Secondary transporters:
describe transport of glucose into intestinal epithelial cells
The cell membrane or the outermost layer of the cell allows the movement of solutes across it. But it is selectively permeable, due to a complex phospholipid bilayer structure. The plasma membrane is selectively permeable; hydrophobic molecules and small polar molecules can diffuse through the lipid layer, but ions and large polar molecules cannot. Integral membrane proteins enable ions and large polar molecules to pass through the membrane by passive or active transport.
The permeability of a membrane is dependent on the organization and characteristics of the membrane lipids and proteins. In this way, cell membranes help maintain a state of homeostasis within cells (and tissues, organs, and organ systems) so that an organism can stay alive and healthy.
The transport across the membrane can either be passive transport (down the concentration gradient, therefore without the input of energy) and the second type of transport is the active transport that occurs against the concentration gradient from a site of lower concentration to a site of a higher concentration, therefore it requires energy in the form of ATP.
A membrane transport protein is a membrane protein involved in the movement of ions, small molecules, and macromolecules, such as another protein, across a biological membrane.
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