Download Cell Organelles

Cell Organelles
Organelles are the components of the cell that synthesize new materials, recycle old materials, transport
molecules, and anything else that is essential to ensure the proper survival of the cell and its propogation.
Organelles incorporate all broad ranges of organic molecules including nucleic acids, amino acids,
carbohydrates, and lipids to produce a viable cell.
Endoplasmic Reticulum is an extensive membranous network in the eukaryotic cells that is
continuous with the outer nuclear membrnce and composed of ribosome-studded (rough) and ribosomefree (smooth regions.
The ER is responsible for the manufactures of the membranes and performs many other biosynethic
functions.
The endoplasmic reticulum (also known as the ER) is made up of a wide system of membranes that make
up over fifty percent of the total membrane in numerous eukaryotic cells, and consists of two sections that
have different functions: the smooth endoplasmic reticulum and the rough endoplasmic reticulum. The
endoplasmic reticulum consist of a system of membranous tubules and pouches called cisternae, which
is held together by the cytoskeleton. The membrane segregates the inside section of the endoplasmic
reticulum from the outside section (cytosol) . The internal section is called the lumen cavity, but is also
known as the cisternal space. The name “endoplasmic” is defined as “within the cytoplasm,” and the Latin
definition for the word “reticulum” is “little net.” The membrane of the endoplasmic reticulum is continuous
throughout the nuclear envelope. Because of this, the area between the two membranes of the envelope
is also continuous to the cisternal space.
The Endoplasmic Reticulum contains a quality control mechanism called chaperones. These special
proteins attach themselves to newly synthesized proteins and assist them in folding into their native
conformations. Charperones also keep mis-folded proteins in the ER to be broken down. These
incorrectly folded proteins are degraded through the process of ER Associated Degredation (ERAD).
These Chaperones can be affected by genetic diseases such as Cystic fibrosis. Cystic fibrosis is a
genetic disease that causes the buildup of chaperones and their incorrectly folded proteins in the ER.
This disease leads to mucus clogging the lungs and digestive track, which can lead to early death.
Rough Endoplasmic Reticulum (RER)
The description of endoplasmic reticulum (ER) being "rough" comes from the organelle being studded
with ribosomes. These ribosomes on the rough ER, are used to synthesize proteins which are destined
to either be inserted into the membrane of the cell or transported out of the cell. Differences between
ribosomes lying on the ER and those lying in the cytosol stem mainly from the destination the protein that
is synthesized by the ribosome. With protein synthesis via ER ribosomes, proteins are synthesized as
they normally would through translation of mRNA. After the protein is synthesized, called
"posttranslation", it then enters the lumen of the endoplasmic reticulum, the inner compartment of the
organelle. It is inside the lumen of the rough endoplasmic reticulum that N- and O- linked glycosidic
bonds are formed between amino acids and sugars, thus this process is called "glycosylation" and the
newly formed protein is called "glycoprotein".
Smooth Endoplasmic Reticulum (SER)
The smooth endoplasmic reticulum, unlike its "rough" counterpart, does not package protein. Instead it
carries out various metabolic reactions within the confines of its membrane. Functions range from
carbohydrate metabolism, lipid synthesis, and even drug detoxification. Drug detoxification is usually
accomplished by adding hydroxyl groups to drug molecules. This makes them more soluble in water and
therefore easier to be flushed from the body. Another important function of the smooth endoplasmic
reticulum is the storage of calcium ions. This is imperative for muscle contraction. When a muscle cell is
stimulated by a nerve impulse, the calcium ions rush into the cytosol and trigger muscle contraction.
Structurally the smooth ER is continuous with the cell's nuclear envelope.
Secretory pathway diagram, including nucleus, endoplasmic reticulum and Golgi apparatus.
1. Nuclear membrane
2. Nuclear pore
3. Rough endoplasmic reticulum (REM)
4. Smooth endoplasmic reticulum
5. Ribosome attached to REM
6. Macromolecules
7. Transport vesicles
8. Golgi apparatus
9. Cis face of Golgi apparatus
10. Trans face of Golgi apparatus
11. Cisternae of Golgi apparatus
12. Secretory vesicle
13. Cell membrane
14. Fused secretory vesicle releasing contents
15. Cell cytoplasm
16. Extracellular environment
Mitochondria
Function
Mitochondria seen through an electron microscope
The purpose of the mitochondria in the eukaryote is to provide cellular respiration to the cell. The
endosymbiotic theory asserts that the mitochondria came to be part of the eukaryote over time through a
symbiotic relationship. The mitochondria consists of two membranes, the inner membrane and the outer
membrane. It is speculated that the outer membrane came about when its ancestor was engulfed by the
host celled via endocytosis, giving it a membrane in addition to the one the mitochondria ancestor already
had. This theory would also explain why the mitochondria had its own DNA and why this DNA is circular.
For some amino acids the genetic code of the mitochondria differ slightly from that of the nucleus (and the
rest of the cell).
Inner Mitochondria & Matrix
H+ ions drive the ATP Synthase to convert ADP + Pi to ATP
Inside the deepest compartments of the mitochondria is the mitochondrian matrix. It is in the matrix that
cellular respiration occurs, where pyruvate (a product of glycolysis in the cytosol) is converted to Carbon
Dioxide and water. The matrix is the site of the the citric acid cycle, whereby the electron transport chain
is used to setup a proton gradient between the inner and outer membrane of the mitochondria, known as
the inter membrane space. The protons in the inter membrane space accumulate to a point that the
concentration gradient causes the protons to flow back into the matrix.
It is the inner membrane that is studded with the proteins necessary for the electron transport chain, such
+
as the cytochrome electron shuttles. Upon reentering the matrix, the H go through ATP synthase, which
in turns powers the synthase to phosphorylate adenosine diphosphate (ADP) to adenosine triphosphate
(ATP). The ATP can be used later on to be coupled with thermodynamically unfavorable reactions to
allow those chemical reactions to proceed. The inner membrane is folded and convoluted which allows
for a greater surface area to utulize for the electron transport chain. These convolutions are what make
up the cristae.
Interestingly enough the matrix of the mitochondria are one of the few locations outside of the nucleus
where genetic information can be found in the cell. Mitochondrial DNA is similar in appearance to that of
bacterial DNA due to its circular shape. The matrix is also known to house tRNA and ribosomes, which
further solidifies the theory that the mitochondria entered the ancestral eukaryotic cell as single celled
organism.
Outer Membrane
The outer mitochondria membrane consist of a phospholipid bilayer, laced throughout with integral
proteins. The lipid bilayer contains porins which allow the passage of molecules which are 10,000 Daltons
