6 Cell Membranes 6 Cell Membranes 6 1

6 Cell Membranes

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. 6 What Are Some Other Functions of Membranes?

6. 1 What Is the Structure of a Biological Membrane? The general structure of membranes is known as the fluid mosaic model. Phospholipids form a bilayer which is like a “lake” in which a variety of proteins “float. ”

Figure 6. 1 The Fluid Mosaic Model

Figure 3. 20 Phospholipids (Part 1)

Figure 3. 20 Phospholipids (Part 2)

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? Membranes may vary in lipid composition. Phospholipids vary in fatty acid chain length, degree of saturation, and phosphate groups. Membranes may be up to 25 percent cholesterol.

6. 1 What Is the Structure of a Biological Membrane? Phospholipid bilayers are flexible, and the interior is fluid, allowing lateral movement of molecules. Fluidity depends on temperature and lipid composition.

6. 1 What Is the Structure of a Biological Membrane? Membranes also contain proteins; the number of proteins varies with cell function.

6. 1 What Is the Structure of a Biological Membrane? Two types of membrane proteins: • 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.

Figure 6. 3 Interactions of Integral Membrane Proteins

6. 1 What Is the Structure of a Biological Membrane? • Freeze-fracturing is a technique that reveals proteins that are 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 in the membrane are independent and only interact noncovalently. But some membrane proteins have fatty acids or other lipid groups covalently attached and are referred to as anchored membrane proteins.

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.

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.

Figure 6. 5 Rapid Diffusion of Membrane Proteins (Part 1)

Figure 6. 5 Rapid Diffusion of Membrane Proteins (Part 2)

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

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.

Figure 6. 6 Cell Recognition and Adhesion

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 (A) Tight junctions help ensure directional movement of materials.

Figure 6. 7 Junctions Link Animal Cells Together (B) Desmosomes are like “spot welds. ”

Figure 6. 7 Junctions Link Animal Cells Together (C) 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.

Figure 6. 8 Integrins Mediate the Attachment of Animal Cells to the Extracellular Matrix

6. 3 What Are the Passive Processes of Membrane Transport? Membranes have selective permeability— some substances can pass through, but not others. Passive transport—no outside energy required (diffusion). Active transport—energy required.

6. 3 What Are the Passive Processes of Membrane Transport? Diffusion: The process of random movement toward equilibrium. Equilibrium—particles continue to move, but there is no net change in distribution.

Figure 6. 9 Diffusion Leads to Uniform Distribution of Solutes

6. 3 What Are the Passive Processes of Membrane Transport? 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. Membrane properties affect the diffusion of solutes. The membrane is permeable to solutes that move easily across it; impermeable to those that can’t.

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. Osmosis depends on the number of solute particles present, not the type of particles.

Figure 6. 10 Osmosis Can Modify the Shapes of Cells (Part 1)

Figure 6. 10 Osmosis Can Modify the Shapes of Cells (Part 2)

Figure 6. 10 Osmosis Can Modify the Shapes of Cells (Part 3)

6. 3 What Are the Passive Processes of Membrane Transport? If two solutions are separated by a membrane that allows water, but not solutes to pass through: Water will diffuse from the region of higher water concentration (lower solute concentration) to the region of lower water concentration (higher solute concentration).

6. 3 What Are the Passive Processes of Membrane Transport? Isotonic solution: Equal solute concentration (and equal water concentration). Hypertonic solution: Higher solute concentration. Hypotonic solution: Lower solute concentration.

6. 3 What Are the Passive Processes of Membrane Transport? Water will diffuse (net movement) from a hypotonic solution across a membrane to a hypertonic solution. Animal cells may burst when placed in a hypotonic solution. Plant cells with rigid cell walls build up internal pressure that keeps more water from entering—turgor pressure.

6. 3 What Are the Passive Processes of Membrane Transport? Facilitated diffusion of polar molecules (passive): • Channel proteins have a central pore lined with polar amino acids. • Carrier proteins—membrane proteins that bind some substances and speed their diffusion through the bilayer.

6. 3 What Are the Passive Processes of Membrane Transport? Ion channels: Specific channel proteins with hydrophilic pores. Most are gated—can be closed or open to ion passage. Gate opens when protein is stimulated to change shape. Stimulus can be a molecule (ligand-gated) or electrical charge resulting from many ions (voltage-gated).

Figure 6. 11 A Gated Channel Protein Opens in Response to a Stimulus

6. 3 What Are the Passive Processes of Membrane Transport? All cells maintain an imbalance of ion concentrations across the plasma membrane; thus a small voltage potential exists. Rate and direction of ion movement through channels depends on the concentration gradient and the distribution of electrical charge.

6. 3 What Are the Passive Processes of Membrane Transport? Membrane potential is a charge imbalance across a membrane. Measured membrane potential of animal cells: – 70 m. V (lots of potential energy)! Membrane potential is related to the concentration imbalance of K+ by the Nernst equation.

6. 3 What Are the Passive Processes of Membrane Transport? Nernst equation:

6. 3 What Are the Passive Processes of Membrane Transport? The potassium channel allows K+ but not Na+ to pass through. K+ passes through in the unhydrated state; hydrated Na+ is too large to pass.

