Cellular and Molecular Biology Exam 2

G proteins are an example of multi-domain proteins A) True B) False

B

In signaling through G proteins and GPCRs, which one of the following is true: A) none of these are correct B) the extracellular signal molecule passes through the plasma membrane to activate the G protein C) the GDP is part of the chemical structure of the G protein D) The GPCR is very rigid and cannot change its shape E) all of these are correct

A

Put the following steps regarding GPCR/G-protein signaling in the correct sequence: (1) The G protein dissociates into an alpha subunit and a beta gamma complex (2) The GTP molecule binds the G a subunit (3) The GPCR undergoes a conformational shape change (4) A signal molecule binds the GPCR (5) The GDP leaves the G a subunit A) 1 - 2 - 3 - 4 - 5 B) 4 - 5 - 3 - 1 - 2 C) 2 - 4 - 5 - 3 - 1 D) none of the above E) 4 - 3 - 2 - 5 - 1

D

Secondary structure is stabilized primarily by hydrogen bonds between the amino acid side groups A) secondary structure is not stable B) True C) False

C

Protein domain is the term used to describe structurally stable independent regions within one polypeptide chain, which are usually connected to each other by relatively short and unstructured polypeptide segments. A) True B) False

A

The alpha helix and beta sheet are two examples of regular folding patterns that are often found in parts of proteins. These patterns result from hydrogen-bonding between the N-H and C=O groups in the polypeptide backbone and do not involve the side groups. A) True B) False

A

Protein secondary structure elements such as α helices and β sheets constitute the major regular folding patterns in proteins. With regard to these elements, A) a certain short amino acid sequence always adopts the same secondary structure. B) hydrogen-bonding between the amino acid side chains defines the type of secondary structure. C) All of the above. D) only a few specific amino acid sequences can adopt these repetitive structures. E) the folding patterns result from hydrogen-bonding between the N-H and C=O groups in the polypeptide backbone

E

Which of the following is NOT true regarding the members of a protein family in general? A) They can functionally replace each other. B) They have similar three-dimensional conformations. C) Over evolutionary time scales, the family has expanded mainly through gene duplication events. D) They share an ancestry; i.e. they are homologs. E) Their gene sequence is less well conserved than their structure.

A

Many viruses have large capsids in the form of a hollow sphere, made of hundreds of identical protein subunits. What are the advantages of having coats made of several copies of only a few subunits? A) All of the above. B) The effect of mistakes in protein synthesis on the overall assembly is minimized. C) Disassembly can be readily regulated. D) Assembly can be readily regulated. E) It requires a smaller amount of genetic information.

A

Stable β-sheet aggregates can form from many proteins, forming intertwined cross-beta strands that have the potential to kill cells or damaged tissues. Which of the following is NOT true regarding these aggregates? A) They form almost exclusively in the cells of the nervous system. B) Some healthy cells form these aggregates to store their secretory proteins. C) Their formation is associated with conditions such as Parkinson's disease D) They can form spontaneously, but also can be triggered to form by an infection with the same aggregate. E) Different types of such aggregates can form from the same protein.

A

The enzyme lysozyme catalyzes the cutting of a polysaccharide chain through hydrolysis. Which of the following is NOT true regarding the catalytic cycle for this enzyme? A) It involves covalent catalysis. B) It involves base catalysis. C) It involves acid catalysis. D) It involves strain catalysis. E) It involves metal ion catalysis.

E

Phosphorylation of a protein by a protein kinase … A) adds two positive charges to the protein. B) requires the hydrolysis of two molecules of ATP per phosphorylated residue. C) activates the protein. D) deactivates the protein. E) can create a binding site for other proteins.

E

Many macromolecular complexes in the cell contain scaffold proteins. What do these proteins do that benefit the cell? A) All of the above. B) They can provide a large macromolecular complex with either flexibility or rigidity. C) They can confine and concentrate a specific set of interacting proteins to a particular cellular location. D) They can enhance the rate of critical cellular reactions. E) They can hold the many subunits of a large complex together.

