Part 2: Build Your Own Planet
Lesson 7: Big Life or Tiny?
Time: approximately 45-50 minutes, depending on length of discussion
Materials: Water samples (see below—at least 1 per group)
Eyedroppers (at least 1 per group)
Blank microscope slides (at least 1 per group)
Microscopes (at least 1 per group)
Petri dishes or other small container capable of holding a few fluid ounces of water (1
per group)
Text: Big Life or Tiny? (from web site – 1 per group)
Overview
Students use microscopes to examine water samples collected from puddles, ponds
or creeks. After identifying any living organisms under the microscope, students estimate
how many living organisms are in their classroom and consider whether those organisms
are large enough to see without a microscope. Students use this information to predict
whether the life forms on their planet are large enough to see without a microscope.
Purpose
The bulk of the biomass on Earth is microscopic, and for most of Earth’s history
all life was microscopic. This lesson is designed to help students appreciate that life
outside of Earth may be too small to see with the naked eye.
Standards
A complete list of the standards covered by this lesson is included in the Appendix
at the end of the lesson.
Procedure
One to two weeks before this lesson: Have students collect samples of water
from puddles, creeks or ponds. Caution them that if they collect a sample from a pond,
lake or deep stream or creek that they should have an adult with them when they collect
it. Have them bring their sample into school in a sealed jar. When they bring their
samples in, place two or three grains of cooked rice in the jar to provide nutrients for any
microorganisms living in the water. The jars should be kept sealed and out of direct
sunlight to prevent overheating.
It is a good idea to collect a few samples yourself in case some groups do not have
any samples containing life. Check your samples occasionally under the microscope to
make sure that there are organisms living in the water.
Begin the lesson. Students should get in their groups and begin reading Lesson 7:
Big Life or Tiny? You may want to read the first paragraph aloud and give the groups
time to answer the question. Students should count all the living things that they can see
in the classroom. Answers should be around 30, depending on whether there are any
plants or pets in the classroom.
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Look around your classroom and count the number of living things that
you see. About how many living things do you see in your classroom?
________
Last week you should have collected some puddle water, brought it to
school and placed two or three grains of cooked rice in the container. What do
you think the purpose of the rice was?* Today we are going to look at the
sample you collected under a microscope.
* The rice was to provide food for anything living in the water.
Distribute microscopes to the groups. One microscope per group will work, but if
there are enough microscopes for each group to have two that is even better. (In this case
give each group two blank slides.) Most conventional optical microscopes have different
magnifications. It is common to have the magnification switchable between 40x, 100x
and 400x. 400x magnification can be very difficult for students to focus and is not
necessary for this activity. Encourage students to work with the lowest magnification
power first. If they want to then increase the magnification they can, but if they can’t get
it focused they should not worry. Even 10x or 25x magnification is usually enough to see
some of the tiny life forms living in the water.
Step 1: Use the eyedropper to get some water from one of your samples. Place
1 drop of water on the microscope slide and place it under the microscope to
examine it. If more than one group member brought in water samples, take the
time to look at all of the samples under the microscope and decide which has
the most interesting objects to look at. Use this sample for the rest of this
exercise.
Step 2: The drop of water is probably around 1/8” high. By carefully adjusting
the focus of the microscope, you can bring different areas of the drop into
sharper focus than other areas. Adjust your focus very slightly so that you can
see different levels in the water come in and out of focus.
It may take some practice before students will be able to see any living organisms
under the microscope. If a group claims that they can not see anything, check to make
sure that they have the microscope adjusted correctly. If the problem is not with the way
microscope is set up, it is possible that the sample does not contain any living organisms.
Offer them some of the sample water that you collected, or see if a group with a good
sample has enough and is willing to share.
Step 3: Do you see anything green in the water? Yes / No Any green things
that you see are most likely a type of plant. Are plants living things? Yes / No
About how many plants do you see? ________
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Step 4: Do you see anything moving? Yes / No Any moving things you see are
most likely living. About how many moving things can you see? _________
Altogether, about how many living things can you see, (including plants)?
