BMB 251 1st EditionExam # 5 Study Guide Lectures: 32-38Midterm Review; Exam 5April 22/23 Problem Set Answers - K+ and Cl- ions are found in greater concentrations outside of the cell, while Na+ ions are found in greater concentrations inside of the cell- For every 3 Na+ ions pushed out of the cell, 2 K+ ions come in. this creates a decreasing potentialinside the cell at first, and occurs through ATP phosphorylation and a conformational change of the ion pump.- Equilibrium means that there is an equal charge and concentraiion balance outside and inside of the cell, and so there is no movement of ions/molecules whatsoever (cells cannot survive like this, there has to be some type of movement). Steady state means that there is still a constant input of energy in order to maintain a membrane potential.- An example of the Na+ gradient used as a source of free energy in the plasma membrane: Na+ moves down its concentration gradient (passive transport) and creates free energy, which can then be coupled to the active transportation of glucose up its concentration gradient and into the cell- The way that a K+ ion interacts with its hydration shell is very similar to that of its interaction with carbonyl-oxygens (4 H-bonds). Once it enters the selective filter, it loses the hydration shell and bonds with carbonyl oxygens. Na+ is smaller than K+ and therefore can only bond with 2 outof the 4 carbonyl-oxygens and cannot pass through- Gated channels:o Voltage-gated: change in concentration of ions on either side changes the cell membrane potential and therefore creates voltage- differs in voltage can cause conformational changes of channels to open/close themo Transmission-gated (mechanical): opens/closes through forceo Ligand-gated: ligand can bind inside or outside of the cell to cause an open/closed conformation of the channel- The steady state constant is known as resting potential. When a ligand bonds to ligand-gated channels, it opens and Na+ are brought in through Na+ channels, increasing the cell voltage potential. Then K+ channels open in response so that repolarization can occur, and K+ rushes outof cell to lower the action potential. - The change in action potential opens voltage-gated channels as a signal moves down an axon. This opening/closing of specific gated-channels causes a single direction of movement, and a refractory period- Neurotransmitters are released from synaptic vesicles in presynaptic cell, move through the synaptic cleft and bind to a transmitter-gated ion channels in order to open the channels. This allows neurotransmitters to move into the postsynaptic-targeted cell- The interior of the endoplasmic reticulum is equivalent to the outside of the cell because of its lumen, which allows selective transport through the space without the molecules having to crossover a membrane- The only organelles that are not “membrane-bound” are cytoplasmic ribosomes; all the others (ER, Golgi, ER ribosomes, lysosomes, endosomes, etc.) are membrane-bound - A signal sequence is a peptide that resides on the end of proteins, and let cell organelles know where the proteins are destined to go throughout the cell, as well as determine the conformation, structure and function of the protein.- A nuclear pore is a huge complex-much bigger than that of protein channels or transporters-and so it can allow passive transport of very small solutes (amino acids, nucleotides, etc.) thorough even when it is in the closed conformation. Macromolecules/ larger solutes must be imported/exported through pore via active transport **the biggest molecule taken out of the cell are the ribosomal subunits - Import into the nucleus: Nuclear import receptors bind with proteins that have nuclear localization signal. This moves from the cytosol to the nucleus, where Ran-GTP binds to the receptor in order to break apart receptor from protein. Protein stays within the nucleus while receptor is taken back out via binding to Ran-GTP - Export out of the nucleus: Nuclear export receptor is brought into the cell and combines with Ran-GTP and the protein carrying the nuclear export signal. All three combined are exported from the nucleus into the cytosol. All three dissociate in the cytosol into a phosphate group, Ran-GTP, protein and the lone nuclear export receptorApril 29/30 Problem Set Answers- The rough ER houses ribosomes, which help to finish out protein synthesis (which are co-translationally sent there) and then send the proteins off to their final destinations. The smooth ER does not contain ribosomes, and is the main location of lipid synthesis, and which then helps to insert the lipids into their proper membrane destinations; the smooth ER can also help to detoxify enzymes. Both ERs can act as stores for Ca+ ions. - Co-translational processes take place where a protein is partially synthesized in cytoplasmic ribosomes and then sent to the rough ER to finish synthesizing. Post-translational translocation takes place when proteins are completely synthesized in the cytosol or finish synthesis in the ER, and then are sent to their respective destinations already completely created. Chaperones are responsible in both instances to properly fold the proteins in the lumen - The signal recognition particle (SRP) is a ribonucleoprotein which can bind to a protein being synthesized on a cytoplasmic membrane; the SRP binds to the ER signal sequence on a protein as soon as it leaves the ribosome, and partially stops the synthesis process so that the ribosome/protein complex can be transported to the ER for the remaining synthesis - A start-transfer sequence is a hydrophobic sequence of amino acids along a polypeptide chain that causes the protein to stop translocation through a translocation pore, and therefore results in the protein remaining in the membrane. A single pass (Type I and Type II) will pass through themembrane once, having one hydrophobic signal sequence. A mutipass protein (Type III) will havemultiple hydrophobic regions that get “stuck” in the membrane as well; the very last region is the stop-transfer sequence-allowing for the next hydrophobic region to act as a new start sequence. - Type I protein (N-terminus luminal): single-pass membrane protein, whose positively charged side of the hydrophobic region is on the N-terminal side, causing the N-terminal to remain in the cytosol (and the C-terminal in the lumen). Type II protein (N-terminal cytosolic): single-pass membrane protein, whose