Content Benchmark L.8.B.1
Students know all organisms are composed of cells, which are the fundamental units of life. E/S
The cell is the fundamental unit of life. All organisms are composed of one or more cells.
Scientists have been studying cells through microscopes for about 350 years, exploring the
structure of organisms, making observations, and drawing conclusion based upon these
observations. The discoveries of these early scientific pioneers led to what is known as the cell
theory.
The components of the cell theory are:
1. All living things are composed of at least one cell.
2. Cells are the most basic unit of structure and function.
3. All cells come from preexisting cells.
A Brief History of the Cell Theory
Robert Hooke (1635 – 1703)
Robert Hooke is famous for his microscopic description of cork (observation XVII of the
Micrographia) that he viewed through a compound light microscope. He described the box-like
structures he observed as “cells.” In fact, Hooke was observing what is now known as the cell
wall of the cork tissue. He called the structure a cell because it reminded him of the cells of a
monastery. Robert Hooke is given credit for discovering the plant cell and coining the term cell.
Figure 1. Hooke devised the compound
microscope and illumination system
shown above, one of the best such
microscopes of his time.
(From
http://www.ucmp.berkeley.edu/history/h
ooke.html)
Figure 2. Robert Hooke's 1664
Micrographia showing a drawing of the
cell like structure of cork, from which the
name cell originated.
(From
http://www.ucmp.berkeley.edu/history/hoo
ke.html)
Anton Van Leeuwenhoek (1632 – 1723)
Anton Van Leeuwenhoek was a Dutch tradesman that had a knack for grinding optical lenses
and is credited with creating over 400 microscopes. He was the first to record microscopic
observation of muscle fibers, bacteria, and blood flow through capillaries. Furthermore, he
described many specific forms of microorganisms (which he called “animalcules” or animal
molecules), including protozoa and other single-celled organism.
Figure 3. Anton Van Leeuwenhoek along side one of his 400 microscopes.
(From http://ez002.k12.sd.us/Chapter%20One%20Science.htm)
Matthias Schleiden (1804 – 1881) & Theodor Schwann (1810 – 1882)
Matthias Schleiden was a German Botanist that viewed plant structures under a microscope. In
1838, he concluded that all plants are composed of cells. The following year, a similar
conclusion was made for animals. A German zoologist, Theodor Schwann, extended Schleiden’s
theory by stating that all living things were composed of cells. Schleiden and Schwann are
credited with the initial formulation of the cell theory.
1. All living things are composed of at least one cell.
2. Cells are the most basic unit of structure and function.
Figure 4. Plant cells magnified 100x.
(From
http://www.moe.gov.sg/edumall/tl/digital_re
sources/biology26.htm)
Figure 5. Human Liver Cells
Magnification 200x.
(From
http://www.histol.chuvashia.com/atlas
-en/cytol-en.htm)
Rudolf Virchow (1821 -1902)
Rudolf Virchow was a German physician who extensively studied pathology (the study of
diseases) and in 1858 published his theory “Omnis cellula e cellula” which translates to “every
cell comes from another existing cell.” Virchow’s conclusion was based on the observation of
cell formation by mitosis and cytokinesis. Virchow’s theory contributed the third part of the cell
theory:
3. All cells arise from pre-existing cells.
Figure 6. Brief diagram of the cell cycle emphasizing mitosis.
(From http://www.biology.iupui.edu/biocourses/N100/images/8mitosiscropped.jpg)
For detailed information on cell division, see MS TIPS Benchmark L.8.B.2
Cell Types: Prokaryotic and Eukaryotic
Prokaryotic cells (from the Greek meaning “before nucleus”) are the simplest cellular organisms.
They have DNA, a cell (plasma) membrane, cytoplasm, and ribosomes but lack a nucleus and
membrane-bound organelles. Examples of prokaryotic cells are bacteria and archaebacteria.
Figure 7. Comparative diagram of the similarities between prokaryotic and eukaryotic cells.
