Nucleus- “the brain” or control center of the cell.
The Nucleus, a
membrane-bound structure of a cell, plays two crucial roles in controlling the
cell. The nucleus carries the cell\'s genetic information that determines if the
organism will develop, for instance, into a tree or a human; and it directs most
cell activities including growth, metabolism, and reproduction by controlling
protein synthesis. The presence of a nucleus distinguishes the more complex
eukaryotic cells of plants and animals from the simpler prokaryotic cells of
bacteria and cyanobacteria that lack a nucleus. The nucleus is the most
predominate structure in the cell. It is typically round and occupies 10% of the
cells total volume. The nucleus is wrapped in a double-layered membrane called
the nuclear envelope. The space between the nuclear envelope layers is called
perinuclear space. The nuclear envelope is attached to a network of
membrane-enclosed tubules that extends throughout the cell called the
endoplasmic reticulum. The nuclear envelope is perforated by many holes, called
nuclear pores, that permit the movement of selected molecules between the
nucleus and the rest of the cell, while blocking the passage of other molecules.
The nucleus contains the nucleolus, which manufactures the organelle known as
the ribosome, or the protein producing organism. Genetic information in the form
of deoxyribonucleic acid(DNA) is stored in threadlike, tangled structures called
chromatin within the nucleus. During the process of cell division known as
mitosis, in which the nucleus divides, the chromatin condense into several
distinct structures called chromosomes. Each time the cell divides, the heredity
information carried in the chromosomes is passed to the two newly formed cells.
The DNA in the nucleus also contains the instructions for regulating the amount
and types of proteins made by the cell. These instructions are copied, or
transcribed, into a type of ribonucleic acid(RNA) called messenger RNA (mRNA).
The mRNA is transported from the nucleus to ribosomes, where proteins are
assembled.
Nuclear Envelope- The nucleus is wrapped in a
double-layered membrane called the nuclear envelope. The space between the
nuclear envelope layers is called perinuclear space. The nuclear envelope is
attached to a network of membrane-enclosed tubules called the endoplasmic
reticulum. The nuclear envelope is perforated by many holes, called nuclear
pores, whose job is to permit the movement of selected molecules between the
nucleus and the rest of the cell, while blocking the passage of other molecules.
Chromatin- a collection of separate structures called Chromosomes.
Within the nucleus the DNA is organized along with proteins into Chromatin.
During Mitosis, the chromosomes condense into what is known as Chromosomes,
which allows the genetic information of the previous cell to be passed on.
Chromosome- Chromosomes are the microscopic structure within cells that
carries the molecule deoxyribonucleic acid. DNA is the hereditary material that
influences the development and characteristics of each organism. In bacteria and
bacteria-like organisms called archaebacteria, chromosomes are simple circles of
DNA that float around in the cell. In more complex cells, or Eukaryotes,
chromosomes are stored within a well developed and defined nucleus. In
eukaryotic cells, chromosomes are highly complex structures in which the shape
of the DNA molecules is linear, rather than circular. Chromosomes consist
chiefly of proteins and DNA. Tiny chemical subunits called nucleotide bases form
the structure of DNA. A sequence of these bases that are along a DNA strand will
create a code for the production of a special protein also known as a gene.
Genes occupy precise locations on the chromosome. Each cell contains enough DNA
to form a thread extending about 2 m (about 7 ft). Proteins called histones play
a key role in packaging DNA within chromosomes. Sections of the DNA molecule
wind around clusters of histones to form units called nucleosomes, which
resemble spools encircled with thread. Another type of protein, called
nonhistone chromosomal protein, will continue to condense nucleosomes into
compact structures. Chromosomes become most condensed when a cell is preparing
to divide. The chromosome structure ensures that even when the DNA is highly
confined, it is still able to carry out cell transcription, or the production of
messenger ribonucleic acid(mRNA). Messenger ribonucleic acid is the molecule
that carries the DNA instructions to the sites where proteins are produced,
indicating which protein is necessary to be made at that specific time. In
addition, chromosomes permit DNA to replicate, or reproduce itself, but still
keep the genetic information that is crucial and unique to that cell and that
entire organism. The chromosomes of nearly all eukaryotic life forms contain two
important structures: centromeres and telomeres. During cell division, the
centromere—visible through a microscope as a knotlike structure—connects to an
apparatus called the spindle. The spindle contains fibers that move the
centromeres around, causing the rest of each chromosome to follow. This is
necessary to make sure that chromosomes will be in the correct spot during
mitosis, when a cell divides to give rise to two cells, or during meiosis, the
process of cell division that creates eggs or sperm. Telomeres are specialized
sequences of DNA that are found at the tips of chromosomes. Telomeres serve as a
kind of protective cover that prevents the ends of chromosomes from attaching to
the ends of other chromosomes. Scientists suspect that telomeres may influence
the activity of nearby genes and may play a role in determining the life span of
a cell.
Nucleolus- This is the most visible structure in the nucleus. It
synthesizes molecular ingredients of ribosomes which will later become proteins.
