KCK ObjectiveWindow 2 Day 4
Cellular Structure and Function
KCK11SCBI0302
Demonstrate an understanding of cellular structure and the interdependence of the organelles for cellular function. (3.1.1, 3.1.4, ▲3.7.2, 301/302/304/318, ACT)
a. Relate the structures of organelles to their specific functions in various cell types.
b. Sequence the movement of materials among intracellular organelles.
c. Model a cell and its organelles and functions.
d. Predict the movement of materials across membranes for both passive and active transport when given information about the concentrations.
e. Relate the concept of homeostasis to membrane transport or other cellular processes.
f. Use a microscope to view cells and identify major organelles (cell membrane, cytoplasm, nucleus).
Lesson Objectives
* identify the cell membrane, central vacuole, cell wall, and chloroplasts in a plant cell
* relate the structure of the cell membrane, central vacuole, cell wall, and chloroplasts to their functions in plant cells
* relate the movement of water from the central vacuole to maintain homeostasis in the plant cell
Materials: cell organelle log sheet, Elodea Osmosis Lab (one per student or group), materials for the lab can be found on the lab worksheet, green construction paper, scissors
Anticipatory Set:
Have students predict how plant cells will look different under a microscope than animal cells. Note their answers on the board, or have them record their answer as bell-work as a writing section. You could then show a projected image of algae cells (image on separate sheet) or have them look at algae under the microscope, if available.
Teacher Input
Present slides in a PPT about the cell structures covered later in the lab (cell membrane, central vacuole, cell wall, and chloroplasts) Or, use video clips that describe these cellular parts. Or, have students read about these organelles on 195, 197, and 198. Discuss both how to identify them and what each one does. As you are covering them, have students directly place the information by writing the descriptions into the cell organelle log for these three terms.
Student Engagement
Begin by having the students complete the rest of the column in their log for the organelles covered today. Once completed, students may then begin the hands-on Elodea Osmosis Lab (directions and explanations on separate sheet).
Any student who is done quickly with the lab and looking for something to do – have that student draw a cell wall around the cell membrane on the class model of the plant cell and a large central vacuole. When other students finish, have them draw and cut-out chloroplasts to add to the class plant cell model.
Closure: Have students make a simple Venn diagram that shows the similarities and differences between plant and animal cells.
Filamentous algae cells
Elodea Osmosis Lab Name: ______
Materials: Elodea leaf (1 per group), microscope slide, cover slip, microscope, tap water, 10% salt solution, paper towel.
Methods: Prepare a wet mount of an Elodea leaf with tap water by putting the leaf on the slide, placing a small drop of tap water, and cover it with the cover slip. Observe the leaf at 40X and record your observations. Increase the magnification to 100X, observe, and record your observations.
Remove the slide from the stage of the microscope. Place 2 drops of the 10% salt solution on the slide at the edge of the cover slip. Tear off a small piece of paper towel and place the torn edge on the slide at the edge of the cover slip that is opposite the side where the salt solution was placed. The piece of towel should begin to soak up water, drawing the salt solution under the cover slip as it does so.
Methods (continued): Return the slide to the microscope stage and repeat the observations of the cells at 40X and 100X. Record your observations. Remove the slide from the stage, clean it and the cover slip, and put it away. Turn off the microscope and put it away.
Observations:
Prepare sketches of a group of Elodea cells under each set of conditions (tap water and salt water). Label the sketches to note the cell structures that you can identify. Be sure to note any changes in the color, size, and shape of the cells. Make your sketches as accurate as possible.
Conclusions: Answer the following questions.
1. What is the shape of the typical Elodea cell?
2. What are the small green blobs found inside the cells? What is their function?
3. What happens to the cells as the salt water flows under the cover slip?
4. Elodea normally lives in fresh water. What changes would you observe in the cells of an Elodea plant that was suddenly moved from fresh water to salt water? Why?
Adapted and Modified from
Elodea Osmosis Lab Activity with Explanation
This section pulled from:
1. Depth of Field: When using a compound microscope this term refers to the thickness of the subject that is in focus. The Elodea leaf is composed of two layers of cells. Only one layer of cells is in focus when using the high power (40x) objective. Therefore, the depth of field is limited to only one layer of cells. You must focus up and down using the fine adjustment knob on the microscope to see both layers of cells.
2. Plasmolysis: Shrinkage of the protoplast or cell contents due to water loss when a drop of salt water (NaCl) is added to the Elodea slide. Because of all the salt ions (Na+ and Cl- ions) outside the cell membrane of each Elodea cell, water molecules move out of the cell membrane causing the cell membrane and it contents to shrink into a blob in the center of the cell wall. The porous, cellulose wall does not shrink because the salt ions easily pass through the wall but cannot pass through the membrane. Therefore, the cell membrane (not the cell wall) loses water molecules and shrinks.
Waterweed (Elodea densa) a submersed South American aquarium plant that is naturalized in ponds, streams and lakes throughout North America. It belongs to the frog's-bit family (Hydrocharitaceae) along with the troublesome Old World aquatic weed called hydrilla (Hydrilla verticillata) that literally clogs the waterways of canals and reservoirs. The highly magnified view of a leaf (right) shows a living photosynthetic cell. Because most of the cell is occupied by a water-filled, large central vacuole, the chloroplasts are displaced around the periphery of the cell, just inside the cell wall and membrane. The chloroplast shown by top red arrow is actually on the outside of this vacuole. Due to the limited depth of field at this magnification (400x), only portions of the viewing plane are in focus at any one time. To see additional chloroplasts you must focus up and down with the fine adjustment knob of the compound microscope. The transparent nucleus is not visible in this image. Please refer to the following illustration:
Elodea leaf cell illustration from a microscope slide. A drop of 10 percent NaCl (sodium chloride) solution was added to the slide. Left: The cell membrane has pulled away from the cell wall marking the onset of plasmolysis called "incipient plasmolysis." Right: The entire cell contents (protoplast) within the membrane has shrunk into a blob in the center of the cell. This phenomenon is called plasmolysis. Salt ions (Na+ and Cl-) have passed through pores in the cellulose cell wall. The ions do not pass through the protein-lipid cell membrane because it is impermeable to them. Due to the concentration differential inside and outside of the membrane, water molecules (shown by blue arrows) move out of the cell membrane, thus causing the cell contents to shrink into a blob. This concentration differential does not occur outside the porous, rigid cell wall, therefore its rectangular shape remains intact.