15. Which molecule isleastlikely to cross a cellularmembrane by simple diffusion?a. waterb. oxygenc. nitrogend.potassium ione. carbon dioxide Show 16. Which explanation best describes the asymmetricallyoriented structure of the proteins in the cell membrane? Get answer to your question and much more d. These proteins pass through the ERmembrane into the ER lumen.e. Enzymes are needed to modify thecarbohydrate chains on these proteins. 17. What does an ABC transporter use to transport largerions and molecules across the cell membrane? Get answer to your question and much more 18. What facilitate therapidtransport of water throughthe plasma membrane?a. osmosisb. uniportersc.aquaporins PCB3023C MOLECULAR & CELL BIOLOGY NAME: ____KEY_____________Exam 2; March 06, 2003
Part 1: Multiple choice. Answer on a scantron form. Each question is worth two points. 1. The “fluid” and “mosaic” terms in the fluid mosaic model of membrane structure refer to the ___ and ___, respectively.
PART 2: Answer in the space provided. Points are in ( ). 1. (10 points) List and describe three types of membrane transport proteins. EXTRA CREDIT: Provide a specific example of each type (1 point each). Carrier proteins - exist in two conformations, altered by high affinity binding of the transported molecule. Moves material in either direction, down concentration gradient (facilitated diffusion). EXAMPLE: GluT1 erythrocyte glucose transporter. Channel proteins - primarily for ion transport. Form an aqueous pore through the lipid bilayer. May be gated. Moves material in either direction, down concentration gradient (facilitated diffusion). EXAMPLES: Voltage-gated sodium channel, erytrhocyte bicarbonate exchange protein. Active transporters - use energy (direct, ATPase; or indirect, ion gradient) to drive molecules across the membrane against a concentration gradient. EXAMPLES: Na+/K+ ATPase, Na+/glucose transporter. 2. (10 points) Describe how a resting membrane potential is established and maintained. The Na+/K+ ATPase pump moves K+ions into the cell and Na+ ions out of the cell to establish strong chemical gradients for each. The cell still maintains near electrical neutrality (K+ balanced inside by large anions, Na+ balanced outside by Cl-). Leaky K+ channels allow some K+ ions to flow out of cell, down chemical concentration gradient. This creates an electrical potential, as positive charges are leaving the cell. This electrical gradient favors movement of K+ back into the cell, setting up an electrochemical equilibrium for K+, typically at about -60 mV.
Closed channels have an internal, voltage sensitive gate that is closed. Na+ ions are prevented from entering the cell by the closed gate. This state exists during the resting membrane potential. The channel is poised to respond to a signal. Open channels have responded to a change in membrane potential by opening the internal gate. This is a protein conformational change in response to electrical changes. Na+ ions rapidly enter the cell, leading to depolarization and potentially to an action potential. Channels are inactivated in response to an action potential. A protein domain blocks the exit to the channel, preventing the flow of Na+ ions.This allows the cell to restore the resting potential, and allows directional travel of the action potential by preventing another signal from occurring too soon.
Primary messengers/signal - bind receptors to intiate a cellular response pathway. Tyrosine kinase receptors - plasma membrane receptors that transmit an external signal to the cell interior by autophosphorylation. G-protein coupled receptors - ligand binding activates intracellular G proteins to trigger a pathway. G proteins - activated by ligand-bound receptors. Trimeric, inactive receptors are induced to uncouple into alpha and beta-gamma subunits, as a result of the alpha subunit exchanging GDP for GTP. SH domain proteins - bind to activated tyrosine kinase receptors to continue a pathway. adenylyl cyclase/phosholipase C - examples of G protein targets that synthesize second messengers. Second messengers - small molecules synthesized in response to a signal. Rapidly spread throughout a cell. Includes cAMP, Ca2+, IP3, DAG, NO, etc. Ser/Thr kinases (MAP kinases) - a cascade of protein activation that amplifies signals and leads to cellular changes. Transcription factors - activated by upstream events to alter cellular gene expression. 5. (15 points) List and describe five different types of molecules that function as part of the extracellular matrix. Collagen - forms high-strength fibers of the ECM to provide a strong support network. Elastin - covalent linkages and stretchable structures provide an elastic component to ECM that undergoes extensive expansion/contraction. Fibronectin - binds to many other cell surface and ECM components, and links with the cytoskeleton to help model cell shape and participate in cell movement. Proteoglycans - composed of polysaccharides and proteins. Carbohydrate components are often acidic and sulfated to attract and retain water. Provides a soluble matrix for other ECM materials, and provides cushioning and lubrication function. Integrins - integral membrane receptors
that bind to components of the ECM. Laminin - an ECM component of the basal lamina, providing a support structure for epithelial cells. Lectins, selectins, N-CAMs, Cadherins, etc. - provide mechanisms for cell-cell interactions. What molecules is least likely to cross a cellular membrane by simple diffusion?Large polar or ionic molecules, which are hydrophilic, cannot easily cross the phospholipid bilayer. Charged atoms or molecules of any size cannot cross the cell membrane via simple diffusion as the charges are repelled by the hydrophobic tails in the interior of the phospholipid bilayer.
Which substance does not pass across the membrane by simple diffusion?This means that ions like sodium, potassium, calcium, and chloride cannot cross membranes to any significant degree by simple diffusion, and must instead be transported by specialized proteins (which we'll discuss later).
What is most likely to cross the cell membrane by simple diffusion?The molecule most likely to be involved in simple diffusion is water - it can easily pass through cell membranes. When water undergoes simple diffusion, it is known as osmosis.
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