Transcript
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Learning Experience 3

No Mat te r Wha tYou r Shape

The easiest traits to describe in an organism are the visible traits. For instance,think about distinguishing between two people. You might use the texture of theirhair as an easily identifiable trait. One might have curly hair, and the otherstraight hair. By using differences in hair texture, you have described variantsof a single trait. In this case, that trait is hair texture.

What is responsible for variants in traits? Are traits and their variants onlycharacteristics that are directly observable? Or are there underlying causes forthese variants? In this learning experience, you use variations in the shape ofpeas as a simple model for investigating the answers to these questions. You then explore a trait in humans, sickle-cell trait, as another example of variations in traits.

Your teacher will distribute two different kinds of peas to you and your partner.Examine them carefully. Discuss the following questions with your partner, andrecord your thinking in your notebook. Be prepared to discuss your ideas withthe class.1. List the traits you can observe in each kind of pea.2. Describe the variations, if any, in each trait you have listed for the two kinds

of peas.3. What do you think might cause the variations in these traits?4. What if you were to soak these peas in water? Do you think there would be a

difference in the amount of water they absorbed? Explain your answer.

Brainstorming

ProloguePrologue

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A Pea by Any Other Name Is Still a SeedHow might you begin to answer the question, “Why is one pea wrinkled andanother round?” Think about ways you could find out more about the differencesbetween round and wrinkled peas. Are there differences in how the peas areconstructed? How might these differences produce a wrinkled shape instead of around shape? Are there differences in what the seed is made of? How would thisaffect the shape?

In this learning experience, you will investigate the causes of variations intraits of organisms. Using these pea variants as a model, you will look at thedifferences between wrinkled peas and round peas (which are actually seeds).But first, you need to understand the general structure of a seed(shown in Figure 2.7). A seed is the part of a plant that resultsfrom the fertilization of the female egg by the male pollen.Following fertilization, the embryonic plant develops within aprotective seed coat. In addition to the embryonic plant and theseed coat, the seed also contains a source of food. The germinatingseedling will use this food until it can carry out photosynthesis. Inone type of plant, this food source is in a separate structure withinthe seed called the endosperm. In another type of plant, theprotein, starch, and fats are stored in two large seed leaves.Regardless of the system of storage, the newly sprouted plantdepends on these stored food sources until it can make its own.

Seeds have a very low water content. During the final stages ofthe development of the seed, cells within the seed dehydrate. Inother words, most of the water in the seed is removed. The resultinglow water content in the seed causes most cellular processes to slowdown or stop. In this dehydrated state, the embryonic plant canremain dormant (inactive) but viable (alive) within the seed. It can stay this wayfor long periods of time without growing or developing. This process enables theseed to delay germination until environmental conditions are suitable for itsgrowth. Germination is the start of growth and development of a plant. This iswhen the embryonic plant breaks out of its seed coat. When conditions becomefavorable, water enters the seed. Rehydration triggers the reactivation of normalmetabolic processes, and germination begins (see Figure 2.8).

As the plant begins to develop, it uses the starch stored in the endosperm orseed leaves as an energy source. A plant can use photosynthesis to provide itsrequired food and energy only after breaking free of the soil and receiving sunlight.

In this activity, you will determine why some seeds (peas) are wrinkled at theend of their development and some are round.

Learning Experience 3 No Matter What Your Shape 165

ACTIVITY

Figure 2.7Structure of a seed.

seed coat

EmbryonicPlant

embryonic stem

embryonic root

seed leaves(cotyledon)

embryonic shoot tip

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For each pair of students:

• 2 pairs of safety goggles• 10 round peas• 10 wrinkled peas• 1 balance• 2 small beakers or containers (50-mL)• 1 wax marking pencil• 2 microscope slides with coverslips• access to a compound microscope• 1 razor blade or scalpel• 1 forceps• 1 dropping bottle of dilute Lugol’s iodine• paper towels• distilled water

Part A1. Develop a hypothesis that explains why some peas are wrinkled

and some are round. Record your hypothesis in your notebook.2. Read steps 3–8. Then create a data chart in your notebook. Fill it in as you

carry out this part of the experiment.3. Weigh all 10 round peas together. Record the weight of the peas on your

data chart. Weigh all 10 wrinkled peas together. Record the weight of thepeas on your data chart.

