Diffusion/ Osmosis/ Water Potential Lab
Notes From the teacher
Day 1
Before class:
· Review the lab – read the background section to the end of PART A (Surface Area and Cell Size).
· Complete the pre-lab for PART A:
o Title and date of the lab à for the title, just say Osmosis/ Diffusion Lab: Part Aà Surface Area and Cell Size (remember to add this lab to your table of contents).
o Purpose à 1-2 sentences describing the overall goal of PART A only; use complete sentences
o Pre-Lab Questionsà you need to number the question, rewrite the question, and then answer it for full credit (see the lab for a list of Pre Lab questions for Part A)
o Lab Procedure à Write a procedure for PART A. First list the materials that you will be using, then use your own words to describe the steps in experiment in paragraph form.
o Data Table à Tape this into your lab notebook (or recopy it)
In class:
· Complete PART A along with all of the data collection, make the graph, and complete the analysis questions. If you do not have time to finish the analysis questions, finish this for homework.
Day 2
Before class:
· Review the lab – read the background section to the end of PART B (Modeling Diffusion and Osmosis).
· Complete the pre-lab for PART B:
o Title and date of the lab à just say for the title - Osmosis/Diffusion Lab: Part B à Modeling Diffusion and Osmosis
o Purpose à 1-2 sentences describing the overall goal of PART B; use complete sentences
o Pre-Lab Questionsà you need to number the question, rewrite the question, and then answer it for full credit
o Lab Procedure à Write a procedure for PART B. First list the materials that you will be using, then use your own words to describe the steps in experiment in paragraph form.
o Data Table à Tape this into your lab notebook (or recopy it)
In class:
· Complete PART B along with all of the data collection, calculations, and analysis questions. If you do not have time to finish the analysis questions, finish this for homework.
Day 3
Before Class:
· Review the lab – read PART C (Understanding Water Potential)
· Complete the pre-lab for PART C:
o Title and date of the lab à just say for the title - Osmosis/Diffusion Lab: Part C à Understanding Water Potential
o Purpose à 1-2 sentences describing the overall goal of PART C; use complete sentences
o Pre-Lab Questionsà you need to number the question, rewrite the question, and then answer it for full credit
In class:
· Review water potential with the teacher.
· Water Potential Problems in lab notebook
Day 4
Before class:
· Review the lab – read PART D (Observing Osmosis in Living Cells)
· Complete the pre-lab for PART D:
o Title and date of the lab à just say for the title - Osmosis/Diffusion Lab: Part D à Observing Osmosis in Living Cells
o Purpose à 1-2 sentences describing the overall goal of PART D; use complete sentences
o Pre-Lab Questionsà you need to number the question, rewrite the question, and then answer it for full credit
o Lab Procedure à Write a procedure for PART D. First list the materials that you will be using, then use your own words to describe the steps in experiment in paragraph form.
o Data Tables/ Graphsà Tape or draw these into your lab notebook.
In class:
· Complete the initial portion of PART D of the lab
Day 5
Before Class: n/a
In class:
· Finish PART D of the lab along with all of the data collection, the graph, and complete the analysis activities and the analysis questions. If you do not have time to finish the analysis questions, finish this for homework. Remember to put both graphs (one for the data part of the lab and the other for one of the analysis questions) in your lab notebook.
Diffusion/ Osmosis/ Water Potential Lab
BACKGROUND
Cells must move materials through membranes and throughout cytoplasm in order to maintain homeostasis. The movement is regulated because cellular membranes, including the plasma and organelle membranes, are selectively permeable. Membranes are phospholipid bilayers containing embedded proteins; the phospholipid fatty acids limit the movement of water because of their hydrophobic characteristics.
The cellular environment is aqueous, meaning that the solvent in which the solutes, such as salts and organic molecules, dissolve is water. Water may pass slowly through the membrane by osmosis or through specialized protein channels called aquaporins. Aquaporins allow the water to move more quickly than it would through osmosis. Most other substances, such as ions, move through protein channels, while larger molecules, including carbohydrates, move through transport proteins.
The simplest form of movement is diffusion, in which solutes move from an area of high concentration to an area of low concentration; diffusion is directly related to molecular kinetic energy. Diffusion does not require energy input by cells. The movement of a solute from an area of low concentration to an area of high concentration requires energy input in the form of ATP and protein carriers called pumps.
Water moves through membranes by diffusion; the movement of water through membranes is called osmosis. Like solutes, water moves down its concentration gradient. Water moves from areas of high potential (high free water concentration) and low solute concentration to areas of low potential (low free water concentration) and high solute concentration. So, water moves towards where there is MORE TOTAL SOLUTE. Solutes decrease the concentration of free water, since water molecules surround the solute molecules. The terms hypertonic, hypotonic, and isotonic are used to describe solutions separated by selectively permeable membranes. A hypertonic solution has a higher solute concentration and a lower water potential as compared to the other solution; therefore, water will move into the hypertonic solution through the membrane by osmosis. A hypotonic solution has a lower solute concentration and a higher water potential than the solution on the other side of the membrane; water will move down its concentration gradient into the other solution. Isotonic solutions have equal water potentials.
