# How Does Our Body Maintain 100 Trillion Cells?

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## Objective

SWBAT explain the growth rate of cells.

#### Big Idea

We have as many as 100 trillion cells in our body. We lose as many as 30,000 skin cells an hour. How does our body maintain such a large number of cells?

## The Need for the Lesson

Cells multiple - fast. According to the site Ask a Biologist we lose between 30,000 and 40,000 skin cells per hour. In our body cells divide nearly two trillion times every day. To explain the concept of cell division, this lesson begins by engaging students with a children's book - One Grain of Rice by Demi.  One Grain of Rice is a folktale that demonstrates what happens when a single grain of rice is given to a clever girl named Rani, then doubled every day for thirty days. Like cell division, the power of doubling is amazing.

Students will then apply this idea to cell division.

## Investigation Summary & Prepartion

5 minutes

Investigation Summary & Preparation

Students will construct a model to show the power of doubling in their quest to understand the concept of cell division and the amazing feat of two trillion cells divisions done each day by the cells in our body. The emphasis of this lesson is on the number of cells and the monumental scale of it as our bodies work to maintain that number.

Students often struggle to understand the ongoing process of growth and repair at the cellular level. As students progress to mastery of the NGSS life science standards, they need to think of the cells as systems and grapple with the Crosscutting Concept of Scale, Proportion and Quantity. (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.) (SP5 Using Mathematics and Computational Thinking)

Additionally, this lesson supports the NGSS cross-cutting concept of Scale, Proportion and Quantity. Students apply logic to count the grains of rice doubled each day for thirty days in a very concrete task, they can then apply this concrete experience to how scientist estimate the number of cell divisions in a human body.

The lesson uses a children's book - One Grain of Rice by Demi to engage students in the mathematical thinking process needed to understand the power of cell division.

A complete materials list is located in the resources section.

## Students in Action

45 minutes

Students in Action

I begin with a read aloud of One Grain of Rice by Demi. The students are going to apply math skills to their build their understanding of cell division.

I pause my reading when the Raja says "Ask me for anything, and you shall have it." I ask students, "Given that the village people are experiencing hunger and famine due to the selfish Raja, what do you think Rani wants for her reward?" I give students a minute or two to discuss possibilities withe their elbow partner and ask groups to share their ideas. Most ideas are to ask for rice.

I continue to read until Rani says, "Today, you will give me a single grain of rice. Then, each day for thirty days you will give me double the rice you gave me the day before. Thus, tomorrow you will give me two grains of rice, the next day four grains of rice, and so on for thirty days." I ask students to estimate the number of grains of rice Rani will have after 30 days.

As I continue to read the book, students fill out the One Grain of Rice table. I remind students that each day must contain a number. The book gives numbers for selected days. As the numbers become large, the author describes the amount of rice in bushels and storehouses.

One Grain of Rice Counting Sheet

Students work with their elbow partner to calculate the number of grains of rice for each day for thirty days. As they progress through the table, they are amazed that one grain of rice, doubled everyday for thirty days, results in such a large number of grains of rice.

As I walk around the classroom as students are working, I ask them if their calculations match their predictions? In most cases their numbers are not very close.

Next we apply the grains of rice example to compare the rate of the cell division cycle. The two cells they are comparing have different rates of division. They are challenged to find a time when both cells will have the same number. The activity is designed to show the power of rapid cell division.

Cell Division in Action

Some of my students struggle with the division of cells. In the Cell A example, they question why there are only two cells and not a total of three cells. This is not the same as the rice example. I use the document camera to model cell division with clay, showing how the first cell divides creating two daughter cells. The first cell is no longer a single cell but two cells. This is the key difference in the rice - cells comparison.

The clay demonstration helps. The connection I want students to make and understand is the rapid growth of cells. Just like they are surprised by the number of rice grains collected by Rani at the end of the month, I am looking for that aha moment when they connect the number of cells for both examples (Cell A and Cell B) are equal to 64 new cells at the end of 120 minutes.

In this short video, I explain the rationale for the Number of Grains of Rice approach to understanding growth rate and cell division.

## Connecting the Learning

15 minutes

Connecting the Learning

At the conclusion of this lesson, I share this informative video where Dr. Bonnie Bassler shares How bacteria talk. I play the video from the beginning and end at 9:00 minutes.

First Dr. Bassler talks about the 100 trillion cells in our human bodies. It turns out that there are more bacteria cells in our body than human cells.  Next she talks about how bacteria talk to each other. Students are genuinely in awe of this paradigm changing information!

Our standards end in scope in the Students in Action section of this lesson. In my reflection I expand on why I selected this video to connect student learning.