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Red Onion Cells

Onion cells shot with QX5 Digital Blue microscope, image by M4K

BRIEF DESCRIPTION

In this simple experiment, students will prepare slides of red onion cells to be viewed under the microscope. Onion cells are easily visible at medium magnification. Their plasma membrane and cytoplasm can be clearly distinguished, and, if a stain is used, their nuclei can be seen, as well. Students will also observe that the cells are uniform in shape and size. This is a consequence of mitotic cell division—which produces genetic clones and contributes to the development of specific tissues, each made up of distinct cell types with distinct functions (in this case, inner epidermal cells). Red onions are preferable because the red pigment in the cells makes them easier to visualize under the microscope.

GRADE LEVELS

4-12

TEACHER BACKGROUND

Bulb onions (Allium cepa) are members of the family Amarylliaceae. The bulbs are formed from modified leaves. Their primary function is to store energy produced during photosynthesis. Glucose that is produced during photosynthesis is converted to starch (a polymer composed of multiple glucose molecules linked together), and transported to the base of the leaves, which results in bulb formation. Because iodine will bind to starch granules, iodine can be used as a stain to visualize the cytoplasm of yellow and white onions (a step that is unnecessary with red onions, which are naturally pigmented).

Students may note that the onion cells lack chloroplasts and chlorophyll (or you can point this out to younger students). Students can be asked (individually, or in groups) to propose hypotheses for why this is (onion plant leaves and shoots, which grow above ground, are green and do have chloroplasts, while the onion bulb, which functions primarily as an energy storage organ, grows below ground, where it has no access to sunlight and thus has no need for chloroplasts).

Germplasm Resources Information Network - (GRIN). "Allium cepa information from NPGS/GRIN" USDA, ARS, National Genetic Resources Program. Retrieved 22 April 2011.

STUDENT LEARNING GOALS

  • Isolate cells from red onions and prepare wet mount slides
  • Examine onion specimens under the microscope
  • Make accurate biological drawings of their specimens
  • Identify larger structures like the plasma membrane and cytoplasm.
  • Optional, or for older and more advanced students:
  • Stain specimens to make the nuclei visible

STUDENT PREREQUISITE KNOWLEDGE

  • Basic cell structure (e.g., plasma membrane, cytoplasm, nucleus)
  • Prior experience with microscopy recommended (e.g., how to focus, how to calculate magnification, parts of the microscope)
  • How to make a biological drawing (See Biological Drawings lesson on this website)

This lab can also be used during a unit on cellular energy (e.g., photosynthesis) or ecology (biogeochemical cycles), in which case you would want to consider covering the following topics prior to doing the lab:

  • Photosynthesis
  • Plant anatomy
  • Carbon cycle


SEQUENCING OF LESSONS

1. Day 1: Microscope structure and function; how to calculate magnification; focusing; etc

2. Day 2: How to do biological drawings

3. Day 3: Basic Cell structure and function

4. Day 4: Preparation and viewing of onion cells


EXPECTED TIME FOR COMPLETION

Preparing, viewing and drawing red onion cells can easily be completed in one 50-minute class period

MATERIALS

(per student team)

  • Microscope
  • Glass slide
  • Tweezers (forceps)
  • Optional: cover slip
  • Optional: Fast Green stain (Carolina Biological #89-2261) or methylene blue stain.
  • Optional: Iodine

PREPARATION

It is recommended that all necessary materials be ordered well in advance of the lab. You will also want to copy lab handouts for students and set up lab stations 1-2 days in advance. Each lab team should have 1 microscope, 1-2 glass slides, 1 forceps or tweezers, and a segment of red onion. If students will be staining specimens themselves, you can aliquot the stains into dropper bottles (1 per student team). These can be stored for several years if kept sealed when not in use. For safety (and to save time) you may wish to cut the onions into wedges in advance (preferably the day of the lab). They can be stored in an air-tight container in the refrigerator).

ANTICIPATORY SET

Ask students to identify some plants that are eaten by people. Have them identify the part of the plant that is typically eaten (e.g., lettuce is a leaf, broccoli and cauliflower are flowers, carrots are roots). Show them an onion bulb and ask if they know its function or its anatomical role for the plant.

You can use this as a way to introduce some basic plant anatomy and to compare onion bulbs to other plant parts.

LESSON PLAN

1. Review microscope basics and safety, how to focus and the rules for making biological drawings

2. Optional: Show the following video, which demonstrates how to obtain a thin layer of onion epidermis, make a wet mount and how to stain it with iodine http://www.youtube.com/watch?v=PrX3h-AflZI

3. Alternatively, demonstrate for students how to obtain the thin layer of cells: How to prepare wet mount slides of onion bulb epidermis. Image by M4K.

4. Go over the lab procedure with students. It is helpful if the teacher demonstrates for the class how to obtain a thin layer and prepare a wet mount slide

5. Monitor and assist students as necessary

Optional: Staining Nuclei

1. After placing onion cells on slide (and before adding the cover slip), put on ONE drop of “Fast Green” or methylene blue stain directly on top of the onion slice.

2. For elementary school kids, teacher should handle the stain (as it is a potential skin toxin). Onion nuclei, stained with Fast Green, seen through QX5 Digital Blue microscope. Image by M4K.

Going Further

Once students have mastered the above procedure, advanced students should be able to design and carry out similar investigations using yellow or white onion (iodine can be used to stain the cells, as it binds to the starch granules in the cytoplasm and nucleus) or other plant cells for comparison.


