Structure and Function of Organelles
Organelles are small structures within cells that perform dedicated functions. As the name implies, you can think of organelles as small organs. There are a dozen different types of organelles commonly found in
eukaryotic cells. In this course we will focus our attentions on only
a handful of organelles. We will examine these organelles with an eye
to their role at a molecular level in the cell, and leave descriptive
cell biology for another course. For example, our interest in the mitochondrion lies in its ability to generate energy in a form that is
useful to the cell, and to that end we are interested in how the
membrane structure allows the energy generation to proceed, but we
will not spend much time naming mitochondrial structures or discussing
their distribution in different cell types.
It is important to know the basic facts about the following
organelles. There are very useful drawings and photographs of these
organelles in Chapter 4 of Purves, and we recommend examining the
images to help comprehend the organelles.
Follow this link to view an animal cell while
reading about the organelles.
Follow this link to view a plant cell while
reading about these 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
- 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.
- Golgi apparatus
- 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 .
There is a very nice WWW site where you can see additional pictures and
descriptions of organelles, entitled The WWW Cell Biology Course .
There are three major structures that somewhat get lost in the shuffle when
we talk about organelles. They are not organelles, but are still
extremely important components of the cellular system. They are the cell
membrane, the cytoplasm, and the cytoskeleton. The cell membrane is so
important that we devote the entire next module to it. The cytoplasm is the
site of many metabolic cycles and synthetic pathways, as well as the location
of protein synthesis. All of these are absolutely necessary to the survival
of the cell, and are spread randomly throughout the cytoplasm. We
will discuss the cytoskeleton now.