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ANALYZE IT

Note: Questions marked with a triangle (∆) are included to enrich students’ understanding. These questions do not appear in students’ SDRs but should be used as additional discussion points during the PostLab.

As a class, discuss the structures that students were able to observe in the six slides they viewed during this investigation.

• ∆ Based on your knowledge of plant and animal cells, which structures should you have been able to see in all of the plant cells? Were you able to see them? Plant cells generally contain a nucleus, a cell wall, a cell membrane, cytoplasm, chloroplasts, nuclear membrane, centrosome, Golgi, endoplasmic reticulum, central vacuole, microfilaments, microtubules, mitochondria, plasma membrane, and plasmodesmata. However, students were most likely unable to view all of these structures in each of the slides. Visible in the Elodea leaf were the cytoplasm, cell wall, nucleus, cell membrane, and chloroplasts, but it is likely that students would be unable to view a difference between the cell wall and cell membrane. They would, however, be able to observe the cytoplasm and nucleus. The onion did not contain chloroplasts, and students would again be unable to distinguish between the cell wall and the cell membrane. The cytoplasm was more clearly visible in cells that were colored or cells that were stained.

• ∆ Why were you unable to distinguish between the cell wall and the cell membrane in the plant cells? Students would have been unable to distinguish between the cell wall and the cell membrane because the space between the two structures is so minimal. Greater magnification or resolution is needed in order to see the separation between the two structures.

• ∆ Why were you unable to view the nuclear membrane, centrosome, Golgi, endoplasmic reticulum, central vacuole, microfilaments, microtubules, mitochondria, plasma membrane, and plasmodesmata? Students were unable to distinguish other organelles and structures because they were unstained and smaller in size. Staining and increased resolution would have been needed to observe these structures.

• ∆ Why were you unable to view any chloroplasts in the onion cell? To fully answer this question, students must understand the function of chloroplasts and the manner in which onions grow. Chloroplasts are structures responsible for photosynthesis in plants, or food production. Photosynthesis requires light in order to occur. The white onion bulb, or the part of the onion we generally eat, grows underground, with the green leaves growing in the sunlight. Because the white part of the onion does not receive significant sunlight, very little photosynthesis occurs within its cells. The green leaves of the onion contain the cells which are responsible for food production. Therefore, very few chloroplasts are found in the white flesh of the onion.

• ∆ Based on your knowledge of plant and animal cells, which structures should you have been able to see in all of the animal cells? Were you able to see them? Animal cells contain cell membranes, nuclei, cytoplasm, endoplasmic reticulum, peroxisomes, centrosomes, mitochondria, lysosomes, microfilaments, microtubules, DNA, nuclear membranes, ribosomes, Golgi apparatus, and plasma membranes. Students were likely able to view some of these structures in all of the specimens, including cell membranes, nuclei, and cytoplasm. Students may have had difficulty seeing the cytoplasm in the cheek cells because the unstained cytoplasm appeared clear. Students also should have noticed that they were able to view nuclei in only some of the blood cells on the blood smear. Inform students that red blood cells do not contain nuclei. The cells with nuclei that students observed were the larger white blood cells rather than the smaller red blood cells. Other organelles and structures may have been difficult to observe because they were unstained or smaller than the resolution of the microscope.

Instruct students to recall one of the questions presented to them at the beginning of Investigation One.

• ∆ How are cells from one organism different from or similar to the cells of another? How might you answer the question after you observed the last set of specimens? Based on our observations, all cells had cell membranes, cytoplasm, and nuclei, with the exception of red blood cells, E. coli, and pneumoniae, which had no nuclei. Cells from plants had cell walls and some had observable chloroplasts, but animal cells did not have either of these structures. Instruct students to answer question #7 from their Investigation Three Data Record.

As a class, discuss whether or not cells from the same organism are identical.

