, the carrier facilitates its transport across the cell membrane. The transport exhibits saturation kinetics because the availability of carrier molecules is finite. Furthermore, this transport can be subject to competitive inhibition.
2.2.2 Canal-mediated facilitated diffusion
In ion transport, the substrate moves through pores in the cell membrane. The presence of specific binding sites on the channels created by proteins ensures that the transport is selective. Additionally, this process can be affected by electrical and chemical factors.
Channel proteins are typically not permanently open; they generally allow diffusion only upon the binding of a molecule or a change in voltage. Once equilibrium is reached or another signal is transmitted, the channels close.
Pores, or porins, perform similar functions to channels in passive transport. The key distinction is that pores can transport significantly larger molecules across the biomembrane. An example of this is aquaporins, which facilitate the rapid passage of water through the membrane. This process represents a specialized form of diffusion known as osmosis.