Cell Biology Exam #2-Spring 2001


Name____________________  Lab section_________________

Multiple choice.  Choose the best answer (2 points each).

1._D OR A____Which of the following factors would increase the fluidity of a cell membrane:  a) lower the amount of cholesterol in the membrane; b) increasing the amount of saturated fatty acids in membrane phospholipids; c) lowering the temperature; d) none of the above.

2.___C__Cells poisoned so that their ATP manufacturing system cannot work are unable to maintain a proper sodium-potassium balance.  Which of the following methods must be involved in maintaining sodium-potassium balance
in the cell?  a) diffusion; b) facilitated diffusion; c) active transport; d) exocytosis; e) endocytosis.

3.___C__If a plant cell is placed in a hypertonic environment:  a) the turgor pressure of the cell will increase; b) the cell will swell but won't burst due to the presence of the cell wall; c) the environment will have a greater solute concentration than is present inside the cell; d) the net
movement of water will be inside the cell.

4.__D___Which of the following is not true about cell membranes:  a) lipids move laterally within the bilayer at speeds of up to 2 um/sec; b) carrier molecules are usually transmembrane proteins; c) glycoproteins are commonly
found associated with the external (environmental) surface of the lipid bilayer; d) lipids can easily flip-flop from the environmental side of the bilayer to the cytoplasmic side.

5.__C___What are the membrane structures that function in active transport?
a)cholesterol  b)phospholipids  c)integral proteins  d)carbohydrates
e)peripheral proteins

6.__D___Of the following functions, the glycoproteins and glycolipids of animal cell membranes are most important for:  a)maintaining membrane fluidity at low temperatures.  b)facilitated diffusion of molecules down their concentration gradients.  c)maintaining the integrity of a fluid-mosaic membrane.  d)the ability of cells to recognize like and different cells  e)active transport of molecules against their concentration gradients.

7.___E__ An organism with a cell wall would have the most difficulty doing which process?
a)active transport  b)osmosis  c)diffusion  d)exocytosis  e)phagocytosis

8.___D__One of the functions of cholesterol in animal cell membranes is to
a)phosphorylate ADP.  b)store energy.  c)speed diffusion.  d)maintain membrane fluidity. e)facilitate transport of ions.

9)___E__Glucose diffuses slowly through artificial phospholipid bilayers. The cells lining the small intestine, however, rapidly move large quantities of glucose from the glucose-rich food into their glucose-poor cytoplasm. Using this information, which transport mechanism is most probably functioning in the intestinal cells? a)simple diffusion  b)active transport pumps  c)exocytosis
d)phagocytosis  e)facilitated diffusion

10.___E__Two similar-sized animal cells are placed in a 0.5% sucrose solution. Cell A enlarges in size for a while, then stops; cell B continues to enlarge and finally ruptures. Which of the following was true at the beginning of the experiment?
a)Cell A was hypertonic to the solution and cell B was hypotonic.  b)Cells A and B were isotonic to each other.  c)Cell A was hypotonic to the solution and cell B was hypertonic.  d)Cell A was hypertonic to cell B.  e)Cell B was hypertonic to cell A.

11.__C___You have a friend with high blood cholesterol levels and the cause is diagnosed as familial hypercholesterolemia. Of the following, the best explanation for this inherited condition is:  a)a poorly formed lipid bilayer that cannot incorporate cholesterol in the membranes of her cells.  b)a general lack of glycolipids in the blood cell membranes.  c)defective LDL receptors on plasma membranes of her cells.  d)inhibition of the cholesterol active transport system in red blood cells. e)poor attachment of the cholesterol to the extracellular matrix of her cells.

12.___A__Which of the following statements about NAD is FALSE?
a)NAD has more chemical energy than NADH.  b)NAD is reduced by the action of dehydrogenases. c)In the absence of NAD, glycolysis cannot function. d) NAD can receive electrons for use in oxidative phosphorylation.  e) NAD is reduced to NADH during both glycolysis and the Krebs cycle.

13.__B___Which metabolic process is most closely associated with intracellular membranes?  a)the Krebs cycle  b)oxidative phosphorylation  c)glycolysis d)substrate-level phosphorylation e)ethanolic fermentation

14.___D__ Which statement below is TRUE?  a)Pyruvate is in a more reduced state than glucose. b)There is more energy in pyruvate than in lactate.  c)Pyruvate is in a more oxidized state than carbon dioxide.  d)There is less energy in two molecules of pyruvate than in one molecule of glucose. e)There is more energy in 6 molecules of carbon dioxide than in 2 molecules of pyruvate.

