MEMBRANE STRUCTURE AND FUNCTION selective permeability permits some substances to cross it more easily than others Figure 7.1 Scientists studying the plasma Reasoned that it must be a phospholipid bilayer Hydrophilic head Hydrophobic tail Figure 7.2 WATER WATER 1
In 1972, Singer and Nicolson Proposed that proteins are dispersed and individually inserted into the phospholipid bilayer Hydrophobic region of protein Phospholipid bilayer Figure 7.3 Hydrophobic region of protein Freeze-fracture studies of the plasma APPLICATION A cell can be split into its two layers, revealing the ultrastructure of the s interior. TECHNIQUE A cell is frozen and fractured with a knife. Extracellular The fracture plane often follows the hydrophobic interior of a, layer splitting the phospholipid bilayer into two separated layers. The proteins go wholly with one of the layers. Knife Proteins Plasma Cytoplasmic layer RESULTS These SEMs show proteins (the bumps ) in the two layers, demonstrating that proteins are embedded in the phospholipid bilayer. Figure 7.4 Extracellular layer Cytoplasmic layer Phospholipids in the plasma Can move within the bilayer Lateral movement (~10 7 times per second) Flip-flop (~ once per month) (a) Movement of phospholipids Figure 7.5 A 2
Affects the fluidity of the plasma Fluid Viscous Unsaturated hydrocarbon tails with kinks Saturated hydro- Carbon tails (b) Membrane fluidity Figure 7.5 B Can drift within the bilayer EXPERIMENT Researchers labeled the plasma mambrane proteins of a mouse cell and a human cell with two different markers and fused the cells. Using a microscope, they observed the markers on the hybrid cell. RESULTS Membrane proteins + Mouse cell Human cell Hybrid cell Mixed proteins after 1 hour Figure 7.6 CONCLUSION The mixing of the mouse and human proteins indicates that at least some proteins move sideways within the plane of the plasma. Has different effects on fluidity at different temperatures Cholesterol Figure 7.5 (c) Cholesterol within the animal cell Low Temperature- Interferes with packing of lipids High Temperature- Hinders transverse movement (fluidity) 3
Glycoprotein Fibers of Carbohydrate extracellular matrix (ECM) Glycolipid EXTRACELLULAR SIDE OF MEMBRANE ure 7.7 Microfilaments of cytoskeleton Cholesterol Peripheral protein Integral CYTOPLASMIC SIDE protein OF MEMBRANE Trans Proteins Structure of trans protein Functions of Trans Proteins Transport Enzymes Example: Na + K + Pump Example: Tyrosine Kinase Receptor 4
Functions of Trans Proteins Signal Transduction Intercellular Joining Example: Hormone Receptor Example: Tight Junction Functions of Trans Proteins Cell-Cell Recognition Cytoskeleton- ECM Transmission of extracellular stimuli Sidedness of Plasma Membranes 5
tendency for molecules of any substance to spread out evenly into the available space (a) Molecules of dye Membrane (cross section) One solute Net diffusion Net diffusion Equilibrium Figure 7.11 A Substances diffuse down their concentration gradient (b) Two solutes Figure 7.11 B Net diffusion Net diffusion Net diffusion Net diffusion Equilibrium Equilibrium 6
Passive transport is diffusion across a Osmosis is the passive transport of water Cell survival depends on balancing water uptake and loss Osmosis The movement of water from a region of high water concentration to a region of lower water concentration through a semipermeable Lower Solute Concentration (Hypotonic) More H 2 O molecules Higher Solute Concentration (Hypertonic) Fewer H 2 O molecules Hypotonic solution is the solution that loses the water Hypertonic solution is the solution that gains the water Isotonic solutions have the same water concentration 7
(a)! Hypotonic solution Isotonic solution Hypertonic solution Animal Cell H 2 O H 2 O H 2 O H 2 O (b) Figure 7.13 Plant Cell Lysed Normal Shriveled H H 2 O H 2 O H 2 O 2 O Turgid (normal) Flaccid Plasmolyzed Filling vacuole 50 µm " (a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm. Contracting vacuole (b) When full, the vacuole and canals contract, expelling fluid from the cell. # Active transport is the pumping of solutes against their gradients 8
Provide corridors that allow a specific molecule or ion to cross the EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM (a) A channel protein (purple) has a channel through which water molecules or a specific solute can pass. Figure 7.15 Undergo a subtle change in shape that translocates the solute-binding site across the Carrier protein Solute (b) A carrier protein alternates between two conformations, moving a solute across the as the shape of the protein changes. The protein can transport the solute in either direction, with the net Figure 7.15 movement being down the concentration gradient of the solute. $ # Some ion pumps generate voltages across s 9
# Couples the transport of one solute to another % & '( & # # # Passive Transport Active Transport Diffusion Facilitated Diffusion ATP Figure 7.17 10
!)# Bulk transport across the plasma occurs by exocytosis and endocytosis Large proteins Cross the by different mechanisms!)# Exocytosis Transport vesicles migrate to the plasma, fuse with it, and release their contents In endocytosis The cell takes in macromolecules by forming new vesicles from the plasma # PHAGOCYTOSIS EXTRACELLULAR CYTOPLASM FLUID Pseudopodium 1 µm Phagocytosis Pseudopodium of amoeba Food or other particle Bacterium Pinocytosis Food vacuole PINOCYTOSIS Plasma Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM). 0.5 µm Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM). Vesicle Figure 7.20 11
# RECEPTOR-MEDIATED ENDOCYTOSIS Receptor Coat protein Coated vesicle Receptor-mediated endocytosis Ligand Coated pit Coat protein A coated pit and a coated vesicle formed during receptormediated endocytosis (TEMs). Plasma 0.25 µm 12