(From http://www.schenectady.k12.ny.us/putman/biology/data/images/cells/allcell.jpg)
Eukaryotic cells (from the Greek meaning “truly nuclear”) are complex cells that contain a
nucleus and membrane bound organelles, such as, an endoplasmic reticulum (smooth and rough),
Golgi apparatus (body), mitochondria, and chloroplasts. Each organelle performs a specific
function in order for the cell to maintain homeostasis. Examples of organisms that contain
eukaryotic cells are animals, plants, fungus, and protists.
Cellular Organelles
Cell Membrane
The cell membrane is a complex structure consisting of a lipid-bilayer embedded with proteins
and carbohydrates. The scientific model used to describe the cell membrane is the Fluid Mosaic
Model (Figure 8). The cell membrane has several functions including: separating the inside of
the cell from the outside environment, regulating what enters and exits the cell, and it is
responsible communication among cells.
Figure 8. Diagram of the Fluid Mosaic Model.
(From http://sun.menloschool.org/~cweaver/cells/c/cell_membrane/)
The cell membrane is semi-permeable, meaning it allows some materials to move through while
preventing other materials from entering or exiting the cell. Movement into or out of a cell along
a concentration gradient is passive transport. Passive transport does not require energy. An
example of passive transport is osmosis (diffusion of water). Active transport requires energy
and occurs when transport is occurring against a concentration gradient, or when the material is
too large to fit through the membrane. Proteins play an integral role in transport through the cell
membrane. The proteins that are embedded and extend through in the cell membrane are also
used for communication. These proteins allow the cell to respond to environmental conditions as
well as communication with other cells.
A resource that provides explanations and analogies for the cell membrane and its functions is
available at http://www.beyondbooks.com/lif71/4b.asp.
For a detailed explanation of cellular transport, see MS TIPS Benchmark L.8.B.2
Cell Wall
The cell wall is found in plant, fungus and bacteria cells. The cell wall provides structural
support and helps to protect the cell. The cell wall is located outside the cell membrane and is
primarily composed of cellulose.
Figure 9. Anatomy of a typical plant cell that shows the cell wall in relation to the cell (plasma) membrane.
(From http://micro.magnet.fsu.edu/cells/plantcell.html)
Nucleus
The nucleus contains and protects the DNA. The DNA within the nucleus controls the cell’s
activity by regulating what proteins are made. The nucleus has two major parts, the nuclear
membrane (nuclear envelope), and the nucleolus. The nuclear membrane regulates what enters
and leaves the nucleus. The nucleolus makes ribosomes which are the site of protein synthesis.
Figure 10. The Structure of the Nucleus. (From
http://www.cartage.org.lb/en/themes/Sciences/Zoology/AnimalPhysiology/Anatomy/AnimalCellStructure/Nucleus/
Nucleus.htm)
Endoplasmic Reticulum
The endoplasmic reticulum helps to make, refine, and transport chemicals used inside and
outside the cell. There are two types of endoplasmic reticulum; the smooth and the rough. The
smooth ER is involved in making fats, breaking down carbohydrates, and removing toxins and
poisons. The rough ER has ribosomes attached to it. Proteins are made at the ribosomes and
transported by the rough ER. Ribosomes can also be free floating in the cytoplasm.
Figure 11. Rough and Smooth Endoplasmic Reticulum.
(From http://student.nu.ac.th/u46410288/endoplasmic.htm)
Golgi Apparatus (Body)
The Golgi apparatus takes the products made in the endoplasmic reticulum, like proteins and
fats, modifies them and prepares them for export outside the cell. Students typically are not
required to know the function of the golgi apparatus until high school.
Figure 12. Diagram shows the relationship between the endoplasmic reticulum and the Golgi apparatus.
(From http://employees.csbsju.edu/hjakubowski/classes/ch331/cho/glycoproteinshtm.htm)
Vacuole
Vacuoles are found in both plant and animal cells. There are several types of vacuoles with
different functions such as storage, digestion, and elimination of waste. The vacuoles in plant
cells are much larger than in animal cells. In plant cells, the vacuole plays a very important role
in helping to maintain the shape and the structure of the plant. When a plant has not been
watered it wilts or sags. This is due to empty vacuoles. The function of vacuoles in animals may
include storage of food and waste.
Figure 13. Comparing plant and animal cells. Notice the difference in the size of the vacuole.