STOP &T H I N K

PPROCEDUREROCEDUREPROCEDURE

MaterialsMaterials

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shoot

root

wateradded

waterremoved

(a) seed development

(b) germination

Figure 2.8(a) During seed development, water is lost from cells. This dehydration slows down metabolic processes. (b) Duringgermination, water enters the seed and the metabolic processes are reactivated. As a result, the embryonic plant begins to grow.

SAFETY NOTE

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4. Use a wax marking pencil to label one beaker “R” and the other beaker “W.”Also mark the beakers with your group name. Place the dried round peas inthe R beaker and the dried wrinkled peas in the W beaker. Add water untilthe beakers are three-quarters full (see Figure 2.9).

5. Place the beakers in the location designated by your teacher until the nextclass session.

6. After soaking the peas overnight, retrieve your 2 beakers. Label one papertowel “R” and another “W.” Pour off the excess water from each beakercarefully. Then empty the peas from each beaker in a pile on theappropriately labeled paper towel.

7. Weigh each pile of peas again to determine the weight after soaking. Recordthe weights in your table.

8. Determine the weight difference for each kind of pea. Calculate the percentageof increase in weight for each kind of pea. Record the results in your table.

9. Think about this experiment and your understanding of seed formation. Do you want to change your hypothesis as to why some

peas wrinkle and others remain round when they are dried? Record yourresponse in your notebook.

Part B1. Use a wax marking pencil to label one microscope slide “R” and another “W.”2. Place 1 drop of dilute iodine on each slide.3. Hold a soaked wrinkled pea with forceps. Cut the pea in half with a scalpel

or razor blade. Cut a very thin segment or slice from the inside of the pea.Gently place the slice into the drop of dilute iodine on the slide labeled W(see Figure 2.10).

4. Carefully wipe the blade of the scalpel or razor blade clean with a papertowel. Repeat step 3 with a soaked round pea. Drop the slice on the slidelabeled R.

5. Place a coverslip at an angle over each drop and gently lower it. Observeeach slice under the microscope.

6. Describe and draw in your notebook what you see. Compare the shapes, colors, patterns, and densities of the starch grains. Look at a

few slides prepared by other class members and compare them with yours.

STOP &T H I N K

STOP &T H I N K

Learning Experience 3 No Matter What Your Shape 167

SAFETY NOTE

Avoid staining your

skin with iodine. Ifiodine is accidentallyingested, seekimmediate medicalattention.

Figure 2.9Lab setup.

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Prepare a laboratory report for this experiment in your notebook. Be sure toinclude the following:a. your initial hypothesis (step 1 in the procedure);b. your revised hypothesis (step 9), if you changed it (if you changed it,

explain why);c. your experimental procedure;d. the purpose of each part of the experiment;e. your data (show all your calculations where appropriate); andf. any conclusions you can draw, based on your data.

Decide whether you have enough data and knowledge to identify the cause of theshape difference between the two different kinds of peas. Explain your answer.

Adding a New Wrinkle to the PictureYou have gathered a great deal of information about the differences betweenwrinkled and round peas. You know some things about these peas at both thevisible and the biochemical levels. But you still may not be able to reach aconclusion about the exact cause of the difference in shape. To identify the exactcause of this difference, you need to understand more about the biochemistry ofseed development.

ANALYSISANALYSISAAA

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forceps

a pea

pea slicedrop of iodine

b

Figure 2.10Hold the pea firmly with theforceps. Carefully slice severalsections (a) until a very thin section is obtained.Using the forceps, transferthe section to the drop ofiodine on the microscopeslide (b).

READING

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In a developing seed, sucrose (a sugar) is converted to a highly branched form ofstarch. This starch is called amylopectin. It serves as a source of food for the developingplant. The conversion from sucrose to starch is facilitated by an enzyme. This enzyme iscalled starch-branching enzyme I (SBEI). Scientists used methods similar to the onesyou carried out in class. They were able to demonstrate that the characteristic ofwrinkled shape was the result of the pea’s inability to synthesize amylopectin fromsucrose. As a result of a faulty SBEI enzyme, these peas cannot make amylopectin. Theunchanged sucrose concentrates in the developing seed. Using a different enzyme,wrinkled peas can synthesize a different, unbranched form of starch. This form of starchis called amylose. Amylose serves as the seed’s food source.

