Honors Biology—Unit 2
Why are cells small?
Name:
Living cells are dependent on the process of diffusion in order to obtain the essential
nutrients they need to survive. As cells take in these nutrients, they break them
down and use the resulting energy and molecular building blocks to make more
cellular components. This causes a cell to grow by increasing in size. However, cells
never get too big, even if the organism is rather large. Cells are always small. In
other word, the cells of a shrew and a whale are, on average, the same size; a whale
just has a lot more of them.
These observations raise an interesting question: Why are cells so small?
Please write a hypothesis to explain why are cells so small?
Hypothesis:
Getting Started
You can test the validity of these different explanations by constructing a model cell
by using agar. Agar is a gel-like substance that you can cut into whatever shape or
size you want. Agar is a useful material because chemicals can diffuse through it. I
have added a chemical indicator, bromothymol blue, to this agar. When
bromothymol blue comes in contact with an acid (such as vinegar) it turns from blue
to yellow. This allows you to see how far an acid diffuses into your model cell over
time.
You will have the following materials:
 Bromothymol blue agar cubes (3cm x 3cm x 5cm)
 Vinegar
 Beakers
 Stopwatch (your phone or clock)
 Ruler
 Plastic spoon
 Plastic knife (to cut your block into different size cubes or cut the cubes open
after they have soaked in vinegar)
Safety note: Wash hands with soap and water after handling the bromothymol blue
cubes.
With your group, determine if your hypothesis is still valid to the research question.
If not revise your hypothesis to match the results you found.
You can use as many of the supplies available to you as needed to test your ideas.
Make sure that you generate the evidence that you will need to support your
explanation as you work. Below record your methods and observations.
METHODS:
OBSERVATIONS:
Argumentation Session
Once your group has decided on a valid hypothesis, prepare a whiteboard that you
can use to share your ideas. Your whiteboard should include all the information
shown in Figure 1.
Figure 1. Components of the Whiteboard
The Research Question
Authors
Hypothesis (Claim)
Analysis of Data/ Evidence
Methods
Data/Evidence
To share your work with others, your entire groups will rotate table to table and
provide feedback using post-it notes. When doing so address the following
questions:
1. Are their methods understandable? If not, leave them a suggestion.
2. Is the analysis logical based upon data/evidence? If not, leave them a
suggestion.
3. If you have additional evidence for their argument, leave them a post-it
note that provides that evidence for them.
Mini-Scientific Paper:
You will write a brief scientific paper explaining your methods and results. See
attached rubric.
Section
4
Introduction
(2 Points)

Hypothesis is testable &
predicts the relationship
between the IV & DV

Hypothesis is stated. It predicts
the influence of one variable on
another but lacks clarity.

Hypothesis is poorly stated &
doesn’t directly mention the
variables.

Hypothesis is incomplete
or inaccurate.
Materials &
Methods
(4 Points)

Complete, detailed list of
materials (size, conc., quantity)
presented in vertical list
format.
Methods accurately test the
hypothesis.
Methods are summarized
clearly enough that the
experiment is reproducible—
Contains an explanation of:
o IV
o DV Measurement
o Constants
o Controls,
o Multiple Trials
o Statistical Analysis
Data table(s) contains accurate,
precise raw data &
manipulated data reported in
correct units with descriptive
titles and labels.
Data is described in a clear,
concise, logical manner.
Patterns are identified &
described, but no conclusions
are drawn.

Most materials are listed &
appropriate.
Methods test hypothesis with
minor errors.
Methods summarize the
experiment, but there are minor
errors or omissions in the
explanation.

Materials are appropriate but list
is incomplete.
Methods attempt to test the
hypothesis but there are major
errors in reasoning.
Methods summarize the
experiment but there are several
errors or omissions.
.

Materials are missing or
inappropriate.
Methods do not address
the hypothesis.
Methods are difficult to
follow &/or there are
major omissions or errors.
Data table is mostly accurate,
but may be missing some raw or
manipulated data. Labels &
units are present or implied
with minor errors. Title is
present but lacks clarity.
Data is accurately described but
may lack clarity. Most patterns
are identified. There may be
attempts to draw conclusions.

Data table contains mostly
accurate data but is missing a
title, important units &/or labels.
Data is described, but there may
be some inaccuracies. Patterns
in data not clearly identified.



Results
(4 Points)


3




2



1



Data table is inaccurate,
confusing, &/or
incomplete.
Data is not logically
described. Patterns in
data not identified.
Discussion
(4 Points)
Communication
(2 Points)

Description and analysis of the
results includes:
o Scientifically valid &
logical
o Well-supported by the
data
o Clearly addressing the
question & hypothesis.
Language skills are superior.
Communication is professional,
fluent, and engaging to the
audience.

Description of analysis of results
is present and scientifically valid
but may lack one of the
following:
o Enough scientific
explanation (too much
is implied)
o Data to support.
Language skills are above
average—demonstrating above
average professionalism,
fluency, and audience
engagement.

Description and analysis of the
results are scientifically valid but
incomplete.
Language skills are inconsistent—
attempting to be professional and
fluent, an inconsistently engaging the
audience.

Description and analysis
of the results are
extremely incomplete
&/or illogical.
Language skills are
lacking—demonstrating a
deficiency in being
professional, fluent, and
engaging to the audience.
_________________________ / 16 Points
Modified from a rubric produced by Brad Williamson. http://www.nabt.org/blog/2010/05/04/mini-posters-authentic-peer-review-in-the-classroom/
Communication modified from a rubric developed by the Rock Bridge High School Studies Department.