Cell Membranes and Diffusion Lab
By Karen Koyko
Science 10, Unit 2 “Energy and Matter in Living Systems”
“The cell is an open system exchanging energy and matter with the environment”
· “describing how materials diffuse across the cell membrane in terms of concentration gradients”
· “describing how the semi-permeable nature of the cell membrane allows the process of osmosis”
Handouts for students (attached)
Rubrics for students (attached)
Rope (optional for pre-lab activity)
Dialysis tubing
Scissors
Distilled water
Salt
Beakers
Stirring sticks
Optional: Weigh scales and thermometers
1) What is a cell membrane?
2) What are some functions of the cell membrane?
3) Does water cross the cell membrane? How?
· Yes. It flows through pores in the cell membrane.
4) What determines the flow of water through the membrane?
5) Will the final concentration of water in and out of the cell the same or different?
6) Does everything cross the cell membrane?
7) What would happen to a cell if the concentration of water in the cell was less then the concentration of water outside the cell?
8) What would happen to a cell if the concentration of water in the cell was greater then the concentration of water outside the cell?
**Have a grape and a raisin as visual aids to go along with questions 8 & 9.
**Questions 8 & 9 are important because they are the basis for the hypotheses for this experiment.
· Split class into small groups, 3 or 4 would be appropriate.
· Explain that each group will be given enough dialysis tubing to “create” 1-4 cells.
· Explain that dialysis tubing acts like a cell membrane in that water can flow through it while some other substances cannot. In our experiment we will have salt available which is a substance that will not flow through dialysis tubing.
· Demonstrate how to use dialysis tubing to create a “cell”.
· Remind students of how they answered questions 8 and 9 (above in guiding questions section) and tell them they are designing an experiment to test their hypotheses based on those questions.
· Let groups split up to design their experiments.
· Bring students back to large group to discuss designs and / or walk around to approve groups’ designs – how will they record their observations? How will they measure their results?
· Remind students that in science it is very important to be able to replicate experiments so they should write out in detail everything they do so that another person could replicate their experiment.
1) What changes if any would you make to your experimental design. Explain.
2) Was your hypothesis supported or refuted? What conclusions can you draw from this experiment?
3) Red blood cells contain proteins, carbohydrates, and other large molecules. Therefore, the concentration of water inside the cell is not 100%. If these cells were placed in a solution of distilled water (100% water) what do you think would happen? Explain.
4) Many unicellular organisms live in pond water. Because the unicellular organism contains many organelles and large molecules, the concentration of water in the cells is less than the concentration of water in the pond. Would water tend to enter, or leave, these organisms? Investigate the water in which unicellular organisms control the amount of water in the cell.
Water enters these organisms. Contractile vacuoles control water balance in many of these organisms.
5) What factors might affect the speed of water movement in or out of a cell?
6) Initially, did your cell float or sink? Specify the conditions under which your cells sunk and/or floated and provide an explanation for your observations.
Cells full of distilled water will float in salt water. Cells full of salt water will sink in distilled. Explanation should include the fact that salt water is more dense that fresh water, and whatever is most dense will sink
Related topics that could should be covered after this activity include:
Active & passive transport
Hyper & hypotonic solutions
Concentration gradient
Additional areas this activity (with modifications) could be utilized:
§ Science 10 – Unit 2, concept 1
Describe the structure and function of a cell membrane in relation to active and passive transport
§ Science 10 - Unit 2, concept 3
Describe how the semipermiable nature of the cell membrane allows the process of osmosis. Students could also design their own experiment with “Ziploc” bags, water, starch, and iodine. An experiment very similar to the one just completed but includes the additional concept of an indicator. In the experiment a starch solution is place in a plastic bag. The plastic bag is placed in an iodine solution. The iodine can cross into the bag but the starch can not escape the bag. A purplish color (result of iodine plus starch) is therefore seen in the bag but not out of it.
