We will now begin our discussion of intracellular organelles. As we have mentioned, only eukaryotic
cells have intracellular sub-divisions, so our discussion will exclude prokaryotic cells. We will also
focus on animal cells, since plant cells have a number of further specialized structures. In this
section we will discuss the importance of the cell nucleus, mitochondria, peroxisomes, endoplasmic
reticulum, golgi apparatus, and lysosome.
The Cell Nucleus
The cell nucleus is one of the largest organelles found in cells and also plays an important biological
role. It composes about 10% of the total volume of the cell and is found near the center of eukaryotic
cells. Its importance lies in its function as a storage site for DNA, our genetic material. The cell
nucleus is composed of two membranes that form a porous nuclear envelope, which allows only
select molecules in and out of the cell.
The DNA that is found in the cell nucleus is packaged into structures called chromosomes.
Chromosomes contain DNA and proteins and carry all the genetic information of an organism. The
nucleus gains support from intermediate filaments that both form the surrounding nuclear lamina and
makes direct contact with the endoplasmic reticulum. The nucleus is also the site of DNA and RNA
Figure %: Location of the cell nucleus, mitochondria, and peroxisomes in a
The mitochondria, with its specialized double-membrane structure, generate adenosine triphosphate
(ATP), a molecule that provides organisms with energy.
Figure %: Mitochondrial structure
The outer and inner membranes of the mitochondria form two sub-
compartments: the internal matrix space and the intermembrane space. Those
few proteins found withn the mitochondria are located within the inner
membrane. Mitochondria synthesize ATP with energy supplied by the electron
transport chain and a process called oxidative phosphorylation.
Peroxisomes are single-membrane structures found in all eukaryotic cells. They are small,
membrane-bound structures that use molecular oxygen to oxidize organic molecules. The structure
is one of the major oxygen utilizing organelles, the other being the mitochondria. Peroxisomes
contain oxidative enzymes and other enzymes that help produce and degrade hydrogen peroxide.
Because of their varying enzymatic compositions, peroxisomes are diverse structures. Their main
function is to help breakdown fatty acids. They perform specific functions in plant cells, which we will
The Endoplasmic Reticulum
The endoplasmic reticulum, or ER, is a very important cellular structure because of its function
in protein synthesis and lipid synthesis. For example, the ER is the site of production of all
transmembrane proteins. Since nearly all proteins that are secreted from a cell pass through it, the
ER is also important in cellular trafficking. In addition to these major roles, the ER plays a role in a number of other biological processes. There are two different types of ER: smooth ER and rough ER
The rough ER has its na