Does (Cell) Size Matter?

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Objective

SWBAT determine the most efficient size of a cell by examining the surface area to volume ratio and explain the need for cell division (mitosis).

Big Idea

What size of cell is the most efficient? Why do cells divide? Students examine models to develop their own claim as to whether "size matters".

Hook - Size Is Essential!

5 minutes

So the age old question, "Does size matter?" is the guiding question for this lesson.  Of course, the lesson will focus on the size of cells.  Your students may misinterpret the title of the lesson, but have fun with it . . . and at least you have their attention.  

The video clip below will give a short tour that compares the size of common everyday items that we can see with our eyes and then contrast their size with microscopic objects that are relevant to our study of Biology.

 

The video clip was created using the content found on the University of Utah Health Science website.

Students will write down the names of each item that they remember discussing throughout this Biology course.  Students will also make two observations or reactions to the size comparisons.  How size 12 point font compares to a mitochondria or how a coffee bean compares to a transfer RNA.   As a follow-up, the class will discuss the size comparison of the items that are viewed on the video clip in a whole-group conversation that focuses on the importance of size differentials.

Direct Instruction - The Importance Of Size

20 minutes

Students will get out a sheet of paper and title it, "Lecture Notes: Cell Growth".  These notes will focus on the cell size and examine cell growth.  The notes will also encourage students to consider the ratio of volume to surface area as it relates to cell size and function.  

The objective of this lesson is for students to conceptualize that once cells grow beyond their optimal volume:surface area ration then they are not as efficient in completing cellular functions.  By regular cell division, the cells constantly create new, healthy cells and remain at the optimal size to complete their cellular function.

As a follow-up, the class will participate in a whole-group discussion using these prompts:

  • Why is cell size important?
  • What determines the most efficient cell size to optimize cell function?
  • How are surface area and volume related to cell size and function?
  • Why do you think cells divide?
  • Do you think all cells divide at the same rate?
  • How do cells divide?

These questions will be written on the front board for students to quickly write a response on their paper.  Once all students have finished, the class will discuss the responses as a segue to the introduction of Mitosis.

The last question will be referred to again at the conclusion of the lesson  to introduce the process of mitosis in preparation of the next lesson on Modeling Mitosis.

Guided Practice - Examining Size

25 minutes

As  follow-up to the lecture notes in the previous section, students will create models using provided Cubes Templates.  Students will use scissors and tape to construct the three different size models by following the instructions:

  1. Cut along the outside boarders 
  2. Fold along the lines
  3. Tape together to make a cube

The students will have created three different sized cubes: 1 cm3, 3 cm3, and a 5cm3.  These cubes will serve as models to demonstrate the relationship between surface area and volume as it relates to cellular function.  

Students will use the Modeling Limits To Cell Size -Student Handout to guide their hands-on experience.  This activity will have students calculate the surface area and volume of each cube and record in the data tables of their handout.  Students are then asked to calculate the ratio of surface area to volume for each of the three different sized cells.  

As an analysis of the activity, students are asked to complete the questions from the handout that contain questions such as:

  • Which measurement from the data table best represents the size of the cell membrane?
  • Which measurement from the data table best represents the cell content (the cytoplasm and the cell organelles inside of the cell)?
  • As the cell grows (use the larger cube model), describe what happens to the surface area to volume ratio.
  • Which cube has the highest surface area to lowest volume ratio?
  • How does this ratio relate to the cell's daily function and its ability to bring in necessary nutrients and expel waste products?
  • Using this lab experience, which cell size is most efficient?

The objective of this activity is for students to realize that the smallest cube (cell model) has the highest surface area to volume ratio (6:1).  The smallest cell has the greatest surface area of 6cm2 to the lowest volume of 1cm3.  This model demonstrates that the smaller the cube the more efficient the surface area to volume ratio which enables the cell to function at a more efficient level for its daily cellular activities.

To facilitate the review and discussion of this activity, teachers can utilize the provided Activity Answer Key.

Close - So Does Size Matter?

5 minutes

As a closing activity for this lesson, students will answer the simple question, "When it comes to cells, does size matter."  Hopefully, after modeling cell size and analyzing the surface area to volume ratio, students will conclude that the smaller the cell the more efficient the surface area to volume ratio.  

In an effort to prepare students for the next lesson on cell division, students will brainstorm as many supporting facts that defend the claim that "cells need to divide so they can remain small and more efficient."

Sample Student Response - Cell Size #1 - This student begins his response with vague details, but then realizes that the cell size needs to remain optimal to ensure that nutrients are able to enter and waste is able to leave the cell.

Sample Student Response - Cell Size #2 - This student is also able to articulate the need for the cell to divide in order to maintain the optimal ratio of volume:surface area in order to obtain nutrients and expel wastes.