§ Biology 30 – Unit 4, concept 2
Excretory systems maintains internal equilibrium with respect to water, PH, and ion concentration
Please see attached rubric.
· Tell students to handle glassware carefully. Remind students to let the teacher know if any glass is broken and how to dispose of glass (ie. In the glass disposal container).
· Warn students not to drink salt water! If a student drinks highly concentrated salt water the student should be taken to the hospital.
Smallwood, William L. (1981). Laboratory investigations in biology record book: Teacher’s edition. Morristown, New Jersey: Silver Burdett Company.
Science 10. (1995). Alberta Learning.
Keywords and Definitions:
Homeostasis: the state of balance that living organisms use to adapt to changing conditions both within themselves and in their environment. At a cellular level, this depends greatly on the movement of materials into and out of the cell.
Cell Membrane: Lipid bilayer with proteins embedded within it. This is the regulatory factor that determines what can and cannot cross into and out of the cell.
Factors affecting whether a substance will cross a membrane:
-the size of the particles
-whether or not the particles dissolve in water – ions/substances which are not
able to dissolve in water usually cannot pass through
-conditions inside or outside the cell
-the structure of the plasma membrane itself – pore size
Permeable: when a substance passes through a membrane, the membrane is said to be permeable to that substance.
Selectively permeable: If the membrane lets some substances pass and not others, it is said to be selectively permeable.
Diffusion: The random movement of molecules leading to the molecules gradually, evenly spreading out until an equilibrium is reached with uniform molecules throughout.
Factors affecting diffusion:
-temperature – the higher the temperature, the greater the molecular motion
-pressure – the higher the pressure, the greater the molecular motion
Osmosis: the diffusion of water through a selectively permeable membrane from an area of greater concentration to an area of lesser concentration.
Hypotonic: the solution with the greater concentration of water molecules vs. the other side of a selectively permeable membrane.
Hypertonic: the solution with the lesser concentration of water molecules vs. the other side of a selectively permeable membrane. Flow would be from hypo to hypertonic down the concentration gradient.
Isotonic: the same concentration of water on both sides of a selectively permeable membrane. Rate of diffusion is constant and equal in both directions.
Passive transport: movement of molecules by their own kinetic (heat/movement) energy.
Active transport: movement of molecules using some form of energy, such as when needed to move molecules against concentration gradients. (ATP usually used)
Manipulated (independent) variable: the condition of an experiment that is controlled or changed in a controlled manner.
Responding (dependent) variable: the condition that changes in an experiment with changes to the manipulated variable.
HANDOUT FOR STUDENTS
Lab: Cell Membranes and Diffusion
Create a hypothesis based on your answers to the following:
Design and write out a procedure that will test your hypotheses.
Questions you might think about . . .
· What is your independent variable? Your dependent variable?
· What evidence will you accept as supporting/rejecting your hypothesis?
· How will you record and / or report your observations?
· What, if any, variables will you need to control?
· What equipment will you need to use?
· Remember to be specific about your procedure, so another scientist could replicate your experiment from the information you include in your lab report.
Available Materials
Dialysis tubing
String
Distilled water
Salt
Beakers
Stirring rods
Scale
Thermometers
4) What changes if any would you make to your experimental design? Explain.
5) Was your hypothesis supported or refuted? Explain your results.
6) Red blood cells contain proteins, carbohydrates, and other large molecules. Therefore, the concentration of water inside the cell is not 100%. If these cells were placed in a solution of distilled water (100% water) what do you think would happen? Explain.
7) Many unicellular organisms live in pond water. Because the unicellular organism contains many organelles and large molecules, the concentration of water in the cells is less than the concentration of water in the pond. Would water tend to enter, or leave, these organisms? Investigate the water in which unicellular organisms control the amount of water in the cell.
8) What factors might affect the speed of water movement in or out of a cell?
9) Initially, did your cell float or sink? Specify the conditions under which your cells sunk and/or floated and provide an explanation for your observations.