Teacher Portal:
Microscopic Exploration
Investigation 4 – PreLab
ASK WHY
Microscopes are one of the most important scientific instruments developed. In fact, in the medical field, microscopes are largely responsible for making modern medicine “modern”!
BRANCH OUT
Cell Biologists study how cells work. Cell Biologists are involved in important research in many different areas of biological sciences. They are employed at universities, hospitals, clinics, and industry.
PRINT IT
Use your browser to download a printable PDF as help during the slide presentation and to make additional notes. In your browser, go to File > Print and then choose to save as PDF.
MINDSET
This Investigation is designed to:
- provide an opportunity for students to learn about the history of science and scientific experiments.
- introduce students to the concept of photosynthesis.
- provide students an opportunity to view examples of eukaryotic plant cells (onion and Elodea leaf cells) with the compound microscope.
- provide students with an additional opportunity to prepare a specimen for microscopic analysis using the Procedural Tool: Wet Mount Slide Preparation.
- help students understand the relationship between the structure and function of cells and their parts.
- review the concept of scale and the importance of accurately representing microscopic observations.
- promote the use of differences in the field of view, magnification and resolution of the low, medium and high power objectives of the compound microscope when observing specimens.
SCIENTIST’S GLOSSARY
- Cell: the smallest structure of living organisms that can perform functions necessary for life. Most cells are microscopic.
- Cell membrane: the outer layer of a cell. The cell membrane helps control what can enter and exit a cell.
- Cell wall: the non-living structure that surrounds the cell membrane of most plant cells. The cell wall helps the plant cell keep its shape.
- Chloroplast: organelle in plant cells where photosynthesis occurs.
- Chlorophyll pigments: green substances found in the chloroplasts of plants. Chlorophylls give leaves their green color and are important for photosynthesis.
- Cytoplasm: the gel-like area of the cell between the nucleus and cell membrane where many of the functions of the cell occur.
- Eukaryotic cell: a cell that contains a defined nucleus.
- Nucleus: small sac inside a cell that contains or contained DNA. One type of organelle in a cell.
- Organelle: a small structure inside a cell that performs a specific function.
- Photosynthesis: The process in which energy from light is converted into the energy of chemical bonds. In plants, water, carbon dioxide and light are used to produce sugar and oxygen. The sugar is used as a source of food for the plant.
- Prokaryotic cell: a cell that does not contain a clearly defined nucleus, such as bacteria.
- Specimen: an example, or a small piece of a larger object, animal or plant.
BE PREPARED
Watch the Investigation 4 Teacher Video and Student Video below to prepare for the PreLab.
SET FOR SUCCESS
- Tell students that they are about to begin their study of plants cells.
- Ask students to share the kinds of things they expect they might learn in these Investigations.
- Tell students they will use the compound microscope to examine plant cells.
Begin the PreLab Concept Slides to start students on their learning journey. Then watch the Pre-Lab Student Video as a class.
NAVIGATE IT
Once the slide presentation is launched
- use your left and right arrows to advance or go back in the slide presentation, and
- hover your mouse over the left edge of the presentation to get a view of the thumbnails for all the slides so that you can quickly move anywhere in the presentation.
- Click HERE to launch the slide presentation for the CELL.
SHARE IT
SLIDE MICRO4-pre-1
In Investigation four, we will learn about plant cells. In the process, students will compare both the difference and the similarities of plant and animal cells.
In addition, students will learn about sub-cellular structures called organelles and in particular, a plant organelle called the chloroplast, and one of the most important chemical reactions that occurs on Earth, photosynthesis.
Review of Microscopic Explorations to this point:
Cells are found in living organisms and contain many different structures. What are some structures that you would expect to find in a cell? Students should indicate that they would expect to find a cell membrane and cytoplasm in a cell. They may suggest that some but not all cells may also contain a nucleus.
How would you describe the size of most cells? Can cells be seen with the unassisted eye? Students should indicate that most cells are microscopic and cannot be viewed with the unassisted eye. A microscope is needed to view most cells.
Note: In general, the cells of most organisms are microscopic. However, there are some exceptions such as the giant squid axon and birds eggs which can be viewed with the unassisted eye.
Do the parts of a cell have a function? What can the structures of a cell tell you about their functions? Students should suggest that the parts of a cell do have a function. Some of the functions of the different parts of a cell can be inferred from their structures. For example, the cell membrane surrounds a cell. It separates the inside from the outside of a cell. Therefore, it is likely that the cell membrane helps to control substances that can enter and exit a cell.
