sis. There are two kinds of endocytosis. These are phagocytosis and pinocytosis.
Phagocytosis engulfs solid substances (large molecules) while pinocytosis
engulfs liquid substances (small molecules). Exocytosisis the process that cells
use to remove large substances, which is the way waste products and useful mate-
rial as hormones and neurotransmitters are expelled from a cell through vesicles.
Cytosol and Cytoplasm
The cytosolis the intracellular fluid of a prokaryotic cell that contains proteins,
lipids, enzymes, ions, waste, and small molecules dissolved in water, com-
monly referred to as semifluid. Substances dissolved in cytosol are involved in
cell metabolism.
The cytosol also contains a region called the nucleoid, which is where the
DNA of the cell is located. Unlike human cells, a prokaryotic microorganism has a
single chromosome that isn’t contained within a nuclear membrane or envelope.
Cytosol is located in the cytoplasm of the cell. Cytoplasmalso contains the
cytoskeleton, ribosomes, and inclusions.
Ribosomes
Aribosomeis an organelle within the cell that synthesizes polypeptide. There
are thousands of ribosomes in the cell. You’ll notice them as the grainy appear-
ance of the cell when viewing the cell with an electron microscope.
Aribosome is comprised of subunits consisting of protein and ribosomal
RNA, which is referred to as rRNA. Ribosomes and their subunits are identified
by their sedimentation rate. Sedimentation rateis the rate at which ribosomes
are drawn to the bottom of a test tube when spun in a centrifuge. Sedimentation
rate is expressed in Svedberg (S) units. A sedimentation rate reflects the mass,
size, and shape of a ribosome and its subunits. It is for this reason why the sed-
imentation rates of subunits of a ribosome do not add up to the ribosome’s
sedimentation rate.
Ribosomes in prokaryotic cells are uniquely identified by the number of pro-
teins and rRNA molecules contained in the ribosome and by sedimentation rate.
Prokaryotic ribosomes are relatively small and less dense than ribosomes of
other microorganisms. For example, bacterial ribosomes have a sedimentation
rate of 70S compared to the 80S sedimentation rate of a eukaryotic ribosome,
which you’ll learn about later in this chapter.
Ribosomes and their subunits are targets for antibiotics that kill a bacterium
by inhibiting the bacterium’s protein synthesis. These antibiotics only kill cells
that have a specific ribosome sedimentation rate. Cells with a different ribosome
CHAPTER 4 Prokaryotic and Eukaryotic Cells^77