<p> Plasma Membrane  Outermost cell component  Maintains cellular integrity  Controls what gets into and out of cell  Gives cell identity: recognize “self”  Membranes found throughout cell  Right now: outermost Plasma Membrane Phospholipid bilayer  Phospholipids have both hydrophilic and hydrophobic regions  Hydrophilic phosphate “head” seeks water  Hydrophobic fatty-acid “tails” avoids water Plasma Membrane Phospholipid bilayer  In water, phospholipids will form bilayers  Two layers of phospholipids  Hydrophobic tails point inward  Hydrophilic heads point outward in both directions  In contact with water on inner (intracellular) and outer (extracellular) surfaces The Plasma Membrane Phospholipid bilayer  Determines which molecules will pass through  Bulk of thickness is hydrophobic  Hydrophobic molecules pass through readily  e.g., Fatty acids, steroid hormones, etc.  Movement of hydrophilic molecules is restricted  e.g., Ions, amino acids, sugars, etc.</p><p>The Plasma Membrane Cholesterol  Nestled between phospholipid molecules  Restricts movement through membrane  Maintains membrane fluidity  Not too rigid in cold temperatures  Not too fluid in high temperatures</p><p>The Plasma Membrane Proteins  Some are embedded within the membrane  Some lie on a surface of the membrane  Various roles  Structural support  Recognition  Communication  Transport  Etc. The Plasma Membrane Proteins  Structural support  Some proteins on the cytoplasmic surface are attached to cytoskeleton  Stabilize the cell  Give animal cells their characteristic shape The Plasma Membrane Proteins  Recognition  Cells in the immune system can discern between self molecules and foreign molecules  This discernment is mediated by cell-surface proteins The Plasma Membrane Proteins  Communication  Cells communicate with one another in various ways  Signals sent to neighboring and distant cells  Various types of signaling molecules  Signals are received through receptor proteins  Each receptor has a binding site that “fits” with a specific signaling molecule  e.g., Insulin receptor, etc. The Plasma Membrane Proteins  Transport  Many materials cannot simply pass through the plasma membrane  Transport proteins move specific molecules across the membrane 2+ + +  e.g., Ca channels, Na /K pump, etc.</p><p>The Plasma Membrane Glycocalyx  Short, branched carbohydrate chains jut from the extracellular face of the membrane  Attached to membrane lipids and proteins  Various functions  Binding sites for signaling molecules  Lubricate cells  Adhesion to adjacent cells</p><p>Diffusion, Gradients, Osmosis  All molecules and ions are in a constant state of random motion  Temperature is a measure of the degree of motion  There is no motion at absolute zero o  -273 C  Movement exists at all temperatures above absolute zero Diffusion, Gradients, Osmosis  Molecules will tend to move from a region of high concentration to a region of lower concentration  Diffusion  Movement of molecules down their concentration gradient  Movement of molecules from an ordered state to a disordered state  Dictated by Laws of Thermodynamics</p><p>Diffusion, Gradients, Osmosis  A membrane can be permeable to some molecules and impermeable to others  Such a membrane is often permeable to water (the solvent), but impermeableimpermeable toto somesome moleculemolecule (the(the solute) dissolved in the water  Molecules able to move across the membrane will move down their concentration gradients Diffusion, Gradients, Osmosis  If a membrane permeable to water but impermeable to salt separates salt water from distilled water, what will happen?  Water will move across the membrane in both directions  More water will move into the salt solution than will move from the salt solution</p><p>Diffusion, Gradients, Osmosis  The plasma membrane is a semipermeable membrane  Somewhat permeable to water and lipids  Impermeable to larger charged substances Osmosis</p><p> Osmosis: movement of water along its concentration gradient across a semi-permeable membrane  Key words:  Water  Semipermeable  Concentration gradient Diffusion, Gradients, Osmosis  Osmosis takes place across the plasma membrane  e.g., Fluid uptake by plants  e.g., Various metabolic processes in animals  e.g., Return of fluid to blood vessels  Etc. Diffusion, Gradients, Osmosis  Two solutions with identical solute and solvent concentrations are termed isotonic  If two solutions have different solute concentrations  The solution with more solute is termed hypertonic  The solution with less solute is termed hypotonic Diffusion, Gradients, Osmosis  What happens to animal and plant cells when placed in an isotonic solution?  