Hands-On Flextension: Microscopic Evidence of Life

Microbiome Lesson Brief Microscopic Evidence of Life

Overview

This hands-on activity builds on and reinforces students’ understanding of organisms and the scale of living things. Students first engage in a discussion about how they would know whether something is living or nonliving, followed by considering what the expected differences might be if they were viewing objects at the microscopic scale. Given specific objects, students then sort and discuss whether they might be living or not. Students prepare slides and observe these objects under a microscope, and that experience helps them identify characteristics of living things. Later, students bring objects from home so they can determine if there is microscopic evidence that shows whether these objects are living or nonliving. The purpose of this lesson is for students to gather evidence that living things are made of cells. You might choose to include this Flextension if you have access to microscopes and would like your students to have firsthand experience with observing cells.

Recommended Placement: between Lessons 1.1 and 1.2 Suggested Time Frame: 90 minutes over two class periods (longer if students need instruction on using a microscope)

Next Generation Science Standards (NGSS)

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

Disciplinary • LS1.A: Structure and Function: Core Ideas °° All living things are made of cells, which is the smallest unit that can be said to be alive. An organism may consist of one single cell (unicellular) or many different numbers and types of cells (multicellular). (MS-LS1-1)

°° Within cells, special structures are responsible for particular functions, and the cell membrane forms the boundary that controls what enters and leaves the cell. (MS-LS1-2)

(continued from previous page)

Disciplinary • LS1.A: Structure and Function (continued) Core Ideas °° In multicellular organisms, the body is a system of multiple interacting subsystems. These subsystems are groups of cells that work together to form tissues and organs that are specialized for particular body functions. (MS-LS1-3)

Science and • Practice 1: Asking Questions Engineering • Practice 3: Planning and Carrying Out Investigations Practices • Practice 8: Obtaining, Evaluating, and Communicating Information

Crosscutting • Scale, Proportion, and Quantity Concepts • Structure and Function

Vocabulary

• cells • microscopic • organism

Materials & Preparation

Materials • thin fabric piece, light-colored and about 2 inches x 2 inches* For the Class • 1 feather, about 3 inches long (or • Hands-On Flextension copymaster: several smaller)* Microscopic Evidence of Life • 1 roll of paper towels* • 1 onion* • water* • 2 teaspoons of dry yeast* • 30 drops 1% methylene blue solution* • 3 teaspoons of salt* • 9 cups (plastic or similar containers, • 3½ teaspoons sugar* one for each type of object)* • several thin plant leaves (elodea, • masking tape* bamboo, or romaine lettuce)* • flat toothpicks (or small wooden • 1 sheet light-colored paper* craft sticks or cotton swabs)*

• 2 spoons (plastic or metal)* Digital Resources for Microbiome • measuring spoon, ½ teaspoon* Lesson 1.1. Make enough copies so • measuring cup, 1 cup* each student will have all four parts. You might, however, want to hold For Each Group of Four Students off on distributing the Part 4 sheets • 1 tray* until the day you conduct the activity • 1 light microscope (at least 100X with the objects from home. magnification)* 2. Prepare dropper bottles. Add • 1 dropper bottle with water and approximately 10 mL of water to methylene blue* each small bottle. Add 3 drops of 1% • 1 dropper bottle with water* methylene blue solution to half the • 9 microscope slides* bottles; the other half should contain • 9 coverslips* only water. Label the bottles. • forceps* 3. Prepare nine different viewing For Each Student objects and place them in cups. Prepare enough objects for all • 1 copy of Hands-On Flextension: classes. The intent is to have nine Microscopic Evidence of Life student cups, each with one type of viewing sheets* object for a central materials station. • 1 pair of gloves*