or less. This permeability in the outer membrane allows for water, ions, and some proteins to flow freely
into the inter membrane space.
Golgi Apparatus
he Golgi apparatus, also called the Golgi complex, is commonly found in eukaryotic cells. The Golgi
complex can be identified by its unique structure which some say looks like a maze, but in fact the
structure is made of stacks of flattened membranous sacs, or cisternae. Unlike the cisternae of the
endoplasmic reticulum orER, these membranes are not connected. The Golgi apparatus is responsible
for the processing and packaging of protein and lipids, as well as processing proteins for secretion. The
golgi apparatus is often thought of as a post office; through the process of protein glycosylation, sugars
are added to the protein which dictate where the protein should travel to. N-linked glycosylation is begun
in the endoplasmic reticulum but is continued in the golgi complex. O-linked glycosylation is solely done in
the golgi complex. After proteins have been modified in the golgi apparatus, they are usually sent to either
the lysosomes, secretory granules, or plasma membrane depending on the signals encoded within the
protein sequence and structure. For this reason, Golgo complex is recognized as "the major sorting
center" of the cell.
Golgi Apparatus as an Independent Organelle
Researchers in Yale show that the Golgi Apparatus is not just an outgrowth of another organelle, the
endoplasmic reticulum, but an independent organelle on its own. The Golgi Apparatus is an organelle
within a cell that has its own autonomy and so must grow and divide to keep pace with the growth and
division that the cell inhibits. It has been found that even without the stacks of membrane compartments
where secretory proteins pass, the Golgi Apparatus still functions. Proteins referred to as matrix proteins
that form a scaffold have been found to be responsible for its growth, division and partitioning.
The Golgi Apparatus has been observed to grow, dived and inherited like other organelles as has been
observed in the regrowth of the motger Golgi Apparatus in daughter cells with the use of matrix proteins.
Structure
The Golgi complex is made several flattened membranes sacs, but can be ultimately divided into two
sections: the Cis golgi and the Trans Golgi Network (TGN). The Cis golgi functions as the receiving end
for newly synthesized proteins from the lumen of the Endoplasmic Reticulum (ER). Vesicles containing
proteins from the ER merge with the Cis golgi allowing the proteins to enter the Golgi complex. As the Cis
golgi received proteins from the ER, the proteins then begin their modification moving along membrane to
membrane towards the TGN. At the other end of the golgi complex, the newly modified protein arrives at
the TGB where it is then send off to different parts of the cell via a transport vesicle.
Vesicle Transport
As soon as modified proteins reach the Trans Golgi Network (TGN), the proteins are separated into
several different transport vesicles for different areas of the cell.
Vesicle Type Description
Exocytotic
vesicles
These vesicles contain protein that are to be sent outside the cell membrane. These vesicles fuse
with the plasma membrane, releasing their contents outside the cell. This process is called
constitutive secretion.
Secretory
These vesicles are also to be sent outside the cell membrane. However what differs these from
exocytotic vesicles is that secretory vesicles need a signal before they are released. When the signal
vesicles
Lysosomal
vesicles
is given, they will fuse with the plasma membrane to release the contents. This process is called
regulated secretion.
These vesicles contain protein that are sent to the lysosome to be digested.
Transport Mechanism
The transport mechanism that proteins use to progress through the Golgi apparatus is still not
clear yet, but there are a number of hypotheses that currently exist. Up until recently, the
vesicular transport mechanism had been the favored mechanism for transport but there is now
more evidence that supports the cisternal maturation. The two models may work in conjunction
with one another rather than being mutually exclusive. This is sometimes referred to as the
combined model. The two theories of how Golgi forms are listed below and described:
Cisternae Maturation Model
In this model, the cisternae of the Golgi apparatus move by being built at the cis face and
destroyed at the trans face. The vesicles from the endoplasmic reticulum fuse together to form a
cisterna at the cis face and this cisternae would appear to move through the Golgi stack when a
new cisterna is formed at the cis face. This model is supported by the fact that structures larger
than the transport vesicles were observed microscopically to progress through the Golgi
apparatus. In summary, packages of processing enzymes and new proteins originating in the
ER fuse together to form the Golgi and as the proteins are processed and mature, the next
Golgi compartment is created.
Vesicular Transport Model
This model views the Golgi apparatus as a very stable organelle that is divided into
compartments in the cis to trans direction. Membrane bound carriers transport material between
the endoplasmic reticulum and the different compartments of the Golgi. Experimental evidence
shows the abundance of small vesicles in close proximity to the Golgi apparatus. Actin filaments
direct the vesicles by connecting packaging proteins to the membrane to ensure that they fuse
with the correct compartment. To summarize, newly made proteins are packaged in the rough
ER and are sent for processing to a pre-existing structure known as the Golgi which is made up
of different compartments.
Centrioles
Centrioles are cylindrical structures that composed of groupings of microtubules arranged in 9+3 pattern.
They are usually exist in pairs that form centrosomes. Centrioles are found in animal cells and play a big
role in cell division. While cell division, the centrosome divides and the centrioles replicate (make new
copies). The result is two centrosomes, each with its own pair of centrioles. The two centrosomes move to
opposite ends of the nucleus, and from each centrosome, microtubules grow into a "spindle" which is
responsible for separating replicated chromosomes into the two daughter cells. Centrioles contain deltatubulin, a protein which is part of the structure of tubulin. Centrioles are barrel-shaped organelles that are
found in most eukaryotic cells, with the exception of vascular plants and fungi. Centrioles consist of nine
triplets of microtubules and are involved in the mitotic spindling, as well as when the cytoplasm of a
eukaryotic cell split to form two daughter cells.
Lysosome
The lysosome is a membranous sac of enzymes that an organism utilizes in the digestion
of macromolecules. The enzymes found within lysosomes are hydrolytic, meaning that during reaction a
single water molecules is slit up into the hydrogen and hydroxide ions. This type of reaction is typically
suited for the break down of polymers with an acid as a catalyst. It is from this reaction that lysosomes
achieve their name because to "lyse" means to break a molecule into two compounds. Lysosomes work
optimally best under acidic conditions and lose their potency if its contents leak into the cytosol, as the pH
of the cytosol is rather neutral. Furthermore, excess leakage of lysosomic contents may cause
autodigestion of the cell.
The most important types of lysosomal enzymes are as listed:

