What are the different types of passive transport?

There are four types of passive transport: simple diffusion, facilitated diffusion, osmosis, and filtration. Simple diffusion is a form of diffusion that doesn’t require the help of membrane proteins. There is a movement of substances in cells from a region of higher concentration to a lower concentration. Simple diffusion is a crucial type of passive transport, as it is required for different life processes. Facilitated diffusion requires carrier and channel proteins. Facilitated diffusion is defined as the passive transport of molecules or ions across the cell membrane through specific transmembrane proteins. The process does not require any cellular or external energy. Some examples of facilitated diffusion are glucose transporter, aquaporins, and ion channels. Facilitated diffusion is particularly important for molecules that are large and insoluble, as the cell membrane is permeable to only smaller and nonpolar molecules. 

In filtration, solids are separated from liquids and gasses. For example, there is the specific absorption of nutrients in the body. No energy is required and the process moves along the concentration gradient. The cell membrane only allows for substances that are soluble and can easily pass through its pores. Kidneys are the organ responsible for filtering toxins from the blood. Osmosis is the process when water molecules move from low solute concentration (high water concentration) to a high solute (low water concentration) across a membrane which is not permeable to that solute. Osmosis is affected by the concentration gradient and temperature. The greater the concentration gradient and temperature, the faster the rate of osmosis.