_________
If you are not seeing any living things in your water then either you have a
problem with your microscope or your water has been sterilized, meaning that
all living things in it have been killed. Ask your teacher for help if you do not see
any living things.
Now your goal is to have the groups estimate how many living things, microscopic
or macroscopic, are in the classroom. They begin by estimating how many living things
are in the drop of water. Remind them that they are seeing only a small part of the drop of
water under the microscope. As they focus through the drop, different planes will come in
and out of focus. They should adjust their focus to estimate how many living things are in
all the planes, then estimate how much larger the drop is than the area of water that is
visible under the microscope.
For example, if a group has focused through the entire drop of water and counted 8
living things, and then moved the slide around and estimated that the drop of water is 10
times larger than the amount of water that can be seen under the microscope at any 1
time, then that group should estimate that there are around 80 living things in the whole
drop of water.
Step 5: While looking through the microscope, slowly move the slide around
until you can see the edge of the drop of water. Then try moving the slide in
the other direction until you see the edge of the other side of the drop. About
how many times bigger is the drop of water than the amount of water that
you see in the microscope? __________________
Step 6: How many living things do you think there are in the whole drop of
water? _____________
After counting out 100 drops of water, groups should assume that there are 100
times as many living things in that volume of water as in the single drop. In our example,
the group should estimate that there are 8,000 living things in 100 drops of water.
Step 7: Using the eyedropper, place 100 drops of water on the plate or Petri
dish. How many living things do you think there are in the water on the plate
or Petri dish? _____________________. (Hint: think about how many living
things you said were in 1 drop of water in Step 6. You now have 100 times as
much water in the Petri dish.)
Finally, groups compare the volume of 100 drops of water to the total amount of
water in their sample(s) and the total volume of all the samples in the classroom to
estimate the number a living organisms in the classroom. In our example the group may
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come up with a total of around 800,000 living things. Although that number is actually
much smaller than the correct number, it is large enough to prove the point that the
overwhelming majority of life in the classroom is microscopic. In reality, however, there
are likely to be hundreds of millions of living microscopic organisms living in a typical
classroom. The overwhelming majority of these organisms are either harmless or
beneficial to humans.
Step 8: Compare the amount of water in the plate or Petri dish to the total
amount of puddle or pond water that the members of the group brought in.
About how many times more water was brought in than is on the plate or Petri
dish? _________
About how many living things do you think there are in all the water samples
that the members of your group brought in? __________________
Step 9: Think about all the water samples in your classroom. About how many
living things do you think there are in your classroom?
___________________________
Are most of the living things in your classroom large enough to see? ______
Explain: ______________________________________________________
_____________________________________________________________
“Microscopic life” is life that is too small to see without a microscope.
Extremely small living things, like bacteria, are often called “microbes.”
“Macroscopic life” is life that is large enough to see without a microscope. Think
about what you have just learned about the life in your classroom. If you were
looking for life on another planet, do you think there would be more
microscopic or macroscopic life? Microscopic / Macroscopic
The groups should conclude that there will be far more microscopic than
macroscopic organisms on their planet. Most scientists think that any life outside of Earth
is much more likely to be microbial. Microbes are very robust, and on Earth they survive
and thrive in a wide range of harsh conditions that would kill more complex life forms.
The word “biomass” means the total mass of all living things. Most of the
biomass on Earth is microscopic. That means that if you were to add up all the
mass of all macroscopic living things—people, animals, insects, trees, plants and
every other living thing that you can see without a microscope—it would still be
less than the total mass that all of the microscopic living things on our planet.
You should probably plan to bring a powerful microscope with you when you
look for life on your planet!
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Appendix
Standards Addressed
Benchmarks (Grades 3 through 5)
1B Scientific Inquiry
Scientific investigations may take many different forms, including observing what things are like
or what is happening somewhere, collecting specimens for analysis, and doing experiments.
Investigations can focus on physical, biological, and social questions.
