Chapter 3 Lecture Slides Part 2.pptx

Composition of the Plasma Membrane The plasma membrane is primarily made of lipids and proteins with a very small amount of carbohydrates. Glycocalyx: combinations of carbohydrates and lipids (glycolipids) and proteins (glycoproteins) on outer surface....

Composition of the Plasma Membrane The plasma membrane is primarily made of lipids and proteins with a very small amount of carbohydrates. Glycocalyx: combinations of carbohydrates and lipids (glycolipids) and proteins (glycoproteins) on outer surface. Access the text alternative for slide images. 1 3.4 Membrane Lipids Phospholipids and cholesterol predominate. Phospholipids: bilayer. Polar heads facing water in the interior and exterior of the cell (hydrophilic); nonpolar tails facing each other on the interior of the membrane (hydrophobic). Cholesterol: interspersed among phospholipids. Amount determines fluid nature of the membrane, providing stability to the membrane. Fluid nature (fluid-mosaic model) provides/allows: Distribution of molecules within the membrane. Phospholipids automatically reassembled if membrane is damaged. Membranes can fuse with each other. Access the text alternative for slide images. 2 3.5 Membrane Proteins Integral membrane proteins. Extend deeply into membrane, extending from one surface to the other. Can form channels through the membrane. Peripheral membrane proteins. Attached to integral proteins at either the inner or outer surfaces of the lipid bilayer or to polar heads of phospholipids. Functioning of membrane proteins depends on 3-D shape and chemical characteristics. May serve as markers, attachment sites, channels, receptors, enzymes, or carriers. 4 Marker Molecules Glycoproteins or glycolipids. Allow cells to identify one another or other molecules. Distinguish between self-cells and foreign cells. Recognition of oocyte by sperm cell. Intercellular communication. 5 Attachment Proteins Considered integral proteins. Cadherins – attach cells to other cells. Integrins – function in pairs to attach to extracellular molecules. Sometimes allow communication due to contact with intracellular molecules. 6 Transport Proteins Includes channel proteins, carrier proteins, and ATP- powered pumps. 7 Carrier Proteins 1 Also called transporters. Integral proteins move ions from one side of membrane to the other. 1. Specific molecule enters the carrier. 2. Molecule attaches to binding site in carrier. 3. Protein changes shape to transport to the other side. Resumes original shape after transport. 8 Carrier Proteins 2 Carrier proteins come in several forms. Uniporters – moves one ion/molecule. Symporters – move two ions/molecules in the same direction at the same time (cotransport). Antiporters – move two ions/molecules in opposite directions at the same time (countertransport). 9 ATP-Powered Pumps Require the breakdown of A TP. 1. ATP-powered pumps have binding sites for specific ions or molecules and ATP. 2. Hydrolysis of ATP to A DP, releases energy to change the shape of the carrier to move the substance across the membrane. 3. The ion and phosphate are released and the pump resumes its original shape. 10 Transport Proteins: Specificity and Competition Involve carrier proteins or channels in the cell membrane. Characteristics. Specificity for a single type of molecule based on shape. Competition among molecules of similar shape. Saturation: rate of transport limited to number of available carrier proteins. 11 Saturation of a Transport Protein Access the text alternative for slide images. 12 Channel Proteins Channel proteins – integral membrane proteins that form tiny channels through membrane. The channel determines the size, shape and charge of what can move through. Hydrophobic regions face outward toward the hydrophobic part of the plasma membrane. Hydrophilic regions face inward and line the tunnel. The charge of the hydrophilic tunnel determines the types of ions that can move through. 13 Leak and Gated Ion Channels Leak ion channels (non-gated ion channels): always open. Responsible for the permeability of the plasma membrane to ions when the plasma membrane is at rest. Gated ion channels: opened or closed by certain stimuli. Ligand-gated ion channel: open in response to small molecules that bind to proteins or glycoproteins. Voltage-gated ion channel: open when there is a change in charge across the plasma membrane. 14 Leak and Gated Membrane Channels 15