Download 5 Eukaryote Cells

COMPOSITION OF EUKARYOTE CELLS
Eukaryotic organisms include algae, Protozoa, fungi, plants, and animals.
1. FLAGELLA AND CILIA
These are used for cellular locomotion or for moving substances along the surface of the
cell. Flagella are few and long in relation to the size of the cell. Cilia are more numerous
and shorter. Algae of the genus Euglena use flagella for locomotion, whereas Protozoa
use cilia for locomotion. Both flagella and cilia are anchored to the plasma membrane by
a basal body, which consists of nine pairs of microtubules arranged in a ring, plus another
two microtubules in the center of the ring, an arrangement called a 9 + 2 array.
Microtubules are made up of a protein called tubulin. Prokaryotic flagella rotate, but the
eukaryotic flagellum moves in a wavelike manner. Ciliated cells of the human
respiratory system move mucous and debris along the surface of the cells in the bronchial
tubes and trachea to clear the lungs.
2. CELL WALL and GLYCOCALYX
Most eukaryotic cells have cell walls, although they are much simple or than those of
prokaryotic cells. Algae and fungi have cellulose in their cell walls, as do all plants.
Eukaryotic cells that lack a cell wall and have direct contact with the environment may
have a coating outside the plasma membrane. In animals, the plasma membrane is covered
by the glycocalyx, which is a sticky carbohydrate. The glycolcalyx strengthens the cell
surface, helps attach cells together, and contributes to cell-cell recognition. Eukaryotic
cells do not contain peptidoglycan. This is medically significant because antibiotics such
as penicillins and cephalosporins only act against peptidoglycan and therefore do not affect
human eukaryotic cells.
3. PLASMA MEMBRANE
The plasma membrane of eukaryotic and prokaryotic cells is very similar in function and
structure. There are, however, differences in the types of proteins found in the
membranes. Eukaryotic membranes also contain carbohydrates, which serve in cell to
cell recognition. Bacteria take advantage of these sites and attach there. Eukaryotic
plasma membranes also contain sterols, which are complex lipids not found in
prokayriotic plasma membranes (with the exception of Mycoplasma). Sterols help the
membranes resist lysis from increased osmotic pressure.
Substances can cross the plasma membrane by diffusion or active transport, or a
mechanism called endocytosis. This occurs when a segment of the plasma membrane
surrounds a particle, encloses it, and brings it into the cell. This process is called
phagocytosis when the plasma membrane projects a pseudopod (false foot), engulfs the
particle and brings it the cell. Phagocytosis is used by white blood cells to destroy
bacteria and foreign substances.
4. CYTOPLASM
The cytoplasm of eukaryotic cells is the watery substance inside the plasma membrane
and outside the nucleus. Cytosol is the fluid portion of the cytoplasm. Unlike
prokaryotic cytoplasm, eukaryotic cytoplasm has a complex internal structure, consisting
of very small rods and cylinders called microfiaments and microtubules. Together, they
form the cytoskeleton. This provides support and shape, and also assists in transporting
substances through the cell. They can also move the entire cell, as in phagocytosis. The
movement of cytoplasm from one part of the cell to another to distribute nutrients is
called cytoplasmic streaming.
Another difference between prokaryotic and eukaryotic cytoplasm is that many of the
important enzymes found in the cytoplasmic fluid of prokaryotes is contained within
organelles of eukaryotes. Some organelles are bound by a membrane and other
organelles are non-membrane bound.
5. MEMBRANE-BOUND ORGANELLES
a. NUCLEUS: this is usually the largest structure in the cell, and contains
almost all of the cell’s hereditary information (DNA). Some DNA is also
found in mitochondria and in the chloroplasts of photosynthetic
organisms. The nucleus is surrounded by a double membrane called the
nuclear envelope. Tiny channels in the membrane called the nuclear pores
allow the nucleus to communicate with the cytoplasm. Within the nucleus
are one or more spherical bodies called nucleoli, which are condensed
regions of chromosomes where ribosomal RNA is being synthesized. The
nucleus also contains some proteins called histones, which wrap around
the DNA and organize it.