What does this inability to make branched starch from sucrose have to dowith shape? During the seed’s development, the high concentration of sucrosecaused water to accumulate inside the seed. The water, as in a water balloon,stretched the seed coat much more than it normally would stretch without theconcentrated sucrose. During the final stages of seed development, dehydrationtakes place. The accumulated water is lost from the seed. This causes thestretched seed coat to collapse, somewhat creating a wrinkled pea. In the peawith the functional SBEI enzyme, sucrose did not accumulate. Thus, the seedcoat was not stretched by excess water. So when the water was lost duringdehydration, the seed coat remained round (see Figure 2.11).

Do round peas have only functional SBEI? Do wrinkled peas have onlynonfunctional SBEI? Would a pea having both kinds of enzymes be a little bitwrinkled? Biochemical analysis demonstrates that peas that have both kinds ofenzymes still appear round. You cannot see any difference from those peas thathave only functional SBEI. Some of the enzyme cannot make amylopectin fromsucrose in these seeds. Even so, the enzyme that can function can convertenough sucrose to amylopectin to prevent water from being retained andstretching the seed coat. They therefore appear round.

Learning Experience 3 No Matter What Your Shape 169

H2O

H2O

H2O

sucrose

amylose

amylopectin

early seed development starch formation dehydration

round peas

wrinkledpeas

1 2 3 Figure 2.11Sucrose changes to starch in seed development: Acomparison of round andwrinkled peas. (1) Sucrose ismade during early seeddevelopment. (2) In roundseeds, SBEI converts sucrose toamylopectin (branched starch).In wrinkled peas, the SBEI doesnot function correctly. So noamylopectin is made. Instead, a different enzyme converts some sucrose to amylose(unbranched starch). The highconcentration of unconvertedsucrose in the wrinkled seedcauses the seed to take up waterand swell. This stretches theseed coat. (3) When the seedundergoes dehydration, thecoat of the round seed remainssmooth. But the coat of thewrinkled seed wrinkles becauseof its stretched seed coat.

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170 Learning Experience 3 No Matter What Your Shape

Think about an enzyme that can function efficiently even in the presence of anonfunctional or dysfunctional enzyme of the same kind. Its activity isconsidered dominant to the activity of the nonfunctional (or recessive) enzyme.An organism that has both kinds of enzymes and displays the trait of thedominant activity (in this case, round) is considered heterozygous for that trait.An organism with only one kind of enzyme is said to be homozygous. Awrinkled pea is always homozygous for the nonfunctional enzyme. But a roundpea could be either homozygous or heterozygous. In this case, you cannotnecessarily judge a pea by its cover.

Am I a Carrier, and What Does That Mean?It was Health Day at Denzel Jones’s high school. “Career Day, Health Day,Environment Day,” fumed Denzel. “When am I going to hear about stuff thatmatters to me?” Denzel’s class filed into the auditorium. The air was buzzing withconversation about music, friends, the last biology exam—everything except thetopic of health. Who cared, anyway? Well, at least it got them out of fifth period.

As several individuals from the local health clinic talked, Denzel foundhimself drawn in by some topics. These included exercise, smoking, and methodsfor the prevention of infectious diseases. One topic in particular caught hisattention because he actually knew a couple of people with the problem. A

physician’s assistant began to talk about somethingcalled “hemoglobinopathies.” He described one inparticular, sickle-cell anemia. Denzel’s uncle Jamal(his father’s brother) had the disorder. He sufferedfrom fatigue and bouts of intense joint pain. Becauseit bothered him to watch his uncle suffer, Denzel wascurious about the cause.

Denzel learned that sickle-cell anemia is a disorderof red blood cells that can run in families. It causesthe red blood cells to collapse into shapes resemblingsickles (see Figure 2.12). This happens when theoxygen level of the blood is low.

