Course: Science 8
Topic: Microscope images of cells (diatoms)
A. Purpose
Name:
Unit: Cells and Systems
Date:
Grade: 8
use instruments effectively and accurately for collecting data (e.g., use a microscope to produce a clear image of cells)
estimate measurements (e.g., estimate the size of an object viewed under a microscope)
observe and record data, and produce simple line drawings (e.g., draw cells and organisms)
organize data, using a format that is appropriate to the task or experiment (e.g., compare the structure and function of two or
more organisms, using charts and drawings)
B. Background
B. Displays/Resources
Phytoplankton are microscopic, single-celled plants that are found in almost every aquatic
environment including fresh and marine waters and even moist soils. As producers
(autotrophs), they form the basis of all aquatic food chains, giving off oxygen as a by-product
of photosynthesis. The oceans produce 20 BILLION tons of phytoplankton per year which
produce 65-75% of the world's oxygen.
Diatoms are one of the most common types of phytoplankton. A characteristic feature
of these single-celled algae is that they are encased within a unique cell wall made of
silica that is almost like a glass house, thus allowing light to pass through to the
chloroplast for photosynthesis. The construction of the cell wall (called the frustule)
consists of two valves that fit into each other like a little pill box, with an amazing
diversity in form that show complex shapes, sizes, and patterns. Some have elaborate
ornamentation or small projections to bolster the plant's ability to float in the water
closer to the sunlight.
Due to the wide diversity in form, beautiful symmetry and exquisite design of diatom
frustules, the cells are ideal subjects for study under the microscope. About a
thousand times smaller than a pinhead, diatoms are commonly between 20-200
microns in length, and due to the intricate markings of certain species they are even
used in testing the resolving power of microscope lens.
Many species of diatoms are restricted to certain regions with specific pH, salinity,
and nutrient levels. When nutrient and light conditions are right, all diatoms are able
to bloom - reproduce very quickly into huge numbers. Knowing what species grow
where, scientists can use diatom communities as a tool for monitoring environmental
conditions and are being used to monitor everything from water quality, effects of
acid rain, and global climate change. Due to their sensitivity to pollutants, diatoms
have frequently been used to evaluate the impact of domestic sewage or industrial
waste on the aquatic environment. In a non-polluted environment there will usually be
a very high diversity of kinds of different diatoms, while a polluted environment will
show a very low diversity with only a few different kinds of diatoms. And because the
cell walls of diatoms are composed of silicon, they are preserved in lake and ocean
sediments. Thus, their fossils can be used to reconstruct past environmental
conditions, such as climatic records, over time.
Under certain conditions great concentrations of diatom frustules can accumulate
and be preserved enough to form sediments. Rock that is largely composed of diatoms
is called diatomite. These deposits are of economic benefit with diatomite used
commercially in high quality chemical filters, sugar refining, toothpaste, fireproof
insulation, silver polish, paint, and as an abrasive agent in flea powder.
There are two basic body shapes of diatoms.
Centric (round) and pennate (thin ellipse).
Scanning electron micrograph of centric
diatom. Diameter of frustrule is ~10 μm.
Scanning electron micrograph of pennate
diatom 5000X magnification.
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1. Introduction
a. How could you distinguish between a plant cell and an animal cell?
b. Can you use the naked eye to see cells?
c. Can you use a light microscope to see cells?
d. What do you think you could see using an scanning electron microscope?
2. Clarifying
Living systems at all levels of organization (unicellular to multicellular) demonstrate
the complementary nature of structure and function. Scientists refer to the overall
shape of the organism as their “morphology”. There is a great deal of diversity in the
morphology of cells from different organisms (plant, animal, bacterial. Cells are so
small, it is difficult to observe cell structure without a microscope.
In this activity, you will view the general shape of single-celled plant cells called
diatoms using a scanning electron microscope (SEM). Using the images produced,
students will estimate the size of the cells and produce a detailed scientific drawing of
the organism.
1.
Labwork
Sample of diatoms - scraping algal slime from the surfaces of rocks and pulling a fine
mesh net through water are two methods for collecting these microscopic organisms.
The best way to make the beautiful structures of the frustule visible is to remove the
content from the cell. Hydrogen peroxide can be used for this. But it is also possible
to find empty frustules of dead diatoms. In this lab activity we will be looking for the
skeletal remains of diatoms on sand collected from a local freshwater lake. Please go
to the listed website to find your samples.
You will need to go to this
website to complete this
activity: King’s University
URL
Data Collection:
You will need to save a series of images (jpeg files) that can be used to create a
composite of low to high magnification images similar to the example below:
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Using the highest magnification image, make a detailed scientific drawing of a single
diatom or a group of diatoms. Label the pores (using a key provided by your
instructor, and be sure to indicate magnification).
Using the scale bar at the bottom left of the highest magnification image, estimate the
size of the diatom in micrometers. Indicate the size of a single diatom on your
drawing.
Analysis:
1. Identify two life functions carried out by diatoms.
2. How are diatoms adapted for making their own food?
3. Although images are in black and white, what color(s) would you expect to see
and why? Note: The colour of the diatom chloroplast is actually yellow-brown instead of
the green we know of other creatures that use light as a source for energy.
4. Identify any additional adaptations the diatom may have related to its shape and
ornamentation.
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3. Summary
Could a magnifying glass or a laboratory microscope have been just as effective?
What other tests might have been more economic? more time-efficient? less
technologically dependent? more environmentally friendly? less collaborative? less
fun?
4. Solitary Practice/Homework
Diatomaceous earth is a naturally occurring siliceous mineral compound made from
the microscopic fossilized remains of diatoms. These plants have been part of the
earth’s ecology since prehistoric times. It is believed that 30 million years ago the
diatoms built up into deep, chalky deposits of diatomite. The fossilized skeleton of the
microscopic diatoms are mined and ground up to a fine, white, crystalline powder. At
the macro-scale, diatomaceous earth looks and feels like talcum powder, yet at the
micro-scale each particle has sharp edges. Lightweight, gritty, and porous,
diatomaceous earth finds a surprising array of uses.
a. Research uses of diatomaceous earth and how it is effective.
Diatomaceous earth
http://www.allbedbugs.com/posts/diatomaceous-earth-and-bed-bugs/
Scanning Electron Microscope
Image of Diatoms (1500X Mag)
http://www.sciencedaily.com/releases/2009/10/091029151621.htm
5. Review
Diatoms are eukaryotic phytoplankton that play a large role in the global carbon
cycle. Each spring, when nutrient levels, light levels, and temperatures are high,
diatom populations soar across the world causing blooms. During these blooms,
increased photosynthesis leads diatoms to sequester huge amounts of CO2.
CO2 levels have risen dramatically since the industrial revolution and blooms play an
important role in sequestering atmospheric that CO2, but little is known about the
specific mechanisms of these blooms, and how their ability to sequester CO2 will
change as atmospheric levels continue to rise. Thus, it’s important for us to
understand how diatoms bloom and what sort of conditions are required for their
growth, in order for us to make accurate predictions about the future of the global
carbon cycle and climate.
Diatoms also rely on Nitrogen and Phosphorus for their growth, and thus play a role
in nitrogen and phosphorus cycles. Nitrogen is required to make amino acids – the
building blocks of proteins. Phosphorus is required for DNA, while they use silica to
make their glass shells. Samples of marine phytoplankton communities have shown
that Nitrogen, Phosphorus and Silica exist in the ratio N:P:Si: 16:1:15.
a. Design an experiment that studies how diatoms grow in varying nutrient and light
conditions.
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