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Organelles
Each cell process is carried out in a specific location in the cell, with
most processes occurring in or around an organelle. Think of an
organelle as a level of organization between macromolecules and the
cell. Organelles carry out specialized tasks within the cell, localizing
functions such as replication, energy production, protein synthesis, and
processing of food and waste. The hundreds of cell types in the body
differ in the arrangement and number of organelles, along with differing
in their physical structure as seen on the previous page.
The focus of this section is to explain where organelles are commonly
found within a cell, how they interact with each other, and how they
function during transport, growth and division in the cell. You will learn
about the controlled chemical environment a cell maintains and what
restrictions this places on the types of chemical reactions it can perform.
This background is vital to understanding key processes such as how a
cell releases energy from glucose, makes and folds proteins, and goes
through growth and cell division.
Think of a city and the various jobs within a city. A cell is similar with
each organelle serving a specific purpose. There are organelles whose
job is to provide shape and structure to the cell, much like the city
streets
and
bridges.
These
protein
rich
organelles
include intermediate
filaments, microtubules,
andmicrofilaments. Some of these actually move other organelles
around the cell or change the shape of the cell. When a muscle cell
contracts or shortens it does so by using the microfilaments made up of
the proteins actin and myosin. One special organelle composed of
microtubules is located in an area near the nucleus, thecentrosome.
The centrosome contains a pair of microtubule bundles known as
the centrioles. Centrioles are important because they move
chromosomes to opposite ends of the cell during cell replication
(mitosis). Neurons do not have centrioles and therefore cannot
replicate.
Other organelles help synthesize the proteins needed by the cell. These
protein factories are calledribosomes. They can be scattered within the
cell or attached to a membrane channel system called theendoplasmic
reticulum or ER. When the ER has ribosomes attached to it, it is
termed the rough ER (the ribosomes give it a rough or grainy
appearance). When the ER lacks ribosomes it is termed the smooth ER
and functions for lipid synthesis and breakdown of toxins. When a
protein is manufactured it often needs be folded into a specific shape to
work. Additional side chains of carbohydrates or lipids may also be
added to these proteins, and then some proteins need to be delivered to
specific locations, such as the cell membrane or another organelle. This
“processing” is done through a system that starts in the ER where the
protein is formed, and then continues as the protein is carried through
the golgi apparatus which is the distributing plant for the cell. The
golgi apparatus completes any protein processing and then packages it
into a vesiclefor transport to its destination. The golgi apparatus also
makes a special type of vesicle termed a lysosome. The lysosome is the
garbage man of the cell. It takes in cell debris and waste and destroys it.
The lysosome contains very powerful hydrolytic enzymes to accomplish
this. It is very important that the enzymes remain in the lysosome or
they would destroy the cell.
The power plant of the cell is the mitochondria. This organelle
generates the ATP or energy for the cell. Mitochondria even have their
own DNA termed mitochondrial DNA (mDNA) and can replicate.
Finally there is the controller of the cell, the nucleus. Not all cells have
a nucleus and those that don't are termed anucleate. If you look at the
image of the red blood cells you will see that they do not have a
nucleaus. The nucleus is ejected when they mature. Some cells have
more than one nucleus and are termedmultinucleate. Skeletal muscle
cells are very large cells and are multinucleate. The nucleus contains the
DNA of the cell as well as the nucleolus. The nucleolus is an organelle
that makes ribosomes while the DNA is your genetic code. The DNA in
the nucleus of your cells contain the genes that provide the instructions
for making every protein in your body. The nucleus is surrounded by it’s
own membrane with tiny holes termednuclear pores. The membrane
is called the nuclear membrane or nuclear envelope.
The interactive diagram below shows a drawing of a eukaryotic cell. The
cell components in the list link to images that highlight these same
structures in a living cell.