*teacher provided a. Onion for cup 1: Cut the onion into pieces and separate pieces into individual scales Preparation (layers). Each scale has a thin, transparent membrane on the Safety Note: Using Chemicals underside. Remove this tissue- It is recommended that students wear like outer layer with a pair of gloves when handling methylene blue forceps or your fingernail, and or iodine. Concentrated methylene blue place them in a viewing object and iodine solutions are toxic when cup. There should be at least one ingested, can irritate the eyes, and stain piece for each group. skin or clothing. b. Yeast for cup 2: In another viewing 1. Print Hands-On Flextension object cup, mix 2 teaspoons (one copymaster: Microscopic packet) of yeast with about ½ Evidence of Life. Locate the teaspoon of sugar and ¼ cup of Hands-On Flextension copymaster: warm water. Mix gently. Microscopic Evidence of Life in

c. Fabric, paper, leaves, feathers • 4 sets of gloves for cups 3–6: Cut or tear these • 12-inch piece of masking tape objects into tiny pieces that will fit 6. Create Living and Nonliving under a coverslip. Place objects T-chart. Make a T-chart on the by type into viewing object cups. board with the headers “Living” and d. Sugar, salt, toothpicks for cups “Nonliving.” You will add to this chart 7–9: Place these items in the throughout class. Remember to remaining three cups: erase entries between class periods.

• 3 teaspoons of sugar and one 7. Optional: Prepare and examine plastic spoon in cup 7 a set of slides before class. In • 3 teaspoons of salt and one order to support your students as plastic spoon in cup 8 they work, it is helpful to anticipate the student experience and view • toothpicks or craft sticks in these objects under a microscope cup 9 beforehand. Prepare a sample set 4. Set up central materials station. of slides with the nine objects and Place the nine filled viewing make your own observations. object cups in a central location so students can easily obtain the 8. Immediately before the lesson, objects they need. have on hand the following materials: 5. Prepare microscope stations and • student sheets trays. Set up microscopes around • microscope stations with trays of the classroom, one microscope slide preparation materials for each group of four students. If • filled viewing object cups in a you have additional microscopes, central location students can work in smaller groups. Prepare one tray of slide preparation 9. Between classes, clean and materials for each microscope. Place replenish microscope tray the following items on each tray: materials. Clean slides, coverslips, and forceps; replenish materials. • 9 microscope slides During Part 4 (another day), each • 9 coverslips group will prepare slides for the • 1 dropper bottle with methylene objects brought from home. blue and water Students will again need the • 1 dropper bottle with water microscopes and slide preparation • 1 pair of forceps materials trays.

Notes

Methylene Blue Substitute Iodine solution can work in place of methylene blue. Typical Lugol’s iodine stock solution should be diluted with a 1:5 dilution ratio before use.

Obtaining Plant Leaves Elodea or a similar aquatic plant can usually be obtained from an aquarium store. In certain areas, you may be able to acquire bamboo from a park or someone’s yard. Bamboo plants can also be purchased from some stores. Romaine lettuce is another easily obtainable option, but the cells are not as well defined.

Science Background

Cells are the basic unit of structure and reproduction in all living things. Some tiny organisms are made of only one cell, and this single cell carries out all the functions of life. Bacteria are one type of single-celled organism. Other examples include unicellular algae and unicellular fungi. Multicellular organisms are made of many cells. In multicellular organisms, cells are specialized to serve particular functions—for example, the cell types in mammals include skin cells, muscle cells, neurons, and blood cells.

In nearly all cases, individual cells are too small to be seen with the naked eye. Tools, such as microscopes, are needed to see cells clearly and are especially necessary to view any details of cellular structure. (There are a few types of cells that are visible to the unaided human eye, including the human egg cell.) Although cells vary in size, they are typically on the scale of micrometers (1 micrometer = 1 millionth of 1 meter). Molecules are more than 1,000 times smaller, most of them less than a nanometer in size (1 nanometer = 1 billionth of 1 meter).

Teacher Reference

Preparing a Wet-Mount Microscope Slide

forceps

object

slide

1. Place the object on the 2. Squeeze one drop of water or slide near the center. diluted methylene blue on top of the object.

coverslip

small piece of paper towel or blotting paper 3. Place the edge of a coverslip on the slide so it touches the edge of the liquid. Slowly lower the coverslip 4. Use a piece of paper towel or onto the slide. blotting paper to remove excess liquid.

