Eukaryotic Cell Structures
Created | Updated Oct 20, 2010
<GUIDE>
<BODY>
<P>Without cells this would be a dead planet as they are the
fundamental building blocks of life. Cells first appeared on earth
over 3.5 billion years ago, less than a billion years after the
planet formed. Although usually thought of as small, there are large
differences in cell size and structure depending on their function. A
nerve cell in a giraffe's neck, for example, can be up to three
metres in length, and even humans can boast of having one metre long
nerve cells. An ostrich egg can also be thought of as a single
cell.</P>
<P>There are two distinct kinds of cell, prokaryotes (pro=before
karyo=nucleus), and eukaryotes (true nucleus) which this page will
mainly focus on.</P>
<P>Prokaryotes were the first type of cell to appear on earth,
probably in a hot mudpool or around a volcanic hyrdrothermal vent
under the ocean. They have a simple cellular structure, consisting of
a membrane enclosing the cytoplasm, a fluid in which all components
of the prokaryotic cell can be found mingling freely. The prokaryotic
DNA is contained in several circular strands. Unlike the eukaryotes,
in general it can be said that prokaryotes have no membrane-bound
internal organelles. The Prokaryotes consist of the bacteria and
archaebacteria and are usually unicellular organisms.</P>
<P>Around two billion years after the appearance of the prokaryotes,
the much more complex eukaryotic cells arose. These cells are on
average around ten times larger than prokaryotic cells and contain
membrane-bound organelles within their cytoplasm which carry out
specific functions. The presence of these membranous organelles has
given rise to the theory that eukaryotes may have arisen from a
symbiotic relationship between prokaryotic cells. If a prokaryotic
cell engulfed another prokaryote which just happened to be more
efficient at photosynthesizing for example, then it would have been
in the interests of both parties to maintain this relationship.</P>
<P>Eukaryotes range from the unicellular yeasts through to organisms,
such as humans, composed of many billions of cells. Eukaryotic cells
may contain the following structures:</P>
<P><B>The Cell or Plasma Membrane<BR/>
</B>Found in all cells, this is a semi-permeable double-membranous
barrier permitting the entrance of nutrients to the cell, and the
exit of waste materials and secretions.</P>
<P><B>Nucleus<BR/>
</B>A double-membraned organelle which contains the cell's DNA and
the nucleolus where the RNA for Ribosome production is formed.
Normally only one nucleus is present in each cell. There is nearly
two metres of DNA within each human cell and it is highly packaged to
fit on the 46 chromosomes present in the nucleus.<BR/>
Human red blood cells lose their nuclei at maturity, although other
animals such as frogs retain nucleated red blood cells.</P>
<P><B><A HREF="http://www.h2g2.com/A461648" TARGET="blank">Mitochondrion</A><BR/>
</B>Mitochondria are rodlike or spherical membranous organelles
responsible for aerobic energy production (respiration) within cells.
The inner membrane of a mitochondrion is highly folded, greatly
increasing its surface area. They are thought to have their origins
as captured prokaryotes, as they contain circular DNA and replicate
themselves by binary fission, like the bacteria. Anaerobic
respiration (fermentation) takes place in the cytoplasm, but is
inefficient and results in a build-up of either ethanol or lactic
acid, the latter responsible for cramp in muscle tissues.</P>
<SCRIPT LANGUAGE="JavaScript">
<!-- Hide
function popup(words) {
win2=window.open("","","height=400,width=200,left=600,top=0");
win2.document.write("<H4>" + words + "</H4>")
win2.document.write("<A HREF='' onMouseOver='self.close()'>Close</A><P>")
win2.focus()
}
// End -->
</script>
<P><B>Chloroplast</B><BR/>
The main <A HREF="" onMouseOver="popup('Plastids: Any of various membrane bound organelles containing photosynthetic pigments, oil, starch, protein, etc. mainly found in plants.')">plastid</A>, found only in plants, membrane-bound chloroplasts
are responsible for the initial steps in converting light energy to
sugars through their chlorophyll. Like the Mitochondria, they have a
double membrane, contain their own DNA and replicate by binary
fission. Other plastids may contain pigments such as carotene, giving
flowers and fruits their characteristic colour, or be used for
storage purposes.</P>
<P><B>Endoplasmic Reticulum<BR/>
</B>Present in all cells, ER consists of a series of flattened
tubules extending throughout the cytoplasm. There are two main types
of ER. Smooth ER is responsible for the production of lipids (fatty
molecules), cholesterol, steroids, and the detoxification of certain
molecules. Rough ER is responsible for the production of proteins and
is so named because of the presence of ribosomes on the outside of
the membrane giving it a rough appearance.</P>
<P><B>Ribosomes<BR/>
</B>Ribosomes are the initial site of protein manufacture (from amino
acids) within the cell. They themselves consist of proteins and
ribosomal RNA manufactured in the nucleolus and can either be found
attached to ER, or free within the cytoplasm. Although prokaryotes
also contain ribosomes, the structure is different from that of
eukaryotes.</P>
<P><B><A HREF="http://www.h2g2.com/A457832" TARGET="blank">The Golgi Apparatus</A><BR/>
