Which of the following outcomes will most likely result from a change in the shape

SCORE: _100__

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.
A. inside of the membrane; outside of the membrane
B. lipids; proteins
C. proteins; lipids
D. fatty acid chains; polar groups

2. Which of the following proteins is likely to contain one or more hydrophobic segments, 20-30 amino acids long?
A. Integral membrane protein.
B. Peripheral membrane protein.
C. Lipid-anchored protein.
D. Cytoplasmic protein.

3. Which of the following would most readily cross a lipid bilayer by simple diffusion?
A. Oxygen
B. Glucose
C. Chloride ions
D. Proteins

4. The voltage-gated potassium channels associated with an action potential provide an example of what type of membrane transport?
A. Simple diffusion.
B. Facilitated diffusion.
C. Coupled transport.
D. Active transport.

5. You are studying the entry of a small molecule into red blood cells. You determine the rate of movement across the membrane under a variety of conditions and make the following observations:
i. The molecules can move across the membrane in either direction.
ii. The molecules always move down their concentration gradient.
iii. No energy source is required for the molecules to move across the membrane.
iv. As the difference in concentration across the membrane increases, the rate of transport reaches a maximum.
The mechanism used to get this molecule across the membrane is most likely:
A. simple diffusion.
B. facilitated diffusion.
C. active transport.
D. There is not enough information to determine a mechanism.

6. A particular cell has an internal chloride ion concentration of 50 mM, while outside the cell the chloride ion concentration is 100 mM. The free energy change associated with chloride transport into the cell (DG) is +970 cal/mol. Which choice below is the best explanation for this data?
A. Cl- ion movement into the cell is energetically favorable.
B. Both the concentration gradient and electrical gradient favor movement of Cl- ions into the cell.
C. The concentration gradient for Cl- ions favors movement into the cell, but the electrical gradient opposes inward movement of Cl-.
D. Both the electrical and chemical gradients for Cl- ions favor outward movement of Cl- ions.

7. Place the following steps in an action potential in the correct order.
1. Sodium channels become inactivated and potassium channels are opened.
2. Sodium and potassium channel gates are closed; membrane potential is –60mV.
3. Sodium channel gates open in response to change in membrane potential.
4. Potassium rapidly leaves the cell; membrane potential drops to –75mV.
5. Sodium rushes into the cell; membrane potential reaches +40mV.
A. 2, 1, 4, 3, 5, 2.
B. 2, 1, 3, 4, 5, 2.
C. 2, 3, 4, 1, 5, 2.
D. 2, 3, 5, 1, 4, 2.

8. How are neurotransmitters released into a synapse in response to an action potential?
A. They pass through voltage-gated neurotransmitter channels.
B. They diffuse through the cell when the action potential reverses membrane potential.
C. They pass through gap junctions into the post-synaptic cell.
D. They are released by membrane fusion of vesicles in response to increased calcium concentration.

9. The neurotransmitter g-amino butyric acid (GABA) binds to receptors that are ligand-gated Cl- ion channels. What affect will this neurotransmitter have on the post-synaptic cell?
A. Cl- ions will rush into the cell leading to hyperpolarization and a reduced likelihood of an action potential.
B. Cl- ions will rush into the cell leading to depolarization and an increase in the chance for an action potential.
C. Cl- ions bind to GABA and inhibit it from interacting with the receptor, stimulating an action potential.
D. There will be no significant effect on the post-synaptic cell; only the pre-synaptic cell is affected by neurotransmitters.

10. Which of the following is the most likely immediate affect of G-protein activation?
A. Receptors are stimulated to bind to their ligands.
B. Enzymes are activated that catalyze second messenger formation.
C. GTP is depleted from the cell.
D. G-proteins bind to DNA and activate gene expression.

11. Proteins with SH2 domains are important in intracellular signaling pathways. What is the function of these domains?
A. They bind to activated tyrosine kinase receptors.
B. They bind to DNA and activate gene transcription.
C. They regulate the activity of voltage-gated ion channels.
D. They hydrolyze GTP to inactivate the pathway.

12. Platelet activation at the site of a wound is a example of:
A. endocrine signaling.
B. paracrine signaling.
C. intracellular receptor activation.
D. apoptosis.

13. Apoptosis is mediated by signal transduction pathways that lead to the programmed death of the cell. How is cell death achieved during apoptosis?
A. Aqueous channels form in the cell membranes leading to inward movement of water and lysis of the cell.
B. Gene expression is activated that leads to the synthesis of inhibitors of respiratory enzymes.
C. The Na+/K+ ATPase is inactivated, leading to the loss of membrane potential which the cell needs to survive.
D. Caspases are activated that lead to hydrolysis of many cellular macromolecules.

14. Which of the following statements is NOT true of tyrosine kinase-linked receptors?
A. Monomeric receptors are often induced to dimerize upon ligand binding.
B. The activated receptors attract and activate G proteins to continue the signal pathway.
C. The cytoplasmic side of the receptor contains a kinase enzyme domain that is activated upon ligand binding.
D. Activated receptors autophosphorylate themselves to attract SH2 domain proteins.

15.What mechanism is used to regulate the spontaneous assembly of collagen protein into a collagen fiber?
A. Different fibroblasts secrete different components for each collagen fiber.
B. Inhibitory protein domains are removed by an extracellular protease.
C. Covalent cross-links between proteins are only made outside the cell.
D. Complete collagen fibers are exported from the cell only as they are needed.

16. Which choice below describes the major function of proteoglycans in the extracellular matrix?
A. They provide a hydrated, gel-like medium for lubrication, cushioning, and embedding other ECM components.
B. They provide high strength fibers required to withstand mechanical stress.
C. They provide a highly elastic support to resist tension.
D. They create the dense, hard support structures of bone tissue.

17. Fibroblasts attach the extracellular matrix to the cytoskeleton via:
A. focal adhesions.
B. tight junctions.
C. hemidesmosomes.
D. gap junctions.

18. Which of the following classes of molecules is not involved in direct cell-to-cell contact?
A. Cadherins
B. N-CAMs
C. Selectins
D. Fibronectins

19. The cell junction that prevents the two different types of glucose transporters from mixing in the plasma membrane of intestinal epithelial cells is the:
A. gap junction.
B. tight junction.
C. adherens junction.
D. desmosome.

20. Polarized epithelial cells:
A. have a reversed membrane potential from most other cells.
B. maintain distinct membrane domains through the action of tight junctions.
C. do not have gap junctions so they are isolated from their neighbors.
D. are found only in animals such as polar bears, walruses and penguins.

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.

3. (10 points) Describe the difference between “open”, “closed” and “inactivated” voltage-gated sodium channels. Include in your answer the role of each state in generating an action potential.

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.

4. (15 points) List and describe five different types of molecules that participate in signal transduction pathways.

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.