• ∆ Were the two cell samples from the Elodea plant identical? No. The slide of the Elodea leaf was comprised of rectangular cells positioned next to one another. The cells contained cytoplasm and nuclei stained dark blue and purple. The Elodea stem contained somewhat circular cells of various sizes that appeared empty. Students saw different structures and different organizations of cells on each of the two slides, in part because the function of the stem is different than the function of the leaf.

• ∆ Compare your observations of the blood smear slide, the cheek cell slide, and the large intestine slide. Were the cells in each of those slides identical? No. The blood cells, cheek cells, and large intestine cells all differed in appearance. The size and shape of the cells varied, as did the structures observed in each cell. The cells in the large intestine, the cheek cells, and the white blood cells contained nuclei, but the red blood cells did not. The red blood cells were disc-shaped and concave, while the cheek cells were irregularly shaped, and the cells in the large intestine were of various shapes.

After students have thought about specific examples, ask the question again.

• Are all cells from the same organism identical? No, the cells in an organism may differ from tissue to tissue. Instruct students to answer question #8 in their Investigation Three Data Record.

Ask students to recall all of the cells they viewed throughout Investigations One through Three.

• ∆ Why do you think cells from the same organism differ? Cells from the same organism may differ as a result of their function within the organism. For example, red blood cells carry oxygen and carbon dioxide into and away from the tissue. In order to do this, they must be small enough (7 microns) to travel through capillaries. Capillaries can have diameters as small as 5 to 10 microns (μm). Other cells that do not travel through capillaries would not need to be as small.

• ∆ How does the arrangement of cells within a tissue affect the function of an organ? Student answers will vary. For example, students may observe differences between the arrangement of cells in the blood smear slide and the large intestine slide. The cells in the blood smear slide seem randomly spread on the slide with little or no organization. However, the cells in the large intestine are very organized in their layout. The organization appears to be specific and the cells create specific structures. These differences may be due to the function of blood and the function of the large intestine. Blood is a liquid that must flow fluidly throughout the body. The seemingly random layout of the cells contributes to this fluidity. The large intestine, however, has the primary function of absorption of water and nutrients. The structures created by the organization of the cells aid in this absorption. Students may attempt to draw conclusions concerning the structure and arrangement of the cheek cells they observed and the function of the cheek tissue. Remind students of the importance of the preparation of the slide when interpreting observations. Given the method used to create the wet mount slide from the cheek cells, it is impossible to know the natural arrangement of the cheek cells within the mouth. In order to draw conclusions about the arrangement and the structure of the cells in the tissue of the mouth, we would need to take a small section of tissue from the cheek and view it, rather than viewing isolated cells. Instruct students to answer question #9 in their Investigation Three Data Record.

Instruct students to complete the Analysis Question in their SDRs then discuss it as a class. Use the suggested response below to guide students’ answers.

• Based on your observations, would you agree or disagree with the following statement? Explain your answer. “The function of a tissue or organism depends only on the cellular organelles in the cells.” Student answers will vary. However, students should begin to see that function at the organism or tissue level depends upon both the structures present in individual cells and the organization of cells within a tissue or organ. In addition, students may infer that other factors such as cell-specific genes and proteins also contribute to the specificity of cellular and tissue function.

GET FOCUSED

Instruct students to complete the Focus Questions in their SDRs then discuss them as a class. Use the suggested responses below to guide students’ answers.

• How are the structures in plant and animal cells similar to and different from each other? Plant cells generally contain a nucleus, a cell wall, a cell membrane, chloroplasts and cytoplasm. However, under a magnification of 1000X, it is not possible to differentiate between the cell wall and the cell membrane. In addition, not all plant cells contain chloroplasts. Animal cells generally contain a nucleus, a cell membrane, and cytoplasm. However, some animal cells, such as red blood cells, do not contain nuclei.

• Are all cells from the same organism the same? No. Based on our observations of different cells from the Elodea plant and human tissue, the cells in an organism may differ from tissue to tissue.