The Cell

Table of Contents

  1. Cell Size
  2. Cell Compartments
  3. Cell Organelles
  4. Mitosis
  5. Protein Characteristics
  6. Essential Nutrients
  7. Adenosine TriPhosphate
  8. Mechanisms for Substance Movement In and Out of a Cell

I. Cell Size

Cell Size
The majority of cells are 10 to 20 mm, but the range of cell size is from 2 mm to 120 mm. However, a nerve cell may be a meter in length. It is necessary to use a microscope to view the typical cell. Approximately 64,000 cells lined up would occupy one inch.

II. Cell Compartments

Human Cell

A. Nucleus

contains the chromosomes; often in center

B. Cytoplasm

  1. Remainder of the cell;

    Cytoplasm and Cytosol
  2. Internal cell membranes further compartmentalizes the cell
  3. Tubules and matrix skeletal materials provide rigidity and movement
    Cytoskeletal Filaments


III. Cell Organelles

Cell organelles are sub-units of the cell that cannot function if removed from the cell; perform specific functions

A. Plasma Membrane

1. Function


It is the outer boundary of the cell and is responsible for regulating whether substances enter or leave the cell; semi-permeable; barrier for material to move into and out of the cell

2. Structure

  1. bounded sheet of fat - lipid bilayer
    Fluid Mosaic
  2. integral associated proteins

3. Location

  1. proteins extending through membrane
    1. stabilize the membrane & physically link cells together
    2. other proteins link protein filaments to the inner surface of the cell membrane
    3. channels and gates
  2. proteins on outer surface of membrane
    1. receptor proteins or "sensitive" spots interact with chemical messengers when present at the outer membrane surface
    2. enzymes
    3. associated with carbohydrates, serve as recognization molecule
  3. proteins on inner surface of membrane - enzymes

4. Junctions between cells

  1. overview constructed of protein; necessary to link cells together to form tissues. A blood cell is an example of a cell that is not linked to another blood cell, but rather it remains as an independent cell suspended in a fluid [plasma]. Most cells are closely associated with like cells to form tissues; e.g., muscle tissue, fat tissue, epithelial tissue, and nerve tissue.
  2. proteins that serve to bind cells to each other include integrins, desmosomes, tight junctions, and gap junctions
    Gap Junction
    1. integrins are cell membrane proteins of the type that either extend all the way through the cell membrane [from the outer surface of the cell to the inner surface of the cell membrane] or of the type that are associated with the outer region of the cell membrane present in all cells that associate to form tissues.

    2. desmosomes hold adjacent cells firmly together in areas that are subject to considerable stretching, such as in the skin. Cells with desmosomes may have movement of substances between the cells.

    3. tight junctions form connections between cells that is much more firm than simple desmosomes. Tight junctions restrict the movement of most organic substances, but there is ionic and water movement between adjacent cells. An example of tight junctions is the epithelial cell layer of the small intestine. Nutrients must be transported to the interior of one of these cells to pass from the intestinal lumen to the extracellular space. The nutrients cannot diffuse between the epithelial cells of the intestinal lining. From the extracellular space, the nutrients move to the blood stream; or in the case of large fatty acids, to the lymph vessels.

    4. junctions that consist of protein channels linking the interiors of two cells are called gap junctions. Small molecules and ions may pass between the cells; however, larger molecules such as proteins cannot. Gap junctions are extremely important in the transmission of electrical activity between some of the muscle cells.

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B. Nucleus

1. general function

Genetic Information

Genetic information is stored in the nucleus. This information is used both to operate the cell and to create more cells. The principal storage medium is a molecule called DNA, which is short for DeoxyriboseNucleic Acid. When the cell divides, mitosis, the DNA is copied such that there are two exact copies of the information, and each set is allocated to one of the daughter cells.

Information to operate the cell is also contained in the nucleus. This information is transcribed on to another molecule called RNA, ribose Nucleic Acid. The RNA leaves the nucleus through nuclear pores and goes into the cytoplasm. This RNA carries a message to the ribosome and is also called mRNA, or messenger RNA.

