Viruses, Cells, and Multicellular Organization: An Overview

Exception to the Cell Theory: Viruses

Is a self-replicating biological entity capable of using cellular machinery. It is a potentially pathogenic agent composed of a capsid protein surrounding the nucleic acid, which can be DNA or RNA. This structure can, in turn, be surrounded by the viral envelope, a lipid layer with different proteins, depending on the virus's life cycle. A virus always needs the metabolic machinery of an invaded cell to replicate its genetic material, producing many copies of the original. This process underlies the destructive power of viruses because they can damage or destroy the cell.

Features of Viruses:

• Size: Extremely small.
• Crystallization Structure: Having a volume and identical form, viral particulates tend to order in a regular, periodic, one-dimensional pattern, i.e., they tend to crystallize.
• Obligate Intracellular Parasites: They need a host, as they do not survive in the wild (alone they might live 40 days).
• Structure: A virus is composed of a molecule of nucleic acid and a protein envelope. This is the basic structure of a virus, although some of them can add an enzyme.

Unicellular and Multicellular Organisms

The cell is the basic unit of life, endowed with its own constitution. It can be isolated as a complete organism (unicellular organism) or grouped with other cells, with which it coordinates its activity to form a unitary body (a multicellular organism). Single-celled organisms may have two types of organization:

• Prokaryotes
• Eukaryotes

All vital organ functions are performed by a single cell, which presents a complex structure (in eukaryotes), developing a large amount of organelles.

Multicellular Organisms: Concept of Cell Specialization

A number of cells are formed, which have a eukaryotic organization. All the cells of a multicellular organism originate from successive divisions of a single germ cell formed from one or two parent organisms.

From the same species, they both carry the same inherited genetic information. As the cells multiply, they undergo a differentiation process consistent with the genetic information carried by the organism. As a consequence of this cell specialization, cells present in each group are morphologically and functionally equal to each other but different from other groups. Each of these groups makes up a tissue, which is defined as a set of morphologically equal cells that perform the same function. Thus, a multicellular organism is characterized by presenting a degree of cell specialization, i.e., all cells are specialized in very specific activities. The way the body operates is not the sum of the activities of every single cell, but the result of a coordinated integration process of the same. The activities lead to cell specialization so that all vital functions in multicellular organisms are distributed among each group of cells in a way that cells perform a determined activity, coordinated with the activities of other groups to ensure this function.

Tissue, Organ, Apparatus, System

The cell is the unit of life. Different classes of small cells perform specific functions, and when grouped, cells make up a tissue of the same type. In the body, there are six kinds of tissues:

• Lining: Skin and mucous membranes
• Connective: Joint capsules and support of the various organs
• Blood: Unique tissue formed by different cells
• Muscular
• Nervous
• Glandular

A group of different tissues jointly organized to perform a special labor makes up a mass called an organ. The group of organs that work together to fulfill a specific function is called an apparatus, and performing a complex function is called a system. All systems and appliances together form the Human Body.