Definition:
A cell is the basic living structural and functional unit of the body.
Cytology - It is a branch of science concerned with the study of cells.
About Cell
a) All living organisms are composed of cell and cell products.
b) Cell is the basic unit of structure & function of all living organisms.
c) All cells come from the division of a pre-existing cell.
d) An organism as a whole can be understood through the collective activities & interactions of its cells.
e)Normal human body cells usually divide at a controlled rate required to replace the dying ones and for growth.
d)Cancer cells are different.
i)They lack the controlling mechanism.
f)Cancer occurs when cells grow and divide at an abnormal rate & then spread beyond the original site.
g)Some of the risk factors for cancer occurrence are radiation, chemicals, extreme pressure, and hormonal therapy.
principal parts of a cell
1. Plasma (cell) membrane: it is the outer lining, limiting membrane separating the cell internal parts from extracellular materials & external environment.
2. Cytoplasm: cytoplasm is the substance that surrounds organelles and is located between the nucleus and plasma membrane.
3. Organelles: these are permanent structures with characteristic morphology that are highly specialized in specific cellular activity.
4. Inclusions: they are the secretions and storage products of cells.
Extracellular materials are also referred to as the matrix, which are substances external to the cell surface.
1. Plasma Membrane
-is a thin outer membrane, which maintains the integrity of the cell.
-It keeps the cell and its contents separate and distinct from the surrounding.
-It is double-layered measuring about 4.5 nm and made of phospholipids, cholesterol, glycolipids, & carbohydrates (oligosaccharides).
-The bi-layer is self-sealing. If a needle is injected and pulled out, it automatically seals.
Functions of Plasma Membrane
1. Separate the cytoplasm inside a cell from the extracellular fluid.
2. Separate cells from one another.
3. Provide an abundant surface on which chemical reactions can occur.
4. Regulate the passage of materials in and out of cells. It also let some things in and keeps others out. The quality selective permeability.
Movement across-cell membrane
- Passive movement
a. Simple diffusion, the random movements of molecules from an area of high concentration to the area of low concentration. Example air in alveoli of the lung.
b. Facilitated diffusion, larger molecules, which are not soluble in lipid need protein channels to pass through the plasma membrane. No direct energy is needed. Example: - Amino acid passes through 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 lower water concentration.
d. Filtration, small molecules pass through a selectively permeable membrane in response to the force of pressure. Example: - filtration in the kidney in the process of urine formation.
- Active movements
Substances move through a selectively permeable membrane from areas of low concentration on side of a membrane to an area of higher concentration on the other side.
This is against a concentration gradient. Therefore, it requires energy.
a) Active Transport: till equilibrium substances could more by passive movement.
But if an equilibrium is reached and still more molecules are needed, they must be pumped through the membrane against a concentration gradient.
b) Endocytosis, pocketing in by plasma membrane. It includes:
Pinocytosis – cell drinking.
Receptor-mediated Endocytosis- Endocytosis with the help of a receptor.
Phagocytosis- cell eating.
c) Exocytosis, opposite to Endocytosis, to remove out undigested particles.
2. Cytoplasm
-The 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, 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. Colloids are particles that remain suspended in the surrounding medium.
3. Organelles
-Organelles are a specialized portion of the cell with a characteristic shape that assumes a specific role in growth, maintenance, repair, and control.
a)Nucleus
The nucleus is separated from other cell structures by a double membrane called a nuclear membrane. Pores over the nuclear membrane allow the nucleus to communicate with the cytoplasm.
-It contains DNA, RNA, and protein, which assist in the construction of ribosomes.
b)Ribosome
-tiny granules, composed of Ribosomal RNA (rRNA). They are the site of protein synthesis
c) Endoplasmic reticulum
-is a double membrane channel.
- It is continuous with the nuclear membrane.
- It is involved in the intracellular exchange of material with the cytoplasm.
-Various products are transported from one portion of the cell to another via the endoplasmic reticulum.
-it is considered intracellular transportation.
-It is also stored for synthesized molecules.
-Together with the Golgi complex, it serves as a synthesis & packaging center.
-The endoplasmic reticulum (ER) is divided into two.
-These are granular E.R. Containing granules and are involved in the synthesis of protein, and agranular E.R. that synthesizes lipid & involved in detoxification.
d) Golgi Complex
-near to the nucleus.
-It consist 4-8 membranous sacs.
-It process, sort, pack & deliver protein to various parts of the cell.
e) Mitochondria
-A small, spherical, rod-shaped, or filamentous structure. It generates energy.
-Each mitochondrion possesses two membranes, one is a smooth (upper) membrane and the other is arranged with a series of folds called cristae.
-The central cavity of a mitochondrion enclosed by the inner membrane is the matrix.
f) Lysosomes
-appear as membrane-enclosed spheres.
-They are formed from Golgi complexes & have a single membrane.
-They contain powerful digestive (hydrolytic enzyme capable of breaking down many kinds of molecules.
-The lysosomal enzyme is believed to be synthesized in the granular endoplasmic reticulum and Golgi complex.
g) The cytoskeleton,
-The cytoplasm has a complex internal structure consisting of a series of exceedingly small microfilaments, microtubule & intermediate filaments together referred to as the cytoskeleton.
h) Centrosome,
-A dense area of cytoplasm is generally spherical and located near the nucleus it contains centrioles.
-It also contains DNA that controls their replication. Centrosomes are made of microtubules, which seam drinking straws. They are involved in the movement of the chromosome during cell division.
i) Cilia/flagella
-The thread-like appendages, which are made of microtubules. When they are beating forms rhythmic movement.
-They are found in the female reproductive organ and upper respiratory tube.
4. Cell inclusion
The large and diverse group of chemicals, which are produced by cells, are cell inclusions.
It is mainly organic and includes melanin, glycogen & Lipids.
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