Genetic Code

Information is sotored in DNA according to a genetic code. This code is comprised of "base pairs" of the double stranded DNA molecule. One strand is read at a time. Three bases of the strand constitue an triplet. The arrangement is known as the triplet code. Each triplet has the capacity to designate the placement of an amino acid on a string of amino acids, a polypeptide, or, if long enough, a protein.

Ligand Binding

Since the order of the bases in the triplet determines the amino acid, if the order is mixed up, the amino acid selected may not be the correct one. Such an event is known as a mutation. There are several types of mutations. If one base is lost, this creates a deletion mutation. this impacts all of the rest of the coding. If a triplet is lost, this causes a "frameshift" mutation, and only one amino acid is lost. Cancer is frequently related to mutations.

Ligand Binding

It is possible to modify the information of the DNA in very controlled ways. This area is called "genetic engineering". This is done naturally by viruses. Genetic engineer utilize the same mechanisms to produce designer genes. Basically, the double strand of DNA is broken apart,(by restriction enzymes) a sequence of "Donor" DNA is inserted, and the breaks are sealed (by ligases). This recombinant DNA now has added properties based upon the Donar DNA.

2. components

a. nucleus contains a fine network of treads called chromatin which is DNA (deoxyribonucleic acid) associated with particular proteins. At the time of cell division, chromatin becomes condensed to form rod-like bodies known as chromosomes b. nucleolus storage site for messenger RNA (m-RNA)

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C. Nucleolus


contains m-RNA that can be thought of as miniature Xerox copies of small segments of the DNA. The membrane surrounding the nucleus contains pores that are large enough to allow diffusion of small molecules; however, m-RNA requires special mechanisms to allow it to pass from the nucleus of the cell to the cytoplasm. Once in the area of the cytoplasm, the m-RNA becomes associated with ribosomes, and the process of protein formation in the cytoplasm begins.


Gene transcription, the process of producing mRNA, startes in a promoter region. The promoter base sequence serves as a binding site for RNA polymerase and transcription factors. The triplet codes, called Codons are read and transcribed onto the growitn chain of mRNA.


The Primary RNA transcript actually contains more codons, information than necessary, introns and extrons. The excess information is excized to create the actual mRNA that will be utilized to create proteins. The RNA is spliced together by the spliceosomes. Subsequently, the mRNA leaves the nucleus by passage through the nuclear pores.

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D. Ribosome

Protein Synthesis

Ribosomes are the machinery that provide a surface location for the assembly of amino acids into proteins. Ribosomes are constructed of protein and ribosomal ribonucleic acid (r-RNA) and are attached to endoplasmic reticulum [an internal cell membrane].

When ribosomes are attached to endoplasmic reticulum, the endoplasmic reticulum is called rough or granular endoplasmic reticulum. Also present are proteins that serve as enzymes that allow the linkage of amino acids to form a polymer of amino acids [protein]

Reading mRNA

The ribosome is comprised of two subunits, a 30 and a 50. The mRNA is cought between the two subunits. The ribosomes slides along the mRNA reading the codons. As it moves along, it starts and adds onto a chain of amino acids, a polypeptide. A series of ribosomes will attach to a strand of mRNA. After the mRNA has been read, the ribosome falls off and is now free. If long enough, the completed polypeptide is called a protein.

Transfer RNA

The actual reading of the codon is accomplished by a cloverleaf shaped piece of RNA known as tRNA. The t stands for transfer. At one end of tRNA is an Anticodon, which has the complementary sequence to the codon. The anticodon binds to the codon. On the opposite end of the tRNA is a binding site for a particular aminoacid. This is the same amino acid for which the codon codes. The amino acid is properly juxtaposed by the ribosome to be placed on the growing strand polypeptide. thereafter, the amino acid is added to the polypeptide, making it grow longer.

Posttranslationaql Splitting

Subsequent to production, the polypeptide is processed. The protein is split, conjugated or otherwised processed into the correct form. Embedded into the genetic code are punctuation commands. These commands are read and the protein is processed according to these commands.

Transcription Control

There are a variety of control mechanism for transcription. A typical on relies upon an extracellular signal such as a hormone. This modulates (see transduction) intracellular factors and signals the initiation of transcription of the DNA and the production and regulation of RNA production.

