Content Benchmark L.8.B.3
Students know some organisms are made of just one cell and that multicellular organisms can
consist of thousands to millions of cells working together. E/S
Single-celled organisms fall in to two specific domains of life: Archaea and Eubacteria.
Organisms in these two domains are prokaryotic, meaning they do not contain a nucleus housing
their genetic material. There are groups of organisms within the domain Eukaryota (cells that
contain a nucleus for their genetic material) that are single-celled, but the majority of phyla
within this domain are multicellular. The domains are a taxonomic level higher than kingdoms.
Figure 1. The Three Domains of Life.
(From http://www.windows.ucar.edu/tour/link=/cool_stuff/tour_evolution_1.html)
Multicellular organisms are everywhere we look… from the largest tree to an insect crawling
across its leaf. Multicellular organisms have two major characteristics in common:
1. The ability of the cells within the organism to adhere together.
2. The lack or reduction in the likelihood of these cells surviving outside the organism.
Cells that make up multicellular organisms have genes that code for proteins allowing cell-cell
adhesion. In addition, the cells making up the organism have specific functions that contribute to
the functioning of the overall organism not to the specific cell’s survival (Gill, 2006).
Humans are multicellular organisms composed of trillions of cells. But each cell within a human
does not perform all functions of the organism, it performs only a few. The few functions that
the cell performs, it performs very well. In other words, the cells are specialized. These cells are
most often organized into groups called tissues. A tissue by definition is a group of cells
performing the same function. These cells rely on one another to survive, and in turn, provide a
valuable function for the organism as a whole. When groups of tissues are organized together
this forms an organ. Each organ such the heart or the intestine performs a different function for
the overall health of the organism.
An interesting theme that comes out in multicelled organisms is the idea of emergent properties.
Basically, this is the idea that the whole is greater than just the sum of the parts. In other words,
as we progress from a single cell to a group of cells and on up to a multicelled organism, there
are properties that emerge that did not exist in the simpler organism.
Unicellular organisms, while capable of forming colonies, are independent organisms. Each cell
has the capability of surviving on its own without other cells. Because of this, each cell has to
perform all the functions necessary for survival. It needs to contain the necessary structures to
obtain nutrients from its environment, utilize those nutrients to transform energy into a useful
form, move the cell toward or away from a stimulus, and in general perform many of the tasks
that a multicelled organism does.
The simplest single-celled organisms (See Figure 2) are called prokaryotes. They are relatively
small cells with no organelles, but a few specializations. They often have cilia or flagella that
help them move toward or away from a stimulus such as food or water.
Figure 2. Prokaryotic Cell.
(From http://www.tpsd.org/ths/sciences/b2eukpro.htm)
There are also single-celled eukaryotic organisms collectively called protists. Kingdom Protista
is a collection of organisms that have few things in common:
1. They are single-celled organisms the majority of their lives.
2. They are eukaryotic (they have a nucleus).
An example of a protist, a paramecium, is illustrated in Figure 3.
Figure 3. Drawing of a Protist (Paramecium).
(From http://www.biology-resources.com/drawing-paramecium.html)
As we progress upward in complexity, we see that the organisms possess more complex tissues
and therefore organ systems with each of the cells performing specialized functions. A tissue is
a collection of cells that look the same and act the same. There are many different types of
tissues. As we investigate the different classifications of animals, we can see that as the
organism becomes more complex, the more types of tissues are present, and the more complex
the tissues become. The various cells found throughout the human body have their own
functions. Scientists categorize all the tissues in the body into four major classifications each
with various subgroups (See Figure 4).
Figure 4. Basic Tissue Types.
(From http://fig.cox.miami.edu/~cmallery/150/physiol/physiology.htm)
The four major classifications of tissues are epithelium, muscle, nervous, and connective tissues.
These different types of tissues are organized in varying amounts into organs that perform
specific functions. In general, epithelium acts as a lining and protective covering, muscle causes
movement, connective tissue connects and supports other tissues, and the nervous tissue is
involved in communicating.
Figure 5. Tissue types found in the heart.
(From http://www.jdaross.cwc.net/heart4.htm)
The human heart’s purpose is to function as a pump that moves blood around in a set of tubes or
blood vessels. In this respect, it has to have a type of tissue that is specialized for movement.
Cardiac muscle (a subtype of muscular tissue) is designed specifically for this function. As the
cells contract they allow for the heart to push blood throughout the body. But, the heart is an
organ meaning there are multiple tissue types. In addition to cardiac muscle, the heart has
copious amounts of connective tissue to support the shape and connect the muscle cells to one
another as well as to its surroundings acting as an anchor. The pericardium anchors the heart to
its surroundings and is made of a couple of different types of connective tissue, dense fibrous
connective tissue, and loose connective tissue. Each type of tissue performs slightly different
functions. The heart is lined with a single layered epithelium called simple squamous
epithelium. This epithelium allows for the blood to move smoothly through the heart and into
the blood vessels.
