Proteins make up a large part of every membrane and carry out most of its specific jobs. They are classified by their position in the bilayer.
Intrinsic proteins (integral proteins) are embedded in the bilayer, and many span it completely, from one side to the other. Their hydrophobic amino acids lie against the fatty acid tails, and their hydrophilic amino acids face the watery surroundings.Extrinsic proteins (peripheral proteins) sit on the inner or outer surface of the membrane, loosely attached to intrinsic proteins or phospholipid heads. They do not enter the hydrophobic core.Roles of membrane proteins
• Channel proteins form water-filled pores that let specific ions or polar molecules diffuse through (facilitated diffusion). Some are gated, opening only when stimulated.
• Carrier proteins bind a specific molecule and change shape to move it across, either by facilitated diffusion or, using ATP, by active transport.
• Receptors have a specific shape that binds a particular hormone or other signal, for example the insulin receptor.
• Glycoproteins (proteins with carbohydrate chains attached) act as antigens for cell recognition and help cells stick together.
• Enzymes are attached to some membranes, for example ATP synthase on the inner membrane of mitochondria.
• Extrinsic proteins on the inner surface can anchor the membrane to the cytoskeleton.Because each protein has a specific shape, the proteins a cell puts in its membrane decide which substances it can take up and which signals it responds to.
Intrinsic proteins (integral proteins) are embedded in the bilayer, and many span it completely, from one side to the other. Their hydrophobic amino acids lie against the fatty acid tails, and their hydrophilic amino acids face the watery surroundings.Extrinsic proteins (peripheral proteins) sit on the inner or outer surface of the membrane, loosely attached to intrinsic proteins or phospholipid heads. They do not enter the hydrophobic core.Roles of membrane proteins• Channel proteins form water-filled pores that let specific ions or polar molecules diffuse through (facilitated diffusion). Some are gated, opening only when stimulated.
• Carrier proteins bind a specific molecule and change shape to move it across, either by facilitated diffusion or, using ATP, by active transport.
• Receptors have a specific shape that binds a particular hormone or other signal, for example the insulin receptor.
• Glycoproteins (proteins with carbohydrate chains attached) act as antigens for cell recognition and help cells stick together.
• Enzymes are attached to some membranes, for example ATP synthase on the inner membrane of mitochondria.
• Extrinsic proteins on the inner surface can anchor the membrane to the cytoskeleton.Because each protein has a specific shape, the proteins a cell puts in its membrane decide which substances it can take up and which signals it responds to.