High School Biology: Cell Organelles
This is where the DNA is kept and RNA is transcribed. RNA is
transported out of the nucleus through the nuclear pores. Proteins
needed inside the nucleus are transported in through the nuclear
pores. The nucleolus is usually visible as a dark spot in the nucleus
(note the dark nucleolus in this electron microscope photo of a nucleus),
and is the site of ribosome formation.
Ribosomes are the sites of protein synthesis, where RNA is
translated into protein. Protein synthesis is extremely important to
cells, and so large numbers of ribosomes are found throughout cells
(often numbering in the hundreds or thousands). Ribosomes exist
floating freely in the cytoplasm, and also bound to the endoplasmic
reticulum (ER). ER bound to ribosomes is called rough ER because the
ribosomes appear as black dots on the ER in electron microscope photos,
giving the ER a rough texture. These organelles are quite small,
made up of 50 proteins and several long RNAs intricately bound together.
Ribosomes have no membrane. Ribosomes disassemble into two subunits
when not actively synthesizing protein..
Mitochondria (singular: mitochondrion) are the sites of aerobic
respiration, and generally are the major energy production center
in eukaryotes. Mitochondria have two membranes, an inner and an outer,
clearly visible in this electron microscope photo of a mitochondrion.
Note the reticulations, or many infoldings, of the inner membrane,
This serves to increase the surface area of membrane on which membrane-
bound reactions can take place. The existence of this double membrane
has led many biologists to theorize that mitochondria are the descendants
of some bacteria that was endocytosed by a larger cell billions of years
ago, but not digested. This fascinating theory of symbiosis, which might
lend an explanation to the development of eukaryotic cells, has
additional supporting evidence. Mitochondria have their own DNA and
their own ribosomes; and those ribosomes are more similar to bacterial
ribosomes than to eukaryotic ribosomes..
These organelles are the site of photosynthesis in plants and other
photosynthesizing organisms. They also have a double membrane. There
is a more complete description of the chloroplast here, in the
chapter on photosynthesis..
Endoplasmic Reticulum (ER)
The ER is the transport network for molecules targeted for certain
modifications and specific final destinations, as opposed to molecules
that are destined to float freely in the cytoplasm. There are two
types of ER, rough and smooth. Rough ER has ribosomes attached to it,
and smooth ER does not.
This organelle modifies molecules and packages them into small membrane
bound sacs called vesicles. These sacs can be targetted at various
locations in the cell and even to its exterior.
This organelle digests waste materials and food within the cell,
breaking down molecules into their base components with strong digestive
enzymes. Here we can see an advantage of the compartmentalization of
the eukaryotic cell: the cell could not support such destructive enzymes
if they were not contained in a membrane-bound lysosome.