A

which group of organic compounds includes the enzymes? A) Proteins B) Carbohydrates C) Starches D) Lipids

A

Enzymes influence chemical reactions in living systems by: A) affecting the rate at which reactions occur B) absorbing water released when polymers are formed C) combining with excess hydrogen to form gaseous wastes D) providing the substrate required for the retain to occur

A

Which chemical is classified as an enzyme? A) Galactose B) Protease C) Lipid D) Manganese dioxide

B

In enzyme regulation, phosphorylation is an example of... A) Covalent modification B) Allosteric regulation C) Product inhibition D) All of the above

A

Enzymes that are allosterically regulated have multiple active sites. When an allosteric inhibitor binds to an enzyme, all active sites on the protein subunits are changed and work less well. A) False B) True

B

Vitamins are essential to the survival of organisms because vitamins usually function as: A) Nucleic acids B) Nucleotides C) Substrates D) Coenzymes

D

Enzymes can catalyze cellular reactions through various mechanisms. Which of the following statements is NOT true regarding enzymes? A) They have a higher affinity for the transition state of the substrate than for its stable form. B) They accelerate a cellular reaction by destabilizing the transition state. C) They can form covalent bonds with the substrate during catalysis. D) They can strain a substrate to force it toward a specific transition state. E) They can provide the chemical groups necessary for simultaneous acid and base catalysis.

B

Which of the following enzymes would digest fat? A) nuclease B) protease C) sucrase D) lipase

D

Competitive inhibitors: A) none of the above B) bind to the active site of an enzyme, competing with the substrate C) bind to the active site of an substrate and compete with the enzyme D) bind to another part (NOT the active site) of an enzyme, causing enzyme shape to change

B

noncompetitive inhibitors: A) none of the above B) bind to the active site of an substrate and compete with the enzyme C) bind to another part (NOT the active site) of an enzyme, causing enzyme shape to change D) bind to the active site of an enzyme, competing with the substrate

C

Allosteric regulation: A) may stimulate enzyme activity B) may inhibit enzyme activity C) all of the above D) occurs when a regulatory molecule binds to a protein at one site and affects protein's function at another site

C

What do antibodies look like? A) antibody forms a Y-shaped molecule B) All of the above C) Each antibody structure consists of two heavy chains and two light chains D) antibody has a different amino acid sequence at the tips of the “Y” E) each antibody is shaped differently

B

A carbohydrate layer is present on the inner leaflet of cell membranes. A) False B) True

A

The actions of a protein kinase to modify a protein can be reversed by a: A) protein GTPase B) protein kinase C) protein dephosphorylase D) protein phosphatase

D

Because the lipid bilayer is thicker and the lipid composition is specialized in lipid rafts, specific membrane proteins can accumulate there. This enables membrane proteins, which are normally highly mobile, to function together. A) False B) True

B

Which of the following is not true of the plasma membrane. A) Phospholipids in the lipid bilayer are free to flip back and forth from one layer to there other. B) The plasma membrane is a continuous double layer of lipid molecules C) The lipid layer is fluid D) Lipid bilayer fluidity depends upon the layer's composition

A

Which of the following is not a class of membrane lipid molecules: A) Glycolipids B) Proteins C) Cholesterol D) Phospholipids

B

What do all β-barrel transmembrane proteins have in common? A) The number of β strands. B) The structural rigidity compared to α-helical transmembrane proteins. C) The general function, i.e. membrane transport. D) The diameter of the barrel.

B

Transmembrane proteins: A) cannot contain β sheets in the part of their structure that interacts with the membrane interior. B) can be released from the membrane by a gentle extraction procedure such as salt treatment. C) are typically exposed only to one side of the membrane. D) are sometimes covalently attached to a fatty acid chain that inserts into the membrane. E) are often further attached to the membrane via a GPI anchor.

D

For each membrane protein in the following schematic drawings, indicate whether the cytoplasmic side of the membrane is more likely to be on the left (L) or on the right (R). Your answer would be a four-letter string composed of letters L and R only, e.g. RRRR. A) RLLR B) LLRL C) RRLR D) LRLR

B

While examining the crystal structure of a membrane protein, you find several phospholipid molecules bound to the protein. You know that these lipids … A) can have head groups of various sizes and charges depending on the protein. B) are thought to help stabilize many membrane proteins. C) All of the above. D) may enhance the crystallization of the bound membrane proteins. E) interact specifically with the protein.

C

Glycolipids such as gangliosides … A) affect the electrical environment of the membrane. B) are found to constitute about 10% of the total lipid mass in the plasma membrane of neurons. C) All of the above D) may contain oligosaccharide chains with negatively charged residues. E) are found in the extracellular leaflet (facing away from the cytosol) in the cellular membranes.