The heart is just one example of how the different tissue types can be organized into a functional
organ. The leaf of a plant also is made of different tissue types. Since plants do not perform the
same functions of animals, their tissue types are a little different. Rather than having four
classifications, plants only have three: dermal, ground, and vascular tissue. Dermal tissue is
similar to epithelial tissue in that it lines the outside of the leaves and stem. Ground tissue forms
the bulk of the plant, and the vascular tissue is responsible for the transport of water, nutrients,
and minerals from one place to the other in the plant.
Dermal
Tissue
Ground
Tissue
Vascular
Tissue
Dermal
Tissue
Figure 6. Tissue types found in a typical leaf.
(From http://www.digitalfrog.com/resources/raingallery.html)
In conclusion, the types of cells that form tissues and the different types of tissues vary widely
across organisms. However, it is these differences in tissue types that allow for the diversity of
organisms we see.
Content Benchmark L.8.B.3
Students know some organisms are made of just one cell and that multicellular organisms can
consist of thousands to millions of cells working together. E/S
Common misconceptions associated with this benchmark
1. Students incorrectly believe that as the size of a multicellular organism increases,
the size of the cells increases rather than there being more cells that accounts for the
increase in size. (Flores, 2003 International Journal of Science Education, 25:2, 269 —
286)
Cell size is in fact quite small in most organisms, not visible to the unaided eye. This can
be related back to the concept of surface area to volume ratios. As the size of a cell gets
bigger, the volume also increases. When the volume of the cell gets too large there will
be difficulty moving/transporting substances throughout the cell. Cells transport many of
the necessities by diffusion and osmosis. If the cell volume is too large, these processes
would take too long and result in cell death.
An article explaining this misconception is available through the following link
http://www.informaworld.com/index/0TBX2JPAE2P6UPH3.pdf
The following links are 2 additional resources to read about surface area to volume ratios
http://www.tiem.utk.edu/bioed/bealsmodules/area_volume.html
http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ce/m2/s1/
2. Some students, although understanding that the cell is the basic structural and
functional unit of life, think that certain parts of the body of multicellular organisms
are not made of cells. (Dreyfus & Jungwirth, 1988).
All organisms are made up completely of cells and cell secretions. When we discuss the
idea that there are parts of the body such as tendons and ligaments, we need to remember
that they are formed partly of cells and largely of cell secretions such as collagen and
fibrin.
Information for misconceptions pertaining to cells as the functional units is available at
http://www.informaworld.com/index/746955314.pdf
3. Often students do not see the connection between subcellular processes and what is
happening at the organ and organism level. (Songer & Mintzes, Journal of Research in
Science Teaching, v31 n6 p621-37 Aug 1994).
An example of this misconception is the connection between cellular respiration and
general respiration or breathing. Students may memorize or even understand the parts of
cellular respiration but often are confused between how the oxygen we take in is
connected to the energy we make and the carbon dioxide we exhale. They fail to take
into account the idea of conservation of matter and how that relates to the cell’s
respiration.
For more information on this misconception, please see
http://homepage.mac.com/vtalsma/misconcept.html
Content Benchmark L.8.B.3
Students know some organisms are made of just one cell and that multicellular organisms can
consist of thousands to millions of cells working together. E/S
Sample Test Questions
Questions and answers to follow on separate document
Content Benchmark L.8.B.3
Students know some organisms are made of just one cell and that multicellular organisms can
consist of thousands to millions of cells working together. E/S
Answers to Sample Test Questions
Questions and answers to follow on separate document
Content Benchmark L.8.B.3
Students know some organisms are made of just one cell and that multicellular organisms can
consist of thousands to millions of cells working together. E/S
Intervention Strategies and Resources
The following is a list of intervention strategies and resources that will facilitate student
understanding of this benchmark.
1. Surface Area to Volume Ratio Activity
This surface area to volume ratio experiment deals with the idea of diffusion, osmosis and the
speed at which the molecules move. This activity can help with the idea of limiting cell size.
The activity can be downloaded from Access Excellence at
http://www.accessexcellence.com/AE//AEC/AEF/1996/deaver_cell.php
There are a series of demonstrations that a teacher could utilize to show the effects of surface
area to volume ratios. This demo is also accessible at the Access Excellence site at
http://www.accessexcellence.org/AE/ATG/data/released/0307-TrumanHoltzclaw/index.php
2. Comparing Animal and Plant Cell Structure
In this activity, students compare their own cheek cells to a layer of red onion. It might help
students comprehend that they are made of trillions of cells.
This activity is available at
http://images.apple.com/education/curriculumlabs/pdf/AnimalsPlantsMS.pdf
3. A New You! Learning How Stem Cells Repair the Body
In this lesson, students research stem cells to learn how they function, the distinguishing
characteristics of types of stem cells, and how stem cells may be manipulated by scientists to
help bodies heal and regenerate unhealthy or damaged cells. This might be interesting for
students to explore the idea of cell differentiation and specialization.
The lesson is available at
http://www.nytimes.com/learning/teachers/lessons/20001107tuesday.html?searchpv=learning
_lessons
4. What’s a Stem Cell?
The Genetic Science Learning Center at the University of Utah also has a portion of their
website that explains stem cells and cell differentiation. A webquest is available in the
teacher resource section of the website which follows along with the animations on the
website.
The “What’s a Stem Cell” animation is available at
http://learn.genetics.utah.edu/units/stemcells/whatissc/