Protein Assembly
Protein secretion

The protein produced by the ribosome is processed and shipped in vesicles to the Golgi apparatus. The golgi apparatus finishes sythesizing complexes, packages them, and then secretes them.

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E. Endoplasmic Reticulum

Endoplasmic Reticulum


- network of internal cell membranes

1. rough (granular) endoplasmic reticulum

ribosomes located on surface

2. smooth endoplasmic reticulum

no ribosomes; Many reactions that occur in the cell do so on the surface of the smooth endoplasmic reticulum; e.g., synthesis of lipid

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F. Golgi Apparatus

Golgi Apparatus


- membrane structure with associated vesicles; for sorting proteins. Once proteins are sorted, they are placed into vesicles to be transported to particular locations in the cell. In addition, for appropriate proteins, the vesicle may fuse with the plasma membrane thereby secreting the protein to the extracellular space

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G. Mitochondria



Mitochondria are rod-shaped organelles with a double outer membrane. The mitochondria contain a special set of enzymes that enable the creation of large amounts of an energy storage molecule called adenosine triphosphate (ATP). Oxygen is required to be present for the enzyme systems to function in the mitochondria.

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H. Lysosomes

Lysosomes are small vesicles containing acid and digestive enzymes that breakdown bacteria, debris from dead cells, and non-functioning organelles from the same cell.

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I. Peroxisomes

Peroxisomes are similar to lysosomes except they contain enzymes.These enzymes are primarily hydrogen peroxide, which is toxic to the cell. Within the cell, there is a variety of filaments including those necessary for cell division and those necessary for movement of the cell.

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IV. Mitosis

Mitosis is the process which produces cell "clones". It is multiplication through division; one cell divides into two identical clone cells and has increased the total cell population. Using the poweres of multiplication; 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, ... it does not take long to produce millions and billions of cells. We call the uncontrolled proliferation of cells in this manner "cancer". Normally, we have a myriad of controls which limit and specify cell growth and division.

Cell Cycle

The "Cell Cycle" is the set of events that a cell typically transverses during its life. After a cell separates from it sibling daughter cell it enters a stage known as G1. (Which is the first growth or gap period.) Some cells tend to hang at this point forever, e.g., liver, nerve, and muscle cells. However, given proper stimuli, they will move on to the next stage.

The second stage is the "S" stage where the synthsis of DNA occurs. Once a cell passes from G1 to S stage, it will divide.


Subsequently the cell proceeds through a second "Growth" stage where it performs the rest of the tasks required in preparation for the actual division, Mitosis.

Mitosis can generally be divided into four phases, Prophase, Metaphase, Anaphase, and Telophase.

During prophase, the chromosomes condense and the nuclear membrane dissappears.

During metaphase, the chromsomes align along a center axis of the cell and the astrocytes align on either side of the equator.

During anaphase, the chromosomes are pulled apart and one set is segregated to each half of the cell.

During telephase, the cell is pinched into two around the equator, the nuclear membrane reforms, the chromosomes disperse, and we are left with two daughter cells.



The majority of cells are 10 to 20 mm, but the range of cell size is from 2 mm to 120 mm. However, a nerve cell may be a meter in length. It is necessary to use a microscope to view the typical cell. Approximately 64,000 cells lined up would occupy one inch.

V. Protein Characteristics

VI. Essential Nutrients

The essental nutrients comprise of the mineral elements, nine of the 20 amino acids, and 2 of the fatty acids.

Essential Nutrients
Mineral ElementsMajor
Mineral ElementsTrace
EssentialAmino Acids
EssentialFatty Acids
VitaminsWater SolubleB - Complex
NiacinPantothenic AcidFolic AcidBiotin
Non - BLipoic acidVitamin C
Fat Soluble
Vitamin AVitamin DVitamin EVitamin K
Other EssentialNutrients

back to Enzyme Description

VII. Adenosine Triphosphate

VIII. Mechanisms for Substance Movement In and Out of Cell

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Copyright © 2000 by M. J. Malachowski, Ph.D.

This page last updated: 2/10/01