(From http://evolution.berkeley.edu/evosite/lines/IIDmolecular.shtml)
Chloroplast and Mitochondria
Chloroplasts are found exclusively in plant cells and are the site of photosynthesis.
Photosynthesis is the process of converting radiant energy into chemical energy in the form of a
simple sugar, glucose. Mitochondria are found in both plant and animal cells and are the site of
cellular respiration. Cellular respiration is the process of breaking down glucose into energy
usable at the cellular level. No matter how much “food” a cell has, without the mitochondria to
make the energy stored in the food usable, the cell will die.
Figure14. Left – Mitochondria, Right – Chloroplast
Diagram depicting the relationship between the mitochondria (site of cellular respiration)
and chloroplast (site of photosynthesis).
(From http://ebiomedia.com/prod/LC/LCcellunit.html)
Table 1: Types of Eukaryotic Cells: Plants and Animals
The following table compares the cellular structures of a typical animal and plant cell.
Cellular Structures
Cell Membrane
Cell Wall
Nucleus
Endoplasmic Reticulum
Golgi Apparatus
Chloroplast
Mitochondria
Ribosome
Vacuole
Plant
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Animal
Yes
No
Yes
Yes
Yes
No
Yes
Yes
Yes
Single-celled (Unicellular) Organisms
(Uni = one; Unicellular = one celled organisms)
A cell is the basic structural and functional unit of all organisms. Cells may exist as independent
units of life, single-celled (unicellular) organism, or may form colonies or tissues as in higher
plants and animals (multicellular). Organisms that are made up of only one cell are unicellular.
Uni- means one, like a unicycle has only one wheel. The smallest living organisms are
unicellular. The four main groups of unicellular organisms are bacteria, protozoa, unicellular
algae, and unicellular fungi.
Figure 15. The 3 bacteria shown
above are unicellular organisms.
(From
www.sustainablesoilcip.org.uk/obj1.ht
m)
Figure 16. The paramecium is a
single-celled organism. This
organism has only one cell.
(From
http://www.ndpteachers.org/perit/
biology_image_gallery1.htm)
Figure 17. Phytoplankton is
unicellular algae. Each organism
above consists of only one cell.
(From
www.waterencyclopedia.com/OcPo/Plankton)
Figure 18. Yeast is a type of
unicellular fungus.
(From
http://lpec.virtualschools.net/fol
ders/learning_zone/ks3_and_ks
4/bread_and_yeast/)
Multicellular Organisms
(Multi = many; Multicellular = composed of many cells)
Multicellular organisms are composed of more than one cell, and have differentiated cells that
are responsible for specific functions. The levels of organization are cells → tissues → organ →
organ system → organism. The main groups of multicellular organisms include: plants, animals
and fungus. Several types of multicellular organisms are illustrated below.
Figure 19. Variety of plants in Costa Rica’s
rainforest.
(From
http://www.casarioblanco.com/rainforest.html)
Figure 20. Variety of animals both living and
extinct.
(From
http://www.ucmp.berkeley.edu/phyla/phyla.html)
Figure 21. Variety of Fungi.
(From
http://www.nationmaster.com/encyclopedia/Fungus)
Content Benchmark L.8.B.1
Students know all organisms are composed of cells, which are the fundamental units of life. E/S
Common misconceptions associated with this benchmark
1. Many students incorrectly believe that simple organisms came from non-living
objects through spontaneous generation.
In 1668, Francesco Redi, an Italian physician and poet, was the first person to address the
notion of spontaneous generation. At that time, it was believed that maggots arose
spontaneously in rotting meat. Redi believed that maggots developed from eggs laid by
flies. To test his hypothesis, he set out meat in a variety of flasks, some open to the air,
some sealed completely, and others covered with gauze. As he had expected, maggots
appeared only in the open flasks in which the flies could reach the meat and lay their
eggs.
In 1860, Louis Pasteur, a French chemist and microbiologist, was one of the first to
disprove spontaneous generation. He concluded through his experiment with swan-neck
flasks and sterile fermentable juice that microorganisms are carried on dust particles and
that air alone could not trigger the growth of microorganisms.
