What life is


Cell structures

There are two basic forms of cells found in nature; the prokaryotic and eukaryotic cell type. The prokaryotic cell type is a cell with just one intracellular compartment that contains all the DNA and proteins necessary for reproduction and growth. They all have a polymer based rigid cell wall. Prokaryotic cells are found in to domains of life, the bacteria and archaea. The eukaryotic cell type is a much larger cell and contains multiple internal organelles that divide these larger cells into specialized compartments. These subcellular compartments include the nucleus (containing genes) and endomembrane systems (secreting, importing and redistribution cellular components) for biosynthesis, the mitochondria and chloroplasts for producing energy, and lysosomes, peroxisome and vacuoles for break down and storage. In addition, eukaryotic cells contain a cytoskeleton structure important for internal movement and overall structure. The table below summarizes cell structure characteristics as determined by domain of life.

Domain Bacteria Archaea Eukarya
Cell type prokaryotic prokaryotic eukayrotic
Ribosomal RNA unique* unique; some match eukarya unique; some match archaea
RNA polymerase one type several types several types
Chromosome structure circular linear linear
Introns rare some common
Histones in chromosomes no rare yes
Responds to antibiotics yes no no
Peptidoglycan in cell wall yes no no
Membrane lipids unbranched; fatty acids branched, isoprenes unbranched; fatty acids
Membrane bound organelles no no yes
Multi cellular organisms no no yes

* Note that the original split of all forms of life into three domains, with prokaryotes composing two domains, came from sequence comparison of ribosomal RNA. Multiple sequence alignment showed that these RNA types come in three large clusters largely congruent with genetic and biochemical features of bacteria, archaea and eukarya listed here. This comparison shows that archaea and eukarya or genetically more closely related to each other and differ from bacteria, considered the oldest form of life on earth.

Multicellular organisms

Multicellular organisms contain only eukaryotic cells. These cells also need surface junctions that allow them to connect and communicate with each other to provide proper stability and physiological integration into the often very large body structures (animals, plants).

For biological macromolecules and cells, and for any hierarchical level for that matter, the basic principle is ‘function follows form’ (*). This means that over evolutionary time, the structure of proteins, DNA, cells and shape of organisms have evolved altering specific functions which may or may not improve the chance for survival of the organism ('survival of the fittest').

* Note: most biology textbooks introduce the notion of 'form follows function', a paradigm of design first articulated by the American architect Louis Sullivan (1856 - 1924). Evolution is not design of a structure for a purpose (e.g. use windows to have natural light inside a house), the rational behind Sullivan's dictum. Rather, evolution is design by trial and error. Evolution does not ‘plan ahead’, but tinkers with changes in structures (mutations) subject to selection ('trial') allowing novel functions (an 'error') to propagate to the next generation (see our discussion of intelligent design theory and why it is not a scientific theory, but theistic belief).

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 Copyright © 1999-2011 Lukas K. Buehler