pre lab questions - carnes ap bio · pre lab questions: ... select lab 1: diffusion & osmosis...
TRANSCRIPT
Name: _______________________________ Date: ____________________________
Virtual Student Guide http://www.phschool.com/sc ience/biology_place/labbench/index.html
PRE LAB QUESTIONS: Before you begin the online lab, log onto the Bozeman videos web page at http://www.bozemanscience.com/ap-
biology/ , and watch 3 videos under the AP Biology Labs section: AP Biology Labs #01 – Diffusion & Osmosis
Osmosis Lab Walkthrough Diffusion Demo
1. What is diffusion?_____________________________________________________________________________
2. What is osmosis?_____________________________________________________________________________
3. What happens to the sugar particles during the 15 minute period in his example?
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4. Why does the water level change in the second example?
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5. Sketch and label the lab setup of the dialysis tubing and the beaker:
6. What is the result when IKI and starch come together?_______________________________________________
7. What does this indicate?_______________________________________________________________________
8. How do you know there is no starch in the beaker contents (outside the tubing)?
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9. What are the 6 concentrations of sugar solutions used in the potato portion of the lab?
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10. Why do the potato cores in 0 M sugar gain mass?
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11. Why do the potato cores in 1 M sugar lose mass?
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12. How do you determine the molarity of the actual potato cores using graphed data?
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AP® Biology Laboratory 1 Diffusion & Osmosis
Prelab: Bozeman AP Labs Osmosis Lab Walkthrough:
Summarize the lab walkthrough:
Prelab: Bozeman AP Labs Diffusion Demo:
Summarize the demo:
Virtual Lab Now log onto the PH School web page at http://www.phschool.com/science/biology_place/labbench/ .
Select Lab 1: Diffusion & Osmosis Complete the virtual lab activity and record your responses in this packet.
Introduction The processes of diffusion and osmosis account for much of the passive movement of molecules at the cellular level.
In this laboratory, you will study some of the basic principles of molecular movement in solution and perform a series of activities to investigate these processes.
Key Concepts I: Diffusion & Osmosis Molecules are in constant motion and tend to move from regions where they are in higher concentration to regions
where they are less concentrated. Diffusion is the net movement of molecules down their concentration gradient.
Diffusion can occur in gases, in liquids, or through solids. An example of diffusion in gases occurs when a bottle of perfume is opened at the front of a room. Within minutes people further and further from the source can smell the
perfume
Osmosis is a specialized case of diffusion that involves the passive transport of water. In osmosis, water moves
through a selectively permeable membrane from a region of its higher concentration to a region of its lower
concentration. The membrane selectively allows passage of certain types of molecules while restricting the movement of others.
The solute concentration in the beaker is higher than that in the bag, and thus the water concentration is lower in the beaker than in the bag. This causes water to move from the bag (left) into the beaker (right).
Movement of Molecules in Solution There are often several different types of molecules in a solution. The motion of each type of molecule is random and
independent of other molecules in the solution. Each molecule moves down its own concentration gradient, from a region of its high concentration to a region of its low concentration.
Though the net movement of molecules is down their concentration gradient, at any time molecules can move in both directions as long as the membrane is permeable to the molecule. Keep this in mind while you take a closer look at
the beaker below.
Notice that the starch molecules are too large to pass through the pores in the membrane. The iodine molecules move across the membrane in both directions, but their net movement is from the bag, where their concentration is higher,
into the beaker, where their concentration is lower. The iodine combines with starch to form a purplish-colored compound.
The net movement of water is into the beaker.
Movement of Molecules in Cells Like dialysis bags, cell membranes are selectively permeable. As you view
the next animation, watch for the selective property of the cell membrane and the two-way diffusion of molecules. Finally, notice the net movement
of the molecules.
The movement of water is influenced by the solute concentrations of the
solutions. Let's review the different types of solutions.
Types of Solutions Based on Solute Concentration The terms hypotonic, hypertonic, and isotonic are used to compare solutions relative to their solute concentrations.
In the illustration, the solution in the bag contains less solute than the solution in the beaker. The solution in the
bag is hypotonic(lower solute concentration) to the solution in the beaker. The solution in the beaker
is hypertonic (higher solute concentration) to the one in the bag. Water will move from the hypotonic solution into the
hypertonic solution.
In this illustration the two solutions are equal in their solute concentrations. We say that they are isotonic to each other.