Cells That Students Observe Note: The cells below may be shown at different magnifications than what students will observe.

human cheek cells stained with onion cells methylene blue

baker’s yeast—Saccharomyces cerevisiae Elodea leaf cells with chloroplasts

Instructional Guide

Explore and Activate Prior Knowledge

1. Review Safety Guidelines. Review each guideline with students. These can be found on page 1 of the Investigation Notebook, which is available in Printable Resources at the unit level of your digital teacher’s guide. In particular, emphasize Guideline 2: “Don’t taste things,” Guideline 4: “Protect your eyes,” and Guideline 5: “Protect your hands.” Inform students that they will be using a staining solution that is toxic if ingested and can stain skin and clothes. 2. Review scale. Remind students that in Lesson 1.1 they thought about scale. They used cards and the Scale Tool to think about a range of small things and sort them by size. Ask students to review some things they sorted and considered in Lesson 1.1. [A grain of salt. An ant. A red blood cell. A water molecule.] 3. Review living things from Lesson 1.1. Remind students that many of the things they sorted were living things, and some of these could be seen with the naked eye, while others were microscopic and required special tools to be seen. Ask students for examples of living things, either from the activities in Lesson 1.1 or from their own experience. [Lesson 1.1 examples: human, ant, bacteria.] 4. Discuss living vs. nonliving. Have students share their ideas about the characteristics of living and nonliving things. As students share, record their ideas on the T-chart you created before class. Accept all responses.

When you see something with the naked eye, how do you know if it is living or nonliving? What are the characteristics of living things?

What would you expect to see in living organisms if you were to zoom in to the microscopic scale? 5. Introduce activity. Explain that students will look at different objects to see if they can find evidence, at the microscopic level, of characteristics that all living things share.

Construct New Ideas

6. Distribute Microscopic Evidence of Life student sheets (at least Parts 1–3) and introduce Part 1. Point out the nine objects listed in Part 1: Initial Ideas About Living and Nonliving Things. Explain that students will decide whether the listed objects are living or nonliving, and place them in the appropriate column. If they are unsure, they can add them to the third column. 7. Students sort objects. Give students a few minutes to discuss each object with a partner. If some students finish quickly, encourage them to explain their thinking and provide evidence about why they sorted each object the way they did.

8. Lead a class discussion about initial ideas. Have students share their ideas. Ask students what evidence helps them know whether something is living or not. If any new ideas emerge, write them on the Living/Nonliving T-chart. Discuss the objects that students are unsure about, but avoid answering whether they are living or not. Explain that students will use microscopes to find additional evidence that will help them figure out if these objects are living or not. 9. Introduce Parts 2 and 3: Observing Objects with a Microscope and Evidence of Living Things. Explain that students will use microscopes to zoom in on the nine objects they just sorted. They will need to prepare a microscope slide for each object, and you’ll discuss the table in Part 2A and the specific procedures in a moment. Focus students on Part 2B, and explain that as they view each prepared slide, they will record their observations by making a detailed drawing of what they see. With the Part 1 and Part 3 student sheets at hand, review the following steps: Step 1) Examine the objects you listed in the “Living” column, and use the information to identify characteristics of living things. Record these characteristics in Part 3. Students will start with the objects that are living (or once were alive, such as part of an onion that was once growing but is no longer in the ground). As students observe, they will try to identify characteristics that all living things have in common. Students will record these common characteristics at the top of the Part 3: Evidence of Living Things student sheet. Step 2) Examine the objects you listed in the “Nonliving” column. See if the characteristics you recorded for living things can also be found in nonliving things. Use your observations to refine the characteristics of living things. Students will use microscopes to observe and sketch the objects they listed as not living. Using their observations, students will compare nonliving to living characteristics. If there is no difference, they will need to revise their evidence in Part 3: Evidence of Living Things. Step 3) Examine the objects you placed in the “Not sure” column. Try to determine whether these are living or nonliving by using the characteristics listed in Part 3 as evidence. Explain your conclusions at the bottom of Part 3. Students will observe and draw the objects they couldn’t categorize and try to assign them to one category or the other, based on the evidence they identified for living things. Point out the space at the bottom of Part 3 where students will explain their thinking. 10. Demonstrate how to use a microscope. If needed, show students how to focus and change magnification on a microscope. 11. Discuss slide-making techniques. Explain that scientists create different kinds of slides depending on the object they want to view. Slides have coverslips to hold the viewing object in place, keep it flat, and protect the microscope lens. Some slides have liquid added, such as water, but others do not, so they are dry. Some objects need to be stained so parts of

