Passive transport is a type of cellular transport in which substances move across a cell membrane without the input of energy. This allows ions and molecules to move down their concentration gradient. In other words, the movement is from an area of higher concentration to an area of lower concentration.
There are three types of passive transport that occur through the plasma membrane – simple diffusion, facilitated diffusion, and filtration.
Simple Diffusion
Simple diffusion refers to the movement of molecules across the plasma membrane from higher to lower concentrations. It is the simplest form of passive transport. It does not require any chemical energy nor does it require the assistance of transport proteins. This form of passive transport is critical to maintaining equilibrium.
Facilitated Diffusion
Facilitated diffusion is similar to simple diffusion in that the movement of molecules is from higher to lower concentrations across the plasma membrane. In this form of passive transport, chemical energy is not required. However, it does involve the assistance of transport proteins to move substances across the plasma membrane through a channel.
The semi-permeable plasma membrane is responsible for the selective movement of substances. Facilitated diffusion is important as it controls what goes in and what goes out of the cell.
Filtration
Filtration refers to the process of segregating suspended particles from a fluid by passing it through a porous filtering device that allows the fluid to pass through while holding back the solid particles.
In this form of passive transport, the selectively permeable plasma membrane acts as the filter. It allows only fluids and substances that are smaller than the size of the membrane pores to pass through while retaining the larger sized particles as a residue over the filter membrane.