</B>This organelle is found in both plant and animal cells and is
closely associated with the endoplasmic reticulum. It consists of a
series of around six fluid-filled sacs and is responsible for
modifying and finalising the structure of molecules transported from
the ER, and despatching them to their destinations within or outside
the cell.</P>
<P><B>Centriole<BR/>
</B>This structure is found in animals near the nucleus in
non-dividing cells, and plays an important role in both types of cell
division, <A HREF="" onMouseOver="popup('Meiosis: A method of cell division where the parent cell divides into four daughter cells each having half the chromosome number of the parent. These haploid cells are necessary for sexual reproduction or each new generation would have double the number of chromosomes of the parent.')"></A><A HREF="" onMouseOver="popup('<P>Meiosis: A method of cell division where the parent cell divides into four daughter cells each having half the chromosome number of the parent. These haploid cells are necessary for sexual reproduction or each new generation would have double the number of chromosomes of the parent.</P>Mitosis: A method of cell division where the parent cell divides into two daughter cells each having the same number of chromosomes as the parent.')">meiosis and mitosis</A>.</P>
<P><B>Lysosome<BR/>
</B>Mainly found in animal cells, these are the waste disposal system
of the cell and contain enzymes for breaking down proteins and other
substances. The interior of the lysosome is more acidic than that of
the cytoplasm, enabling its enzymes to function more efficiently. The
lysis membrane is resistant to the actions of the enzymes.</P>
<P><B>Vacuole<BR/>
</B>This is a fluid-filled single-membraned cavity within plant and
animal cells. It may be used for storage, digestion or secretion.
Plant cells have a large central vacuole, where chemicals which would
otherwise be harmful to the cell are stored.</P>
<P><B>Cell Wall</B><BR/>
A rigid structure found in plant cells, the cell wall supports the
plant structure and gives plant cells their shape. It also prevents
plant cells from bursting due to taking up too much water through
osmosis.</P>
<P><B>Cytoskeleton<BR/>
</B>A framework of fibrous protein filaments and microtubules
responsible for shape (animal cells), anchoring of organelles, and
transport within the cell. Microtubules are also found in cilia,
hairlike structures used in movement. In humans, cilia can be found
in the epithelial cells lining the respiratory tract, they are
responsible for wafting mucus and dirt out of the lungs.</P>
<P>Thus, as a human is composed of many different organs and tissues,
each having its own vital role to play in the life of the
macro-organism, so the eukaryotic cell has its own organelles and
structures which are equally necessary for its existence.</P>
</BODY>
</GUIDE>
<BODY>
<P>Without cells this would be a dead planet as they are the
fundamental building blocks of life. Cells first appeared on earth
over 3.5 billion years ago, less than a billion years after the
planet formed. Although usually thought of as small, there are large
differences in cell size and structure depending on their function. A
nerve cell in a giraffe's neck, for example, can be up to three
metres in length, and even humans can boast of having one metre long
nerve cells. An ostrich egg can also be thought of as a single
cell.</P>
<P>There are two distinct kinds of cell, prokaryotes (pro=before
karyo=nucleus), and eukaryotes (true nucleus) which this page will
mainly focus on.</P>
<P>Prokaryotes were the first type of cell to appear on earth,
probably in a hot mudpool or around a volcanic hyrdrothermal vent
under the ocean. They have a simple cellular structure, consisting of
a membrane enclosing the cytoplasm, a fluid in which all components
of the prokaryotic cell can be found mingling freely. The prokaryotic
DNA is contained in several circular strands. Unlike the eukaryotes,
in general it can be said that prokaryotes have no membrane-bound
internal organelles. The Prokaryotes consist of the bacteria and
archaebacteria and are usually unicellular organisms.</P>
<P>Around two billion years after the appearance of the prokaryotes,
the much more complex eukaryotic cells arose. These cells are on
average around ten times larger than prokaryotic cells and contain
membrane-bound organelles within their cytoplasm which carry out
specific functions. The presence of these membranous organelles has
given rise to the theory that eukaryotes may have arisen from a
symbiotic relationship between prokaryotic cells. If a prokaryotic
cell engulfed another prokaryote which just happened to be more
efficient at photosynthesizing for example, then it would have been
in the interests of both parties to maintain this relationship.</P>
<P>Eukaryotes range from the unicellular yeasts through to organisms,
such as humans, composed of many billions of cells. Eukaryotic cells
may contain the following structures:</P>
<P><B>The Cell or Plasma Membrane<BR/>
</B>Found in all cells, this is a semi-permeable double-membranous
barrier permitting the entrance of nutrients to the cell, and the
exit of waste materials and secretions.</P>
<P><B>Nucleus<BR/>
</B>A double-membraned organelle which contains the cell's DNA and
the nucleolus where the RNA for Ribosome production is formed.