C

The motion of lipid molecules in a synthetic bilayer can be studied by various techniques. Which of the following has been observed in these systems? A) The flip-flops are very rare for phospholipids but cholesterol molecules flip-flop more often. B) Within a bilayer, lipid molecules rarely rotate about their long axis, but diffuse laterally at very high rates. C) Phospholipids diffuse rapidly within and between the two leaflets of a bilayer. D) All of the above.

A

The two monolayers of the plasma membrane in a human red blood cell... A) have the same abundance of phosphatidylinositol. B) have different overall electrical charges, with negatively charged phospholipids (e.g. phosphatidylserine) normally enriched in the inner monolayer. C) both contain glycolipids. D) exchange phospholipids only through spontaneous flip-flops.

B

Many cells store lipids in droplets of varying sizes. These droplets … A) have mostly protein-free bilayer membranes. B) mostly store cholesterol and phospholipids. C) are composed primarily of charged amphiphilic lipids. D) are enclosed by a phospholipid monolayer (instead of a bilayer). E) are produced by and released from the Golgi apparatus.

D

In the following schematic drawing of an abundant plasma membrane phosphoglyceride, which part is positively charged? A) E B) A C) C D) B E) D

B

Which of the following is correct regarding the molecule Cholesterol in cell membrane? A) All of the above B) It makes the membrane less permeable to small hydrophilic molecules. C) It is a sterol. D) It is an amphiphilic molecule E) It affects the fluidity of the lipid bilayer.

A

In contrast to transporters, the channel proteins in cellular membranes … A) can only mediate passive transport. B) interact strongly with the solute(s) that they transport. C) undergo a conformational change every time they transport a solute. D) form pores that are always open.

A

Which of the following types of proteins do not extend into the bilayer? A) Monolayer-associated proteins B) Peripheral proteins C) beta-barrels D) Transmembrane proteins

B

Which of the following describes membrane protein function? A) All of the above B) They transduce external signals to the inside of the cell C) They transport ions, nutrients and other substances across the membrane D) They anchor cells to each other

A

Transmembrane alpha helices have side chains that are: A) Hydrophobic B) Charged C) Amphipathic D) Hydrophilic

A

Which of the following terms describes transmembrane proteins? A) None of the above B) They have alpha helices with hydrophilic side chains C) They are amphipathic with both hydrophilic and hydrophobic portions D) They associate with only one side of the phospholipid bilayer

C

Which intermolecular process primarily drives the formation of a bilayer when phospholipids are added to water? A) The ordered arrangement of a bilayer is more favorable than the disordered state of individual free-floating phospholipids B) Phospholipid self-assemble into a bilayer due to the strong affinity they have for each otehr C) Lipids cause water to arrange in an ordered, unfavorable cage-like structure. Forcing lipids into a bilayer reduces this effect. D) A bilayer arrangement maximizes the strength of Van der waals forces among phospholipids

C

Why do cells not have membrane transport proteins for O2? A) Because oxygen is transported in and out of the cell in special oxygen-carrying proteins such as hemoglobin. B) Because oxygen can dissolve in the lipid bilayer and diffuse in and out rapidly without the need for a transporter. C) Because they need to keep the oxygen concentration low inside the reducing environment of the cell. D) Because oxygen transport across a membrane is energetically unfavorable.

B

An ion channel … A) always mediates passive transport. B) is ion-selective. C) is usually gated. D) All of the above. E) is typically several orders of magnitude faster than a transporter.

D

ATP is required in the transport of A) molecules through a protein channel B) molecules to areas of lower concentrations C) all molecules across a membrane D) molecules to areas of higher concentrations

D

The net movement of uncharged, polar molecules across a semipermeable membrane from a low concentration to a high concentration occurs by facilitated diffusion. A) False B) True

A

Red blood cells have a characteristic concave shape because of A) iron B) hemocyanin C) hemoglobin D) cytoskeletal spectrin

D

Principal classes of membrane proteins include all of the following except A) cell surface markers B) transport proteins C) receptors D) Spectrins

D

A, B, and O blood groups are marked by surface A) glycoproteins B) glycocarbohydrates C) glycolipids D) glycerol

C

Transmembrane proteins are A) always fixed in position B) none of the above C) always abundant D) never abundant

B

In each cycle the sodium-potassium pumps transfer A) two potassium ions in and two sodium ions out B) three sodium ions out and two potassium ions in C) one sodium ion out and one potassium ion in D) one potassium ion out and two sodium ions in

B