Do you think that animal cells contain different structures than plant cells? Why? Student answers will vary. Students may use their knowledge of the structure and function of animal and plant parts to infer that there may be differences between the two types of organisms. Accept all answers that are provided with an explanation.
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SLIDE MICRO4-pre-2
This slide was first seen by students in Microscopic Explorations Investigation 3 PreLab and shows the division in life on Earth between eukaryotic and prokaryotic organisms.
Note: An important observation can be made from this simple slide. That is that, while animal cells lack the ability to photosynthesize, some protozoans and even some prokaryotes (like blue-green algae) can perform photosynthesis.
Eukaryotes:
- Notice that, while some eukaryotes are single cells, like the protozoans, eukaryotes also include multicellular plants and animals. You are a eukaryote (“you-karyote”), for example.
- Every eukaryotic cell, both single-celled and multicellular organisms, and both plants and animals have a nucleus that is surrounded by a membrane, the nuclear membrane. In fact, the root karyon is Greek and refers to the nucleus.
- Eukaryotic organisms are also much larger than prokaryotic cells.
Prokaryotes:
- All prokaryotes are unicellular.
- Although prokaryotes have DNA, it is not surrounded by a nuclear membrane to separate it from the rest of the cytoplasm. Thus, we say that prokaryotes have no nucleus.
- Prokaryotes appeared on Earth way before eukaryotic organisms.
- Prokaryotic cells are much smaller than eukaryotic cells.
- Unlike eukaryotes, prokaryotes do not have organelles (mitochondria, lysosomes, etc.).
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PRELAB EXERCISE
Tell students that during Investigation Four they will have an opportunity to expand their knowledge of cells by working with and observing plant cells. Explain that scientists have been investigating plants, their functions, and plant cells for several centuries.
Encourage students to imagine what it must have been like to be the first to discover some of the functions of plants. Direct students to Problem 1 in their SDRs. Tell students that the paragraph in Problem 1 is similar to one that they may have read if they lived in the 18th century and were friends of a man named Joseph Priestly. Explain that Joseph Priestly was a minister and a scientist and the passage is similar to one that he may have written in his science journal or in a letter to another scientist friend.
Note: Although the passage is fictional, it accurately represents the experiments, data, and observations of Joseph Priestly.
Direct students to read the passage with a partner. Explain that after students read the passage they should work with their partner to record at least five of Joseph Priestly’s observations. Students should record the observations in Problem 1a of their SDRs.
Once students have read the passage and recorded Joseph Priestly’s observations, ask several student pairs to share their list of observations. Review the observations and if necessary help students find any they may have overlooked.
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- The candle flame went out when the jar was sealed.
- The mouse could not survive in the jar once the flame died.
- When the mint plant was added to the jar, the candle flame could burn.
- When the mint plant was added to the jar, the mouse could live.
- When the mint plants were placed in water, bubbles left the stems of the plants.
- The “air” inside the bubbles was oxygen.
- More oxygen was made by the stems if the jar with the plants was sitting in the sun.
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After compiling the list of observations, discuss the conclusions that Joseph Priestly may have made from them. The following questions may be helpful during the discussion:
Joseph Priestly observed that when a candle was placed in the jar, the flame went out. He also observed that when the mouse was placed in the jar with the candle, the mouse died. What conclusions do you think Joseph Priestly drew from these observations? What conclusions can you draw from them? Students may suggest several conclusions. For example, one conclusion is that there may have been a substance in the jar that prevented the candle from burning and killed the mouse. Another conclusion is that there was one substance in the jar that the candle needed to burn and the mouse needed to live. Once that substance was used up, the candle went out and the mouse died. Another conclusion is that the burning candled produced some substance that caused the mouse to die or used up a substance that the mouse needed to live.
Joseph Priestly observed that when a mint plant was added to the jar with the candle and mouse, the candle still burned and the mouse lived. What conclusions do you think Joseph Priestly drew from these observations? What conclusions can you draw from them? Students may suggest several conclusions. For example, one conclusion is that the mint plant produced a substance that the candle needed to burn and that the mouse needed to live. Another conclusion is that the plant used up the substance that had prevented the candle from burning and the mouse from living.
What other conclusions about the function of plants can be drawn by studying all of Joseph Priestly’s observations? Students should record their conclusions in Problem 1b of their SDRs. Students may suggest several conclusions. For example:
The roots and stems of the mint plants produced bubbles that contained mostly oxygen. Therefore, plants produce a substance called oxygen. Oxygen may be the substance that allowed the candle to burn and the mouse to live.
