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Content Benchmark L.8.B.3
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Life Science
Structure of Life
Organisms and Their Environment
Diversity of Life
Content Areas
Nature of Science (NOS)
Life Science
Earth Science
Physical Science

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.

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. 

Prokaryotic <a href=cell">

Figure 2. Prokaryotic Cell.

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.

Structure of paramecium
Figure 3. Drawing of a Protist (Paramecium).

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.

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.

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.

Figure 6. Tissue types found in a typical leaf.

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.

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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 cellCells 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

The following links are 2 additional resources to read about surface area to volume ratios

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

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

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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

1st Item Specification: Describe the specialization of cells in multicellular organisms (e.g., skeletal muscles, nerve cells, epidermal cells, cardiac muscle).

Depth of Knowledge Level 1

  1. All unicellular organisms
    1. are bacteria.
    2. carry on all functions of life.
    3. exist as parasites.
    4. are prokaryotic cells.
  1. The cells within an organism that carry out different tasks are called
    1. separated.
    2. organized.
    3. specialized.
    4. distinguished.
  1. Specialized cells within an organism carry out
    1. similar functions which are unnecessary for the survival of an organism.
    2. all functions necessary for the survival of an organism.
    3. identical functions to every other cell in the organism.
    4. specific functions that aid in the survival of an organism.
  1. Which of the following is NOT true of multicellular organisms?
    1. All cells perform the same functions.
    2. All cells are contained within a membrane.
    3. All cells work together for the survival of the organism.
    4. All cells originated from one cell.
  1. Which of the following is NOT an example of cell specialization?
    1. A bacterium that carries out photosynthesis.
    2. A muscle cell that maintains a heart beat.
    3. A red blood cell that carries oxygen throughout an organism.
    4. A skin cell that helps form a barrier around an organism.

Depth of Knowledge Level 2

  1. The DNA in every cell of a multicellular organism is genetically identical, but some cells are specialized because those cells
    1. contain certain genes and not others.
    2. eliminate certain genes and not others.
    3. express specific genes and not others.
    4. replicate certain genes and not others.
  1. A tissue sample is composed of cells with long projections to send signals to other cells. This tissue is most likely composed of
    1. muscle cells.
    2. nerve cells.
    3. adipose cells.
    4. bone cells.
  1. In which cell would you expect to find the most mitochondria?
    1. A muscle cell in the leg muscle of a sprinter.
    2. A cell of the stomach lining that manufactures digestive enzymes.
    3. A red blood cell that transports oxygen.
    4. A nerve cell that transmit signals to the brain stem
  1. Which of the following is the BEST explanation for why a unicellular organism has no need of a circulatory system? A unicellular organism
    1. can exchange materials directly with its environment.
    2. does not transport materials within itself.
    3. can surround itself in all the nutrients that are required for life and does not need to excrete wastes.
    4. has indirect contact with the external environment and does not require the transport of nutrients.
  1. What organism contains specialized cells?
    1. Amoeba
    2. Paramecium
    3. E. coli
    4. Fungi

Constructed Response L.8.B.3

  1. The data in the table below indicates the average number of cells in five different organisms. 


Average Number of Cells


5.0 x 1013

blue whale

1.0 x 1017

tomato fruit

1.6 x 106

nematode worm

2.0 x 103

green algae


Use the data table and your knowledge of cell structure and organization to answer the following questions.

    1. Define a unicellular organism.
    2. Compare and contrast cell organization and function in unicellular organisms and multicellular organisms.
    3. Humans are composed of a large number of cells which are specialized. Why must some human cells be specialized? Use the muscle cell as an example.

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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

  1. B, DOK Level 1
  2. C, DOK Level 1
  3. D, DOK Level 1
  4. A, DOK Level 1
  5. A, DOK Level 1
  6. C, DOK Level 2
  7. B, DOK Level 2
  8. A, DOK Level 2
  9. A, DOK Level 2
  10. D, DOK Level 2

Constructed Response L.8.B.3 Score Rubric:

 3 points

Response addresses all parts of the question clearly and correctly.

  1. Students must define a unicellular organism as an organism which carries on all the functions of life, but is composed of only one cell.
  2. Students should include the fact that unicellular organisms consist of one cell and multicellular organisms are composed of more than one cell.  Both types of organisms carry on all the functions of life such as metabolism, growth and development, response to stimuli, and reproduction.  However, multicellular organisms may contain specialized cells for increase efficiency of these roles.
  3. Students should note that the specialization of cells in the human body allows for more efficient functioning for survival. Muscle cells in humans allow for locomotion and the movement of various materials like air, blood, wastes, and nutrients throughout the body.  Unicellular organisms have no need of this specialization.  A unicellular organism may use a flagellum, cilia, or the cytoskeleton for locomotion.  Also, the cell can move things around the inside of the cell via the cytoskeleton.

2 points

Response addresses all parts of the question and includes only minor errors.

1 point

 Response does not address all parts of the question.

0 points

The response is totally incorrect or no response.

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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

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

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

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

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

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Benchmark Related Vocabulary