Cell

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').