Red blood cells sickle because they containhemoglobin that is biochemically a little differentfrom the normal hemoglobin protein. Normalhemoglobin (or hemoglobin A) is found in solution in

red blood cells. It binds oxygen and transports it throughout the body. Once itreleases the oxygen, the hemoglobin remains in solution in the red blood cell.Sickling hemoglobin (designated S) is a variant form of hemoglobin. It differsfrom normal hemoglobin by only a single amino acid. That slight difference instructure, however, alters its function. Hemoglobin S binds oxygen and carries itto where it is needed. But a problem arises when the oxygen is released and theconcentration of oxygen around the hemoglobin is reduced. Normal hemoglobinremains in solution under these conditions. But the sickling hemoglobin comesout of solution. Its molecules bind together into long fibrous chains (crystallizes).These fibers push out against the inside of the membrane of the red blood cell.This produces the characteristic sickle shape (see Figure 2.13).

CASE STUDY

Figure 2.12Normal red blood cells areshaped like disks. Some redblood cells of sickle-cellpatients become stiff andsickled (see arrows). Themisshapen cells often getstuck in small blood vessels.This causes extreme pain anddamage.© Dr. Gladden Willis/VisualsUnlimited

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Because of their shape, these cells cannot flow easilythrough the tiny capillaries. (Capillaries are thesmallest passageways of the circulatory system.) Thecells get stuck and clog the flow of blood. Thisblockage decreases the blood supply to the vitalorgans—such as the heart, spleen, kidneys, and brain.These organs can be damaged. The buildup of pressurebehind the blockage also can cause small blood vesselsto burst. This results in internal bleeding and pain.

The symptoms of sickle-cell anemia are quitevariable. But some general features include jaundice,anemia, and pain. (Jaundice is yellowing of the skinand other tissues due to the breakdown products of redblood cells.) Infants and children may have apredisposition to infection. In later years, blood-richorgans such as the heart, spleen, and liver are damagedby the restricted blood flow. The disease may cause legulcers, anemia, kidney failure, stroke, and heart failure.The severity of the symptoms varies from individual toindividual. Some show few symptoms; others dieyoung.

The physician’s assistant explained that sickle-cellanemia is an inherited disease. The variant can run infamilies. Individuals can pass the variant to theirchildren without having symptoms themselves. Parentswho do not have sickle-cell anemia can have childrenwith the disorder and children without the disorder.About 2.5 million, or one in every 12 African Americans carry the sickling traitwithout having the disease. (This group is the most affected population in theUnited States.) They have both kinds of hemoglobin in their red blood cells.Individuals who have both kinds of proteins are called carriers. Approximately80,000 African Americans have only sickling hemoglobin. These peopledemonstrate the characteristics or symptoms of sickle-cell anemia.

Denzel began to wonder whether anyone else in his family besides UncleJamal had sickle-cell anemia. Could he be one of the individuals who had thesickle hemoglobin variant but didn’t show it? The physician’s assistant told thegroup that an easy test for sickling hemoglobin could be done at the clinic. Heencouraged the students to have it done.

Denzel decided he wanted to be tested.After the assembly, Denzel approached the physician’s assistant to ask

questions about the test. He told Denzel that there is a test to distinguish normalhemoglobin (hemoglobin A) from sickling hemoglobin (hemoglobin S). Thistest is based on the understanding that the difference between the two types ofhemoglobin is only one amino acid. This amino acid changes the electricalcharge on the molecule. This charge difference causes the two different forms ofhemoglobin to separate in an electric field. In a solution through which anelectrical current is passed, hemoglobin A will move in one direction;hemoglobin S will travel the opposite way.

Denzel was amazed. The difference between being healthy and having thesymptoms of sickle-cell anemia was a single amino acid. And, through a fairlysimple blood test, Denzel could learn whether he had any hemoglobin S.

