AccelePrep for the ACT Test 2nd Edition Student Text

P OWER U P ! T HE E XCLUSIVE C AMBRIDGE S AMPLE E XAM • 291

Table 1: Plasmolyzed Onion Cell Counts Sugar Solution Concentration (mol/kg)* Number of Cells Analyzed Number of Plasmolyzed Cells 0.55 50 50 100 0.50 50 46 92.0 0.45 64 50 78.1 0.40 64 44 68.8 0.35 50 25 50.0 0.30 62 12 19.4 0.25 48 5 10.4 0.20 55 3 5.5 0.15 57 2 3.5 0.10 41 1 2.4 0.05 40 0 0.0 0.00 50 0 0.0 *moles of sugar per kilogram of water Experiment 2 The students conduct a second experiment with beets. Beakers are prepared with sugar solutions of the same concentrations as in Experiment 1. Using an 8-millimeter diameter cork borer, cylinders of tissue are extracted from the beets. The tissue cylinders are sliced into disks approximately 3 millimeters thick. Each disk is weighed before being submerged in one of the prepared beakers. After 75 minutes, the students extract, blot dry, and weigh each of the disks. The percentage of weight change in the disks are calculated. Table 2 summarizes the experimental results. Table 2: Changes in Beet Tissue Weights 0.55 2.865 2.460 –14.14 0.50 2.732 2.407 –11.90 0.45 2.807 2.666 –5.02 0.40 2.474 2.422 –2.10 0.35 3.101 3.152 1.64 0.30 3.060 3.118 1.90 0.25 2.549 2.642 3.65 0.20 2.801 2.889 3.50 0.15 2.357 2.428 3.01 0.10 2.675 2.754 2.95 0.05 2.413 2.528 4.77 0.00 2.880 3.060 6.25 Sugar Solution Concentration (mol/kg) Original Weight (g) Final Weight (g)

PASSAGE III Osmosis is the spontaneous movement of solvent molecules through a semi-permeable membrane into a region of higher solute concentration. The molecules move in the direction that tends to equalize solute concentration on both sides of the membrane. Figure 1 shows a semi-permeable membrane between two sugar (sucrose) solutions of different concentrations. Osmosis occurs as the water molecules pass through the membrane to the side with higher sugar concentration. The sugar molecules, on the other hand, are too large to pass through the pore spaces in the semi-permeable membrane. To observe the effect of osmosis, a group of students conducts experiments using plant tissues. In the experiments, the walls of the cells in the plant tissues function as the semi-permeable membrane separating regions of different solution concentrations. Experiment 1 The students prepare shallow dishes with sugar solutions of varying concentrations. Using tweezers, the students gently pull off small strips of tissue, 3 to 10 millimeters in length and one cell layer thick, from the outer layer of an onion. Each strip is submerged in one of the prepared dishes. After 45 minutes, the students mount the strips on slides and, using a microscope, examine the tissues for evidence of plasmolysis—the shrinkage of a cell’s protoplasm and separation from the cell wall due to water loss from the cell. Forty to sixty- ϐ‹˜‡…‡ŽŽ• ˆ”‘ ‡ƒ…Š ‘‹‘ •–”‹’ ƒ”‡ ƒƒŽ›œ‡† ƒ† scored as either plasmolyzed or not plasmolyzed. ›…‡ŽŽ• ‘–…Ž‡ƒ”Ž› ’Žƒ•‘Ž›œ‡† ƒ”‡…Žƒ••‹ϐ‹‡† as not plasmolyzed. Table 1 summarizes the experimental results. Figure 1

Percentage of Plasmolyzed Cells

Percentage of Weight Change

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