The cell, which is the
fundamental structural and functional unit of the body, is the focus of
cytology, a branch of science that studies cells.
The Cell Theory
provides insights into the nature of cells, stating that:
a)
All living organisms are made up of cells and cell products.
b)
Cells are the basic units of structure and function in all living organisms.
c)
Cells arise from the division of preexisting cells.
d)
The collective activities and interactions of cells contribute to the
understanding of the entire organism.
To gain a better
understanding of cells, they can be divided into four main components:
Plasma
(cell) membrane: This is the outer lining that separates
the internal parts of the cell from the extracellular materials and the
external environment.
Cytoplasm:
This is the substance that surrounds organelles and is located between the
nucleus and the plasma membrane.
Organelles:
These are permanent structures with distinctive morphology that are highly specialized
for specific cellular activities.
Inclusions:
These are secretions and storage products of cells.
The extracellular
materials, also known as the matrix, are substances found outside the cell
surface.
The plasma membrane, a
thin outer membrane, maintains the integrity of the cell by keeping its
contents separate and distinct from the surrounding environment. It is a double
layer composed of phospholipids, cholesterol, glycolipids, and carbohydrates
(oligosaccharides) and is self-sealing, automatically sealing if punctured by a
needle.
Functions
of the plasma membrane include
Separating the
cytoplasm inside the cell from the extracellular fluid.
Separating cells from
one another.
Providing an abundant
surface for chemical reactions to occur.
Regulating the passage
of materials into and out of the cell, exhibiting selective permeability.
Movement
across the cell membrane occurs in two ways: passive and
active movements. Passive movement does not require energy, while active
movement consumes ATP (adenosine triphosphate) energy.
Passive
movement includes:
a)
Simple diffusion: the random movement of molecules from
areas of high concentration to areas of low concentration. An example is the
exchange of gases in the alveoli of the lungs.
b)
Facilitated diffusion: larger molecules that are not
soluble in lipids require protein channels to pass through the plasma membrane.
No direct energy is needed. An example is the transport of amino acids across
the cell membrane.
c)
Osmosis: a special type of diffusion referring to the
passage of water through a selectively permeable membrane from an area of high
water concentration to an area of low water concentration.
d)
Filtration: small molecules pass through a
selectively permeable membrane in response to pressure. An example is the
filtration process in the kidneys during urine formation.
Active movements across
membranes involve substances moving from areas of low concentration on one side
of the membrane to areas of higher concentration on the other side. This
movement occurs against the concentration gradient and requires energy.
Active transport occurs
when equilibrium is reached and more molecules are needed. Substances are
pumped through the membrane against the concentration gradient, requiring the
use of ATP. Examples of active transport processes include the sodium-potassium
pump and the calcium pump. In these processes, molecules bind to carrier
proteins, and the molecule-carrier complex passes through the membrane,
assisted by an enzyme and ATP. The carrier protein then returns to its original
shape and repeats the process.
Endocytosis,
which involves pocketing of the plasma membrane, includes:
a)
Pinocytosis: cell drinking.
b)
Receptor-mediated endocytosis: endocytosis facilitated
by receptors.
c)
Phagocytosis: cell eating.
Exocytosis is the
opposite of endocytosis and involves the removal of undigested particles."
Cytoplasm
Cytoplasm is a matrix
or ground substance in which various cellular components are found. It is a
thick, semi-transparent, elastic fluid containing suspended particles and a
series of minute tubules and filaments that form the cytoskeleton. Water
constitutes 75-90% of the cytoplasm. It also contains solid components such as
proteins, carbohydrates, lipids, and inorganic substances. The inorganic
components exist as solutions because they are soluble in water. The majority
of organic substances, however, are found as colloids, which are particles that
remain suspended in the surrounding medium.
Organelles
Organelles are
specialized portions of the cell with characteristic shapes that assume
specific roles in growth, maintenance, repair, and control.