Lipase: water-soluble enzymes involved in the catalysis of the hydrolysis reaction (by which a water
molecule is broken) that break the ester bonds which hold lipids together. This enzyme is present in
almost all living organisms.

Carbohydrase: the enzymes which hydrolyze polysaccharides into disaccharides. Some common
carbohydrases are sucrase (which hydrolyzes sugars), lactase (which hydrolyzes lactose), and
amylase (which hydrolyzes starches).

Nuclease: the enzymes that break the phosphodiester bonds that hold nucleotides together in nucleic
acids. This type of enzyme is vital in the study of DNA.

Protease: the enzymes that hydrolyze the peptide bonds in proteins. This enzyme is present in all
living organisms.
Through use of the aforementioned types of enzymes, lysosomes are responsible for intracellular
digestion. In the process of phagocytosis, digestion products, such as simple sugars and amino acids, are
passed into the cell's cytosol and serve as the nutrients for further reactions performed by other cell
compartments. Lysosomes may also use this process is the recycling of the cell's own organic materials
during autophagy. During this process, a small amount of cytosol or damaged organelle is surrounded by
a membrane that the lysosome fuses to. This becomes broken down by lysosomal enzymes.
In lysosomal diseases, the lysosome lacks a functioning enzyme, causing imbalance in the cell. In TaySachs Disease, for example, a lipase is in malfunction or missing, and the brain malfunctions due to overaccumulation of lipids in the cells.
Peroxisome
Peroxisomes are the centers by which the cell may be rid of potential toxins. The peroxisome is in itself a
receptacle of oxidative enzymes that remove hydrogen atoms from certain organic substances to produce
peroxides, which in itself is harmful. This peroxide serves to oxidate the potentially harmful substances,
such as alcohol. In reaction, a hydrogen leaves the peroxide compound, and the result is the R group of
the reactant compound in question and molecules of water.