Ribosomes- There are two different forms of Ribosomes, Free ribosomes,
and bound ribosomes. Free Ribosomes are suspended in the cytosplasm, and their
job is to carry out protein synthesis. Bound Ribosomes are attached to the
outside of the endoplasmic reticulum. Bound Ribosomes usually synthesize
proteins that are going to be shipped out of the cell. Even though their jobs
are different, Bound and free ribosomes are structurally the same. The faster
the organism can synthesize proteins, the more ribosomes they have in their
cells.
Vesicles- membrane enclosed sacs.
Endomembrane system- Different membranes of the Eukaryotic cells,
together are formed into one group called the Endomembrane system. These
membranes are related either because their composition is the same or if they
transfer segments of the membrane through the membrane of small vesicles.
Despite the common system that they share, it does not mean that they are
similar in structure or function, and only really need to be membranes to be
considered in the Endomembrane system. Included in the endomembrane system is
the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, the
lysosomes, some kinds of vacuoles, and the plasma membrane. Even though the
plasma membrane is not an ENDOmembrane, it is still in the group because it
shares functions similar to the likes of edoplasmic reticulum and other internal
membranous structures.
Endoplasmic Reticulum- There are two forms of
Endoplasmic reticulum, rough and smooth, but together they form a membranous
labyrinth so long and extensive that is accounts for more than half the total
membrane in many eukaryotic cells. It does this by folding over and over itself
many times thereby forming many membranous sacs. It consists of a network of
membranous tubules and scas called cisternae. There are two distinct regrions of
Endoplasmic reticulum that differ in structure and function. Smooth Endoplasmic
Reticulum lacks ribosomes, and appears to have a smooth surface when looking
under a light microscope or electron microscope. On the other hand Rough
Endoplasmic reticulum consists of many ribosomes, which look bumpy and rough
under a microscope, which was where Rough Endoplasmic reticulum got its name.
Smooth Endoplasmic reticulum funtions in diverse metaolic processes like
sythesis of lipids, and is found commonly in places such as the liver. Smooth
Endoplasmic reticulum is used in the liver to detoxify poisonous substances.
Rough Endoplasmic reticulum is important for protein synthesis because of its
ribosomes. As discussed earlier, the ribosomes that are connected to the Rough
Endoplasmic Reticulum differ from the ribosomes in the cytoplasm, for they are
responsible for producing proteins that will eventually be shipped out of the
cell. It is also responsible for producing membranes.
Golgi
Apparatus- proteins from both the free and bound ribosomes of the cell are sent
to the Golgi apparatus, the center of manufacturing, sorting and shipping in the
cell. The Golgi apparatus is an organelle that resembles a stack of deflated
balloons. The Golgi apparatus is filled with enzymes that allow it to finish the
processing of the protein. These enzymes will add sulfur or phosphorus to
certain regions of the proteins, or it might even chop off a small piece of the
protein. Once it is completed, the protein leaves the Golgi apparatus and goes
to its destination either inside or outside of the cell wall. The Golgi
apparatus also produces ribosomes, by using 4 to 100 amino acids it has
collected, known as a signal. This therefore showing the evolution that a Gogli
apparatus was present before the ribosome, proving that the ribosome was
produced to most likely make the process more efficient.
Lysosomes- the lysosomes is a small spherical organelle that
functions as the cells recycling center and garbage disposal. A lysosomes is a
membrane elcosed sac, filled with hydrolytic enzymes that the cells uses to
digest macromolecules. Basically, the lysosome uses its powerful digestive
enzymes to break down old worn out organelles, and put tiny particles of them
back in the cytoplasm. The hope is that they will later be used to make new
organelles. Lysosomes do the same thing to proteins, and hydrolyses them, along
with lipids and other molecules. In fact, the lysosomes digestive power is so
strong that it can destroy the cell that harbors it through a process called
autodigestion. Lysosomes will also use the hydrolytic enzymes mentioned earlier
to recycles the cells organic materials. Judging by the function of the
lysosomes, it is most likely true that is was one of the last organelles to
evolve, because its job is based around breaking down OTHER things in the cell,
implying they had to exist before the lysosome was needed.
Mitochondria- the powerhouse of the cell. Shaped long and slender or
like a bean, and is the place where enzymes turn the sugar glucose and other
nutrients into Adenosine Triphosphate, or ATP, this process is also known as
aerobic respiration. In this process glucose is broken down in the cells
cytoplasm to form pyruvic acid which is transported into the mitochondrion.