When the cell is not reproducing, the DNA appears as a thread-like mass
called chromatin. During nuclear division, the chromatin becomes shorter
and thicker, and they are now called chromosomes. Prokaryotic
chromosomes do not undergo this process, do not have histones, and are
not enclosed in a nuclear envelope. Eukaryotic cells divide by mitosis and
meiosis; these processes do not occur in prokaryotic cells.
b. ER: within the cytoplasm of eukaryotic cells is the endoplasmic
reticulum, an extensive network of channels which are continuous with the
nuclear envelope. Most eukaryotic cells contain two distinct forms of ER
that differ and structure and function. The rough ER is studded with
ribosomes, the sites of protein synthesis. Proteins synthesized by
ribosomes that are attached to rough ER enter the channels within the ER
to be processed and sorted. These proteins may be incorporated into the
cell’s organelles or plasma membrane. Thus, rough ER are protein
factories.
Smooth ER extends from the rough ER to form a separate network.
Smooth ER does not have any ribosomes. However smooth ER contains
unique enzymes; it synthesizes phospholipids, fats, and steroids such as
estrogen and testosterone. In liver cells, the enzymes of smooth ER
detoxify drugs.
c. GOLGI COMPLEX: most of the proteins synthesized by ribosomes
from rough ER are transported to other regions of the cell. The first step
in the transport pathway is through an organelle called the Golgi complex.
Proteins synthesized by ribosomes on the rough ER are surrounded by a
portion of the ER membrane, which eventually buds from the membrane
surface to form a transport vesicle. This transport vesicle fuses with the
Golgi complex, releasing the proteins into the channels of the Golgi
complex. Within the Golgi complex, the proteins are modified. For
instance, enzymes in the Golgi complex can modify proteins to form
glycoproteins, glycolipids, and lipoproteins. Some of the processed
proteins leave the Golgi complex in secretory vesicles, which detach from
the Golgi membrane and deliver the proteins to the plasma membrane,
where they are discharged from the cell. Some of the processed proteins
leave the Golgi complex in vesicles that are called storage vesicles. The
major storage vesicle is a lysosome.
d. LYSOSOMES: lysosomes are formed from the Golgi complexes and
look like membrane-enclosed spheres. Unlike mitochondria, lysosomes
have only one membrane and lack internal structure. They contain as
many as 40 different kinds of powerful digestive enzymes capable of
breaking down various molecules. They can also digest bacteria that enter
the cell. Human white blood cells, which use phagocytosis to ingest
bacteria, contain large numbers of lysosomes.
e. VACUOLES: a vacuole is a space or cavity in the cytoplasm that is
enclosed by a membrane. In plant cells, vacuoles may occupy 5 to 90% of
the cell volume. Vacuoles are made by the Golgi complex and have
several functions. Some vacuoles serve as temporary storage organelles
for proteins, sugars, etc. Some plant cells also store wastes and poisons to
prevent toxicity to the cytoplasm. Vacuoles may also take up water,
enabling plant cells to increase in size and also provide rigidity to leaves
and stems.
f. MITOCHONDRIA: these are rod-shaped organelles which appear
throughout the cytoplasm of most eukaryotic cells. There can be as many
as 2000 mitochondria and one cell. Mitochondria have a double
membrane; the outer membrane is smooth but the inner membrane is
arranged in a series of folds called cristae. The center of the
mitochondrion is a semi-fluid substance called the matrix. The
convolutions of the cristae provide an enormous surface area on which
chemical reactions can occur. Some proteins that function in cellular
respiration, including the enzyme that makes ATP, are located on the
cristae, and many of the metabolic steps involved in cellular respiration
occur in the matrix. Mitochondria are called the powerhouses of the cell
because of their central role in ATP production. Mitochondria contain
their own ribosomes and DNA and are able to replicate themselves and
make their own proteins.
It is theorized in the endosymbiotic theory that eukaryotic cells evolved
from bacteria millions of years ago. According to this theory, larger
bacterial cells lost their cell walls and engulfed smaller bacterial cells.
