The Host Cell

The cell, plant, animal, human, or otherwise, is the host and environment in which the virus lives and reproduces itself. Just as a virus is built from repeating structures of proteins, so multicelluar life - everything from plants to people - is built from repeating structures of cells. The cell is the most basic component of any complex organism - human beings included.

In turn, a cell is composed of a variety smaller parts packaged together to form a fully functioning whole, in the same way that the human body is composed of its organs and other basic materials - muscle, blood, and skin - that make it a functioning whole.These cellular "organs" (called subcellular organelles) each have their own unique functions, and are packaged together with other basic materials that comprise the cell.

The heart of any cell is ultimately its genetic information - the information needed to replicate the cell and control its behavior and processes. (These cell-level processes, in turn, aggregate to control the larger organism as well.)

This long strands of celluar DNA are wrapped around a core of proteins, producing a small compact unit. This genetic core of information controls the activity of the rest of the cell, and is walled off from the rest of the cell in a container called the nucleus. The wall that surrounds the nucleus is known as the nuclear membrane, and allows only a select group of molecules to pass in and out of the nucleus, mostly in the form of information used to communicate with the rest of the cell.

Outside the nucleus lies the cell's cytoplasm - a plasma medium in which the contents of the cell are suspended - which is in turn contained within the cell's "skin", the plasma membrane. You could think of a cell as essentially like a jelly-filled bag, with the various organelles suspended within it.

Traversing the cytoplasm of the cell is a complex network of tubes called the endoplasmic reticulum. This "plumbing system" connects all of the different locations in the cell together, and seems to be used for the transport of molecules around the cell. Scattered throughout the cell are various organelles - cell structures which, much like the organs of the human body, each serve some specific purpose or function within the cell.

Chief among these are the ribosomes, small organelles that line the inside portions of the endoplasmic reticulum (the cell's

plumbing system). Ribosomes are the cell's "factories" - they are the place

The structure of a typical animal cell

where all protein synthesis occurs. Messenger RNA (mRNA) messages from the nucleus are translated by the ribosomes, which build the encoded proteins. Newly made proteins are carried in vesicles, small bubbles of lipid, through the plumbing system of the endoplasmic reticulum to various locations throughout the cell.

Some of these proteins are carried to the Golgi apparatus, another cell structure where complex sets of enzymes modify the created proteins and other chemicals for use outside the cell. Once modified, they are transported by vesicles to the cell surface, the plasma membrane, where they are secreted into the outside environment. The plasma membrane is constantly creating small buds on its surface, releasing material manufactured by the cell into the outside environent, and creating new vesicles that carry elements from the outside environment into the cell, collecting information about what is happening outside. In this way, cells can communicate with each other, and have the ability to form complex life forms like human begins.

Elsewhere in the cell lie the two other basic structures of the cell: the mitochondria, the cell's "power plants" where energy is produced for use in the rest of the cell, and the lysosomes, specialized vesicles which break down proteins, mRNA, and other molecules and recycle them for use elsewhere in the cell.

Between them, these structures account for the most important parts of cell function - information and control of cell functions, transport of materials and information within the cell, synthesis of material for use within or outside the cell, communication with the outside world, energy production, and waste disposal and recycling of material within the cell.

However, these structures represent only a small portion of the varied

A pituitary rat cell.The endoplasmic reticulum is clearly visible in the cell.

Ribosomes in a human liver cell

Mitochondria in a human liver cell

structures in a typical cell. Many cells have specialized features which give them their particular characteristics or allow them to perform a certain function - like the organelles in plant cells that allow them to harness sunlight. Bacteria lack some of the basic structures discussed here, and are classified separately from plants and animals. However, it is the cell structures and processes discussed here that viruses use to subvert the normal operations of cells - and turn them against us.