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 Can anything or everything move in or out of the cell? No. It is the semipermeable plasma membrane that determines what can enter and leave the cell. So, if not everything can cross the membrane, how do certain things get across? Membrane ProteinsThe plasma membrane contains molecules other than phospholipids, primarily other lipids and proteins. The green molecules in Figure below, for example, are the lipid cholesterol. Molecules of cholesterol help the plasma membrane keep its shape. Many of the proteins in the plasma membrane assist other substances in crossing the membrane. The plasma membranes also contain certain types of proteins. A membrane protein is a protein molecule that is attached to, or associated with, the membrane of a cell or an organelle. Membrane proteins can be put into two groups based on how the protein is associated with the membrane. Integral membrane proteins are permanently embedded within the plasma membrane. They have a range of important functions. Such functions include channeling or transporting molecules across the membrane. Other integral proteins act as cell receptors. Integral membrane proteins can be classified according to their relationship with the bilayer:
Some integral membrane proteins are responsible for cell adhesion (sticking of a cell to another cell or surface). On the outside of cell membranes and attached to some of the proteins are carbohydrate chains that act as labels that identify the cell type. Shown in Figure below are two different types of membrane proteins and associated molecules. Peripheral membrane proteins are proteins that are only temporarily associated with the membrane. They can be easily removed, which allows them to be involved in cell signaling. Peripheral proteins can also be attached to integral membrane proteins, or they can stick into a small portion of the lipid bilayer by themselves. Peripheral membrane proteins are often associated with ion channels and transmembrane receptors. Most peripheral membrane proteins are hydrophilic. Some of the membrane proteins make up a major transport system that moves molecules and ions through the polar phospholipid bilayer. The Fluid Mosaic ModelIn 1972 S.J. Singer and G.L. Nicolson proposed the now widely accepted Fluid Mosaic Modelof the structure of cell membranes. The model proposes that integral membrane proteins are embedded in the phospholipid bilayer, as seen in Figure above. Some of these proteins extend all the way through the bilayer, and some only partially across it. These membrane proteins act as transport proteins and receptors proteins. Their model also proposed that the membrane behaves like a fluid, rather than a solid. The proteins and lipids of the membrane move around the membrane, much like buoys in water. Such movement causes a constant change in the "mosaic pattern" of the plasma membrane. A further description of the Fluid Mosaic Model can be viewed athttp://www.youtube.com/watch?v=Qqsf_UJcfBc (1:27). Extensions of the Plasma MembraneThe plasma membrane may have extensions, such as whip-like flagella or brush-like cilia. In single-celled organisms, like those shown in Figure below, the membrane extensions may help the organisms move. In multicellular organisms, the extensions have other functions. For example, the cilia on human lung cells sweep foreign particles and mucus toward the mouth and nose. Flagella and Cilia. Cilia and flagella are extensions of the plasma membrane of many cells. Summary
Explore MoreUse these resources to answer the questions that follow. Explore More I
Explore More II
Review
This page titled 2.6: Membrane Proteins is shared under a CK-12 license and was authored, remixed, and/or curated by CK-12 Foundation via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request. What are 4 functions of proteins in the cell membrane?Membrane proteins serve a range of important functions that helps cells to communicate, maintain their shape, carry out changes triggered by chemical messengers, and transport and share material.
What are five major functions of proteins in the plasma membrane?While membrane lipids form the basic structure of the lipid bilayer, the active functions of the membrane are dependent on the proteins. Cell adhesion, energy transduction, signaling, cell recognition and transport are just some of the important biological processes carried out by membrane proteins.
What are the 3 functions of membrane proteins?Membrane proteins mediate processes that are fundamental for the flourishing of biological cells. Membrane-embedded transporters move ions and larger solutes across membranes, receptors mediate communication between the cell and its environment and membrane-embedded enzymes catalyze chemical reactions.
What are the functions and types of plasma membrane proteins?Plasma membrane proteins that are exposed on the cell surface have important biological functions, such as signaling into and out of the cells, ion transport, and cell-cell and cell-matrix interactions.
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Function of protein in the plasma membrane
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