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Plasma membrane is the biological membrane which separates the contents of the cell from the outside environment. It is made up of a phospholipid bilayer. Plasma membrane serves as a selectively permeable barrier which only allows certain molecules to pass through the membrane. Different types of proteins are embedded in the plasma membrane as well. Integral and peripheral proteins are two types of such membrane proteins. The main difference between integral and peripheral proteins is that integral proteins are embedded in the whole bilayer whereas peripheral proteins are located on the inner or outer surface of the phospholipid bilayer. Key Areas Covered1. What are Integral Proteins Key Terms: Integral Proteins, Hydrophobic Core, Membrane Proteins, Peripheral Proteins, Phospholipid Bilayer, Plasma Membrane, Transmembrane Protein What are Integral ProteinsIntegral proteins are proteins that are permanently attached to the plasma membrane. They penetrate through the phospholipid bilayer. Therefore, these proteins are attached to the lipid bilayer through hydrophobic, electrostatic or non-covalent interactions. On that account, detergents should be used to remove integral proteins from the lipid bilayer. This will destroy the hydrophobic interactions of the whole lipid bilayer. Transmembrane proteins, which completely penetrate the lipid bilayer, are integral proteins. All transmembrane proteins are integral proteins, but all integral proteins are not transmembrane proteins. This means some integral proteins can partially penetrate the lipid bilayer. Since most integral proteins are transmembrane proteins, they serve as membrane transport proteins such as carrier proteins and channel proteins. Other integral proteins serve as receptors and enzymes. Na+ – K+ ATPase, ion channels and gates, glucose permease, hormone receptors, histocompatibility antigens, certain enzymes of the electron transport chains, and gap junction proteins are types of integral proteins. What are Peripheral ProteinsPeripheral proteins are proteins that are temporarily attached to the plasma membrane. This means they do not penetrate the hydrophobic core of the phospholipid bilayer. Most peripheral proteins are associated with the proteins. Thereby, the removal of peripheral proteins is easier than integral proteins. The removal does not interrupt the hydrophobic interactions of the lipid bilayer as well. The structure of both integral and peripheral proteins are shown in figure 2. Most peripheral proteins serve as receptors, antigens, and enzymes. Cytochrome c in the electron transport chain is an example of a peripheral protein. Similarities Between Integral and Peripheral Proteins
Difference Between Integral and Peripheral ProteinsDefinitionIntegral Proteins: Integral proteins are proteins that are permanently attached to the plasma membrane. Peripheral Proteins: Peripheral proteins are proteins that are temporarily attached to the plasma membrane. Alternative NamesIntegral Proteins: Integral proteins are called intrinsic proteins. Peripheral Proteins: Peripheral proteins are called extrinsic proteins. LocationIntegral Proteins: Integral proteins are embedded in the whole membrane. Peripheral Proteins: Peripheral proteins are located on the inner or outer surface of the phospholipid bilayer. Interaction with the Hydrophobic Core of the Lipid BilayerIntegral Proteins: Integral proteins highly interact with the hydrophobic core of the lipid bilayer. Peripheral Proteins: Peripheral proteins interact less with the hydrophobic core of the lipid bilayer. Types of Interactions with Lipid BilayerIntegral Proteins: Integral proteins bind to the lipid bilayer by hydrophobic, electrostatic or non-covalent interactions. Peripheral Proteins: Peripheral proteins on the inner surface of the lipid bilayer are held by the cytoskeleton. Constituent of the Membrane ProteinIntegral Proteins: Integral proteins constitute 70% of the total membrane proteins. Peripheral Proteins: Peripheral proteins constitute 30% of the total membrane proteins. Hydrophilic/HydrophobicIntegral Proteins: Integral proteins contain both hydrophilic and hydrophobic parts. Peripheral Proteins: peripheral proteins contain hydrophilic parts. FunctionIntegral Proteins: Integral proteins serve as carrier proteins, channel proteins, and enzymes. Peripheral Proteins: Peripheral proteins serve as receptors and surface antigens. Protein RemovalIntegral Proteins: Detergents should be used to remove integral proteins from the plasma membrane. Peripheral Proteins: Dilute salt solutions can be used to remove peripheral proteins from the plasma membrane. ExamplesIntegral Proteins: Glycophorin, rhodopsin, and NADH dehydrogenase are examples of integral proteins. Peripheral Proteins: Mitochondrial cytochrome c and erythrocyte spectrin are examples of peripheral proteins. ConclusionIntegral and peripheral proteins are two types of membrane proteins in the phospholipid bilayer. Integral proteins penetrate the hydrophobic core of the lipid bilayer while peripheral proteins are attached to the intracellular or extracellular surface of the lipid bilayer. Transmembrane proteins are a type of integral proteins. The main difference between integral and peripheral proteins is the penetrance of the hydrophobic core of the lipid bilayer. Reference:1. Lodish, Harvey. “Membrane Proteins.” Molecular Cell Biology. 4th edition., U.S. National Library of Medicine, 1 Jan. 1970, Available here. Image Courtesy:1. “Transmembrane receptor” By Mouagip (talk) (CC BY-SA 3.0) via Commons Wikimedia |