15.__E___ Which type of enzyme in cellular respiration is primarily responsible for removing electrons from organic molecules? a) phosphofructokinase  b)deaminase c)decarboxylase d)ATPase  e)dehydrogenase

16.___B__The direct energy source that drives ATP synthesis during respiratory oxidative phosphorylation is  a)oxidation of glucose to CO2 and water.  b)the difference in H+ concentrations on opposite sides of the inner mitochondrial membrane. c)the final transfer of electrons to oxygen. d)the thermodynamically favorable flow of electrons from NADH to the mitochondrial electron transport carriers. e)thermodynamically favorable transfer of phosphate from glycolysis and Krebs cycle intermediate molecules of ADP.

17.__C___The ATP made during fermentation is generated by which of the following?
a)oxidatve phosphorylation b)oxidation of NADH c)substrate-level phosphorylation
d) the sodium/potassium ATPase e)electron transport

18.___C__The Krebs cycle produces which of the following molecules that then transfer energy to the electron transport system? a)ATP and CO2 b) NADH, FADH2, and ATP  c)  FADH2 and NADH d)CO2 and FAD  e) NADH and ATP

19.___E__Which of the following intermediary metabolites enters the Krebs cycle and is formed, in part, by the removal of CO2 from a molecule of pyruvate?
a) citric acid  b)phosphoenol pyruvate (PEP) c) oxaloacetic acid d)lactate
e)acetyl CoA

20.___A__You have a friend who lost 15 pounds of fat on a diet. Where did the fat go (how was it lost)?  a)It was released as CO2 and H2O. b)It was converted to urine and eliminated from the body. c)Chemical energy was converted to heat and then released. d)It was broken down to amino acids and eliminated from the body.
e)It was converted to ATP, which weighs much less than fat.

21.___A__Which of the following reactions contains a step which requires an input of ATP before it can take place?  a) glycolysis; b) TCA cycle; c) electron transport system; d) conversion of pyruvate to acetyl Co-A.

22.___D__The greatest percentage of the total amount of ATP produced by the aerobic respiration of 1 glucose molecule is produced by this system:
a) glycolysis; b) fermentation; c) TCA cycle; d) the electron transport system.

23.___D__Which of the following is not true about the mitochondrial electron transport system?  a) hydrogen carriers alternate with electron carriers;b) electrons are passed to oxygen to help reduce it into water; c) FADH2 is oxidized into FAD; d) 4 protons are added to the proton pool for every
NADH2 that is oxidized by the system; e) none of the above.

24.___C__Without the vitamin niacin (B-12) in your diet, your body will lack the ability to manufacture this compound, which must be present in order for cell respiration to occur:  a) ADP; b) pyruvate; c) NAD; d) cytochromes; e) enzymes.

25. (6 points) What happens as the result of the sodium/potassium pump?  Describe two specific instances of conformational changes that occur in the sodium/potassium ATPase protein.  Describe what stimulates the specific conformational change and the result of that conformational change.

The pump results in the movement of 3 Na+ out of the cell and 2 K+ into the cell.  This contributes to the negative membrane potential of the cell membrane.

Pick any two of these for the conformational changes asked for

1) Na+ binds tot he ATPase.  This activates the ATPase and results in ATP hydrolysis.

2) The Atpase becomes phosphorylated.  This opens the channel protein to the environmental side.  It also changes the binding sites for Na+ so Na+ ions can no longer bind (thus releasing them to the outside). It also changes the shape of the K+ binding sites so that K+ can now bind.

3) K= binds to the sites on the ATPase.  This causes dephosphorylation of the ATPase.

4. Dephosphorylation of ATPase. This opens the channel protein to the cytoplasmic  side.  It also changes the binding sites for K+ so K+ ions can no longer bind (thus releasing them to the inside). It also changes the shape of the Na+ binding sites so that Na+ can now bind.
26. (8 points) Using diagrams and accompanying explanations, describe the structure of the cell membrane (including what molecules it is made up of and how they are arranged) according to the fluid mosaic model of Singer and Nicholson.  Describe how this model answered the problems of the Davson-Danelli model of membrane structure.

The diagram and explanations should show the following elements:

Phospholipid bilayer, lipids and proteins able to move laterally within the bilayer, integral proteins, peripheral proteins, cholesterol, gylcoproteins, the membrane being bifacial (different on the outer and inner surfaces).

The Singer-Nicholson model places ampipathic proteins in their most stable arrangements.   The hydrophobic portions are inside the nonpolar part of the membrane and the polar portions are on the surface of the membrane.  The Davson-Danielli model places ampipathic proteins on the surface of the membrane.  The hydrophobic portions would be exposed to water in this arrangement.  This is not stable.

27. (3 points each) Agree or disagree with the following statements.  In either case, fully defend your answer.

A. The presence of proteins in the cellís membranes is strictly related the membraneís structural role of holding the contents of the cell in place.

