Biomedical Engineering Reference
In-Depth Information
20 µm
10 µm
Cell wall
Starch granules
Mitochondridria
Plasma
membrane
Chloroplast
Nucleus
Endoplasmic
Reticulum
Golgi complex
Tonoplast
Central vacuole
Lysosome
Rough
Endoplasmic
Reticulum
Animal
Plant
FIGURE 2.6 Sketches of the two primary types of higher eukaryotic cells. Neither sketch is complete, but each
summarizes the principal differences and similarities of such cells.
nucleus and a number of cellular organelles inside the cytoplasm. Figure 2.6 is a schematic of
two typical eukaryotic cells.
In cell wall and cell membrane structure, eukaryotes are similar to prokaryotes. The
plasma membrane is made of proteins and phospholipids that form a bilayer structure. Major
proteins of the membrane are hydrophobic and are embedded in the phospholipid matrix.
One major difference is the presence of sterols in the cytoplasmic membrane of eukaryotes.
Sterols strengthen the structure and make the membrane less flexible. The cell wall of eukary-
otic cells shows considerable variations. Some eukaryotes have a peptidoglycan layer in their
cell wall; some have polysaccharides and cellulose (e.g. algae). The plant cell wall is
composed of cellulose fibers embedded in pectin aggregates, which impart strength to the
cell wall. Animal cells do not have a cell wall but only a cytoplasmic membrane. For this
reason, animal cells are very shear-sensitive and fragile. This factor significantly complicates
the design of large-scale bioreactors for animal cells.
The nucleus of eukaryotic cells contains chromosomes as nuclear material (DNAmolecules
with some closely associated small proteins), surrounded by a membrane. The nuclear
membrane consists of a pair of concentric and porous membranes. The nucleolus is an
area in the nucleus that stains differently and is the site of ribosome synthesis. Many chro-
mosomes contain small amounts of RNA and basic proteins called histones attached to the
DNA. Each chromosome contains a single linear DNA molecule on which the histones are
attached.
Cell division (asexual) in eukaryotes involves several major steps: DNA synthesis, nuclear
division, cell division, and cell separation. Sexual reproduction in eukaryotic cells involves
the conjugation of two cells called gametes (egg and sperm cells). The single cell formed
from the conjugation of gametes is called a zygote. The zygote has twice as many chromo-
somes as the gamete. Gametes are haploid cells, while zygotes are diploid. For humans,
a haploid cell contains 23 chromosomes, and diploid cells have 46. The cell-division cycle
(asexual reproduction) in a eukaryotic cell is depicted in Fig. 2.7 .
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