N Controls What Gets Into and out of Cell
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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.
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
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.
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
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
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
Diffusion, Gradients, Osmosis The plasma membrane is a semipermeable membrane Somewhat permeable to water and lipids Impermeable to larger charged substances Osmosis
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?
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
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
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
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
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
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
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
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