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Course : Biology
Topic : cell and its organization
Somatic cells are the cells that make up all the tissues of living organisms, excluding germ
cells
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In the case of animals, the egg continues cell
proliferation by mitosis and completes ontogenesis from early development forming three layers
of germ layers (outer, middle and endoderm) to tissue formation and organogenesis
...
The
timing at which the somatic cells die differs depending on the characteristics of the tissue
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In contrast, in tissues such as the skin blood, and intestinal
epithelium, which are functionally depleted of cells, special cells that retain the ability to divide,
called stem cells, exist in the tissue and divide regularly
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Such tissue is called regenerated
tissue
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For example, human skin takes about one month, red blood cells take 120 days, and small
intestinal epithelium takes only a few days to replace tissue cells, and old cells are gradually
lost
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In these regenerated tissues, stem
cells divide at regular intervals almost every day, and about half of the proliferated stem cells
undergo functional differentiation
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Therefore, cell division of stem cells is
fully regulated, and cell proliferation and cell functional differentiation form a cycle that repeats
at regular intervals
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, is the greatest attraction for tissue regenerative medicine
technology
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Cells are classified into prokaryotic cells
and eukaryotic cells based on differences in structure and function, and eukaryotic organisms
include unicellular organisms and multicellular organisms
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Prokaryotic cells have a cell membrane but no nuclear membrane
or chromosomal structure, an almost naked DNA molecule in the center, and are structurally
indistinguishable from the cytoplasm
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Unlike
eukaryotic cells, the cytoplasm does not contain organelles such as mitochondria

or chloroplasts , nor does cytoplasmic streaming
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Among eukaryotic cells, there are unicellular organisms
such as amoeba and paramecium, in which one cell is an individual, and multicellular
organisms, in which an individual is composed of many cells
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Cells are surrounded by a cell membrane (plasma membrane)
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The part of the cell other than the nucleus is called the cytoplasm, which contains
proteins, carbohydrates, vitamins, and other substances necessary for living organisms
suspended in a water-soluble cytoplasmic matrix
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there is Cell membranes, nuclear membranes, and
organelles are separated from each other by a membrane consisting of a bilayer of lipids called
a biological membrane
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The nucleus has a
bag-like structure surrounded by two layers of membranes, and inside the nucleus there is
something called chromatin, which is composed of the main DNA of genes and basic proteins
bound together
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Cellular functions are
carried out by an extremely large variety of enzymes and proteins, and genetic DNA is the
substance that synthesizes these proteins at the required times and in the required
amounts
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Transfer RNA plays a role in
transporting the amino acids necessary for protein synthesis in the cytoplasm
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In the nucleus, these RNA and DNA synthetic enzymes, DNA replication enzymes,
repair enzymes when DNA is damaged, and the like are present
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Numerous small pores called nuclear pores are present in the nuclear envelope,
and various substances such as synthesized RNA are transported into and out of the nucleus
through these pores
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The inner membrane has an
enzyme system called an electron transfer system and an enzyme system involved in oxidative
phosphorylation, and through these systems ATP, which is the most important energy source
for life activities, is synthesized
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There are rough endoplasmic reticulum and smooth endoplasmic reticulum, and the
rough endoplasmic reticulum binds ribosomal particles that serve as sites for protein
synthesis
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The role of the endoplasmic reticulum is to act as a site for protein synthesis using
messenger RNA that has copied genetic information from nuclear DNA
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Especially in the liver, there are a large number of drug-metabolizing enzyme
systems that metabolize and detoxify countless low-molecular-weight toxic substances that
enter the body from the outside, such as drugs and poisons
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Carcinogenic substances
are also decomposed here
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It has the role of chemically modifying the synthesized
secretory protein to create a structure that allows the protein to be secreted outside the cell
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(5) Lysosome:
A single-layer membrane-enclosed vesicle containing a group of enzymes that hydrolyze

proteins, nucleic acids , polysaccharides, lipids, etc
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It fuses with vesicles
containing substances that cells have taken up from the outside, hydrolyzes those substances,
and makes them available to cells as necessary substances
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If the
substance taken into the cell is a bacterium, it is also sterilized
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It also performs uric acid metabolism and lipid metabolism
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For plants, it is
extremely important as a site for ATP synthesis, which is the source of energy, along with
mitochondria
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Although it is rarely observed in normal animal
cells, in plants, the cell size increases with aging, and in old cells, vacuoles occupy most of the
cell, and the nucleus and cytoplasm may be relegated to the periphery
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Vacuoles generate turgor pressure due to their
water absorption to keep the cell walls in a tense state
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Not only ribosomes bound to the rough
endoplasmic reticulum, but also ribosomes not bound to the membrane are located here and
synthesize the proteins necessary for the cell
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(10) Cell membrane (plasma membrane):
A one-layer membrane structure directly enveloping the outer surface of the
cytoplasm
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It plays important roles such as
membrane selective permeability, phagocytosis, immune response, intracellular and
extracellular transport of various substances
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These roles are played by receptors bound to the cell membrane and
various intramembrane ion transport systems including ATPases
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Each component of these cells carries out its own functions and roles, and performs cellular
activities by functioning in a mutually related manner
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Cells are the structural and functional
minimum units of life phenomena

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