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Arginine is one of the 20 most common natural amino acids. Most healthy people do not need to supplementwith arginine because the body usually produces sufficient quantities. The pathway for arginine synthesis wasstudied using cells from a red bread mold. This natural form of arginine is illustrated below.The red bread mold Neurospora crassa grows well on a cultural plate with "minimal" medium which is a fluidcontaining only a few simple sugars, inorganic salts, and vitamin. Neurospora that grows normally in nature(wild type) has enzymes that convert these simple substances into the amino acids necessary for growth.Mutating any one of the genes that makes an enzyme can produce a Neurospora strain that cannot grow onminimal medium. The mutant would only grow if the enzyme product were to be added as a supplement. On theother hand, if a "complete" medium is provided, containing all required amino acids, then Neurospora wouldgrow, with or without mutation.Figure 1 A synthesis pathway for the amino acid arginine. Each gene in italics in the diagram produces oneenzyme necessary for the synthesis of this essential amino acid required for growth.Table 1 Growth response of mutant strains in “minimal” media with supplements (ornithine, citrulline,argininosuccinate, and arginine) as indicated. Strain growth is indicated by (+) and no strain growth is indicatedby (−).Which of the following is NOT an accurate description of naturally-occurring arginine?Section: Physical Sciences 

It is critical for the human body blood to maintain its pH at approximately 7.4. Decreased or increased blood pHare called acidosis and alkalosis respectively; both are serious metabolic problems that can cause death. Thetable below lists the major buffers found in the blood and/or kidneys.Table 1BufferpKa of a typical conjugate acid:*Histidine side chainsOrganic phosphatesN-terminal amino groups 6.16.36.87.08.09.2*For buffers in many of these categories, there is a range of actual pKa values.The relationship between blood pH and the pKa of any buffer can be described by the Henderson-Hasselbalchequation:pH = pKa + log([conjugate base] / [conjugate acid])Equation 1Bicarbonate, the most important buffer in the plasma, enters the blood in the form of carbon dioxide, abyproduct of metabolism, and leaves in two forms: exhaled CO2 and excreted bicarbonate. Blood pH can beadjusted rapidly by changes in the rate of CO2 exhalation. The reaction given below, which is catalyzed bycarbonic anhydrase in the erythrocytes, describes how bicarbonate and CO2 interact in the blood.Reaction 1What would be the order of conjugate acid strength in the following buffers?Section: Physical Sciences 

Four major blood types exist in the human ABO blood system: types A, B, AB, and O; and there are threealleles that code for them. The A and B alleles are codominant, and the O allele is recessive. Blood types arederived from the presence of specific polysaccharide antigens that lie on the outer surface of the red blood cellmembrane. The A allele codes for the production of the A antigen; the B allele codes for the production of the Bantigen; the O allele does not code for any antigen.While there are many other antigens found on red blood cell membranes, the second most important antigen isthe Rh antigen. Rh is an autosomally dominant trait coded for by 2 alleles. If this antigen is present, anindividual is Rh+; if it is absent, an individual is Rh−. For example, a person with type AB blood with the Rhantigen is said to be AB+.These antigens become most important when an individual comes into contact with foreign blood. Because ofthe presence of naturally occurring substances that closely mimic the A and B antigens, individuals who do nothave these antigens on their red blood cells will form antibodies against them. This is inconsequential untilsituations such as blood transfusion, organ transplant, or pregnancy occur.Erythroblastosis fetalis is a condition in which the red blood cells of an Rh+ fetus are attached by antibodiesproduced by its Rh− mother. Unlike ABO incompatibility, in which there are naturally occurring antibodies toforeign antigens, the Rh system requires prior sensitization to the Rh antigen before antibodies are produced.This sensitization usually occurs during the delivery of an Rh+ baby. So while the first baby will not be harmed,any further Rh+ fetuses are at risk.The Coombs tests provide a method for determining whether a mother has mounted an immune responseagain her baby’s blood. The tests are based on whether or not agglutination occurs when Coombs reagent isadded to a sample. Coombs reagent contains antibodies against the anti-Rh antibodies produced by themother. The indirect Coombs test takes the mother’s serum, which contains her antibodies but no red bloodcells, and mixes it with Rh+ red blood cells. Coombs reagent is then added. If agglutination occurs, the test ispositive, and the mother must be producing anti-Rh antibodies. The direct Coombs test mixes the baby’s redblood cells with Coombs reagent. If agglutination occurs, the test is positive, and the baby’s red blood cellsmust have been attacked by its mother’s anti-Rh antibodies.How might one most practically assess the risk of erythroblastosis fetalis in a pregnant woman?Section: Biological Sciences 

