Active transport requires cellular energy to achieve this movement. There are two types of active transport: primary active transport that uses adenosine triphosphate (ATP), and secondary active transport that uses an electrochemical gradient.

Passive Transport using Membrane Proteins These molecules need special transport proteins to help them move across the membrane, a process known as facilitative diffusion. These special proteins are called channel proteins or carrier proteins (Figure below), and they are attached to the cell membrane.

Carrier Proteins

Transport proteins function in both active and passive transport to move molecules across the plasma membrane. These channel proteins are responsible for bringing in ions and other small molecules into the cell. It’s important to remember that each channel protein can only bring in a specific molecule.

Instead, the cell must bring in more glucose molecules via active transport. In active transport, unlike passive transport, the cell expends energy (for example, in the form of ATP) to move a substance against its concentration gradient.

Facilitated diffusion does not require high energy molecules such as ATP. This type of diffusion uses channel proteins and carrier proteins to transport molecules across the plasma membrane. Facilitated diffusion involves the movement of molecules from areas of higher concentration to areas of lower concentration.

The movement of macromolecules such as proteins or polysaccharides into or out of the cell is called bulk transport. There are two types of bulk transport, exocytosis and endocytosis, and both require the expenditure of energy (ATP).

Secondary active transport is a type of active transport that moves two different molecules across a transport membrane. An example of secondary active transport is the movement of glucose in the proximal convoluted tubule.

There are two classes of membrane transport proteins—carriers and channels. Both form continuous protein pathways across the lipid bilayer. Whereas transport by carriers can be either active or passive, solute flow through channel proteins is always passive.

Aquaporins, also called water channels, are channel proteins from a larger family of major intrinsic proteins that form pores in the membrane of biological cells, mainly facilitating transport of water between cells….