The cell is the structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living. The cell is the smallest living unit with the ability to feed itself, interact, and reproduce. New cells arise only from other living cells by the process of cell division.

                Until the microscope was invented, at the beginning of the seventeenth century, it was not possible to identify the cells, as its diminutive size prevents that can be seen with the naked eye.
Living things can be divided into:

                Unicellular organisms:

                They are made up of a single cell that performs all functions. They can form colonies in which each cell continues carring out all cell functions. Example: Bacteria, green algae, paramecium, amoeba, etc...                                                                                         


             Green algae 






                Multicellular organisms:

                They are made up of more than one cell. Each cell perform a certain function. Together, all cells work so that the organism can carry out its functions in order to live. Most life that can be seen with the naked eye is multicellular. Example: Animals, plants, several fungi, etc...                                                                                    







                All cells have a structure very similar.

               Plasma membrane or cell membrane:

                Wraps the cell and regulates the entry and exit of substances. In plant cells, there are often also other coarse and rigid membrane, made up of cellulose, called cell wall that gives the cells support and rigidness.


                Space inside the cell, and is the area in which most cell activities take place and is the matter in which the organelles (small organs such as vacuoles, mitochondrias, chloroplasts, etc) are embedded. These include vacuoles, mitochondria, chloroplasts, and the nucleus.

                                Vacuoles: It may be a reservoir for fluids that the cell will secrete to the outside or may be filled with storage products. Plants cells usually have a large central vacuole.

                                Mitochondrias: Creates energy for the cell.

                                Chloroplasts: They are only in the cells of green plants and green algae that contain chlorophylls ( green pigment )and carry out photosynthesis.



                Controls and regulate the cell's activities and carries DNA and genes.
                Plant cells have vacuoles largest than animal cells, chloroplasts (for photosynthesis) and cell wall
                There are two types of cells:
                Prokaryotic cells: These cells DO NOT have a nucleus and genetic material is dispersed in the cytoplasm. Example: Bacteria
                Eukaryotic cells: These cells have a nucleus. Example Animals and plants.

                Prokaryotic cells are usually independent, while eukaryotic cells are usually found in multicellular organisms.



                Cells are sensitive and can detect changes (example: temperature) in the medium and react in response to these changes. Cells can respond in several ways, for example producing substances or doing movements. There are several types of cellular movements:

Amoeboid movement:

They move and feed by means of temporary cytoplasmic projections, called pseudopods (false feet). Examples: Amoeba and white blood cells.



Movement with the use of flagela:

                They are organelles on the surface of some cells. Flagella use a whip-like action to create movement of the whole cell. Flagela are much longer than cilia. Examples: Spermatozoid and trypanosoma.




Movement with the use of cilia:

                They are more numerous than flagela, move in a waving action. Example: Paramecium.

            Paramecium              Cilia





Contractile movement:

                It is stretched or contracted as elastic fibers. Examples: muscle cells and vorticella.

               Muscle cells                  Vorticella




                Cell reproduction is asexual. One cell (the "mother cell") grows and divides to produce two "daughter cells". Cell reproduction is more complex in eukaryotes than in other organisms. Prokaryotic cells such as bacterial cells reproduce by simple binary fission, while eukaryotes cells must divide too the nucleus. This process can be of two types: mitosis or meiosis.



                Cell populations go through a type of exponential growth called doubling. Thus, each generation of cells should be twice as numerous as the previous generation. However, the number of generations only gives a maximum figure as not all cells survive in each generation.


          A series of growth disorders can occur at the cellular level and these underpins much of the subsequent course in cancer, in which a group of cells display uncontrolled growth and division beyond the normal limits, invasion (intrusion on and destruction of adjacent tissues), and sometimes metastasis (spread to other locations in the body via lymph or blood).

When cells are damaged beyond repair, they are eliminated ( A)

Cancer cells avoid elimination and continue to multiply in an unregulated manner (B)