membranes, penetrated by numerous protein pores. (In prokaryotes, the DNA is not iso-
lated from the cytoplasm by a membrane, and the structure is called a nuclear region
rather than a nucleus.) Each of the DNA molecules in the nucleus is contained in a
chromosome. A chromosome is a complex of a DNA molecule and associa ted proteins.
Chromosomal DNA forms a template for protein synthesis.
Protein synthesis takes place outside the nucleus in the cytoplasm. The nucleus
synthesizes RNA molecules, each having the code for production of a protein. The
RNA then passes out through the nuclear pores to the cytoplasm. There, the RNA interacts
with particles in the cytoplasm. These particles are the ribosomes, in which amino acids
are linked up to form proteins, using the RNA templates as a guide. Ribosomes are made
of several proteins and RNA molecules in two subparticles called 50S and 30S in prokar-
yotes, and 60S and 40S in eukaryotes. The designation refers to their size in Svedberg
units as measured by the settling velocity in a centrifuge.
In eukaryotes, the ribosomes line a folded membrane structure called the endoplasmic
reticulum (ER), which occupies a large portion of the cytoplasm. ERs serve as channels
to transpor t newly synthesized substances within a cel l. The part studded with ribosomes,
called the rough ER, is responsible for the synthesis of proteins, including enzymes.
Another portion of the ER, the smooth ER, lacks ribosomes. The smooth ER is associated
with synthesis of lipids and the detoxification of lipid-soluble toxins. The liver has abun-
dant smooth ER. It is the location of the enzyme complex cytochrome P450 system, which
is responsible for much of the liver’s detoxification activity as well as other biotransfor-
mation functions (see Section 18.5)
The endoplasmic reticulum pinches off small, self-enclosed sacs of synthesized com-
pounds called vesicles for transpor t elsewhere in the cell. Often, the vesicles go to the
Golgi apparatus, a structure consisting of flattened disks of membrane structures. The
Golgi apparatus may perform finishing touches on the products, then pinch them off in
another set of vesicles that go toward their final destination. For example, some digestive
enzymes are activated in the Golgi apparatus, and migrate in vesicles to the plasma mem-
brane, where their contents are discharged outside the cell.
Other vesicles, including lysosomes, remain in the cytoplasm. Lysosomes contain
enzymes that can digest particles taken in by endocytosis or, interestingly, can be used
by the cell to commit ‘‘suicide,’’ when the cell is damaged or otherwise unneeded.
Other vesicles contain enzymes that brea k down water-soluble toxins in the cytoplasm.
Peroxisomes destroy peroxides in the cell, and peroxisomes in the liver and kidney
perform about half of the work of ethanol detoxification in the body. Microsomes,
vesicles originating in the smooth ER, contain enzymes for detoxification.
The mitochondria are the site where eukaryotes perform respiration, the oxidation of
substances using inorganics such as oxygen as ultimate electron acceptors, for the produc-
tion of energy. Thus, the mitochondria are often described as the cell’s ‘‘powerhouse.’’
They are the place where organics and oxygen are reacted to form carbon dioxide and
water, with the capture of energy in the biologically useful form of ATP molecules.
The structure and function of mitochondria are discussed in detail in Section 5.4.3 (see
also Figure 5.6).
Several organelles are unique to plants and plantlike protists. Chief among these is the
chloroplast. The chloroplast is the site where energy from light is captured in photosynthesis.
Photosynthesis is the production of carbohydrates from CO
2
and H
2
O using light energy;
it is the reverse reaction to respiration and is the basis for the production of almost
all organic matter by the biosphere. The chloroplast and the mitochondrion have many
EUKARYOTIC CELL STRUCTURE AND FUNCTI ON 73