In cells without a cell wall, such as animal cells, the movement of water into and out of a cell is affected by the relative solute concentration on either side of the plasma membrane. As water moves out of the cell, the cell shrinks; if water moves into the cell, it swells and may eventually burst. In cells with a cell wall, including fungal and plant cells, osmosis is affected not only by the solute concentration, but also by the resistance to water movement in the cell by the cell wall. This resistance is called turgor pressure. The presence of a cell wall prevents the cells from bursting as water enters; however, pressure builds up inside the cell and affects the rate of osmosis. Water movement in plants is important in water transport from the roots into the shoots and leaves. You likely will explore this specialized movement called transpiration in another lab investigation later in this course.
General Safety Precautions
You must wear safety glasses or goggles, aprons, and gloves because you will be working with acids and caustic chemicals. The HCl and NaOH solutions will cause chemical burns, and you should use these solutions in spill-proof trays or pans. Follow your teacher’s instructions carefully. Do not work in the laboratory without your teacher’s supervision. Talk to your teacher if you have any questions or concerns about the experiments.
THE INVESTIGATIONS
This investigation consists of three parts. In PART A, you use artificial cells (cubes of agar) to study the relationship of surface area and volume. In PART B, you create models of living cells (by using dialysis tubing to simulate cell membranes) to explore osmosis and diffusion. Finally, in PART C, you will be observing osmosis in living cells (potato) that are submerged in different concentrations of sugar solutions. You will also be learning how to solve water potential problems during this lab.
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PART A: Surface Area and Cell Size
Overview – Part A:
Cell size and shape are important factors in determining the rate of diffusion. Think about cells with specialized functions, such as the epithelial cells that line the small intestine OR plant root hairs. How do they aid in absorbing nutrients for the plant or the animal? In this section of the lab you are going to create 3 different size “cells” and look at how fast diffusion happens in each one. The cells will be made out of agar that has been treated with a pH indicator called bromothymol blue. When this indicator comes into contact with an acid (we are using vinegar to act as the acid), it will change from the blue color to a yellowish/clear color. Depending on how fast each block changes color completely, we will be able to figure out the diffusion rate and see how cell size affects that.
PreLab Questions – Part A:
Read the Background information for this lab and answer the prelab questions below in your lab notebook. Don’t forget to copy the question before answering it. Your answers do NOT need to be in complete sentences, but you do need to show your work for the calculations. You may need to refer to your text or an online source to answer some of the questions.
1. How do small intestinal epithelial and root hair cells function in absorbing nutrients and how does their structure aid in this?
2. What is Bromothymol Blue and how are we using it in Part A of this lab?
3. What is diffusion?
4. What is kinetic energy?
5. Why are most cells small?
6. Calculate the surface area of this cube.
7. Calculate the volume of this cube.
8. Calculate the surface area to volume ratio of this cube
(See Procedure for help with how to do these calculations!)
Materials – Part A:
Scalpel
Metric rulers
Dishes
Bromothymol Blue agar preparation*
Vinegar
Procedure – Part A:
1. Cut your agar into 3 different size CUBES making sure that all the sides of each cube are the same size. You should end up with a small, medium, and large size cube. Do not make any cubes where the sides are larger than 1.5 cm.
2. Measure your cubes and enter the height, length, and width for each one into the Data Table 1.1.
3. Think about how size affects diffusion rate and hypothesize which cube will turn totally yellow the fastest, the second fastest, and then slowest. Record your hypothesis in Data Table 1.1.
4. Drop all 3 cubes into a cup with vinegar in it and note the time in your data table. As the vinegar (acid) diffuses into the agar, the pH indicator in the agar (bromothymol blue) changes from blue to yellow. When each cube turns totally yellow, note the time and record it in Table 1.1.
5. While you are waiting for the vinegar to diffuse, calculate surface area and volume for each of your cubes. Show your work in your lab notebook and put your answers in the data table.
Surface Area = L x W x # of sides
Volume = L x W x H
6. Calculate the ratio of SA:Vol by dividing the surface area by volume for each one. This should be a DECIMAL, not a FRACTION. Show your work in your lab notebook and put your answers in the data table.
7. After all your cubes are done, figure out how many minutes it took for the vinegar to diffuse all the way through each of your cubes.
8. Calculate the distance the vinegar traveled for each of the cubes:
Distance = .5 x (smallest measurement of height, length or width)
For example, if my cube is 1.4 cm on each side, the distance traveled would be .7cm.
9. Calculate the rate of diffusion in each of the cubes:
Rate of Diffusion = Distance / Minutes to completion
Data Table 1.1: Diffusion Rate of Vinegar in Agar
Cube #1Biggest / Cube #2
Medium / Cube #3
Smallest
Height (cm)
Length (cm)
Width (cm)
Hypothesis
Time into Vinegar
Time Finished
# minutes for completed diffusion
Surface Area (show calculations in lab notebook)
Volume (show calculations in lab notebook)
SA/Volume Ratio
Distance Vinegar Traveled
Rate of Diffusion (cm/min)
Graph – Part A:
Using a ruler, create a graph of your data. Time (min) goes on the x-axis (this will be the # of minutes it took to complete diffusion) and SA/V ratio (no units) goes on the y-axis. You will have 3 data points (one for each agar cube) and you will connect them to make a line graph. Label each of your data points as “Large,” “Medium,” or “Small” for the size of the cube the point represents.
Analysis Questions – Part A:
1. Looking at your graph, what can you tell me about the correlation between the size of a cell and the rate of diffusion?
2. Calculate the following data values for the cells shown below. Make sure you SHOW YOUR WORK in your lab manual and clearly identify what the numbers mean.
a. Surface area, volume and ratio for the one large cell
b. Surface area, volume and ratio for 1 small cell