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What do these have in common? • • • • • • HIV infection Transplanted organs Communication between neurons Drug addiction Cystic fibrosis hypercholesteremia Membranes organize the chemical activities of cells • selectively permeable • hold teams of enzymes Cytoplasm Figure 5.10 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Plasma membrane • Contact between cell and environment • Keeps useful materials inside and harmful stuff outside • Allows transport, communication in both directions • Anchors energy-converting enzymes Plasma membrane components Phospholipid bilayer Cholesterol Proteins Glycocalyx polar head P – hydrophobic molecules nonpolar tails Phospholipid bilayer hydrophilic molecules cytosol THE PLASMA MEMBRANE phospholipids cholesterol cytoskeleton peripheral protein integral protein Cholesterol blocks some small molecules, adds fluidity • Membrane Proteins – span entire membrane or lie on either side – Purposes • Structural Support • Recognition • Communication • Transport • Glycocalyx – Composed of sugars protruding from lipids and proteins – Functions • Binding sites for proteins • Lubricate cells. • Stick cells down. • Many membrane proteins are enzymes • Some proteins function as receptors for chemical messages from other cells – The binding of a messenger to a receptor may trigger signal transduction Messenger molecule Receptor Activated molecule Figure 5.13 Enzyme activity Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Signal transduction • The plasma membrane of an animal cell Glycoprotein Carbohydrate (of glycoprotein) Fibers of the extracellular matrix Glycolipid Phospholipid Cholesterol Microfilaments of the cytoskeleton Proteins CYTOPLASM Figure 5.12 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Diffusion and Gradients – Diffusion = movement of molecules from region of higher to lower concentration. – Osmosis = diffusion of water across a membrane • In passive transport, substances diffuse through membranes without work by the cell Molecule of dye Membrane EQUILIBRIUM EQUILIBRIUM Figure 5.14A & B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings (a) selectively permeable membrane H2O free water molecule: can fit through pore sugar bound water molecules clustered around sugar: cannot fit through pore pore (b) selectively permeable membrane sugar molecule water molecule pure water bag bursts Osmosis = diffusion of water across a membrane • water travels from an area of higher concentration to an area of lower water concentration Hypotonic solution Hypertonic solution Selectively permeable membrane Solute molecule HYPOTONIC SOLUTION HYPERTONIC SOLUTION Water molecule Selectively permeable membrane Solute molecule with cluster of water molecules NET FLOW OF WATER Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 5.15 Water balance between cells and their surroundings is crucial to organisms osmoregulation = control of water balance • Osmosis causes cells to shrink in a hypertonic solution and swell in a hypotonic solution Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 10 microns isotonic solution equal movement of water into and out of cells hypertonic solution net water movement out of cells hypotonic solution net water movement into cells Passive transport = diffusion across membranes • Small nonpolar molecules - simple diffusion • Many molecules pass through protein pores by facilitated diffusion Solute molecule Transport protein Figure 5.17 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Active transport • transport proteins needed • against a concentration gradient • requires energy (ATP) Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Active transport in two solutes across a membrane FLUID OUTSIDE CELL Transport protein First solute 1 • Na+/K+ pump Phosphorylated transport protein First solute, inside cell, binds to protein 2 ATP transfers phosphate to protein 3 Protein releases solute outside cell 5 Phosphate detaches from protein 6 Protein releases second solute into cell Second solute • Protein shape change 4 Second solute binds to protein Figure 5.18 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Exocytosis and endocytosis transport large molecules exocytosis = vesicle fuses with the membrane and expels its contents FLUID OUTSIDE CELL Figure 5.19A CYTOPLASM Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings b – or the membrane may fold inward, trapping material from the outside (endocytosis) Figure 5.19B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Phagocytosis, “cell eating” —How the human immune system ingests whole bacteria or one-celled creatures eat. phagocytosis food particle 1 2 3 particle enclosed in vesicle pinocytosis 2 vesicle containing extracellular fluid (cytoplasm) extracellular fluid cytosol receptors vesicle captured molecules coated pit vesicle bacterium pseudopodium vesicle Receptor-mediated endocytosis • Cholesterol can accumulate in the blood if membranes lack cholesterol receptors LDL PARTICLE Phospholipid outer layer Receptor protein Protein Cholesterol Plasma membrane Vesicle CYTOPLASM Figure 5.20 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings What do these have in common? • • • • • • HIV infection Transplanted organs Communication between neurons Drug addiction Cystic fibrosis hypercholesteremia
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