Hypertonic?  Hypotonic?</p><p>Moving Small Stuff In and Out  Molecules can move across a plasma membrane in a variety of ways  Directly across the phospholipid bilayer  Via proteins embedded within the bilayer  Without the expenditure of energy  With the expenditure of energy Moving Small Stuff In and Out  Movement of molecules across a membrane without the expenditure of energy is termed passive transport  Movement directly through the bilayer  Movement facilitated by membrane proteins  Movement of molecules across a membrane with the expenditure of energy is termed active transport  “Paid” movement via membrane proteins Moving Small Stuff In and Out Passive Transport: Simple Diffusion</p><p> Molecules such as O2, CO2, and steroid hormones move down their concentration gradients directly across the bilayer  No protein channel is required  No energy expenditure is required</p><p>Moving Small Stuff In and Out Passive Transport: Facilitated Diffusion  Molecules such as sugars, amino acids, and various ions cannot directly traverse the bilayer  These molecules can move down their concentration gradients across the membrane through special protein channels  No energy expenditure is required</p><p>Moving Small Stuff In and Out Active Transport  Molecules can move across membranes against their concentration gradients  Special protein channels are required  Energy expenditure is required</p><p>Moving Big Stuff In and Out  Relatively small molecules can cross membranes in the ways we have discussed  Sometimes it is necessary to move large molecules across membranes  Large molecules cannot be moved by the same mechanisms as small molecules  Channels and pumps are too small  Movement employs vesicles Moving Big Stuff In and Out Movement Out: Exocytosis  Movement of materials out of a cell by fusing a vesicle with the plasma membrane  Vesicle’’s contents released into extracellular fluid  e.g., Vesicles budding from Golgi complex fuse with plasma membrane to export proteins  e.g., Waste products released</p><p>Moving Big Stuff In and Out Movement In: Endocytosis  Movement of large materials into a cell  e.g., Ingestion of an entire bacterial cell  Accomplished by enclosing them within vesicles derived from the plasma membrane  Essentially, exocytosis in reverse  Three forms  Pinocytosis  Receptor-mediate endocytosis  Phagocytosis Moving Big Stuff In and Out Movement In: Endocytosis  Pinocytosis ( “ cell drinking ” )  Cell folds inward  “Harbor” is formed  Membrane fuses, harbor pinches off  Vesicle is formed  Material within harbor brought into cell  Small volume of extracellular fluid  Materials dissolved in fluid Moving Big Stuff In and Out Movement In: Endocytosis  Receptor-mediated endocytosis  Groups of receptors congregate in depression in cell membrane  “Coated pit”  Receptors specific for a particular molecule  e.g., Cholesterol  Pit deepens and pinches off  Vesicle is formed</p><p>Moving Big Stuff In and Out Movement In: Endocytosis  Phagocytosis ( “ cell eating ” )  Can bring even larger materials into the cell  e.g., Amoeba and various white blood cells engulf entire cells  Cell sends out pseudopodia  “False feet”  Pseudopodia surround food, then fuse  Vesicle is formed  Material often digested following fusion of vesicle with lysosome</p><p>Moving Big Stuff In and Out Endosymbiosis  Bacteria ancestral to mitochondria were engulfed by phagocytosis  Chloroplasts arose in the same manner  Bacterium now surrounded by two membranes  Outer membrane of host (eukaryote) origin  Inner membrane of bacterial origin  “Vesicle” maintained rather than digested Moving Big Stuff In and Out  The plasma membrane is constantly being “remade”  Membrane is lost via endocytosis  Membrane is gained via exocytosis Application  Blood and tissue typing  Familial hypercholesteremia  Cystic fibrosis Cystic Fibrosis  Hereditary  Impacts all cells in body  Progressive disability, early death  Difficulty breathing, immune system impairment  Sinus infection, failure to thrive, diarrhea  1 in 3900 kids in US CF  1 in 22 of European descent are carriers  Failure to produce protein found in outer cell membrane  Channel membrane move Cl- into cell  + ions attracted to – ion, don’’t move into cell  Sweat glands, pancreas, lung Cell Video  Inner workings of a cell</p>