Learning Experience 3 No Matter What Your Shape 171

Figure 2.13(a) Normal hemoglobin (A)remains dissolved in the cellafter the release of oxygen.Cells remain disk-shaped. (b) Sickling hemoglobin (S)comes out of solution afterthe release of oxygen. Itforms long crystals anddistorts the cell shape.© Dr. Stanley Flegler/VisualsUnlimited

a

b

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At dinner that night, Denzel told his family what he had learned that dayabout the sickle-cell trait. He said he would like to be tested. He also thoughtthat it might be a good idea for everyone to be tested, to know whether theycarried the trait. Denzel’s father was not so sure. He worried that if he carried thetrait and someone at work found out, they might think he wasn’t healthy enoughto operate the forklift he drove every day. And what if he applied for morehealth insurance? What impact would being a carrier have on that? Denzelassured him that the physician’s assistant said that individuals who carried thetrait rarely exhibited any symptoms of the disease and were never considered“sick.” Anyway, the results of the test were confidential. No one was eversupposed to know.

Tara, Denzel’s older sister, was worried for a different reason. She was planningto be married soon and very much wanted to have children. What if she and herfiancé, Carlos Jackson, were both carriers? What would that mean for thechildren they might have? She wasn’t sure she wanted to know.

In the end, everyone in the family decided to be tested. This includedDenzel’s four grandparents, Grandpa and Grandma Jones and Grandma andGrandpa Beausejour; his sisters, Tara and Tabitha; and Uncle Jamal. Even Tara’sfiancé, Carlos, wanted to find out whether he carried the trait.

Everyone nervously waited a week for the blood test results. The data in Table 2.1were collected on the Jones and Beausejour families. (A + sign indicates theindividual has that form of hemoglobin; a – sign indicates it was not present.)

Record your responses to questions 1–5 in your notebook.1. Earlier in this learning experience, you found out that the difference in the

shape of peas is the result of a difference in a single enzyme that functionsduring pea development. Explain the differences in the biochemistry ofsickling and normal hemoglobin. How do these differences result in thevisible trait (as seen under the microscope)?

ANALYSISANALYSISAAA

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Individual Hemoglobin A Hemoglobin S

Grandpa Jones + +

Grandma Jones + +

Grandpa Beausejour + –

Grandma Beausejour + –

Mr. Jones + +

Mrs. Beausejour-Jones + –

Uncle Jamal Jones – +

Tara Jones + +

Tabitha Jones + –

Denzel Jones + –

Carlos Jackson + +

Table 2.1 Hemoglobin Data

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2. The results of Tara’s test, as well as some of the others tested, indicate thatsome of her red blood cells carry hemoglobin S as well as normalhemoglobin. These individuals are carriers of this variant form of protein.Yet none of them has shown any symptoms of sickle-cell anemia undernormal circumstances. How do you explain this?

3. Carlos enjoys mountain climbing. On occasion, at very high altitudes he hassuffered fatigue and severe cramps in his joints. What do you think is thereason for this? Base your answer on his test results.

4. Scientists use family trees or pedigrees as a tool to record and track inheritedcharacteristics in families. What specific characteristics have you seen inmembers of a family that help identify them as belonging to that family?

5. Create a pedigree for Denzel’s family. Indicate how members are related and howthe sickle-cell trait runs in the family. Use the test results shown in Table 2.1.

To help you diagram the trait of sickle cell in Denzel’s family, you will need touse the symbols shown in Figure 2.14 to create a pedigree of his family.

The generations of a family are marked with Roman numerals. Begin with thefirst generation listed in Table 2.1. Each individual within a generation is labeledwith an Arabic numeral (1, 2, 3, 4, etc.). Within the children of a particularcouple, the first born child is usually placed to the far left. Subsequent childrenfollow to the right. Figure 2.15 is one example of a pedigree. Examine thepedigree. What can you tell about the relationships in this family? Who has thedisease? Who are the carriers?6. Tara and Carlos hope to marry soon and to have children. What do you

think the test results mean for them? Write responses to the following:a. List all of the choices that Tara and Carlos have with respect to having

children.b. Describe all of the consequences for each of the choices you listed in item 6a.c. Describe in a short paragraph what choice you might make if you were in

the same situation as Tara and Carlos. Include your reasons for makingthat choice.

d. What do you think would happen if everyone who was confronted withthis situation made the same choice you made? Write a short paragraphdescribing what this future might look like.

e. List four important values that influenced your decision. Explain howthey influenced you. For example, some values might include religiousreasons, your view of community, your sense of responsibility, your ownpersonal health issues, and your sense of family.