Read about Redi’s experiment and address some of the questions at the following
website, http://www.kent.k12.wa.us/staff/TimLynch/sci_class/chap01/redi.html
2. Many students incorrectly believe that photosynthesis is a plant process and
respiration is a process carried out exclusively by animals.
Plants have chloroplasts which are the organelles where photosynthesis takes place. The
process of photosynthesis takes the carbon from carbon dioxide and the hydrogen from
water to convert radiant energy into chemical energy in the form of carbohydrates.
Cellular respiration, or the process of breaking carbohydrates down into usable energy for
the cell, takes place in the mitochondria. Both plants and animals have mitochondria,
both plants and animals must respire in order to breakdown sugar into ATP. Animal cells
do not contain chloroplast and therefore, do not have the ability to photosynthesize.
To access a lab on plant respiration, go to
http://www.teachervision.fen.com/tv/printables/SRPA03201_3.pdf
The following website is a resource provided by the Missouri Department of Elementary
and Secondary Education that informs teachers of common misconceptions students have
about specific science concepts. Page 5 of the document highlights misconceptions
related to this benchmark.
http://www.dese.mo.gov/divimprove/curriculum/science/SciMisconc11.05.pdf
3. Many students have difficulty with the concept that living things contain cells
(rather than they are made up or composed of cells).
Cells are considered the smallest structure that is alive. Cells are often too small to be
seen without a microscope. After 150 years of research, scientists concluded that
organisms are composed or made up of cells. The smallest organisms are composed of a
single cell, while larger organisms are composed of multiple cells that have specialized
functions. In order to understand this concept, use the analogy of a brick wall. It does not
contain bricks, it is made of bricks.
The following website is a resource provided by the Missouri Department of Elementary
and Secondary Education that informs teachers of common misconceptions students have
about specific science concepts. Page 6 of the document highlights this misconception.
http://www.dese.mo.gov/divimprove/curriculum/science/SciMisconc11.05.pdf
Content Benchmark L.8.B.1
Students know some characteristics of an organism are composed of cells, which are the
fundamental unit of life. E/S
Sample Test Questions
Questions will be inserted once they are finalized.
Content Benchmark L.8.B.1
Students know some characteristics of an organism are composed of cells, which are the
fundamental unit of life. E/S
Answers to Sample Test Questions
Questions will be inserted once they are finalized.
Content Benchmark L.8.B.1
Students know all organisms are composed of cells, which are the fundamental units of life. 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. Tutor Vista – Cells as a Unit of Life: Why are organisms composed of cells?
This website gives a brief explanation of the hierarchical organization of cells in
multicellular organisms.
To access the website, go to
http://www.tutorvista.com/content/biology/biology-iii/cell-unit-life/cells-organisms.php
2. Tutor Vista – Cells as a Unit of Life: Cell Theory
The cell theory is the foundation of cell biology. This website reviews the cell theory and
provides statements for the modern cell principle.
To view this information, go to http://www.tutorvista.com/content/biology/biology-iii/cellunit-life/cell-theory-and-principle.php
3. The Cell Theory Rap
This is a fun website that includes the lyrics to a rap song. The song incorporates the major
components of the cell theory into a fun format.
To access the song, go to
http://www.biologycorner.com/worksheets/cellrap.html
4. Cells Alive! Mitosis
This website is an animated tutorial reviewing the phases of mitosis. This is the process
Virchow observed that led to his conclusion that all cell arise from other cells.
To view the animation, go to
http://www.cellsalive.com/mitosis.htm
5. Unlocking the Mystery: Kingdoms
This website provides a general outline review of the following Kingdoms: Monera, Protista,
Fungi, Plantae, and Animalia.
For the outline, visit
http://www.usoe.k12.ut.us/curr/Science/sciber00/7th/classify/sciber/5king2.htm
6. Cells Alive!
This website describes the function of cell organelles and provides an interactive cell model
for both plant and animal cells. This is a great interactive website for introducing and
reviewing cells and cellular structures.
To access the website, go to
http://www.cellsalive.com/cells/cell_model.htm
7. Levels of Organization
Multicellular organisms have levels of organization. This website provides an outline
overview of the levels of organization.
To access the website, go to
http://www.usoe.k12.ut.us/curr/Science/sciber00/7th/cells/sciber/levelorg.htm