Figure 6. 12 The Potassium Channel

6. 3 What Are the Passive Processes of Membrane Transport? Water can cross a membrane by “hitchhiking” with hydrated ions, or moving through special water channels called aquaporins. The function of these proteins was determined by injecting the aquaporin m. RNA into an oocyte.

Figure 6. 13 Aquaporin Increases Membrane Permeability to Water (Part 1)

Figure 6. 13 Aquaporin Increases Membrane Permeability to Water (Part 2)

6. 3 What Are the Passive Processes of Membrane Transport? In facilitated diffusion, carrier proteins transport polar molecules such as glucose across membranes in both directions. Glucose binds to the protein, which causes it to change shape and release glucose on the other side.

Figure 6. 14 A Carrier Protein Facilitates Diffusion (Part 1)

Figure 6. 14 A Carrier Protein Facilitates Diffusion (Part 2)

6. 4 What Are the Active Processes of Membrane Transport? Active transport: Moves substances against a concentration and/or electrical gradient—requires energy. The energy source is often adenosine triphosphate (ATP).

6. 4 What Are the Active Processes of Membrane Transport? Active transport is directional. It involves three kinds of proteins: • Uniporters • Symporters • Antiporters

Figure 6. 15 Three Types of Proteins for Active Transport

6. 4 What Are the Active Processes of Membrane Transport? Primary active transport: Requires direct hydrolysis of ATP. Secondary active transport: Energy comes from an ion concentration gradient that is established by primary active transport.

6. 4 What Are the Active Processes of Membrane Transport? The sodium–potassium (Na+–K+) pump is primary active transport. Found in all animal cells. The pump is an integral membrane glycoprotein (an antiporter).

Figure 6. 16 Primary Active Transport: The Sodium–Potassium Pump

6. 4 What Are the Active Processes of Membrane Transport? In secondary active transport, energy can be “regained” by letting ions move across a membrane with the concentration gradient. • Aids in uptake of amino acids and sugars. • Uses symporters and antiporters.

Figure 6. 17 Secondary Active Transport

6. 5 How Do Large Molecules Enter and Leave a Cell? Macromolecules (proteins, polysaccharides, nucleic acids) are too large to cross the membrane. They can be taken in or secreted by means of membrane vesicles.

6. 5 How Do Large Molecules Enter and Leave a Cell? Endocytosis: Processes that bring molecules and cells into a eukaryotic cell. The plasma membrane folds in or invaginates around the material, forming a vesicle.

Figure 6. 18 Endocytosis and Exocytosis (A)

6. 5 How Do Large Molecules Enter and Leave a Cell? Phagocytosis: Molecules or entire cells are engulfed. Some protists feed in this way. Some white blood cells engulf foreign substances A food vacuole or phagosome forms, which fuses with a lysosome.

6. 5 How Do Large Molecules Enter and Leave a Cell? Pinocytosis: A vesicle forms to bring small dissolved substances or fluids into a cell. Vesicles are much smaller than in phagocytosis. Pinocytosis is constant in endothelial (capillary) cells.

6. 5 How Do Large Molecules Enter and Leave a Cell? Receptor mediated endocytosis is highly specific: Depends on receptor proteins—integral membrane proteins—to bind to specific substances. Sites are called coated pits—coated with other proteins such as clathrin.

Figure 6. 19 Receptor-Mediated Endocytosis (Part 1)

Figure 6. 19 Receptor-Mediated Endocytosis (Part 2)

6. 5 How Do Large Molecules Enter and Leave a Cell? Mammalian cells take in cholesterol by receptor-mediated endocytosis. In the liver, cholesterol is packaged into low-density lipoprotein, or LDL, and secreted to the bloodstream. Cells that need cholesterol have receptors for the LDLs in clathrin-coated pits.

6. 5 How Do Large Molecules Enter and Leave a Cell? Exocytosis: Material in vesicles is expelled from a cell. Indigestible materials are expelled. Other materials leave cells such as digestive enzymes and neurotransmitters.

Figure 6. 18 Endocytosis and Exocytosis (B)

6. 6 What Are Some Other Functions of Membranes? Some membranes are electrically excitable: • The plasma membrane of neurons conducts nerve impulses. • In muscle cells, electrical excitation results in muscle contraction.

6. 6 What Are Some Other Functions of Membranes? Some membranes transform energy: • Inner mitochondrial membranes— energy from fuel molecules is transformed to ATP. • Thylakoid membranes of chloroplasts transform light energy to chemical bonds.

Figure 6. 20 Other Membrane Functions (Part 1)

6. 6 What Are Some Other Functions of Membranes? Some membrane proteins can organize chemical reactions: • Many cellular processes involve a series of enzyme-catalyzed reactions—all the molecules must come together for these to occur. Forms an “assembly line” of enzymes.

Figure 6. 20 Other Membrane Functions (Part 2)

6. 6 What Are Some Other Functions of Membranes? Some membrane proteins process information: • Binding of a specific ligand can initiate, stop, or modify cell functions.

Figure 6. 20 Other Membrane Functions (Part 3)