QUESTIONS FOR DISCUSSION

Grades 4-5

1. Which plant organ is modified to produce onion bulbs? (Ans: Leaf)

2. Why can onion bulbs be considered a separate plant organ? (Ans: They have a different function than leaves (bulbs are an emergency energy storage organ, whereas leaves convert solar energy to glucose for immediate use by the plant)

3. What biological function do onion bulbs serve for the onion plant? (Ans: Emergency energy storage)


Grades 6-8

1. Which plant organ is modified to produce onion bulbs? (Ans: Leaf)

2. Why can onion bulbs be considered a separate plant organ? (Ans: They have a different function than leaves (bulbs are an emergency energy storage organ, whereas leaves convert solar energy to glucose for immediate use by the plant)

3. What biological function do onion bulbs serve for the onion plant? (Ans: Emergency energy storage)

4. Were you able to see individual cells? Which parts of the cell were you able to identify? (Ans: Without stain, the cytoplasm and plasma membrane should be visible. With stain, the nucleus is also visible.)

5. Did you see one type of cell or different types of cells? Provide evidence to support your claim. (Ans: There is only 1 type of cell present. The evidence is that the cells are fairly uniform in size and shape, suggesting they are the same type of cell.)</font>

6. What magnification did you use to observe your specimen? (Ans: Answers will vary depending on the type of microscopes used, but it is recommended that students use magnifications ranging from 40-60x and up to 100-200x)</font>

7. Can you identify structures that were likely present in the cells that might have been visible at higher magnification? (Ans: Answers will vary depending on how much cell biology has been covered prior to the experiment. Possible answers include: cell wall, DNA or chromosomes, organelles (e.g., lysosomes, chloroplasts, mitochondria, lysosomes, vacuoles, ribosomes, Golgi bodies, endoplasmic reticula).)</font>


Grades 9-12

1. Which plant organ is modified to produce onion bulbs? (Ans: Leaf)

2. Why can onion bulbs be considered a separate plant organ? (Ans: They have a different function than leaves (bulbs are an emergency energy storage organ, whereas leaves convert solar energy to glucose for immediate use by the plant)

3. What biological function do onion bulbs serve for the onion plant? (Ans: Emergency energy storage)

4. Which observations or evidence from this experiment supports this? (Ans: If you used iodine to stain the cells, it would have bound to and stained the starch granules in the cytoplasm. This would be good supporting evidence since the role of the bulbs is to store starch as an emergency energy source.)

5. Were you able to see individual cells? Which parts of the cell were you able to identify? (Ans: Without stain, the cytoplasm and plasma membrane should be visible. With stain, the nucleus is also visible.)

6. Did you see one type of cell or different types of cells? Provide evidence to support your claim. (Ans: There is only 1 type of cell present. The evidence is that the cells are fairly uniform in size and shape, suggesting they are the same type of cell.)

7. What magnification did you use to observe your specimen? (Ans: Answers will vary depending on the type of microscopes used, but it is recommended that students use magnifications ranging from 40-60x and up to 100-200x)

8. Can you identify structures that were likely present in the cells that might have been visible at higher magnification? (Ans: Answers will vary depending on how much cell biology has been covered prior to the experiment. Possible answers include: cell wall, DNA or chromosomes, organelles (e.g., lysosomes, chloroplasts, mitochondria, lysosomes, vacuoles, ribosomes, Golgi bodies, endoplasmic reticula).)

9. How does iodine work to make onion bulb cytoplasm and nucleus visible? (Ans: Iodine binds to starch present in the cytoplasm.)

10. Why would you expect iodine to be more effective in onion bulbs than in roots or leaves? (Ans: Iodine should be more effective in the bulbs because the bulb's role is to store starch and you would expect there to be more starch there than in other parts of the plant.)

11. Were the cells you observed the product of mitosis or meiosis? Provide evidence to support your claim.

12. Which observations or evidence from this experiment supports


STANDARDS

Next Generation Science Standards (NGSS)

4-LS1-1. Construct an argument that plants and animals have internal and external structures that function to support survival, growth, behavior, and reproduction.

MS-LS1-1. Conduct an investigation to provide evidence that living things are made of cells; either one cell or many different numbers and types of cells.

MS-LS1-2. Develop and use a model to describe the function of a cell as a whole and ways parts of cells contribute to the function.

HS-LS1-4. Use a model to illustrate the role of cellular division (mitosis) and differentiation in producing and maintaining complex organisms.

HS-LS1-2. Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms.

HS-LS1-6. Construct and revise an explanation based on evidence for how carbon, hydrogen, and oxygen from sugar molecules may combine with other elements to form amino acids and/or other large carbon-based molecules.

Common Core State Standards

CCSS.ELA-Literacy.RST.6-8.3 Follow precisely a multistep procedure when carrying out experiments, taking measurements, or performing technical tasks. (Grades 6-8)

CCSS.ELA-Literacy.RST.9-10.3 Follow precisely a complex multistep procedure when carrying out experiments, taking measurements, or performing technical tasks, attending to special cases or exceptions defined in the text. (Grades 9-10)

CCSS.ELA-Literacy.RST.11-12.3 Follow precisely a complex multistep procedure when carrying out experiments, taking measurements, or performing technical tasks; analyze the specific results based on explanations in the text. (Grades 11-12)

CCSS.ELA-Literacy.RST.6-8.9 Compare and contrast the information gained from experiments, simulations, video, or multimedia sources with that gained from reading a text on the same topic. (Grades 6-8)

CCSS.ELA-Literacy.RST.9-10.9 Compare and contrast findings presented in a text to those from other sources (including their own experiments), noting when the findings support or contradict previous explanations or accounts. (Grades 9-10)


Video Cytoplasmic streaming http://www.youtube.com/watch?v=VXbQpRpUDmQ

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