Definitions of Cellular Organelles

Definitions of Cellular Organelles

Cell Membrane

Objectives

• 
  summarize the six important functions of the cell membrane;
  
• 
  diagram the fluid mosaic model of the cell membrane;
  
• 
  differentiate between osmosis and diffusion in terms of the types of substances involved and the role of a semi-permeable membrane (from Organizing Topic — Investigating Biochemistry).
  

Materials needed

• 
  Student-selected materials, such as wire, tubes, straws, string, pipe cleaners, beads, glue guns and glue sticks, mini marshmallows, magnets, small ball bearings, rubber sheets, toothpicks
  
• 
  Glue
  
• 
  Markers
  
• 
  Poster board
  
• 
  Graph paper or graphing software and printer
  
• 
  Fresh onion, perfume, or other odorous substances that are acceptable for use in the classroom
  
• 
  Water
  
• 
  Tincture of iodine (CAUTION!)
  
• 
  Container for iodine mixture
  
• 
  Starch suspension — water into which spray starch has been mixed
  
• 
  Plastic sandwich bags with ties
  

Instructional activity

Content/Teacher Notes

Membrane components may

• 
  be protective;
  
• 
  regulate transport in and out of the cell or subcellular domain;
  
• 
  allow selective receptivity and signal transduction by providing transmembrane receptors that bind signaling molecules;
  
• 
  allow cell recognition;
  
• 
  provide anchoring sites for cytoskeletal filaments or components of the extracellular matrix. This allows the cell to maintain its shape and perhaps move to distant sites.
  
• 
  help compartmentalize subcellular domains or microdomains;
  
• 
  provide a stable site for the binding and catalysis of enzymes;
  
• 
  regulate the fusion of the membrane with other membranes in the cell via specialized junctions;
  
• 
  provide a passageway across the membrane for certain molecules, such as in gap junctions;
  
• 
  allow directed cell or organelle motility.
  

Introduction

2. After the fumes have diffused, begin a discussion of diffusion and associated terms such as kinetic energy and Brownian motion. Discuss the importance of being able to control what goes out (keeping the onion odor in one place) and comes in (getting water when you need it). This will lead to a differentiation between osmosis and diffusion. Relate the discussion to what is observed as the odor diffuses throughout the room. Segue into the assignment.

Procedure

1. Construct a model of the cell membrane that illustrates or demonstrates the following:

• 
  Phospholipid bilayer molecules
  
• 
  Glycoprotein markers
  
• 
  Channel proteins
  
• 
  Selective permeability
  
• 
  Cholesterol
  
• 
  Receptor proteins
  

2. For good representations and explanations of a cell membrane see the following Web sites:

• 
  http://library.thinkquest.org/C004535/cell_membranes.html
  
• 
  http://sun.menloschool.org/~cweaver/cells/c/cell_membrane/
  
• 
  http://telstar.ote.cmu.edu/Hughes/tutorial/cellmembranes/
  
• 
  http://www.johnkyrk.com/cellmembrane.html
  

1. Fill a 500-mL beaker half full of water.

2. Add tincture of iodine solution until the water is noticeably colored. Mix well.

3. In a separate space away from possible contamination, put 100-200 mL of water in a sandwich bag.

4. Add 5–10 mL of the starch suspension. Squeeze gently to mix.

5. Tie off the bag.

6. Rinse the outside of the bag well.

7. Place the bag in the beaker of the iodine solution.

8. Set up a bag without starch as a control.

9. Wait and watch. (It may take overnight.)

Observations and Conclusions

1. Obviously, the models in Part 1 will vary. Student creativity should be encouraged, but all constructions should be safe and sturdy enough to hold up to use in a presentation of what the parts represent and how they show some characteristic of the structure of the components modeled. If the function extension is included, then the part should actually function by modeling the process as well as represent the component’s structure.

2. In Part 2, the students should observe (based on the iodine/starch reaction shown in the biomolecules activities) that in the time frame and under the conditions of the activity, iodine went into the bag and starch did not come out.

Sample assessment

• 
  Provide students with analogies of cell membrane parts, and have them apply metaphorical analyses to identify the parts, which map onto the analogy.
  
• 
  Have students discuss why they saw what they saw in Part 2, based on size, polarity, and concentrations.
  
• 
  Have students discuss what happened, if anything, to the water itself in Part 2.
  

Follow-up/extension

1. Most of the models will be static. An optional extension is to have at least one part function to model a dynamic process. When presented with this challenge, students have been known to be rather creative in producing functioning analogs.

2. You may wish to have students differentiate between the types of transport proteins by including distinctive models of these transport varieties:

• 
  Uniport
  
• 
  Synport
  
• 
  Antiport
  
• 
  Active
  
• 
  Passive
  
• 
  Facilitated diffusion
  

3. Engage students in a discussion of the practical value of an understanding of osmosis and diffusion (separation science, dialysis, bactericidal effects). Have students look at homeostasis life processes that depend on transport mechanisms, such as waste removal, prevention of freezing in some winter survival mechanisms, or gas exchange. (This may be delayed until later in the course, if deemed more appropriate.)

Resources

Suggested Web sites with graphics and explanations of the cell membrane:

• 
  Cell Anatomy: Cell Membrane. http://library.thinkquest.org/C004535/cell_membranes.html.
  
• 
  Cell Membranes. http://www.johnkyrk.com/cellmembrane.html.
  
• 
  Geobel, Greg. The Cell Membrane. http://sun.menloschool.org/~cweaver/cells/c/cell_membrane/.
  
• 
  The Structure and Function of the Cell Membrane. http://telstar.ote.cmu.edu/Hughes/tutorial/cellmembranes/.
  

Mitosis and Cell Cycle

Organizing Topic Investigating Cells

Overview Students investigate the phases of the typical cell cycle, including nuclear and cytoplasmic division.

Related Standards of Learning BIO.4a; BIO.6a

Objectives

The students will

• 
  discuss the different types of cells that undergo mitosis and cytokinesis and their rates of cell division;
  
• 
  describe the events that occur during the cell cycle, emphasizing mitosis and cytokinesis;
  
• 
  diagram the different phases of the cell cycle, labeling the parts of the cell that are pertinent. Records may include the percentage of the time cells spend in each phase.
  

Materials needed

• 
  Prepared slides of onion root tip mitosis
  
• 
  Prepared slides of animal cell mitosis
  
• 
  Microscopes
  
• 
  Video of the mitosis process (See http://www.cellsalive.com, which is an excellent site for both phases of mitosis and the cell cycle.)
  
• 
  Drawing materials or microphotographic hardware and photo-editing software
  

Optional lab

• 
  Fresh onion root tips
  
• 
  Toluidine blue, 2%
  
• 
  Water
  
• 
  A
  M
  
  (mitosis)
  pron or lab coat
  
• 
  Carnoy fluid with chloroform
  
• 
  C
  G2
  
  (Gap 2)
  
  G1
  
  (Gap 1)
  ompound microscope
  
• 
  Coverslips
  
• 
  Eyedropper
  
• 
  Hydrochloric acid, 18%
  
• 
  Latex gloves
  
• 
  Safety glasses
  
• 
  Slides
  

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