Disagree-Cell membrane proteins play a number of important functional roles in addition to their structural roles.  These include enzymatic activity, cell-cell recognition, transport, cell adhesion, and  receptors

B. A genetic defect that resulted in a drastically decreased ability of a cell to synthesize unsaturated fatty acids would likely have more effect on an organism that lived in an arctic environment than an organism that lived in a tropical environment.

Agree-Colder temperatures make the membrane less fluid.  Unsaturated phospholipids help maintain fluidity because their hydrocarbon chains do not pack closely together because of the kinks in the chain produced by the double bonds.   Thus, a deficiency in unsaturated fatty acids would be more of a problem to a cell that was at a colder temperature.

C. An inhibitor that completely inhibited the function of decarboxylase enzymes would completely stop the ability of human cells to produce ATP from glucose.

Decarboxylase enzymes remove C02 from molecules.  Glycolysis has no decarboxylase enzymes yet still produces 2 ATP by substrate level phosphorylation.  Thus human cells could ferment and still produce ATP even in the presence of inhibitors of decarboxylase enzymes.

D. If the mitochondrial inner membrane became permeable to H+, the electron trasnport system would stop.

The ETS casues the H= gradient to develop and is not a result or dependent on the H+ gradient.  If the membrane became permeable to H+, the gradient would dissipate and no ATP would be made.  However, the cell would keep the ETS running to recycle coenzymes.  The ETS would not be affects, only the H+ gradient.

28. (8 points) Using diagrams and accompanying explanations, describe how the oxidation of NADH and FADH2 by the electron transport system results in the formation of significant amounts of ATP.  What role does oxygen play in this process?

Your diagram and accompanying explanations should include the following:

There are two types of ETS carriers (electron only and hydrogen only)in the inner membrane, the two types of carriers are assymetrically distributed within the mitochondrial inner membrane, passage of electrons through the ETS causes a high H+ to accumulate in the intermembrane space, passage of electrons through the ETS causes a low H+ to accumulate in the matrix, the dissociation of water provides H+ ions which are needed to help reduce hydrogen only carriers, the inner membrane is impermeable to H+, there is a channel protein (ATPase) inside the inner membrane that allows H+ to flow into the matrix, as two H+ flow through the ATPase, one ATP is made.

29. (5 points) If all of the NAD in the cell were converted into NADH2, cellular respiration would stop.  Explain why.  How does the cell prevent this from happening in the presence of oxygen?  In the absence of oxygen?

The enzymes of glycolysis and TCA cycle that are dehydrogenases require oxidized coenzymes (including NAD) to accept the electrons that are removed from the oxidation of glucose. If the coenzyme is not in the proper state (oxidized) then the enzyme will not work and the reactions will halt.  Thus the cell needs a recycling mechanism to recycle NADH2 back to NAD.  In the presence of oxygen, NADH@ donates its electrons to the ETS and they are eventually passed to oxygen to make water.  In the absence of oxygen, the cell can recycle NADH2 by donating its electrons to pyruvate to make ethanol or lactic acid (fermentation).

30. (3 points) List three differences between oxidative and substrate-level phosphorylation.

Substrate level phosphorylation uses organic phosphate to phosphorylate ADP to make ATP, requires no membranes, and makes significant amounts of ATP(32/glucose).  Oxidative phosphorylation uses inorganic phosphate to make ATP, requires no membranes and makes a small amount of ATP )4 net per glucose)


Bonus questions.  The lowest score you can get on this bonus section is 0.  Bonus points will be awarded by giving one point for each correct answer and subtracting one point for each incorrect answer (there is a bonus penalty for guessing on these questions).

1)__A____Plasmodesmata in plant cells are MOST similar in function to which of the following structures in animal cells?  a)gap junctions b) glycocalyx c) tight junctions d)desmosomes e) peroxisomes

2)___D__Ions can travel directly from the cytoplasm of one animal cell to the cytoplasm of an adjacent cell through a) desmosomes. b)plasmodesmata. c)tight junctions. d) gap junctions. e) intermediate filaments.

3)__C___Which of the following contain the 9 + 2 arrangement of microtubules?
a) basal bodies  b)centrioles c)cilia d)nuclei e)microfilaments

4)___E__Which of the following is NOT a known function of the cytoskeleton?
a)to provide mechanical support to the cell  b)to maintain characteristic shape of the cell  c)to assist in cell motility by interacting with specialized motor proteins d)to hold mitochondria and other organelles in place within the cytosol
e)to maintain a critical limit on cell size

5)___C__A cell lacking the ability to make and secrete glycoproteins would most likely be deficient in itís: a)microtubules.  b)nuclear DNA.  c)extracellular matrix. d)cytoskeleton.

6)__B___Cilia and flagella move:  a)when the central pair of microtubules produces ATP. b)when the bending of microtubules is powered by ATP.  c)when contraction of basal body triplets is powered by ATP. d)in opposite directions on the plasma membrane.