Just as the ingestion of nutrients is mandatory for human life, so is the excretion of metabolic waste products.One of these nutrients, protein, is used for building muscle, nucleic acids, and countless compounds integral tohomeostasis. However, the catabolism of the amino acids generated from protein digestion produces ammonia,which, if not further degraded, can become toxic. Similarly, if the same salts that provide energy and chemicalbalance to cells are in excess, fluid retention will occur, damaging the circulatory, cardiac, and pulmonarysystems.One of the most important homeostatic organs is the kidney, which closely regulates the excretion andreabsorption of many essential ions and molecules. One mechanism of renal function involves the secretion ofantidiuretic hormone (ADH).Diabetes insipidus (DI), is the condition that occurs when ADH is ineffective. As a result, the kidneys are unableto concentrate urine, leading to excessive water loss. There are two types of DI – central and nephrogenic.Central DI occurs when there is a deficiency in the quantity or quality of ADH produced. Nephrogenic DI occurswhen the kidney tubules are unresponsive to ADH. To differentiate between these two conditions, a patient’surine osmolarity is measured both prior to therapy and after a 24-hour restriction on fluid intake. ExogenousADH is then administered and urine osmolarity is measured again. The table below gives the results of testingon four patients. Assume that a urine osmolarity of 285 mOsm/L of H2O is normal.Which of the following would you most likely expect to find in a patient with diabetes insipidus?Section: Biological Sciences 

Due to ever-increasing paranoia about the transmission of hepatitis and AIDS via blood transfusions and thefrequent difficulty of procuring matching blood donors for patients, researchers have been working at a feverishpace to produce disease-free and easy-to-use blood substitutes. The difficulty most synthetic blood researcheshave had is in formulating a substance that combines qualities of sterility, high capacity for carrying oxygen tobody tissues, and versatility within the human body. Three major substitute technologies have been developedto date; each has certain advantages and shortcomings.“Red blood,” the first of the blood substitute technologies, is derived from hemoglobin which has been recycledfrom old, dead, or worn-out red blood cells and modified so that it can carry oxygen outside the red blood cell.Hemoglobin, a complex protein, is the blood’s natural oxygen carrier and is attractive to scientists for use insynthetic blood because of its oxygen-carrying capacity. However, hemoglobin can sometimes constitute a twofold threat to humans when it is extracted from the red blood cell and introduced to the body in its naked form.First, hemoglobin molecules are rarely sterile and often remain contaminated by viruses to which they wereexposed in the cell. Second, naked hemoglobin is extremely dangerous to the kidneys, causing blood flow atthese organs to shut down and leading, ultimately, to renal failure. Additional problems arise from the fact thathemoglobin is adapted to operate optimally within the intricate environment of the red blood cell. Stripped of theprotection of the cell, the hemoglobin molecule tends to suffer breakdown within several hours. Althoughmodification has produced more durable hemoglobin molecules which do not cause renal failure, undesiredside effects continue to plague patients and hinder the development of hemoglobin-based blood substitutes.Another synthetic blood alternative, “white blood”, is dependent on laboratory synthesized chemicals calledperfluorocarbons (PFCs). Unlike blood, PFCs are clear oil like liquids, yet they are capable of absorbingquantities of oxygen up to 50% of their volume, enough of an oxygen carrying potential for oxygen-dependentorganisms to survive submerged in the liquid for hours by “breathing” it. Although PFCs imitate real blood byeffectively absorbing oxygen, scientists are primarily interested in them as constituents of blood substitutesbecause they are inherently safer to use than hemoglobin-based substitutes. PFCs do not interact with anychemicals in the body and can be manufactured in near-perfect sterility. The primary pitfall of PFCs is in theirtendency to form globules in plasma that can block circulation. Dissolving PFCs in solution can mitigateglobulation; however, this procedure also seriously curtails the PFCs’ oxygen capacity.The final and perhaps most ambitious attempt to form a blood substitute involves the synthesis of a modifiedversion of human hemoglobin by genetically-altered bacteria. Fortunately, this synthetic hemoglobin seems toclosely mimic the qualities of sterility, and durability outside the cellular environment, and the oxygen-carryingefficiency of blood. Furthermore, researchers have found that if modified hemoglobin genes are added tobacterial DNA, the bacteria will produce the desired product in copious quantities. This procedure is extremelychallenging, however, because it requires the isolation of the human gene for the production of hemoglobin,and the modification of the gene to express a molecule that works without support from a living cell.While all the above technologies have serious drawbacks and difficulties, work to perfect an ideal bloodsubstitute continues. Scientists hope that in the near future safe synthetic blood transfusions may ease bloodshortages and resolve the unavailability of various blood types.It can be inferred from the passage that the difficulty of producing an ideal blood substitute is compounded byall of the following EXCEPT:Section: Verbal Reasoning