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The Arizona STEM Acceleration Project

Creating A Dichotomous Key for Cells

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Creating a Dichotomous Key for Cells

A 7th Grade STEM Lesson

Amanda Sibley

3/20/2023

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Notes for Teachers

This lesson is intended to help students make sense of the different organelles in both a plant and animal cell.

It is also intended to help students decide on what it means to be living and nonliving.

List of Materials

microscopes
plant & animal cell prepared slides *You could use images as well.
pencils
different types of pasta (or images)
Worksheet (only doing #1 and #2 activities.)
Article for reading

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Standards

Arizona Science Standards 7.L1U1.8

Obtain, evaluate, and communicate information to provide evidence that all living things are made of cells, cells come from existing cells, and cells are the basic structural and functional unit of all living things.

Core Idea: All living things are made up of cells, which is the smallest unit that can be said to be alive.

Science and Engineering Practice: Element: Communicate scientific information and/or technical information (e.g. about a proposed object, tool, process, system) in different formats (e.g., verbally, graphically, textually, and mathematically).

Cross-Cutting Concept: Patterns: Observed patterns of forms and events guide organization and classification and prompt questions about relationships and the factors that influence them.

Standards

Arizona Science Standards- Engineering Practices

Constructing Explanations and Designing Solutions

Element: Apply scientific knowledge to design, construct, and test a design of an object, tool, process or system.

Element: Optimize performance of a design by prioritizing criteria, making tradeoffs, testing, revising, and re-testing.

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Objective(s):

I can explore the meaning of living and nonliving through the creation of a dichotomous key.

I can create a dichotomous key to help me identify living and nonliving and identify a cell as plant or animal.

I can test my dichotomous key with another groups images to see if the design works and make changes if it does not.

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Agenda (3-4 days)

Day One:

Class discussion on what it means to be living and nonliving.

Create public document of what it means to be living and nonliving with current class definitions.

Time to use a dichotomous key and introduction to how a dichotomous key works.

Day Two:

As a class- create the first part of the dichotomous key (living and nonliving). In small groups test the system designed.

Day Three:

In small groups- add on plant and animal cells to the dichotomous key. Test the system in partner pairs.

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Introduction

What does it mean to be living?

Intro/Anticipatory Set/Opening

This slide to to get students thinking and talking to each other. Scientists have several different ideas of what it means to be living. Make sure before you start this lesson that you have decided which criteria you will use and create questions that will help guide students to this thinking in case they do not begin discussing it on their own. Knowing beforehand will also help you identify when students are on the right track and might need just a little nudge.

Pose the question to your students and ask them to talk to their shoulder partner or their small group (4 students in my classroom).

If you feel that students will struggle with this show them an image (or images) of a nonliving and living item and begin the conversations with that. You could also gather samples and distribute to your small groups.

Keep track of their ideas and create a public document. Title it LIVING. This is where you record the student created criteria. Ideally your students will have some moments where they disagree with each other. This might be good place to highlight the nature of science. Scientists disagree respectfully and different fields might have different definitions of living.

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Day 1: Hands-on Activity Instructions

Link to activity (Only use 1st and 2nd activity)
Groups of 2-4
Use the link above to introduce the topic of dichotomous keys to students.
Start with something familiar to students (pasta in the above link).
After each section, pull students together to talk about what a dichotomous key is and compare the different types.
Bring it back to the topic of living and nonliving. Talk about how a dichotomous key could help other people decide if something is living or nonliving.

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Day 2: Hands-on Activity Instructions

Small groups (2-4 students)
Review yesterday's learning and big ideas.
Review the public document where we decided on criteria from Day 1.
Work as a class to create a step-by-step diagram - similar to the one on the right.

Explain to students that we are going to add to our dichotomous key. We need to be able to recognize if the cells are plant or animal.

Complete the close reading assignment with this article.
Ask students to think about what the differences between plant and animal cells.

As you work with your students on creating this key, think aloud and wonder aloud. It can be different than the one pictured here and you could use the blank one on the practice activity (slide 8). Take special note of students who are struggling and who seems to be understanding the process to switch up groups tomorrow.

Ask students -

Can we have a yes to one of the characteristics and have it be living? Why or why not?

How do we show that even if they are a yes to a couple of these criteria that it still is nonliving?

Have students complete a close reading of this article. We are paying close attention to what we could include as another step of our key to tell the difference between plant and animal cells. This article will also help set you up to introduce students to the idea of organelles and structure and function.

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Day 3: Hands-on Activity Instructions

Mixed ability groups (2-4 students)
Ask students to share key takeaways from the reading. Specifically - what did they notice was a difference between plant and animal cells?
Allow students time to work in small groups and add on to their flow charts.

Once students have completed their charts with a way to figure out how to guide a decision between a plant and animal cell.

Ask students trade prepared samples. Ask students to test their key with another groups samples.

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Assessment

Link to possible assessment

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Differentiation

Mixed ability grouping through the different levels
Close reading assignment - in whichever technique your school implements (scaffolding to support understanding)
Teacher modeling as whole class
Working in pairs and small groups
Testing models and then making adjustments based on results

Remediation

Extension/Enrichment

Students who are moving quickly can create a key based on different types of cells

skin cells
bone cells
muscle cells

Make sure they have the prepared slides and/or the printed photos of the cells. More than one.