CELL SIZE

Introduction

  • Multicellular organisms grow by making more cells, not bigger cells.
  • Unicellular organisms are microscopic.

Both of these facts are due to size limits imposed on a cell in order to move materials in and out of the cell at a rate that is efficient enough to maintain homeostasis. Most of this movement is due to diffusion, a type of passive transport that by definition requires no energy expenditure by the cell, but is a very slow process. In order for diffusion to occur fast enough to meet a cell’s needs, the distance must be very small. This highlights an important principle in cells . . . living, functioning units of life . . . the relationship of surface area to volume.

  • Surface area is represented by the cell membrane . . . how much space is there for diffusion?
  • Volume is the amount of cytoplasm contained within the cell . . . how much “stuff” is there inside the cell?

A cell is only able to meet its needs if this ratio is equal to or greater than a critical value. In Part A, we will observe the effect of changing surface area versus volume on this ratio. In Part B, you will create a “cell” that maximizes mass, but minimizes diffusion time.

Pre-Lab

  • Write the title of the lab.
  • Use a ruler to construct the following data tables. Be sure to allow enough space in the Part B Data Table to sketch your “cell”.

Data Table A – Surface Area/Volume

Cube Size / Surface Area
(cm2) / Volume
(cm3) / S A / Volume
Ratio / Time / Extent of Diffusion
1 x 1 x 1
2 x 2 x 2
1 x 1 x8

Data Table B – Our Ex – cell – ent Designer Cell

Cell Design / Cell Mass
(g) / Time for Complete Diffusion / Mass / Time
(g/min)

Procedure:

Part A - Determining the Relationship between Surface Area and Volume

  1. Work together to cut three agar cubes as precisely as possiblein the dimensions given in Data Table A.
  2. Gently place the cubes in the vinegar solution and record the time.
  3. The cubes contain bromothymol blue, a pH indicator that turns yellow in the presence of an acid. As the vinegar (acetic acid) diffuses into the cube, a color change will be observed. Diffusion is complete when the blue color completely disappears from the center of the cube. Record the amount of time required.
  4. Please note: If diffusion is not completed at the end of 20 minutes, calculate the extent of diffusion using the following formula:

Total cube volume (B3) – Volume of cube that did not change (A3) X 100

Total cube volume (B3)

  1. Discard the cubes. Discard the vinegar and rinse the container.

Part B - The Most Ex-CELL-ent Cell Design 

Your task is to create a cell that maximizes mass while minimizing diffusion time. Every table will receive two identical agar cubes. You will design the most efficient cell, let it soak in the vinegar until diffusion is complete, mass your cell, then determine the mass/diffusion time ratio. The group with the greatest ratio earns the title, MOST EX-CELL-ENT DESIGNERS!  The following rules must be observed:

  • No donut-like holes through the agar cell . . . this is biologically impossible.
  • Once the agar cell is in the container of vinegar, no poking, prodding, or touching the container.
  • I will determine when 100% diffusion takes place. The blue color must be completely gone from the center of the cell.
  • You must mass the “cell” at the end of the race and it must not break when handled. Any breakage results in disqualification.
  • Winner = Highest Ratio of mass divided by time!!

Questions – Please Answer in Complete Sentences in Your Lab Notebook

  1. Calculate the Surface Area/Volume ratio of a cube that is 3cmX3cmX3cm. Compare this value with the 1cmX1cmX1cm and 2cmX2cmX2cm cubes. As the cube increases in size, specifically, what happens to the surface area to volume ratio?
  2. The 2X2X2 cell and 1X1X8 have the same volume. Were the diffusion times the same? Explain.
  3. Is diffusion occurring at a different speed in each of the “cells”? Explain.
  4. Describe your cell design. What principles were you basing your design on to increase diffusion time?
  5. Choose an example of a plant or animal cell type that is shaped differently than the stereotypical textbook shape. Explain the purpose behind its shape and correlate it to cell function.

Reprinted and revised with permission from Kim B. Foglia