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Explore the fluid mosaic model, lipid composition, protein interactions, and dynamic nature of biological membranes. Learn about membrane proteins and their functions in cell adhesion and recognition.
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6 Cell Membranes • 6.1 What Is the Structure of a Biological Membrane? • 6.2 How Is the Plasma Membrane Involved in Cell Adhesion and Recognition? • 6.3 What Are the Passive Processes of Membrane Transport? • 6.4 What Are the Active Processes of Membrane Transport? • 6.5 How Do Large Molecules Enter and Leave a Cell?
6 Cell Membranes The cell membrane regulates what enters and leaves the cytoplasm. Some cell membranes have pores called aquaporins that allow water to pass freely. Opening Question: Water purity is a worldwide problem. Can aquaporin membrane channels be used in water purification?
6.1 What Is the Structure of a Biological Membrane? • The general structure of biological membranes is known as the fluid mosaic model. • Phospholipids form a bilayer, which is like a “lake” in which a variety of proteins “float.”
6.1 What Is the Structure of a Biological Membrane? • Phospholipids have a polar, hydrophilic “head” and hydrophobic fatty acid “tails.” • In an aqueous environment, phospholipids form a bilayer.
6.1 What Is the Structure of a Biological Membrane? • Artificial bilayers can be made in the laboratory. • Lipids maintain a bilayer organization spontaneously. This helps membranes fuse during phagocytosis, vesicle formation, etc.
6.1 What Is the Structure of a Biological Membrane? • Lipid composition of membranes vary. • Phospholipids vary in fatty acid chain length, degree of saturation, and phosphate groups.
6.1 What Is the Structure of a Biological Membrane? • Animal cell membranes may be up to 25% cholesterol, which is important for membrane integrity.
6.1 What Is the Structure of a Biological Membrane? • The fatty acid tails make the interior somewhat fluid, allowing lateral movement of molecules. • Fluidity depends on temperature and lipid composition.
6.1 What Is the Structure of a Biological Membrane? • Cholesterol and long-chain, saturated fatty acids pack tightly, making a less-fluid membrane. • As temperature decreases, movement of molecules and cellular processes slow. Some organisms change the lipid content of the cell membranes when they get cold.
6.1 What Is the Structure of a Biological Membrane? • Membranes also contain proteins; the number varies depending on membrane function. • Peripheral membrane proteins lack exposed hydrophobic groups and do not penetrate the bilayer.
6.1 What Is the Structure of a Biological Membrane? • Integral membrane proteins have hydrophobic and hydrophilic regions or domains. • Some extend across the lipid bilayer; others are partially embedded.
6.1 What Is the Structure of a Biological Membrane? • Freeze-fracturing is a technique that reveals proteins embedded in the phospholipid bilayers of cellular membranes.
Figure 6.4 Membrane Proteins Revealed by the Freeze-Fracture Technique
6.1 What Is the Structure of a Biological Membrane? • The proteins and lipids interact noncovalently. • But some membrane proteins have lipid groups covalently attached and are tethered to the lipid bilayer.
6.1 What Is the Structure of a Biological Membrane? • Transmembrane proteins extend all the way through the phospholipid bilayer. • They have one or more transmembrane domains, and the domains on the inner and outer sides of the membrane can have specific functions. • Peripheral membrane proteins are located on one side of the membrane.
6.1 What Is the Structure of a Biological Membrane? • Some membrane proteins can move freely within the bilayer, while some are anchored to a specific region. • When cells are fused experimentally, some proteins from each cell distribute themselves uniformly around the membrane.
6.1 What Is the Structure of a Biological Membrane? • Membranes are dynamic and are constantly forming, transforming, fusing, and breaking down.
6.1 What Is the Structure of a Biological Membrane? • Membranes also have carbohydrates on the outer surface that serve as recognition sites for other cells and molecules. • Glycolipids—carbohydrate + lipid • Glycoproteins—carbohydrate + protein
Working with Data • A key experiment providing evidence for the fluid mosaic model used the technique of cell fusion to show that membrane proteins rapidly diffuse within the cell membrane.
Working with Data 6.1: Rapid Diffusion of Membrane Proteins • Question 1: • Plot the percentage of fully mixed cells over time. • How long did it take for complete mixing?
Working with Data 6.1: Rapid Diffusion of Membrane Proteins • Question 2: • What does your answer to Question 1 indicate about the rate of diffusion of the mouse and human proteins?
6.2 How Is the Plasma Membrane Involved In Cell Adhesion and Recognition? • Cells arrange themselves in groups by cell recognition and cell adhesion. • These processes can be studied in sponge cells—the cells are easily separated and will come back together again.
6.2 How Is the Plasma Membrane Involved In Cell Adhesion and Recognition? • Molecules involved in cell recognition and binding are glycoproteins. • Binding of cells is usually homotypic: The same molecule sticks out from both cells and forms a bond. • Some binding is heterotypic: The cells have different proteins.
6.2 How Is the Plasma Membrane Involved In Cell Adhesion and Recognition? • Cell junctions are specialized structures that hold cells together: • • Tight junctions • • Desmosomes • • Gap junctions
Figure 6.7 Junctions Link Animal Cells Together (Part 1) • Tight junctions help ensure directional movement of materials.
Figure 6.7 Junctions Link Animal Cells Together (Part 2) • Desmosomes are like “spot welds.”
Figure 6.7 Junctions Link Animal Cells Together (Part 3) • Gap junctions allow communication.
6.2 How Is the Plasma Membrane Involved In Cell Adhesion and Recognition? • Cell membranes also adhere to the extracellular matrix. • The transmembrane protein integrin binds to the matrix outside epithelial cells, and to actin filaments inside the cells. • The binding is noncovalent and reversible.
6.2 How Is the Plasma Membrane Involved In Cell Adhesion and Recognition? • Cells can move within a tissue by the binding and reattaching of integrin to the extracellular matrix. • This is important for cell movement within developing embryos and for the spread of cancer cells.
6.3 What Are the Passive Processes of Membrane Transport? • Membranes have selective permeability—some substances can pass through, but not others. • Passivetransport—no outside energy required (diffusion). • Activetransport—energy required.
6.3 What Are the Passive Processes of Membrane Transport? • Energy for passive transport comes from the concentration gradient: the difference in concentration between one side of the membrane and the other.
6.3 What Are the Passive Processes of Membrane Transport? • Particles in a solution move randomly until they are evenly distributed. • At equilibrium, the particles continue to move, but there is no net change in distribution.
6.3 What Are the Passive Processes of Membrane Transport? • Diffusion: the process of random movement toward equilibrium. • Net movement is directional until equilibrium is reached. • Diffusion is the net movement from regions of greater concentration to regions of lesser concentration.
6.3 What Are the Passive Processes of Membrane Transport? • Diffusion rate depends on: • Diameter of the molecules or ions • Temperature of the solution • Concentration gradient
6.3 What Are the Passive Processes of Membrane Transport? • Diffusion works very well over short distances (e.g., within a cell). • Membrane properties affect the diffusion of solutes. • A membrane is permeable to solutes that move easily across it; impermeable to those that cannot.
6.3 What Are the Passive Processes of Membrane Transport? • Simple diffusion: Small molecules pass through the lipid bilayer. • Water and lipid-soluble molecules can diffuse across the membrane. • Electrically charged and polar molecules can not pass through easily.
6.3 What Are the Passive Processes of Membrane Transport? • Osmosis: the diffusion of water. • It depends on the relative concentrations of water molecules on each side of the membrane. • Hypertonic: higher solute concentration • Isotonic: equal solute concentrations • Hypotonic: lower solute concentration
