The Cell as a Functional Unit
Sound knowledge of the structure and functions of the human body starts from the very smallest components, moves through increasingly larger units, and ends with the whole organism.The smallest independent functional unit of any organism is the cell.The structure and function of the cell are described in general cytology (cytos = cell, cellula = small chamber). The cell as an independent functional unit displays the characteristics of life:
- Metabolism: The ability to absorb substances and use them to build up the body's own cell substance and obtain heat. In plant cells, metabolism mainly takes place via photosynthesis, in which carbohydrates are formed from carbon dioxide and hydrogen by the chlorophyll molecules. In animal cells, metabolism mainly takes place with oxygen, in which complex carbohydrate compounds are broken down into the smallest basic building blocks and carbon dioxide is released.
- Growth: Enlargement of the cell substance made up of the products of metabolism, where certain protein compounds reduced during catabolism are used for protein synthesis.
- Reproduction: In cells, the re-creation of their own cell substance by cell division or some other principle of reproduction. Reproduction means that all the skills, characteristics, and external form of the cell are passed on.
- Movement: The ability of certain parts of cells to perform specific motions as a reaction to stimuli.
- Regulatory ability and reactivity: Keeping the living conditions inside the cell constant by an exchange of substances with and, to a certain extent, reacting to changes in theexternal environment.
Organs arise from the joining together of similar types of cells geared to a specific function. In the human body, the cells of the various tissues and organs can be classified by form and size:
- Largest cell: egg cell (oocyte) — 200 to 252 |jm
- Smallest cell: lymphocyte—4 jm
- Average cell size: 10 to 20 jm
- Longest cell: nerve cell (due to the axon) — up to approximately 1 m
Despite the differences in form and function of specialized cells, there is an underlying structural principle: Cells have a cell membrane that encases the cellular fluid and the organelles it contains (Fig 2-1). The nucleus, demarcated by a nuclear membrane, contains the cell's genetic material. The organelles are highly specialized subcellular structures with their own membrane border. They are small, specially structured, separate reaction areas within the cell for the specific function of metabolism.
From simple to complex
The simplest animal life is single cells that perform all the functions of life. One example of such unicellular organisms or protozoa—of which there are about 20,000 species—is the amoeba, which moves by extending and retracting so-called pseudopodia (false feet).
Multicellular organisms without any specialization of individual cells are networks or colonies of cells in which each cell is still capable of living and therefore possesses all the characteristics of life. These colonies of cells are the precursor to true multicellular organisms. Organisms made up of networks of cells, or metazoa, have cells that have specialized in certain functions, so that specialized cells undertake partial functions of the whole organism. In networks of cells, however, not every living cell has all the defined abilities; the cells have given up certain skills for the sake of the network functioning as a whole.
In multicellular living beings, only the cells that have specially developed flagella (whiplike structures) have the ability of independent movement, while cells that have developed an ocellus (eye-spot) take on a function of orientation.
Viruses are not cells, but they have the capacity to exploit the reproductive mechanism of other cells to reproduce a viral body. They are made up of a protein envelope for a DNA molecule, which contains the instruction to utilize the reproductive apparatus of a living cell and to prepare the receptacle and the blueprint itself. Viruses are dependent on living cells for all their functional characteristics.