Normally only one nucleus is present in each cell. There is nearly
two metres of DNA within each human cell and it is highly packaged to
fit on the 46 chromosomes present in the nucleus.<BR/>
Human red blood cells lose their nuclei at maturity, although other
animals such as frogs retain nucleated red blood cells.</P>
<P><B><A HREF="http://www.h2g2.com/A461648" TARGET="blank">Mitochondrion</A><BR/>
</B>Mitochondria are rodlike or spherical membranous organelles
responsible for aerobic energy production (respiration) within cells.
The inner membrane of a mitochondrion is highly folded, greatly
increasing its surface area. They are thought to have their origins
as captured prokaryotes, as they contain circular DNA and replicate
themselves by binary fission, like the bacteria. Anaerobic
respiration (fermentation) takes place in the cytoplasm, but is
inefficient and results in a build-up of either ethanol or lactic
acid, the latter responsible for cramp in muscle tissues.</P>
<SCRIPT LANGUAGE="JavaScript">
<!-- Hide
function popup(words) {
win2=window.open("","","height=400,width=200,left=600,top=0");
win2.document.write("<H4>" + words + "</H4>")
win2.document.write("<A HREF='' onMouseOver='self.close()'>Close</A><P>")
win2.focus()
}
// End -->
</script>
<P><B>Chloroplast</B><BR/>
The main <A HREF="" onMouseOver="popup('Plastids: Any of various membrane bound organelles containing photosynthetic pigments, oil, starch, protein, etc. mainly found in plants.')">plastid</A>, found only in plants, membrane-bound chloroplasts
are responsible for the initial steps in converting light energy to
sugars through their chlorophyll. Like the Mitochondria, they have a
double membrane, contain their own DNA and replicate by binary
fission. Other plastids may contain pigments such as carotene, giving
flowers and fruits their characteristic colour, or be used for
storage purposes.</P>
<P><B>Endoplasmic Reticulum<BR/>
</B>Present in all cells, ER consists of a series of flattened
tubules extending throughout the cytoplasm. There are two main types
of ER. Smooth ER is responsible for the production of lipids (fatty
molecules), cholesterol, steroids, and the detoxification of certain
molecules. Rough ER is responsible for the production of proteins and
is so named because of the presence of ribosomes on the outside of
the membrane giving it a rough appearance.</P>
<P><B>Ribosomes<BR/>
</B>Ribosomes are the initial site of protein manufacture (from amino
acids) within the cell. They themselves consist of proteins and
ribosomal RNA manufactured in the nucleolus and can either be found
attached to ER, or free within the cytoplasm. Although prokaryotes
also contain ribosomes, the structure is different from that of
eukaryotes.</P>
<P><B><A HREF="http://www.h2g2.com/A457832" TARGET="blank">The Golgi Apparatus</A><BR/>
</B>This organelle is found in both plant and animal cells and is
closely associated with the endoplasmic reticulum. It consists of a
series of around six fluid-filled sacs and is responsible for
modifying and finalising the structure of molecules transported from
the ER, and despatching them to their destinations within or outside
the cell.</P>
<P><B>Centriole<BR/>
</B>This structure is found in animals near the nucleus in
non-dividing cells, and plays an important role in both types of cell
division, <A HREF="" onMouseOver="popup('Meiosis: A method of cell division where the parent cell divides into four daughter cells each having half the chromosome number of the parent. These haploid cells are necessary for sexual reproduction or each new generation would have double the number of chromosomes of the parent.')"></A><A HREF="" onMouseOver="popup('<P>Meiosis: A method of cell division where the parent cell divides into four daughter cells each having half the chromosome number of the parent. These haploid cells are necessary for sexual reproduction or each new generation would have double the number of chromosomes of the parent.</P>Mitosis: A method of cell division where the parent cell divides into two daughter cells each having the same number of chromosomes as the parent.')">meiosis and mitosis</A>.</P>
<P><B>Lysosome<BR/>
</B>Mainly found in animal cells, these are the waste disposal system
of the cell and contain enzymes for breaking down proteins and other
substances. The interior of the lysosome is more acidic than that of
the cytoplasm, enabling its enzymes to function more efficiently. The
lysis membrane is resistant to the actions of the enzymes.</P>
<P><B>Vacuole<BR/>
</B>This is a fluid-filled single-membraned cavity within plant and
animal cells. It may be used for storage, digestion or secretion.
Plant cells have a large central vacuole, where chemicals which would
otherwise be harmful to the cell are stored.</P>
<P><B>Cell Wall</B><BR/>
A rigid structure found in plant cells, the cell wall supports the
plant structure and gives plant cells their shape. It also prevents
plant cells from bursting due to taking up too much water through
osmosis.</P>
<P><B>Cytoskeleton<BR/>
</B>A framework of fibrous protein filaments and microtubules
responsible for shape (animal cells), anchoring of organelles, and
transport within the cell. Microtubules are also found in cilia,
hairlike structures used in movement. In humans, cilia can be found
in the epithelial cells lining the respiratory tract, they are
responsible for wafting mucus and dirt out of the lungs.</P>
<P>Thus, as a human is composed of many different organs and tissues,
each having its own vital role to play in the life of the
macro-organism, so the eukaryotic cell has its own organelles and
structures which are equally necessary for its existence.</P>
</BODY>
</GUIDE>