the object stand out. The stain that students will use colors some parts blue, but those parts are not actually blue unless the stain is added. Point out the chart at the top of Part 2: Observing Objects with a Microscope that shows whether to create wet or dry slides and whether to use water or stain on the wet slides. 12. Demonstrate wet-mount slide preparation. There are different techniques for preparing wet mounts. Use the technique you are familiar with or what is described (the Teacher Reference section has step-by-step diagrams). a. Place an object on the slide: Inner cheek skin: Gently scrape the inside of your cheek with the flat, rounded end of a toothpick (NOT the pointy end). Then, smear the toothpick on the center of the microscope slide for a few seconds. Remind students to throw away the used toothpicks immediately. Onion, fabric, feather, leaf, and paper: Use forceps to place one object on the slide. Yeast: Dip one end of a toothpick into the container at the central station. Smear the toothpick on the center of the slide for a few seconds. b. Add water or methylene blue: Methylene blue: The cheek, onion, and yeast slides need one drop of methylene blue. Students may need to use the forceps to straighten out any wrinkles on the onion piece. Water: The fabric, feather, leaf, and paper slides need one drop of water, although the feather and paper may need a little more than one drop. c. Place the coverslip on top gently. Put one edge of the coverslip on the slide and lower it at an angle to reduce air bubbles. d. Use a piece of paper towel to remove excess liquid. 13. Demonstrate dry-mount slide preparation. If needed, demonstrate how to prepare dry mounts. Place a tiny amount of salt or sugar on the center of a slide. Use a coverslip to hold the substance in place. 14. Encourage students to label all the slides. Point out that students have tape on their trays. They can label each slide with a small piece of tape near the edge of the slide. This will help them keep track of each object. 15. Students examine objects using microscopes. Direct students to the student sheets. Give students about 40 minutes to examine and draw all nine objects. If some groups do not finish, you can have other groups share their observations. Remind students to start with the “living” objects first so they have characteristics to use as evidence for the objects they’re not sure about. They will record characteristics of living objects and their decisions about the unknown objects in Part 3. Circulate and assist students with the microscopes as needed.

16. Lead a class discussion about findings. Invite students to share characteristics of living things. Explain that these characteristics can provide evidence that things are living or were once living. Highlight observations that describe cells or are related to cells. If students do not describe any characteristics related to cells, you can project the images in the Teacher Reference section and guide students to notice the cells in the images. 17. Introduce cells. Explain that all living things are made of smaller, microscopic structures called cells; they may remember seeing some examples of different kinds of human cells in Lesson 1.1. Many cells have a nucleus where DNA is stored. The nucleus is the small dot they might have seen in the middle of the cells they observed. Organisms, like humans or plants, are made of many cells. Students saw a few cells that they are made of when they scraped their inner cheek. Other organisms, like bacteria or yeast, are a single cell. Add “made of cells” to the Living/Nonliving T-chart. 18. Discuss “not sure” objects. Have students share if they found additional evidence that would help them determine if the objects they were not sure about are living or nonliving. If there is some disagreement, lead a discussion about whether or not they saw cells. If needed, explain that some things that are not living can also come from living things (feathers). Feathers are not living because they are not made of cells, but they come from living things. 19. Have students share remaining questions. Invite students to share any questions they still have about the objects they examined or about evidence that things are alive. Lead a class discussion about how they could answer those questions. In the next part of the activity, Part 4, students can focus on those questions when they examine objects from home. 20. Instruct students to bring objects from home for microscopic observation. Inform students that they may each bring two objects. Explain that they will make slides and observe the objects under a microscope to determine whether they are living or nonliving. If students need some direction, you could suggest small quantities of hair (their own or a pet’s), a spice or herb, kosher salt, chalk dust, yarn, a thin slice of cheese or meat, pond water sample, a dead fly, a fish scale, a flower petal or stamen, grains of sand, a shell, or a button.

