Functions of membrane proteins Some integral proteins form ion channels pores or holes that specific ions such as potassium can flow through to get into or out of the cell Other integral proteins act as carriers transporters selectively moving a polar substance or ion from one side of the membrane to the other Integral proteins called receptors serve as cellular recognition sites o Each receptor recognizes and binds a specific type of molecule o A specific molecule that binds to a receptor is called a ligand Some integral proteins are enzymes that catalyze specific chemical reactions at the inside or outside surface of the cell Integral proteins also serve as linkers that anchor proteins in the plasma membranes of neighboring cells to one another or to protein filaments inside and outside the cell Membrane glycoproteins and glycolipids often serve as cell identity markers o May enable cell to recognize other cells of the same kind during tissue formation o May recognize and respond to potentially dangerous foreign cells Transport Passive transport o Ex simple diffusion Active transport o Substance moves downhill its concentration electrical gradient to cross membrane using only its own kinetic energy no energy o ATP is used to drive substance uphill against its concentration gradient Vesicles spherical membrane sacs for transport requires energy o Endocytosis o Exocytosis Materials move into a cell in a vesicle formed from the plasma membrane Materials move out of a cell by the fusion with the plasma membrane of vesicles formed inside the cell Secretory cells and nerve cells o Pinocytosis drinking taken up A form of endocytosis in which tiny droplets of extracellular fluid are Plasma membrane folds inward and forms a vesicle containing a droplet of extracellular fluid The vesicle pinches off from the plasma membrane and enters the cytosol Vesicle fuses with lysosome where enzymes degrade the engulfed solutes o Phagocytosis eating as bacteria A form of endocytosis in which the cell engulfs large solid particles such Begins when particle binds to a plasma membrane receptor causing it to extend pseudopods projections of its plasma membrane and cytoplasm Pseudopods surround the particle outside the cell and membranes fuse to form a vesicle called a phagosome which enters the cytoplasm Phagosome fuses with one or more lysosomes and enzymes break down ingested material Diffusion Simple diffusion gases Facilitated diffusion o Passive process where substances move freely through the lipid bilayer of the plasma membranes of cells without the help of membrane transport proteins Ex nonpolar hydrophobic molecules oxygen carbon dioxide nitrogen o Passive process where integral protein assists a specific substance across the membrane when solutes are too polar or highly charged Channel mediated facilitated diffusion When a solute moves down its concentration gradient across the lipid bilayer through a membrane channel potassium ions through ion channel Carrier mediated facilitated diffusion When a carrier moves a solute down its concentration gradient across the plasma membrane Solute binds more often to the carrier on the side of the membrane with a higher concentration of solute Ex glucose fructose Osmosis o When there is a net movement of a solvent through a selectively permeable membrane passive process Moving from areas of higher water concentration to areas of lower water concentration concentration Moving from areas of lower solute concentration to areas of higher solute o Water molecules pass through the plasma membrane either by moving between neighboring phospholipid molecules in the lipid bilayer via simple diffusion or by moving through aquaporins integral membrane proteins that function as water channels o Hydrostatic pressure o Osmotic pressure Pressure exerted by a liquid that forces water molecules to move back and forth when equilibrium is reached The force exerted by the solution with the impermeable solute The higher the solute concentration the higher the osmotic pressure o A measure of the solutions ability to change the volume of cells by altering their Tonicity water content Isotonic solution Red blood cell maintaining normal shape and volume Hypotonic solution A solution that has a lower concentration of solutes than the cytosol inside the red blood cells Water enters the cells faster than they leave causing it to burst hemolysis of red blood cells Hypertonic solution A solution that has a higher concentration of solutes than does the cytosol inside red blood cells Water moves outside the cells faster than they enter causing it to shrink crenation Active processes Active transport a concentration gradient Primary active transport o Requires energy for carrier proteins to move solutes across the membrane against o Energy derived from hydrolysis of ATP changes the shape of a carrier protein which pumps a substance across a plasma membrane against its concentration gradient o Sodium potassium pump potassium ions in Most prevalent mechanism that expels sodium ions from cells and brings Pump maintains a low concentration of Na in the cytosol by pumping these ions into the extracellular fluid against the Na concentration gradient while also moving K into cells against the K concentration gradient Works nonstop to maintain a low concentration of Na and a high concentration of K in the cytosol Maintains normal tonicity on each side of the plasma membrane Secondary active transport o Indirectly uses energy obtained from hydrolysis of ATP o The energy stored in a Na or H concentration gradient is used to drive other substances across the membrane against their concentration gradient o Secondary active transport proteins harness the energy in the Na concentration gradient by providing routes for Na to leak into cells A carrier protein simultaneously binds to Na and another substance and then changed its shape so that both substances cross the membrane at the same time If these transporters move 2 substances in the same direction they are called symporters Whereas antiporters move 2 substances in opposite directions across the membrane Protein synthesis Transcription messenger RNA copies genetic info from DNA o Takes place in the nucleus where the genetic code is transcribed into mRNA Cytosine pairs with Guanine Adenine pairs with Thymine o mRNA leaves the nucleus via nuclear pore and travels to cytoplasm o mRNA then binds to ribosome via rRNA the code on the mRNA is translated
View Full Document
Unlocking...