3A – Technology and Society
Technology enables scientists and others to observe things that are too small or too far away to
be seen without them and to study the motion of objects that are moving very rapidly or are
hardly moving at all
5C – Cells
Some living things consist of a single cell. Like familiar organisms, they need food, water, and
air; a way to dispose of waste; and an environment they can live in.
Microscopes make it possible to see that living things are made mostly of cells. Some organisms
are made of a collection of similar cells that benefit from cooperating. Some organisms' cells
vary greatly in appearance and perform very different roles in the organism.
5D – Interdependence of Life
For any particular environment, some kinds of plants and animals survive well, some survive
less well, and some cannot survive at all.
Most microorganisms do not cause disease, and many are beneficial.
5E – Flow of Matter and Energy
Some source of "energy" is needed for all organisms to stay alive and grow.
12B – Computation and Estimation
Judge whether measurements and computations of quantities such as length, area, volume,
weight, or time are reasonable in a familiar context by comparing them to typical values.
12D – Communication Skills
Use numerical data in describing and comparing objects and events.
Benchmarks (Grades 6 through 8)
3A – Technology and Society
Technology is essential to science for such purposes as access to outer space and other remote
locations, sample collection and treatment, measurement, data collection and storage,
computation, and communication of information.
5A – Diversity of Life
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Animals and plants have a great variety of body plans and internal structures that contribute to
their being able to make or find food and reproduce.
5C – Cells
All living things are composed of cells, from just one to many millions, whose details usually are
visible only through a microscope. Different body tissues and organs are made up of different
kinds of cells. The cells in similar tissues and organs in other animals are similar to those in
human beings but differ somewhat from cells found in plants.
Benchmarks (Grades 9 through 12)
5C – Cells
Within every cell are specialized parts for the transport of materials, energy transfer, protein
building, waste disposal, information feedback, and even movement. In addition, most cells in
multicellular organisms perform some special functions that others do not.
12D – Communication Skills
Participate in group discussions on scientific topics by restating or summarizing accurately what
others have said, asking for clarification or elaboration, and expressing alternative positions.
National Standards (Grades 5-8)
Understandings about Scientific Inquiry
Different kinds of questions suggest different kinds of scientific investigations. Some
investigations involve observing and describing objects, organisms, or events; some involve
collecting specimens; some involve experiments; some involve seeking more information; some
involve discovery of new objects and phenomena; and some involve making models.
Technology used to gather data enhances accuracy and allows scientists to analyze and quantify
results of investigations.
Structure and Function in Living Systems
All organisms are composed of cells--the fundamental unit of life. Most organisms are single
cells; other organisms, including humans, are multicellular.
Regulation and Behavior
All organisms must be able to obtain and use resources, grow, reproduce, and maintain stable
internal conditions while living in a constantly changing external environment.
National Standards (Grades 9-12)
Understandings about Scientific Inquiry
Scientists rely on technology to enhance the gathering and manipulation of data. New techniques
and tools provide new evidence to guide inquiry and new methods to gather data, thereby
contributing to the advance of science. The accuracy and precision of the data, and therefore the
quality of the exploration, depends on the technology used.
Indiana Standards
Grade 5
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Mathematics – Problem Solving
5.7.2 – Decide when and how to break a problem into simpler parts.
5.7.3 – Apply strategies and results from simpler problems to solve more complex problems.
5.7.5 – Recognize the relative advantages of exact and approximate solutions to problems and
give answers to a specified degree of accuracy.
5.7.6 – Know and apply appropriate methods for estimating results of rational-number
computations.
5.7.8 – Decide whether a solution is reasonable in the context of the original situation.
Science – Technology and Science
5.1.4 – Give examples of technology, such as telescopes, microscopes, and cameras, that enable
scientists and others to observe things that are too small or too far away to be seen without them
and to study the motion of objects that are moving very rapidly or are hardly moving.
Diversity of Life
5.4.2 – Observe and describe that some living things consist of a single cell that needs food,
water, air, a way to dispose of waste, and an environment in which to live.
