Membrane proteins determine functions of cell membranes, including serving as pumps, gates, receptors, cell adhesion molecules, energy transducers, and enzymes. Peripheral membrane proteins are associated with the surfaces of membranes while integral membrane proteins are embedded in the membrane and may pass through the lipid bilayer one or more times.
Carbohydrates covalently linked to proteins glycoproteins or lipids glycolipids are also a part of cell membranes, and function as adhesion and address loci for cells. The Fluid Mosaic Model describes membranes as a fluid lipid bilayer with floating proteins and carbohydrates. Cell junctions are a special set of proteins that anchor cells together desmosomes , occlude water passing between cells tight junctions , and allow cell to cell direct communication gap junctions.
Because they contain phosphoric acid, they are also classified as phospholipids. Sphingomyelins are important constituents of the myelin sheath surrounding the axon of a nerve cell. Multiple sclerosis is one of several diseases resulting from damage to the myelin sheath. Cerebrosides are composed of sphingosine, a fatty acid, and galactose or glucose. They therefore resemble sphingomyelins but have a sugar unit in place of the choline phosphate group.
Cerebrosides are important constituents of the membranes of nerve and brain cells. Because of considerable variation in their sugar components, about varieties of gangliosides have been identified. Most cell-to-cell recognition and communication processes e. Gangliosides are most prevalent in the outer membranes of nerve cells, although they also occur in smaller quantities in the outer membranes of most other cells.
Because cerebrosides and gangliosides contain sugar groups, they are also classified as glycolipids. However, certain charged and polar species do cross the membrane, aided by proteins that move about in the lipid bilayer. Peripheral proteins may be attached to integral proteins, to the polar head groups of phospholipids, or to both by hydrogen bonding and electrostatic forces.
Small ions and molecules soluble in water enter and leave the cell by way of channels through the integral proteins. Some proteins, called carrier proteins , facilitate the passage of certain molecules, such as hormones and neurotransmitters, by specific interactions between the protein and the molecule being transported. Lipids are important components of biological membranes.
These lipids have dual characteristics: part of the molecule is hydrophilic, and part of the molecule is hydrophobic.
Proteins are another important component of biological membranes. Integral proteins span the lipid bilayer, while peripheral proteins are more loosely associated with the surface of the membrane. Why is it important that membrane lipids have dual character—part of the molecule is hydrophilic and part of the molecule is hydrophobic?
Why do you suppose lecithins phosphatidylcholines are often added to processed foods such as hot cocoa mix? The dual character is critical for the formation of the lipid bilayer. The hydrophilic portions of the molecule are in contact with the aqueous environment of the cell, while the hydrophobic portion of the lipids is in the interior of the bilayer and provides a barrier to the passive diffusion of most molecules.
Like transport proteins, receptor proteins are specific and selective for the molecules they bind Figure 4. Figure 4: Examples of the action of transmembrane proteins Transporters carry a molecule such as glucose from one side of the plasma membrane to the other.
Receptors can bind an extracellular molecule triangle , and this activates an intracellular process. Enzymes in the membrane can do the same thing they do in the cytoplasm of a cell: transform a molecule into another form. Anchor proteins can physically link intracellular structures with extracellular structures.
Figure Detail. Peripheral membrane proteins are associated with the membrane but are not inserted into the bilayer. Rather, they are usually bound to other proteins in the membrane. Some peripheral proteins form a filamentous network just under the membrane that provides attachment sites for transmembrane proteins. Other peripheral proteins are secreted by the cell and form an extracellular matrix that functions in cell recognition.
In contrast to prokaryotes, eukaryotic cells have not only a plasma membrane that encases the entire cell, but also intracellular membranes that surround various organelles. In such cells, the plasma membrane is part of an extensive endomembrane system that includes the endoplasmic reticulum ER , the nuclear membrane, the Golgi apparatus , and lysosomes. Membrane components are exchanged throughout the endomembrane system in an organized fashion. For instance, the membranes of the ER and the Golgi apparatus have different compositions, and the proteins that are found in these membranes contain sorting signals, which are like molecular zip codes that specify their final destination.
Mitochondria and chloroplasts are also surrounded by membranes, but they have unusual membrane structures — specifically, each of these organelles has two surrounding membranes instead of just one. The outer membrane of mitochondria and chloroplasts has pores that allow small molecules to pass easily.
The inner membrane is loaded with the proteins that make up the electron transport chain and help generate energy for the cell. The double membrane enclosures of mitochondria and chloroplasts are similar to certain modern-day prokaryotes and are thought to reflect these organelles' evolutionary origins. This page appears in the following eBook. Aa Aa Aa. Cell Membranes.
Figure 1: The lipid bilayer and the structure and composition of a glycerophospholipid molecule. A The plasma membrane of a cell is a bilayer of glycerophospholipid molecules. Figure 2: The glycerophospholipid bilayer with embedded transmembrane proteins. What Do Membranes Do? Figure 3: Selective transport. Specialized proteins in the cell membrane regulate the concentration of specific molecules inside the cell. Figure 4: Examples of the action of transmembrane proteins. Transporters carry a molecule such as glucose from one side of the plasma membrane to the other.
How Diverse Are Cell Membranes? Membranes are made of lipids and proteins, and they serve a variety of barrier functions for cells and intracellular organelles. Membranes keep the outside "out" and the inside "in," allowing only certain molecules to cross and relaying messages via a chain of molecular events. Cell Biology for Seminars, Unit 3. Topic rooms within Cell Biology Close.
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