Cell And Molecular Bio. Ch 11/12
|What is a lipid bilayer?||Thin bimolecular sheet of mainly phospholipid molecules that gorms the structural basis for all cell membranes. The two layers of lipid molecules are packed with their hydrophobic tails pointing inward and their hydrophillic heads outward, exposed to the water.|
|How do molecules move across a lipid bilayer?||by transport proteins and by the permeability characteristics of the lipid bilayer itself. |
-small nonpolar molecules rapidly move across.
-uncharged polar molecules also diffuse rapidly
-ions and charged molecules, lipid bilayers membrane is impermeable
two types of transport proteins:
-channels: discriminate mainly on size and electric charge.
-transporter: allows passage only to those molecules or ions that fit into a binding site on the protein; then transfers these molecules across the membrane one at a time by changing its own conformation.
|How do carrier proteins function?|| bind a solute on one side of the membrane and deliver it to the other side by a conformational change.|
-contain alpha helices that traverse the membrane multiple times creating a pore.
|How do channel proteins function?||form tiny hydrophilic pores in the membranes through which solutes (ions) can pass by diffusion.|
|What is a gradient or electrochemical gradient?|| -driving force that causes an ion to move across a membrane.|
-caused by differences in ion concentration and in electrical charge on either side of the membrane.
|What is osmotic pressure?||Pressure that must be exerted on the low-solute concentration side of a semipermeable membrane to prevent the flow of water across the membrane as a result of osmosis.|
|What is active transport verses passive transport?|| -passive transport: molecules move spontaneously down the concentration gradient.|
-active transport: movement of a molecule across a membrane driven by ATP hydrolysis or another form of metabolic energy.
|What are examples of active transporters?||-coupled transporters: couple "uphill" transport of one molecule to "downhill" transport of another molecule.|
-ATP driven pump: couple "uphill" transport of a molecule to the hydrolysis of ATP.
-Light driven pump: couple "uphill" transport of a molecule to the input of light energy.
examples: animal cells use the energy of ATP hydrolysis to pump out Na+; the Na+ K+ pump is driven by the transient addition of a phosphate group.
|What are the steps in the Na+ K+ pump?||1. Na+ binds to the pump on inside of cell activating ATPase activity of the pump. |
2. Pump hydrolyzes ATP into transferring the phosphate to the pump.
3. Phosphorylation of pump causes a conformational change releasing NA+ to the outside of the cell.
4. K+ on outside of the cell binds to the exposed binding site.
5. Dephosphoylation of pump causes conformational change releasing K+ into the cell.
|What are the types of coupled transporters?|| uniport: only one solute transported.|
symport: 2 solutes transferred in the same direction.
antiport: 2 solutes transferred in opposite directions.
|How does the Ca2+ pump function?|| -can act as a signal, so it is pumped out of the cell (ATP-driven) to make the cell more sensitive in intracellular changes in Ca2+.|
-Function similar to Na+ K+ pump, but it returns to its original conformation without binding and transporting a second ion.
|How does the H+ (proton) pump function?|| -plants, bacteria, and fungi do not have Na+ K+ pumps, so they use H+ pumps. |
-pumping H+ out of the cell sets up a proton gradient making the inside of the cell neutral.
-Lysosomes and vacuoles also utilize H+ pumps to keep the inside of the organelle acidic.
|What is a gated ion channel?||-means that a specific stimulus triggers them to switch between a closed and an open state by a change in their conformation.|
|What is a membrane potential?||-voltage difference across a membrane due to a slight excess of positive ions on one side and of negative ions on the other. A typical membrane potential for an animal cell plasma membrane is -6 mV (inside negative), measured relative to the surrounding fluid.|
|What is patch clamp recording?||-technique in which the tip of a small glass electrode is sealed onto a patch of cell membrane, thereby making it possible to record the flow of current through individual ion channels in the patch.|
|How is an action potential made and propagated?||-a nerve impulse.|
-neurons solve this long-distance communication problem by employing an active signaling mechanism: a local electrical stimulus of sufficient strength triggers an explosion of electrical activity in the plasma membrane that is propagated rapidly along the membrane of the axon and sustained by automatic renewal all along the way.
|What is depolarization?||-a shift in the membrane potential to a less negative value. (a shift towards zero).|
|What are the membrane potentials (mV) at the different stages in an action potential?||-resting potential is -60 mV, and must be raised to -40 mV to trigger the channel to open. Once the ions flow in, the membrane potential can raise to +40 mV|
|How can electrical signals be converted into chemical signals?||??|
|How do voltage gated channels work?||1. Na+ channels open transferring Na+ into the cell and resulting in depolarization of the membrane.|
2. Within milliseconds, the channel will adopt an inactive state in which the channel is closed and cannot open. This inactive state prevents the impulse from moving backward.
3. The channel then adopts the closed state when the membrane potential returns to its resting state.
|How do ligand-gated channels work?|| -a molecule binds to the channel causing a conformational change to open the channel.|
-extracellular ligand=ligand binds to the gated channel on the outside of the cell.
-intracellular ligand=ligand binds to the gated channel on the inside of the cell.
|How do mechanical (stress-activated) channels work?|| -channels open due to a mechanical force applied.|
- examples include auditory hair cells, the mimosa leaf closing, and the venus fly trap.