Interdependence of Life and Evolution
5.4.6 – Recognize and explain that most microorganisms do not cause disease and many are
beneficial.
Grade 6
Mathematics – Computation
6.2.6 – Interpret and use ratios to show the relative sizes of two quantities. Use the notations:
a/b, a to b, a:b.
6.2.7 – Understand proportions and use them to solve problems.
Problem Solving
6.7.2 – Make and justify mathematical conjectures based on a general description of a
mathematical question or problem.
6.7.3 – Decide when and how to break a problem into simpler parts.
6.7.4 – Apply strategies and results from simpler problems to solve more complex problems.
6.7.5 – Recognize the relative advantages of exact and approximate solutions to problems and
give answers to a specified degree of accuracy.
6.7.10 – Decide whether a solution is reasonable in the context of the original situation.
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Science – Technology and Society
6.1.7 – Explain that technology is essential to science for such purposes as access to outer space
and other remote locations, sample collection and treatment, measurement, data collection and
storage, computation, and communication of information.
Communication Skills
Diversity of Life
6.4.2 – Give examples of organisms that cannot be neatly classified as either plants or animals,
such as fungi and bacteria.
6.4.5 – Investigate and explain that all living things are composed of cells whose details are
usually visible only through a microscope.
Interdependence of Life and Evolution
6.4.8 – Explain that in all environments, such as freshwater, marine, forest, desert, grassland,
mountain, and others, organisms with similar needs may compete with one another for
resources, including food, space, water, air, and shelter. Note that in any environment, the
growth and survival of organisms depend on the physical conditions.
Grade 7
Mathematics – Problem Solving
7.7.2 – Make and justify mathematical conjectures based on a general description of a
mathematical question or problem.
7.7.3 Decide when and how to break a problem into simpler parts.
7.7.4 – Apply strategies and results from simpler problems to solve more complex problems.
7.7.7 – Recognize the relative advantages of exact and approximate solutions to problems and
give answers to a specified degree of accuracy.
7.7.8 – Select and apply appropriate methods for estimating results of rational-number
computations.
7.7.11 – Decide whether a solution is reasonable in the context of the original situation.
Science – Scientific Inquiry
Diversity of Life
7.4.5 – Explain that the basic functions of organisms, such as extracting energy from food and
getting rid of wastes, are carried out within the cell and understand that the way in which cells
function is similar in all organisms.
Interdependence of Life and Evolution
7.4.6 – Explain how food provides the fuel and the building material for all organisms.
Grade 8
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Mathematics – Computation
8.2.3 – Use estimation techniques to decide whether answers to computations on a calculator are
reasonable.
Measurement
8.5.3 – Solve problems involving scale factors, area, and volume using ratio and proportion.
Data Analysis and Probability
8.6.1 – Identify claims based on statistical data and, in simple cases, evaluate the reasonableness
of the claims. Design a study to investigate the claim.
Problem Solving
8.7.2 – Make and justify mathematical conjectures based on a general description of a
mathematical question or problem.
8.7.3 – Decide when and how to divide a problem into simpler parts.
8.7.4 – Apply strategies and results from simpler problems to solve more complex problems.
8.7.7 – Recognize the relative advantages of exact and approximate solutions to problems and
give answers to a specified degree of accuracy.
8.7.8 – Select and apply appropriate methods for estimating results of rational-number
computations.
8.7.11 – Decide whether a solution is reasonable in the context of the original situation.
Science – Manipulation and Observation
8.2.3 – Use proportional reasoning to solve problems.
Communication
8.2.7 – Participate in group discussions on scientific topics by restating or summarizing
accurately what others have said, asking for clarification or elaboration, and expressing
alternative positions.
Critical Response Skills
Interdependence of Life and Evolution
8.4.5 – Explain that energy can be transferred from one form to another in living things.
Reasoning and Uncertainty
8.5.7 – Recognize and describe the danger of making over-generalizations when inventing a
general rule based on a few observations.