When the molecules size are large, and they are insoluble in lipids, at this time facilitated diffusion is necessary to transport such molecules across the plasma membrane.
Explanation:
Facilitated diffusion is one type of passive transport. It occurs from higher concentration to lower concentration. This diffusion depends on the size of the particle, the solubility of the membrane. The smaller size particles can move faster across the membrane but the bigger particles can't pass the plasma membrane. Another fact is the entire plasma membrane is made up of lipids. It can't allow the lipid insoluble particles to the membrane. For this, there is a special facility to carry such polar molecules like water, such as channels, carrier proteins, porins. These channels facilitate the bigger polar molecules to enter into the membrane and cross the membrane. The enzymes like permease act as channel proteins that bind with the bigger particles and carry them through the channels. This type of diffusion also needs ATP like passive diffusion.
In biology, diffusion is the movement of substances with the concentration gradient, which means they move from an area of high concentration to an area of low concentration. Facilitated diffusion is diffusion that is performed with the help of a carrier protein that can move particles of the substance.
