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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.
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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.
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Figure 2.7Structure of a seed.
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 Lugols 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 38. 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
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(a) seed development
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.
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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
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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.
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pea slicedrop of iodine
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).
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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