Then the larger bacterial cell evolved into a eukaryotic cell as its plasma
membrane folded over the DNA and created a nucleus. As the nucleus was
formed, it also engulfed aerobic bacteria (require air). The host cell
supplied nutrients to the ingested bacteria, which in turn supplied energy
to the cell. These aerobic bacteria are now known as mitochondria. There
is current research on mitochondrial DNA (mtDNA), which determines
the distant ancestry of each human being. The nuclear DNA is from both
parents, but all the mitochondrial DNA is inherited by the mother.
Unlike nuclear DNA, whose genes are rearranged in the process of
recombination, there is usually no change in mDNA from parent to
offspring. Because of this, mtDNA is a powerful tool for tracking
matrilineage (lineage of the mother), and can track the female ancestry of
many species back hundreds of generations. This process was used to
prove that all domestic dogs are descendant from wolves.
The concept of the Mitochondrial Eve is based on the same type of
analysis, attempting to discover the origin of humanity by tracking the
lineage back in time. Mitochondrial Eve is the name given by researchers
to the woman who is the matrilineal most recent common ancestor
(MRCA) for all living humans. Passed down from mothers to offspring for
over a hundred thousand years, her mitochondrial DNA (mtDNA) is now
found in all living humans: every mtDNA in every living person is derived
from hers. She is believed to have lived about 140,000 years ago in what is
now Ethiopia, Kenya or Tanzania. The time she lived is calculated based
on the molecular clock technique of correlating elapsed time with
observed genetic drift.
g. CHLOROPLASTS: only algae and green plants contain this unique
organelle. It contains the pigment chlorophyll plus enzymes required for
photosynthesis. Like mitochondria, chloroplasts contain ribosomes, DNA,
and enzymes involved in protein synthesis. They are capable of
multiplying on their own within the cell. Again, chloroplasts and
mitochondria are strikingly similar to bacteria in their method of
multiplication.
h. PEROXISOMES: these organelles are similar to lysosomes but they are
smaller. They contain one or more enzymes that can oxidize various
substances. For example, amino acids are oxidized in peroxisomes as part
of normal metabolism. In addition, enzymes in peroxisomes oxidize toxic
substances such as alcohol. The end product of the oxidation reaction is
hydrogen peroxide, which is a very toxic compound. However
peroxisomes also contain the enzyme catalase, which decomposes
hydrogen peroxide, so it is safe within the cell. These peroxisomes can
also be used to digest bacteria that have invaded the cell.
i. CENTROSOME: the centrosome is located near the nucleus. It consists
of proteins fibers and two centrioles, which are small fibers. This area
organizes the spindles that appear during mitosis to help the duplicated
chromosomes move towards opposite ends of the cell. Therefore, the
centrosome plays a critical role in cell division. Each of the two centrioles
in the centrosome is arranged so that the long axis of one centriole is at a
right angle to the long axis of the other.
6. NON-MEMBRANE-BOUND STRUCTURES
a. RIBOSOMES: attached to the outer surface of rough ER are ribosomes;
they are also found floating free in the cytoplasm. They are the sites of
protein synthesis in the cell. They are larger (80S instead of 70S) and denser
than the ribosomes of prokaryotic cells. The free ribosomes synthesize
proteins which are used inside the cell, and they do not have a membrane.
The membrane-bound ribosomes are the ones attached to the rough ER.
These ribosomes synthesize proteins destined for insertion in the plasma
membrane or for export from the cell. Ribosomes within the mitochondria
synthesize special mitochondrial proteins.
PROKARYOTIC
One circular chromosome, not
membrane-bound
No histones
No organelles
Peptidoglycan cell walls
Reproduce by binary fission
No true nucleus; no nuclear membrane
Glycocalyx present as capsule or slime
layer
Plasma membrane has no carbohydrates
and lack sterols
No cytoskeleton
Ribosomes are small (70S)
EUKARYOTIC
Paired chromosomes, membrane-bound
Histones present
Organelles present: Golgi complex, ER,
mitochondria, chloroplasts
Polysaccharide cell walls
Reproduce by mitosis
True nucleus; nuclear membrane; also has
nucleoli
Present in some cells that lack a cell wall
Plasma membrane has carbohydrates and
sterols
Has a cytoskeleton
Ribosomes are large (80S)