The Structure of Life
Within the scope of this section, the intention is only to provide a brief introduction to the following topics:
- The Structure of Cells
- The Structure of DNA
- The Life Cycle of Cells
- The Nature of Life
- The Nature of Death
It has been shown that the taxonomy of life has essentially been built on two basic types of cells, the prokaryotic and eukaryotic cells. In the evolution of more complex life forms, the eukaryotic cells have also successfully adopted a multi-cellular structure. Within this approach, organisms have evolved both germinal and somatic cells. The 'somatic' cells meet the demands of physiology, while the 'germinal' cells have become specialized in the process of reproduction that leads to the next generation of the species. Somewhere along the path of eucarya evolution, somatic and germinal cells entered into a complex symbiotic relationship with various primitive cell types. This was a fundamental divergence from the survival of individual cells to the survival of group of cells. This feature of the eucarya taxonomy is shared by its descendents from mushrooms to people. In fact, a great deal of basic human biochemistry was already set in place by the time multi-cellular organisms appeared on the Earth.
Note: The cell is a fundamental structure in biology, equivalent to the atom in physics. However, like particle physicists, molecular biologists are also discovering that these fundamental units have a myriad of sub-structures. A eukaryotic cell can perform the functions of respiration, movement, digestion and reproduction, although not every cell has to perform all functions. As we will see in more detail, the eukaryotic cell contain organelles, or little organs, which perform specialized functions within the boundary of the cell; defined by its outer membrane. Within the cell, the organelles `float` in cytoplasm that is a jelly-like substance predominately (90%) made of water and within the organelles are various enzymes, amino acids and other molecular substances required for cell functions.
As indicated, one of the primary advantages of a multi-cellular structure is that not every cell has to perform all functions so; as a consequence, they could start to specialize. Groups of specialized cells are called `tissue` and are described as `differentiated`. There are many types of tissue, which includes bone, muscles and cardiovascular tissue plus nerve and connective tissue, such as tendons and ligaments. Other tissue types include the digestive, respiratory, urinary and reproductive systems. Equally, skin and blood are both classed as types of tissue, as is lymphatic tissue that makes up the immune system. The hormone-producing glands of the endocrine system are another type of tissue, while epithelium tissue lines the cavities of the body, some capable of absorbing water and nutrients. Overall, there are more than 200 different types of specialized cells in a typical vertebrate like Homo Sapien. Some cells are large, others small. Some divide quickly, while others do not. For example, bone marrow cells divide every few hours, while adult nerve cells can live 100 years without dividing. The mechanism in which cells divide depends on whether they are germinal or somatic:
The division of somatic cells ends in the production of two identical daughter cells. While mutation can occur, differences only affect daughter cells, not the offspring of the organism as a whole.
This process relates to germinal cells, which includes both the sperm and egg cells, each having only half the genetic material to be inherited by the offspring.
Once a group of cells are differentiated, they cannot change from one type to another, even though they all have exactly the same genetic code. The difference is described as `gene expression` and is a mechanism that can turn on and off sequence of genes.