Water Potential Because you will be working with potato cells in the laboratory, you need to understand the concept of water
potential. Biologists use this term to describe the tendency of water to leave one place in favor of another. Water
always moves from an area of higher water potential to an area of lower water potential.
Water potential is affected by two factors: pressure and the amount of solute. For example, imagine a red blood cell dropped into distilled water. Water will move into the red blood cell and cause the cell to expand, stretching the
flexible membrane. At some point, the pressure of the incoming water will cause the cell to pop, just like an over-filled
balloon.
If a plant cell is placed in distilled water, water will enter the cell
and the cell contents will expand. However, the elastic cell wall
exerts a back pressure, which will limit the net gain of water.
Calculating Water Potential Water potential is calculated using the following formula:
Water Potential The water potential of pure water in an open container is zero because there is no solute and the pressure in the container is zero. Adding solute lowers the water potential. When a solution is enclosed by a rigid cell wall, the
movement of water into the cell will exert pressure on the ce ll wall. This increase in pressure within the cell will raise
the water potential.
Look again at the equation for water potential:
Design of the Experiment I In this laboratory you do a number of exercises to demonstrate diffusion and osmosis.
* There are no new techniques
for you to learn in these exercises, so we present just brief overviews here. If you don't want to review, proceed to
Analysis of Results, where we focus on calculating water potential. Exercise 1: Diffusion Exercise 2: Osmosis Exercise 3: Water Potential of Potato Cores Exercise 4: Water Potential Exercise 5: Plasmolysis
Exercise 1: Diffusion In this activity, you fill a dialysis bag with a sugar/starch solution and immerse the bag in a dilute iodine solution.
Water, sugar, starch, and iodine molecules will all be in motion, and each molecule will move to a region of its lower
concentration, unless the molecule is too large to pass through the membrane. Your task is to determine relative size of the various molecules and gather evidence of molecular movement. Hint: One piece of information that will help
you is to recall that when iodine comes in contact with starch, it changes from an orange-brown color to blue-black.
Exercise 2: Osmosis In this activity you investigate the relationship between solute concentration and water movement by filling six
different dialysis bags with increasing concentrations of sucrose and placing the bags into distilled water. After the time for the experiment has elapsed, you compare the initial weight of each bag with its final weight, calculate the
percent change in mass, pool your data with that of your classmates, and graph your results. Hint: Be sure that the entire dialysis bag and tie are submerged!
Exercise 3: Water Potential of Potato Cores This activity is very similar to Exercise 2, except that you use cores from potatoes instead of dialysis bags. You submerge the cores in solutions of varying sucrose concentrations. When you calculate the percent change in mass,
some of the cores will have gained weight while others will have lost weight, depending on the movement of water.
You then graph this data and determine which concentration of the sucrose solution is in equilibrium with the cores. Since you know that the pressure potential of the surrounding solution in an open beaker is zero, you can now
calculate the water potential.
Exercises 4 & 5: Water Potential & Plasmolysis Exercise 4: Calculation of Water Potential from Experimental Data
In this exercise, you use the value for the molar concentration of the potato cores that you obtain in Exercise 3 to
determine the water potential for the potato cells.
Exercise 5: Onion Cell Plasmolysis In this activity, you watch the effect of placing a living cell into a solution that has a lower or higher concentration of
water than the cell.
Analysis of Results So that you might better understand the procedure for calculating water potential, here is a practice problem. Once
you know the solute concentration, you can calculate solute potential using the following formula:
Lab Quiz
Conclusion Essay Prompt An experiment was designed to test whether different concentration gradients affect the rate of diffusion. In this experiment, four solutions (0% NaCl, 1% NaCl, 5% NaCl, and 10% NaCl) were tested under identical conditions. Fifteen milliliters (mL) of 0% NaCl were put into a bag formed of dialysis tubing that is permeable to Na+, Cl -, and water. The same was done for each NaCl solution. Each bag was submerged in a separate beaker containing 300 mL of distilled water. The concentration of NaCl in mg/L in the water outside each bag was measured at 40-second intervals. The results from the 5% bag are shown in the table below.
(a) On the axis provided, graph the data for the 5% NaCl solution. (b) Using the same set of axes, draw and label three additional lines representing the results that you would predict
for the 0% NaCl, 1% NaCl, and 10% NaCl solutions. Explain your predictions. (c) Farmlands located near coastal regions are being threatened by encroaching seawater seeping into the soil. In
terms of water movement into or out of plant cells, explain why seawater could decrease crop production. Include a discussion of water potential in your answer.
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