Nucleus
The nucleus is oval in
shape and is the largest structure in the cell. It contains the hereditary
factors in the cell; hence it controls cell activity and structure. Most cells
contain a single nucleus, but some, like matured red blood cells, do not contain
a nucleus. However, muscle cells contain several nuclei. The nucleus is
separated from other cell structures by a double membrane called the nuclear
membrane. Pores in the nuclear membrane allow the nucleus to communicate with
the cytoplasm. The nucleus contains a jelly-like fluid that fills the nucleus
called karyolymph (nucleoplasm), which contains the genetic material called
chromosomes. The nucleus also contains a dark, somewhat spherical,
non-membrane-bound mass called the nucleolus, which contains DNA, RNA, and
protein that assist in the construction of ribosomes.
Ribosomes
Ribosomes are tiny
granules composed of Ribosomal RNA (rRNA). They are the site of protein
synthesis in the cell, where they use information from the DNA in the nucleus
to synthesize proteins. Ribosomes can be found free in the cytoplasm or
attached to the endoplasmic reticulum.
Endoplasmic
Reticulum
The endoplasmic
reticulum (ER) is a double-membrane channel system that is involved in protein
synthesis, lipid metabolism, and detoxification of drugs and toxins. It can be
classified into two types: rough endoplasmic reticulum (RER) and smooth
endoplasmic reticulum (SER). RER has ribosomes attached to its surface, giving
it a rough appearance, and is involved in protein synthesis and protein
folding. SER lacks ribosomes and is involved in lipid metabolism,
detoxification, and calcium storage.
Golgi
Apparatus
The Golgi apparatus,
also known as the Golgi complex or Golgi body, is a stack of flattened sacs
that processes, modifies, and packages proteins and lipids synthesized in the
ER for transport to their final destinations within or outside the cell. It
plays a crucial role in intracellular transportation and secretion.
Mitochondria
Mitochondria are
double-membraned organelles often referred to as the "powerhouses of the
cell" because they are responsible for generating energy in the form of
ATP through cellular respiration. They contain their own DNA and can
self-replicate, suggesting that they originated from a symbiotic relationship
between early eukaryotic cells and bacteria.
Lysosomes
Lysosomes are
membrane-bound organelles that contain enzymes capable of breaking down
cellular waste materials, cellular debris, and macromolecules. They play a
crucial role in cellular waste disposal, recycling of cellular components, and
the immune response.
Inclusions
Inclusions are
secretions and storage products of cells. They can include pigments, crystals,
and other substances that are not bound by a membrane. Inclusions are important
for various cellular functions such as energy storage, pigment production, and
detoxification.
Extracellular
Extracellular refers to
the space outside the cell. It includes the interstitial fluid, which is the
fluid that surrounds and bathes the cells in tissues, as well as other
extracellular fluids such as blood plasma and lymph. The extracellular
environment plays a critical role in cellular communication, nutrient exchange,
waste elimination, and overall tissue function.
Cell
Membrane
The cell membrane, also
known as the plasma membrane, is a selectively permeable barrier that separates
the interior of the cell from the extracellular environment. It consists of a
phospholipid bilayer embedded with proteins and other molecules. The cell
membrane controls the passage of substances in and out of the cell, allowing
for the exchange of nutrients, gases, and waste products, as well as
communication with neighboring cells and the external environment.
Cell
Junctions
Cell junctions are
specialized structures that connect cells together and play a role in tissue
integrity and communication. There are several types of cell junctions,
including tight junctions, desmosomes, and gap junctions. Tight junctions are
found in epithelial tissues and form a seal between adjacent cells, preventing
the passage of substances between them. Desmosomes are spot-like adhesions that
anchor cells together, providing mechanical stability to tissues subjected to
mechanical stress. Gap junctions are channels that allow for direct
communication between adjacent cells, allowing for the exchange of ions and
small molecules.
Extracellular
Matrix
The extracellular
matrix (ECM) is a complex network of proteins and carbohydrates that surrounds
cells in tissues and provides structural support, mechanical strength, and
signaling cues for cellular functions. The ECM is composed of various
components, including fibrous proteins such as collagen and elastin,
proteoglycans, glycoproteins, and other molecules. The ECM plays a crucial role
in tissue development, maintenance, and repair, as well as cell behavior and
function, including cell adhesion, migration, proliferation, and
differentiation.
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| The Cell |