Plants may use sunlight to produce oxygen because more oxygen was produced when the jar with the water and mint plants was placed in the sun.d.
Can you think of an experiment that was not described in the passage about Joseph Priestly that would have helped him draw more conclusions about plants? Students should record their answers in Problem 1c. Student answers may vary. However, students should be able to provide support for their suggestions. A sample answer is provided below:
Joseph Priestly could have prepared two jars, each with a mint plant, a candle, and a mouse. One jar could have been placed in the sun and the second in the dark. He could have then observed if there was a difference in how long the candle remained lit and the mouse lived between the two jars. This would have helped to verify that sunlight was responsible for oxygen production and that it was the oxygen that the candle needed to burn and the mouse to live.
KEYS: PRELAB EXERCISE
SHARE IT
SLIDE MICRO4-pre-3
Tell students that Joseph Priestly completed many more experiments and that other scientists also continued to study plants. From the experiments of Joseph Priestly and other scientists, there is a great deal of information we know about the structure and function of plants and plant cells and a process known as photosynthesis. Explain that students will be examining plant cells in the Lab, and so it is important that they understand something about photosynthesis.
Direct students to problem 2a of their Scientist Data Record which contains a sketch of a plant. Encourage students to work with a partner to complete the activity. Use the Parts of a Plant on this slide as a student aid in labeling three main parts of a plant: the stem, the roots and the leaves.
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SLIDE MICRO4-pre-4
Continue the discussion by explaining to students that each of the three parts they labeled are composed of cells. Use the leaf as an example.
Draw students’ attention to the sketch of the plant cell in Problem 2b. Encourage students to use their Scientist’s Glossary to identify some typical structures found in the cells of plants. Encourage students to work with a partner to complete the activity.
Discuss the relationship between the structures of a plant cell and a plant and the process of photosynthesis.
- Draw students’ attention to one of the organelles within the diagram of the plant cell, the chloroplast. Explain that it is important for plants because it contains several colored substances or pigments inside it. The pigments are called chlorophylls. Encourage students to find the terms chloroplast and chlorophyll in their Scientist’s Glossary.
- Remind students that light is made of different wavelengths and that we see these wavelengths as different colors. If necessary, ask students to imagine the colors in a rainbow to illustrate the different colors that make up the visible spectrum of light.
- Explain to students that as light shines on a plant, chlorophyll absorbs and reflects different wavelengths of light. They absorb light of red and blue wavelengths and reflect light that corresponds to green wavelengths. As a result, chlorophyll appears as green pigments and the chloroplasts that contain the chlorophylls also appear green.
Ask students: Can you hypothesize as to why the leaves of plants appear green? Student answers will vary. Students should suggest that if leaves contain chloroplasts and chloroplasts contain chlorophyll pigments that appear green, then the green color of leaves may be the result of those chlorophyll pigments.
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SLIDE MICRO3-pre-5
Introduce students to the basic steps involved in the process of photosynthesis. These steps are also located in Problem 3a of their SDRs.
Tell students that one of the functions that plants can perform is the ability to make sugar and starch which they use for food. The process by which plants perform this function is called photosynthesis.
- During photosynthesis, light energy is converted into chemical energy. Photosynthesis occurs inside the chloroplast and involves the chlorophyll pigments. Photosynthesis also requires carbon dioxide and water.
- Plants absorb water through their roots and carbon dioxide through their leaves. Chlorophyll pigments are contained within the chloroplast.
- During the process of photosynthesis, light is absorbed by chlorophyll pigments and the energy is used by the chloroplast to produce sugar and oxygen from carbon dioxide and water through a chemical reaction.
- Some of the sugar produced in the chloroplast is used by the plant right away for energy and to build different plant structures. However, some is stored within different areas of the plant as a substance called starch. Much of the starch is used by the plant at night, when there is no light and photosynthesis does not occur. Animals that eat plants gain energy from both the stored glucose and starch.
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SLIDE MICRO4-pre-6
This is a very advanced slide that LabLearner Students will see again in middle school. It is included here simply to have students anticipate that there is more to learn about photosynthesis and that they will return to this topic in several years.
Each of the reactants and products of the photosynthesis reaction is shown in three formats:
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KEYS:
WATCH IT
Play the following Student Video in preparation for the lab. Discuss as necessary to answer student questions.