Membrane Structure and Function Chapter 5 Membrane Models Fluid-Mosaic Outline Plasma Membrane Structure and Function Protein Functions Plasma Membrane Permeability! Diffusion! Osmosis! Transport Via Carrier Proteins Cell Surface Modifications 1 2 Fluid-Mosaic Model Membrane Models Membrane is a fluid phospholipid bilayer in which protein molecules are either partially or wholly embedded. Fig. 5.1 3 4
Fluid-Mosaic Model Plasma Membrane Structure and Function Plasma membrane separates internal environment from the external environment. Hydrophilic polar heads face outside, and hydrophobic nonpolar tails face each other. 5 6 Plasma Membrane Structure and Function Proteins may be peripheral or integral. Peripheral proteins are found on the inner membrane surface. Integral proteins are embedded in the membrane. Fig. 5.3 7 8
Protein Functions Channel Proteins - Involved in passage of molecules through membrane. Carrier Proteins - Combine with substance to aid in passage through membrane. Cell Recognition Proteins - Help body recognize foreign substances. 9 10 Protein Functions Receptor Proteins - Allow molecule binding, causing protein to change shape and bring about cellular change. Enzymatic Proteins - Carry out metabolic reactions directly. Plasma Membrane Permeability Plasma membrane is differentially permeable. Passive Transport - No ATP requirement.! Molecules follow concentration gradient. Active Transport - Requires carrier protein and ATP. 11 12
Crossing Plasma Membrane Diffusion Diffusion - Movement of molecules from a higher to a lower concentration until equilibrium is reached. Down concentration gradient A solution contains a solute (solid) and a solvent (liquid). 13 14 Diffusion 15 16
Osmosis Osmosis Osmosis - Diffusion of water across a differentially (selectively) permeable membrane due to concentration differences. Osmotic pressure is the pressure that develops due to osmosis.! The greater the osmotic pressure, the more likely water will diffuse in that direction. 17 18 Osmosis Isotonic Solution - Solute and water concentrations both inside and outside the membrane are equal. Hypotonic Solution - Solution with a lower concentration of solute than the solution on the other side of the membrane. Cells placed in a hypotonic solution will swell.! Lysis 19 20
Osmosis Hypertonic Solution - Solution with a higher concentration of solute than the solution on the other side of the membrane. Cells placed in a hypertonic solution will shrink.! Plasmolysis Transport by Carrier Proteins Carrier proteins combine with a certain molecules which are then transported through the membrane. Facilitated Transport! Small molecules follow concentration gradient by combining with carrier proteins. 21 22 Transport by Carrier Proteins Active Transport! Small molecules move against their concentration gradient by combining with carrier proteins. 23 24
Membrane-Assisted Transport Large marcomolecules are transported into or out of the cell by vesicle formation. Exocytosis - Vesicles fuse with plasma membrane as secretion occurs. Membrane-Assisted Transport Endocytosis - Cells take in substances by vesicle formation.! Phagocytosis - Large, solid material.! Pinocytosis - Liquid or small, solid particles.! Receptor-Mediated - Specific form of pinocytosis using a coated pit. 25 26 Cell Surface Modifications Cell Surfaces in Animals Junctions Between Cells! Adhesion Junctions " Intercellular filaments between cells.! Tight Junctions " Form impermeable barriers.! Gap Junctions " Plasma membrane channels are joined (allows communication). 27 28
Cell Surface Modifications Extracellular Matrix! Meshwork of polysaccharides and proteins in close association with the cell that produced them. Plant Cell Walls Plants have freely permeable cell wall, with cellulose as the main component.! Joined by plasmodesmata that spans cell wall and contains strands of cytoplasm that allow passage of material between cells. 29 30 Membrane Models Fluid-Mosaic Review Plasma Membrane Structure and Function Protein Functions Plasma Membrane Permeability! Diffusion! Osmosis! Transport Via Carrier Proteins Cell Surface Modifications 31