After going through the Kreb Cycle, the pyruvic acid reacts with water to form
10 hydrogen atoms. These atoms are then transported to the cristae of the
mitochondrion by means of coenzymes. After going through the electron transport
chain, the cell produces energy in the form of ATP. ATP then acts as an energy
source for countless processes within the cell, including transport across the
plasma membrane, the building and transport of proteins and lipids, the
recycling of molecules and organelles, and the dividing of cells. Muscle and
liver cells are so active that they required dozens, if not hundreds of
mitochondria to meet with their energy needs. Mitochondria are unique for
several reasons, the most obvious being that they contain their own DNA in the
form of prokaryote-like circular chromosomes. Mitochondria also contain their
own ribosomes which resemble prokaryote ribosomes, and divide independently of
the cell. The evolution of the mitochondria can be explained by what some
scientists call the endosymbiosis hypothesis. This hypothesis states that
millions of years ago, prokaryotic cells that were capable of aerobic
respiration were engulfed by other prokaryotic cells but not digested. The host
cell than became dependent of the other cell where the host cell provided
nutrients, and the engulfed cell provided the other cell with ATP. Therefore
this is how the evolution most likely occurred.
Vacuoles- Found in both
plant and animal cells, but serve a more important function in plant cells. A
vacuole takes up most of the room in a plant cell, and takes up a much smaller
amount of space in an animal cell. The Vacuole, a membranous bag, pushes the
organelles and cytoplasm to the edges of the cell, in plants, it remains by
itself alone in animal cells. The vacuole holds water, salts, sugars, proteins,
and other nutrients. In plant cells, it also stores blue, red and purple
pigments that give a flower its color. In plants, vacuoles also store plant
wastes, that taste bitter to certain insects, thereby discouraging them from
eating it.
Peroxisomes- a specialzed metabolic compartment bounded
by a single membrane. Peroxisomes contain enzymes that transfer hydrogen from
various substrates to oxygen-producing hydrogen peroxide. They grow by
incorporating proteins and lipids.
Chloroplast- Found only in plants and
eukaryotic algae. Structure in the cell where phosynthesis takes place.
Chloroplasts are small disc shaped structures found most commonly in leaf cells
so that they are closest to light. Being close to light is essential for
photsynthesis to take place because the process involved the transformation of
light energy into chemical energy used in the cell. Each chloroplasts consists
of a double membrane, with a ground up material inside called the stroma. The
stroma is crossed by a complex network of interconnected disks called
thykaloids. Thykaloids will stack up like saucers into what is known as grana.
Chlorophyll molecules, the molecules that actually absorb the light for
photosynthesis are attached to the thykaloids. Chlorophyll is produced in the
presence of light by small colorless organelles called proplastids. The
evolution of the Chloroplasts is similar to that of the mitochondria, in plant
cells. The self-reproducing ability of chloroplasts, their bacteria-like DNA and
ribosomes, and their close similarity regardless of the type of cell they
inhabit, suggest that they were once independent organisms that come to exist in
symbiosis with the plant cell as host.
Cytoskeleton- Unlike Prokaryotic
cells that are small, large Eukaryotic cells require support. The cytoskeleton
is a large network of tubes made out of proteins, that support the cell and
allow it to retain its desired shape. Protein tubes, filaments and fiber, cross
all over the cytoplasm and hook on to organelles, and keep them in their place.
The structure can also be used as a track for proteins and other items to travel
across the cell. Scientist believe the cytoskeleton may have also evolved as a
result of the nucleus’s desire to have a structure that could keep the
organelles where the nucleus wants them.
Cilia and Flagella- In
Eukaryotic and Prokaryotic cells, these locomotive appendages that protrude from
the cell help with movement. Swimming organism, usually move by means of
Flagellum, a long tail like structure made of protein. Bacteria can have one,
two, or many flagella to propell them in the water. But a Eukaryotic cell will
have a longer and larger single flagellum. The cell with flagella propells
itself by flapping around the flagella like a whip, in the same manner that a
sperm cell with its flagella will propel itself toward a female egg. Movement in
Eukaryotic cells also is accomplished by use of Cilia. Cilia are short hairlike
proteins built by centrioles, which are barrel like structures located in the
cytoplasm that assemble and break down protein filaments. The cell covered in
the cilia appears to be hairy, and the cell propels itself by having all the
hairs move like the oars of a boat and move where desired. But Cilia is not only
used for movement, Cilia is found in humans to prevent dust, and smog from
entering the lungs, and does this by sweeping them into mucus where they are
swallowed, as opposed to inhaled.
Cell Wall- The most
predominate feature that distinguishes plant cells from animal cells, is the
cell wall. The Cell wall surrounds and protects the plasma membrane located
within it, and helps it to maintain its shape. The pores in the cell wall allow
objects to flow freely through the walls, into and out of the cell. The strength
of the wall also allows for the central vacuole to be filled with water, or in a
turgid state, without bursting. The strength of the cell walls is portrayed in
the firmness of stems, leaves and flowers. It is also divided into a primary and
a secondary cell wall.
Extracellular matrix- Functions in support,
adhesion and movement and development. In animal cells like cell walls of
plants. It also functions in a cells dynamic behavior. It Helps to control the
activity of the genes in the nucleus.
Intercellular Junctions- integrate
cells into higher levels of structure and function. The cell wall of plants are
perforated by plasmodesmata which allow cytoplasm to pass through. This allows
water and small solutes to pass freely from cell to cell.