Transport Across a Cell Membrane

Transport Across a Cell Membrane Passive Transport & Active Transport Across a Cell Membrane: Passive Transport: molecules and ions going from areas of higher concentration to areas of lower concentration (when water does this it is known as osmosis) Simple Diffusion: molecules move down their gradient through the membrane (must be small and nonpolar) Facilitated Diffusion: molecule need help to move down their concentration gradient (i.e. through an ion channel) Active Transport: molecule and ions move up their concentration gradients and therefore need energy in order to do so Primary Active Transport: transport that directly uses ATP to pump ions against their concentration gradient Secondary Active Transport: moves multiple ions at once using the energy from bringing an ion down its concentration gradient to power the pump of an ion against its concentration gradient (there are two kinds, symporters and antiporters) Na+ is often used as an ion to power the movement of other ions against their concentration gradients (that’s why the sodium potassium pump is so important) How do things move across a cell membrane? Vesicles are simply little plasma membrane cells that are use during exocytosis or endocytosis Passive Transport by Facilitated Diffusion: Facilitated Diffusion: brings ions across a membrane down their concentration gradient through a protein (transmembrane protein). Basically when the ion binds to the protein, it causes a conformational change in the protein allowing it to bring the cell into the cell, and when its empty, it becomes uncomfortable and snaps back to the original conformation Diffusion & Osmosis: Solvent: whatever molecule/substance there is more of (typically is water) Solute: whatever is dissolved in the solvent Solution: the solvent and solute combined Even with gases, gases will still expand to take up as much space as possible Hypertonic Solution: higher solute concentration on the outside of the cell Hypotonic Solution: lower concentration on the outside of the cell Remember, water is able to diffuse through the membrane (since it's a small molecule) Osmosis: water seeping through membranes in order to equalize solute concentrations on either side of the membrane Glomerular Filtration in the Nephron: Nephron: functional unit of filtration and collection in the kidney Afferent arterioles come off of an artery and form a spiral structure known as the glomerulus of the nephron. The glomerulus leaks out fluid and is caught by Bowman’s Capsule Erythrocytes, proteins, white blood cells, etc. won’t leak out of the glomerulus because they are too large. Ions like sodium, amino acids, and glucose can leak out into Bowman’s Capsule Endothelial Cells: line the blood vessel and are fenestrated (have a lot of holes which allow small molecules like the ones listed above to pass through) Basement Membrane: outside of the endothelial cell that makes sure that only small things like ions, amino acids, and sometimes glucose can pass through and nothing else Podocytes and Tubules line the Bowman's Capsule and hug around the arteriole which keeps the connection close Sodium-Potassium Pump: Remember, there is a higher [Na+] on the outside and a higher [K+] on the interior of the cell, and the inside of the cell is less positive than the outside of the cell Sodium-Potassium Pump (Na+/K+ ATPase): pumps 3 Na+ out of the cell and 2 K+ into the cell (against their concentration gradients by using ATP) and this 3:2 ratio between Na+ and K+, there is more positive charge accumulating inside the cell (resting membrane potential of a neuron is about -70 mV) Leak Channels maintain this resting membrane potential stable at around -70 mV Secondary Active Transport in the Nephron: Capillaries surround the nephron allowing for certain ions to be reabsorbed by the body Sodium is being pumped back into the blood and potassium into the space between the basolateral and apical sides of the proximal convoluted tubule by sodium-potassium pumps (primary active transport) Since sodium is being pumped out, there is a low concentration of sodium generated in this region, and allows for sodium to be pumped down its concentration gradient from the lumen into this area and take glucose with it as a symporter (secondary active transport) Same thing occurs in the ascending loop of henle where sodium takes other ions (like potassium and chlorine) back into the region between the basolateral and apical sides In the distal convoluted tubule calcium is reabsorbed into the capillaries by an antiporter (sodium is brought down its concentration gradient and uses its energy known as secondary active transport) Exocytosis: Exocytosis: the removal of contents from a cell by fusion of a vesicle with the plasma membrane Remember, the cytoskeleton provides framework and structure for the cell and these vesicles can ride on motor proteins which use ATP on the cytoskeleton in order to move the vesicles to the extremities of the cell Phagocytosis: Viruses and other objects are too large to go through protein channels/pumps and therefore the need for phagocytosis arises (a process where binds to an item it wants to engulf and draws the item inwards while engulfing it; often occurs when the cell is trying to destroy something). The most common cell receptors for phagocytosis are: Opsonin Receptors: used to bind bacteria or other particles that have been coated with immunoglobulin G (IgG) antibodies by the immune system Scavenger Receptors: bind to molecules that are produced by bacteria and aren’t very specific (most cells produce a matrix of proteins around them called an extracellular matrix) Toll-Like Receptors: bind to specific molecules produced by bacteria and therefore recognize specific bacteria in order to activate an immune response Antibodies: some immune cells make this which bind to specific antigens The steps of Phagocytosis engulfing a virus: 1. The cell and the virus must be in contact with one another 2. The virus binds to cell surface receptors on the macrophage 3. Macrophage starts to surround the virus and engulf the cell (doesn’t bring it through the plasma membrane but instead traps it in its own vesicle (pseudopods protrude from the cell and bind to the surface receptors of the virus) 4. The virus is completely surrounded by cell membrane (phagosome) and is within the cell’s cytoplasm 5. The phagosome fuses with a lysosome that process waste within the cell (forming a phagolysosome) 6. Phagolysosomes lower the pH in order to break down its contents 7. Once neutralized, the phagolysosome forms a residual body that contains the waste products and is eventually discharged from the cell Other things that can be destroy pathogens in the phagolysosome: Oxygen Radicals: during moments of stress, oxygen radical concentration can increase and therefore destroy cell structures Nitric Oxide: reactive substance that reacts with superoxide to create molecules that cause all sorts of biological damage Antimicrobial Proteins: proteins that specifically damage or kill bacteria Antimicrobial Peptides: proteins that attack or kill bacteria Binding Proteins: important players in the innate immune system since they competitively bind to proteins/ions that could have otherwise been beneficial to bacteria or viral replication Some species use phagocytosis as a mechanism to obtain nutrients (such as amoebas, algae, and other single-celled organisms) Membrane Potentials Part 1: K+ typically wants to move out of the cell since its concentration is lower on the outside On the other hand, K+ also wants to move in because of the anions present within the cell Eventually, these two forces equilibrate at around -92 mV (known as the equilibrium potential) and is the voltage where K+ will leave as much as it comes in Membrane Potentials Part 2: Permeability is how the potassium ions leave and concentration gradient is the desire for the potassium ions to leave Membrane potential equation: Vm = 61.5𝑙𝑜𝑔 $ ["!"# ] $] ["%& and can be used for other ions like Na+, Cl-, and Ca2+ (61.5 → 30.75 because calcium has a 2+ charge) Permeability & Membrane Potentials: Remember, K+ typically has more on inside wanting to get out of the cell and Na+, Cl-, and Ca2+ typically has more on the outside wanting to get into the cell In most cells, the potassium ion is the most permeable to the cell If a certain percentage is permeable to a specific ion, multiple that percentage by the membrane potential of that ion and add them all together (shown below) In other words, you could change permeability to certain ions and change the membrane potential substantially