Learning Experience 3 No Matter What Your Shape 173

male

female

affectedindividual

carrier

deceasedindividual

adopted intoa family

marriage

divorce

female twins

Figure 2.14Pedigree symbols.

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174 Learning Experience 3 No Matter What Your Shape

Figure 2.15An example of a pedigree.

John Isabelle

MariaAl Elizabeth

John

John

Sandy

Joshua

TanyaElaine Sally Dan Jack

Sue EricLucy

Charles

Luke Joan Evelyn Derrick Phil

I

II

III

IV

1 2

1 2 3 4 5

1 2 3 4 5 6 7 8 9 10

1 2 3 4 5 6

In the 1970s, Susan Perrine was a young doctor working in Saudi Arabia.She observed that many of the Arab patients who came to her clinic hadsurprisingly mild cases of sickle-cell anemia. In fact, many of them displayedno symptoms, even though their blood showed the characteristic sicklingeffect under conditions of low oxygen. When their hemoglobin wasexamined, the patients displayed high levels of fetal hemoglobin. Fetalhemoglobin is the kind of hemoglobin that all humans produce before birthbut generally is replaced after birth by adult hemoglobin. Fetal hemoglobinhas a higher affinity for oxygen. That means it binds oxygen more tightly thanadult hemoglobin does. Apparently in these Arab patients for some reasonthe red blood cells had not completely switched from making fetalhemoglobin to adult hemoglobin. And surprisingly, the presence of this fetalhemoglobin reduced or eliminated the problem found when an individualmakes only hemoglobin S. Explain why the presence of fetal hemoglobin maymask or dominate the effects of the sickling hemoglobin. Describe how thisinformation might be used to treat sickle-cell patients.

The study of genealogy, that is, tracing a family’s history, can be fascinating.Some people track their ancestors when an unfortunate illness shows up inthe immediate family. They are concerned about whether they or theirchildren may inherit the disease. Others search for the names and places oforigin in their mother’s and father’s pasts for clues to their heritage. Createyour own family tree. You may want to interview your oldest relatives. Askthem for their views of life and of family in past times to help you recapturefamily history that is often lost.

E X T E N D I N G

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Learning Experience 3 No Matter What Your Shape 175

CAREERFocusPhlebotomist It’s early in the morning, but the hospital is already busy.

Metal trays covered with vials, syringes, tourniquets, and doctors’ orders arebeing wheeled from room to room. One of these trays is followed closely byArzu, a phlebotomist. On her morning rounds, she has orders to draw bloodfrom an elderly woman being treated for a blood clot. She will also take a bloodsample from a middle-aged man needing tests to find out why he has beenfeeling so ill and a young girl who is in the hospital for gall bladder surgery.

Arzu loves meeting new people. Many of those she deals with aren’t toohappy to see her, because it is common for people to be afraid of needles.But Arzu comforts them by educating them about what is going to happenand describing each step. She can usually quell patients’ fears and takesamples of their blood without any problem. Long-time patients are relievedwhen they see her face in the morning. They know she cares.

Patients are thankful for Arzu’s gentle touch, but they are often unaware ofher great range of knowledge. She is very skilled. To complete her certificateprogram in phlebotomy, Arzu was trained in collecting, transporting, handling,and processing blood samples; identifying and selecting equipment, supplies,and additives used in blood collection; recognizing and adhering to infectioncontrol and safety procedures; and recognizing the importance of each stepfrom drawing blood to analysis and seeing how her part fits into the wholepicture of a specific person’s medical care.

Arzu’s expertise in the field is in drawing blood for analysis. She translatesthe doctors’ orders for the lab technicians who do the analysis. Whendoctors, physician’s assistants, and nurses receive the results, they use thedata from these blood tests to prescribe medication and a plan for care. Bybeing specifically trained in bloodletting procedures, Arzu allows doctors andnurses the time to complete important paperwork, update records, andcontinue patient care toward a speedy recovery.

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