Cell Membrane: a Phospholipid Bilayer. Membrane Structure and Function. Fluid Mosaic Model. Chapter 5

Transcription

1 Membrane Structure and Function Chapter 5 Cell Membrane: a Phospholipid Bilayer Phospholipid 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.

2 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. 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. 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 Protein Mosaic within the Membrane Five major types of proteins: Receptor proteins Recognition proteins Enzymatic proteins Attachment proteins Transport proteins

3 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: Immune system and bacteria Red blood cells Enzymatic Proteins Proteins attached to the inner surface of membranes Promote chemical reactions Synthesize new molecules Break apart biological molecules Enzyme Reactant Enzyme Reactant Enzyme Product

4 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.

5 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) No energy needed Simple Diffusion Small molecules take advantage of the selective permeability of the membrane. O 2, CO 2, H 2 O Lipid-soluble molecules

6 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 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. Aquaporin in the Membrane Osmosis Diffusion of water across the selectively permeable membrane from high to low concentration. water

7 Hypertonic: more solute than solvent outside the membrane. Isotonic: equal concentrations of solute and solvent across the membrane. Hypotonic: more solvent than solute outside the membrane. Osmosis in Plants Water flows in by osmosis to fill the central vacuole. A full central vacuole exerts turgor pressure, pushing the cytosol against the cell wall. Provides support and rigidity for non-woody plants. Plants deprived of water shrink the central vacuole. The lack of turgor pressure causes the plant to wilt - plasmolysis. Transport Across the Membrane Passive transport Down the concentration gradient No energy required Active transport Against the concentration gradient, from low to high concentration. Energy required! energy Active Transport Using a gated channel, cells use up energy to move molecules against the concentration gradient.

8 Endocytosis Acquiring extracellular fluid or particles by engulfing in a membrane sac. Three types: Pinocytosis Receptor-mediated endocytosis Phagocytosis Pinocytosis Cell drinking The plasma membrane dimples inward and pinches off into the cytosol. Creates a vesicle filled with extracellular fluid. 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.

9 Out of the Cell Exocytosis 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 Tight Junctions: attachment proteins link cells to make them watertight. Cell Communication Gap Junctions: channel proteins link cells to allow transfer of chemical signals.

10 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