which is called endocytosis (a type of active transport). There are two types of
endocytosis. These are phagocytosis (eat) and pinocytosis (drink). In phago-
cytosis,solid particles are engulfed by the cell. An example is when a white
blood cell engulfs and destroys a bacteria cell. In pinocytosis,liquid particles are
brought into the cell. An example is when extracellular fluid containing a sub-
stance is destroyed by the cell.
CYTOPLASM AND NUCLEUS
The cytoplasm of a eukaryotic cell contains cytosol, organelles, and inclusions,
which is similar to the cytoplasm of the prokaryotic cell. Eukaryotic cytoplasm
also contains a cytoskeleton that gives structure and shape to the cell and assists
in transporting substances throughout the cell.
The nucleus of a eukarytoic cell contains DNA (hereditary information) and
is contained within a nuclear envelope. DNA is also found in the mitochondria
and chloroplasts. Depending on the organism, there can be one or more nucleoli
within the nuclear envelope. Anucleolus(little nucleus) is the site of ribosomal
RNA synthesis, which is necessary for ribosomes to function properly.
In the nucleus, the cell’s DNA is combined to form several proteins called
histones. The combination of about 165 pairs of DNA and nine molecular of his-
tones make up the nucleosome. When a eukaryotic cell is not in the reproduc-
tion phase, the DNA and its proteins look like a threaded mass called chromatin.
When the cell goes through nuclear division, the strands of chromatin condense
and coil together, producing rod-shaped bodies called chromosomes.
Aeukaryotic cell uses a method of cell division during reproduction called
mitosis. This is the formation of two daughter cells from a parent cell.
ENDOPLASMIC RETICULUM
The endoplasmic reticulumcontributes to the mechanical support and distribu-
tion of the cytoplasm and is the pathway for transporting lipids and proteins
throughout the cell. The endoplasmic reticulum also provides the surface area
for the chemical reaction that synthesizes lipids, it stores lipids and proteins until
the cell needs them.
The endoplasmic reticulum consists of cisterns, which are a network of flat-
tened membranous sacs. The end of these cisterns can be pinched off to become
membrane-enclosed sacs called secretory vesicles. Vesicles transport synthe-
sized material in the cell.
CHAPTER 4 Prokaryotic and Eukaryotic Cells^81