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Transcript
Cell Membrane: a Phospholipid Bilayer Membrane Structure and Function Phospholipid Chapter 5 Hydrophilic “Head” Hydrophobic “Tail” Lipid Bilayer Fluid Mosaic Model • Mixture of saturated and unsaturated fatty acid tails create fluidity. • Proteins in the membrane constantly shift and flow within the fluid membrane. Cell Membrane • Isolates the cell’s contents from the external environment. • Regulates exchange of substance across the membrane. • Communicates with other cells. • Creates attachments within and between cells. • Regulates many biochemical reactions. Cholesterol Benefits Membrane Structure • Makes the bilayer – – – – – – Stronger More flexible Less fluid at high temperatures Less solid at low temperatures Slows phospholipid movement Less permeable to water-soluble substances Isolate Cell Contents • The cell is surrounded by an external aqueous environment. • The cell’s interior, the cytosol, is composed mostly of water. • Spontaneous arrangement of the lipid bilayer: – Hydrophilic heads outside (interacting with water) – Hydrophobic tails inside (interacting with themselves) • Most substances contacting the cell membrane are hydrophilic and cannot penetrate the hydrophobic interior of the membrane. Protein Mosaic within the Membrane • Five major types of proteins: – Receptor proteins – Recognition proteins – Enzymatic proteins – Attachment proteins – Transport proteins Receptor Proteins • Have a binding site, or receptor, for a specific molecule. • Binding to the receptor activates the protein. • Activation leads to a conformational change in the protein, triggering a sequence of reactions. Recognition Proteins • Glycoprotein, a membrane protein with a carbohydrate group attached to the extracellular side. • The carbohydrate acts as an identification tag for the cell. • Examples: Enzymatic Proteins • Proteins attached to the inner surface of membranes • Promote chemical reactions – Synthesize new molecules – Break apart biological molecules – Immune system and bacteria – Red blood cells Reactant Product Reactant Enzyme Enzyme Enzyme Enzymatic Proteins Example: Dehydration Synthesis Attachment Proteins • Anchor the cell membrane by – Binding to the cytoskeleton – Binding to external structures – Binding to other cells Enzyme An enzyme is a protein that catalyzes the formation of a bond between two molecules. Enzyme Transport Proteins • Channel proteins: form channels to allow specific ions or molecules to pass through the membrane. • Carrier proteins: bind substrates to move them through the membrane. • Movement through these proteins occurs by both active and passive transport Movement across the Membrane Responds to Gradients • Concentration: the number of molecules of a substance in a given volume of fluid. • Gradient: physical difference between two different regions of space. – Temperature, concentration, pressure, etc. – Cells use energy and their cell membranes to generate concentration gradients. Diffusion • Random movement of molecules from regions of high to low concentration. • Slow process. • The greater the concentration gradient, the faster the rate of diffusion. Transport Across the Membrane • The cell uses concentration gradients across the cell membrane to move molecules. • Two types of movement: – Passive Transport (no energy required) • Simple Diffusion • Facilitated Diffusion • Osmosis – Active Transport (energy required) Simple Diffusion • No energy needed • Small molecules take advantage of the selective permeability of the membrane. • O2, CO2, H2O • Lipid-soluble molecules Facilitated Diffusion • Small molecules and ions diffuse across the membrane with the help of channel and carrier proteins. – Channel proteins create hydrophilic channels for ions. – Carrier proteins have receptors to recognize certain small molecules Aquaporin in the Membrane Aquaporins • Water channels within the membrane. • Respond to osmotic stress. •Open when cells are in a hypotonic environment. •Close when cells are in a hypertonic environment. Osmosis • Diffusion of water across the selectively permeable membrane from high to low concentration. water Osmosis in Plants • Hypertonic: more solute than solvent outside the membrane. • Water flows in by osmosis to fill the central vacuole. • A full central vacuole exerts turgor pressure, pushing the cytosol against the cell wall. • Isotonic: equal concentrations of solute and solvent across the membrane. – Provides support and rigidity for non-woody plants. • Plants deprived of water shrink the central vacuole. • Hypotonic: more solvent than solute outside the membrane. – The lack of turgor pressure causes the plant to wilt - plasmolysis. Active Transport Transport Across the Membrane • Using a gated channel, cells use up energy to move molecules against the concentration gradient. • Passive transport – Down the concentration gradient – No energy required • Active transport – Against the concentration gradient, from low to high concentration. – Energy required! energy Pinocytosis Endocytosis • “Cell drinking” • The plasma membrane “dimples” inward and pinches off into the cytosol. • Creates a vesicle filled with extracellular fluid. • Acquiring extracellular fluid or particles by engulfing in a membrane sac. •Three types: –Pinocytosis –Receptor-mediated endocytosis –Phagocytosis Receptor-mediated Endocytosis • Selectively concentrates certain molecules within a vesicle. • Receptors concentrate on the extracellular membrane in coated pits. • Binding of the targeted molecule causes the pit to deepen and pinch off. Phagocytosis • “Cell Eating” • Cells extend part of their membrane, creating pseudopods, to engulf large food particles, prey, and pathogens. Exocytosis • “Out of the Cell” • Disposal of unwanted materials through vesicle fusion with the extracellular membrane. Cell Communication • Desmosomes: intermediate filaments link the plasma membranes of adjacent cells. Removal of: • Products of digestion • Hormones • Toxins Cell Communication Cell Communication • Tight Junctions: attachment proteins link cells to make them watertight. • Gap Junctions: channel proteins link cells to allow transfer of chemical signals. Cell Communication • Plasmodesmata: "holes" in the cell walls of plants to allow transfer of water and nutrients. Homework Thinking Through the Concepts, Review Question #2 Applying the Concepts #3