Apply New Ideas

21. Students make predictions. Distribute student sheets or direct students to Part 4: Examining Objects from Home. Have students record the names of the two objects they brought from home, their partner’s two objects, and predict whether the items are living or nonliving. 22. Students prepare slides and examine objects. Give students about 20 minutes to complete the activity. Have students first determine if they would need to prepare a wet or dry mount for each object. As needed, help students prepare their slides. Remind them to make detailed sketches of what they see. If time permits, have students make predictions about the objects from another group and trade slides. 23. Lead a class discussion. Invite students to share anything they found surprising or anything they are still unsure about.

Microscopic Evidence of Life

Part 1: Initial Ideas About Living and Nonliving Things

Decide whether the following objects are living or nonliving. Place them in the appropriate column. If you are not sure about an item, add it to the third column.

• inner cheek skin • fabric • feather • leaf • onion • paper • salt • sugar • yeast Answers will vary. Students are not expected to sort all objects correctly. Example:

Living Nonliving Not sure

• leaf • sugar • feather

• onion • salt • yeast

• inner cheek • fabric skin • paper

Explain your reasoning for sorting the objects in the living and nonliving columns the way you did.

______Responses will vary.

______

Microscopic Evidence of Life (continued)

Part 2: Observing Objects with a Microscope

A. Prepare Slides Prepare one slide for each object you will examine under the microscope. The table below indicates whether you should make a dry or wet slide and whether the wet slide should use methylene blue or water.

Wet Dry Methylene Blue Water • sugar • inner • onion • fabric • leaf cheek skin • salt • yeast • feather • paper

Sketches and annotations will vary. B. Examine Objects See cell images in the Teacher Reference section. Follow steps 1–3 in order. Make detailed sketches of what you see for each object. Annotate your sketches with labels. Object: ______

1. Examine the objects you listed in the “Living” column, and use the information to identify characteristics of living things. Record these characteristics in Part 3.

2. Examine the objects you listed in the “Nonliving” column. See if the characteristics you recorded for living things can also be found in nonliving things. Use your observations to refine the characteristics of living things.

3. Examine the objects you placed in the “Not sure” column. Try to determine whether these are living or nonliving ______by using the characteristics listed magnification: in Part 3 as evidence. Explain your conclusions at the bottom of Part 3.

Microscopic Evidence of Life (continued) Sketches and Part 2: Observing Objects with a Microscope (continued) annotations will vary.

Object: ______Object: ______

See cell images in the Teacher Reference section.

magnification: ______magnification: ______

Object: ______Object: ______

magnification: ______magnification: ______

Microscopic Evidence of Life (continued)

Part 2: Observing Objects with a Microscope (continued) Sketches and annotations will vary.

Object: ______Object: ______

See cell images in the Teacher Reference section.

magnification: ______magnification: ______

Object: ______Object: ______

magnification: ______magnification: ______

Microscopic Evidence of Life (continued)

Part 3: Evidence of Living Things Answers will vary.

Using your observations and diagrams, list some common characteristics that you can use as evidence of living things.

______Students should notice that all the living objects are made of ______smaller structures. They should describe cells, although they ______may not yet use the term “cell.” ______Example: Made of oval, circular, or rectangular structures ______that sometimes have smaller things in the middle.

Use the characteristics listed above to help you identify the unknown objects as living or nonliving. List the objects you weren’t sure about and explain why you have now identified them as living or nonliving.

______Answers will vary. ______

______

Microscopic Evidence of Life (continued)

Part 4: Examining Objects from Home

Predict: List the objects from home that you will be examining. Place a check in the appropriate column to predict whether the object is living or nonliving.

Object Living Nonliving

A: Objects will vary. Predictions will vary.

Analyze results: Describe any evidence you found that could help you determine whether these objects are living.

______Answers will vary. Students should mention whether or not ______they found cells. ______

______

Microscopic Evidence of Life (continued)

Part 4: Examining Objects from Home (continued)

Object A: ______Object B: ______

magnification: ______magnification: ______

Object C: ______Object D: ______

magnification: ______magnification: ______