Making a Wet Mount Slide

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SWBAT make a wet mount slide to examine onion cells and cheek cells.

Big Idea

What do plant cells (onion cells) and animals cells (cheek cells) look like under the microscope?

The Need for the Lesson

Students will learn to make a wet mount slide so they can view plant cells (onion cells) and animal cells (cheek cells). Students will have a better understanding of cells when they can actually see them.

Making their own slides and examining them under the microscope empowers students to work as scientists. They are moving from just looking at cell images in a book or on a screen to seeing the real thing. It is an amazing experience for students. They are fascinated by what they see under the microscope. 

While not a required standard, working with microscopes engages students in further exploration of cells and cellular organelle because they have an anchor event that interest them.

Investigation Summary & Preparation

15 minutes

This lesson prepares students to use the microscope to examine cells. In this lesson we use the microscope to look at onion cells and cheek cells. Students will see several cells together when viewing the onion skin and single cells when viewing the cheek cells. The distinctive shapes will be visible. (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.)

We are preparing students to tackle the standard - LS1.A: Structure and Function - All living things are made up 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). 

When students are using the microscope, they should be asking the question, what does __________ look like if I magnify it to see it better? And, what else can I investigate using the microscope? (SP1 - Asking questions and defining problems).

Students will be examining plant cells (onions) and animal cells (check cells) under the microscope. They will be investigating the differences between the two types of cells as they are seen using a microscope. (SP3 - Planning and carrying out investigations)

Students will compare plant cells (onions) and animal cells (check cells) using a microscope. (SP4 - Analyzing and interpreting data)

Recording exactly what they see is a learning goal for students in the lesson. Student observations can be evaluated for their attention to detail as they view the objects and their precision as they draw the representations to be shared with other scientists. (SP8 - Obtaining, evaluating and communicating information)

Students will make their own slides in this lesson.  I cut up the onions and store them in ice water. This seems to make the onions easier to pull apart and of course keeps them fresh throughout the day.

The MSDS (Material Safety Data Sheet) for Methylene Blue can be accessed through the link. We use Methylene Blue to stain the samples to see more detail.

A material list can be found in the resources section.

Students in Action

45 minutes

This lesson begins with the review of the procedures for creating a wet mount slide. I model each step for the students since for most of the students, this is the first slide they have ever created.

Take a small slice of onion and gently pull apart a layer of the onion skin. The layer should be thin and transparent.

Place the onion skin on the microscope slide. Make sure the skin is as flat as possible. Use the round end of the toothpick to help you remove folds.

Place a small drop of water on the onion skin.

Hold the cover slip at a 45⁰ angle and let it drop gently onto the onion skill. Tap the cover slip to remove air bubbles.

Record what you see at low, medium and high magnifications.

Students may struggle to see details without the Methylene blue stain. The purpose of the first part of the lab is to show students the benefit of using the stain to cell details.

We review the procedures to add the Methylene blue to the onion cells slides.

Remove the cover slip.  Add a drop of methylene blue. Replace the coverslip, again place the coverslip at a 45⁰ angle and let it drop gently onto the slip.

Tap the coverslip to remove air bubbles.

Record what you see at low, medium and high magnifications.

Students should see more details with the staining.

As students view the slides at the microscope I check their views. We are looking for possible evidence of mitosis. Mostly the students will see the cell wall, and the nucleus. 

The students should draw the onion cells adjacent to one another.

Next we review how to make the cheek cell slide. Students will reuse the onion cells slide. 

Place a drop of Methylene blue on the clean slide.

Use a new toothpick to gently scrape the inside of your cheek.

Swirl the toothpick with your cheek cells into the methylene blue liquid. Place a cover slip on the slide.

Record your observations at low, medium and high magnifications.

 After recording the observations, I have students discard the slide with their cheek cell. 

In this short video, I share how to make a wet mount slide using onion skin and iodine.