Ideas about cell structure have changed dramatically over the years. Early biologists saw cells as simple membranous sacs that contain a liquid and some floating particles. Biologists today know that cells are infinitely more complex than this.
What is a cell.
The cell, in biology, is the basic, membrane-bound unit that contains the basic molecules of life and from which all living things are made.
A single cell is often an entire organism in its own right, such as a bacterium or yeast. Other cells acquire specialized functions as they mature.
These cells cooperate with other specialized cells and become the building blocks of large multicellular organisms, such as humans and other animals. Even though cells are much larger than atoms, they are still very small.
The smallest cells known are the mycoplasma microorganism group. Some unicellular organisms are small spheres up to 0.2 micrometers in diameter (1 micrometer = approximately 0.000039 inch), with a total mass of 10-14 grams – equal to 8,000,000,000 hydrogen atoms.
Typically, human cells have a mass 400,000 times the mass of a single mycoplasma bacterium, even human cells which are only about 20 μm.
It would take a sheet of about 10,000 human cells to cover the pinhead, and every human being consists of more than 30,000,000,000,000 cells.
The nature and function of cells.
The cell is surrounded by a plasma membrane, which forms a selective barrier that allows nutrients to enter and empty the products.The inner part of the cell is organized into many specialized compartments or organelles, each surrounded by a separate membrane.
The nucleus, which is one of the major organelles, contains the genetic information needed for cell growth and reproduction. Each cell contains only one nucleus, while other types of organelles are found in multiple copies in the cellular contents, or the cytoplasm.
Organelles include mitochondria, which are responsible for the energy parameters required for cell survival; Lysosomes, which digest unwanted substances inside the cell; The endoplasmic reticulum and the Golgi apparatus, which play important roles in the internal organization of the cell by synthesizing the selected molecules and then processing, sorting and directing them to their correct locations.
(In addition, plant cells contain chloroplasts responsible for photosynthesis, as the energy of sunlight is used to convert carbon dioxide (CO2) and water (H2O) into carbohydrates.)
Cytosol: is the main substance of the cytoplasm, and it is a suspended fluid found inside cells, in which chemical and inorganic substances, molecules, chromosomes, crystals, and cellular organelles are suspended or dissolved in it.
The cytosol contains an organized framework of fibrous particles that make up the cytoskeleton, which gives the cell its shape, enables organelles to move within the cell, and provides a mechanism by which the cell itself can move.
Cytosol also contains more than 10,000 different types of molecules that are involved in cellular biosynthesis, which is the process of making large biological molecules from small molecules.
Specialized organelles are a feature of the cells of organisms known as eukaryotes. In contrast, cells of organisms known as prokaryotes do not contain organelles and are generally smaller than eukaryotic cells. However, all cells share strong similarities in biochemical function.
_ Cell molecules.
Cells contain a special set of membrane-bound molecules. These molecules give cells the ability to grow and reproduce.
The overall process of cellular reproduction occurs in two steps:
(Cell growth and cell division).
During cell growth, the cell engulfs specific molecules from its surroundings by selectively carrying them across the cell membrane. Once inside the cell, these molecules are exposed to the action of highly specialized, large, elaborately folded molecules called enzymes.
Enzymes act as catalysts by binding to the ingested molecules and regulating their rate of change chemically. These chemical modifications make the molecules more beneficial to the cell. Unlike the ingested molecules, the catalysts are not chemically altered during the reaction, allowing a single catalyst to regulate a specific chemical reaction in many molecules.
Biological stimuli create chains of reactions. In other words, a chemically transformed molecule by one catalyst acts as a raw material, or a substrate, for a second catalyst and so on. In this way, the catalysts use small molecules brought into the cell from the external environment to produce increasingly complex reaction products. These products are used for cell growth and for replicating genetic material.
Once the genetic material is copied and there are enough molecules to support cell division, the cell divides to form two daughter cells (daughter cells).
Through many cycles of cell growth and division, each original cell can give rise to millions of daughter cells, in the process of converting large amounts of non-living matter into biologically active molecules.
Cell biology / www.britannica.com.