All multi-cellular lifeforms are made up of eukaryotic cells. Some single-cell lifeforms are also eukaryotic, but most are prokaryotic. All organisms on Earth, that are considered to be living, have cells that are either one or the other. The disease causing pathogens called viruses are neither prokaryotic nor eukaryotic, but they are not considered to actually be lifeforms.
Viruses are unable to replicate independently, have “babies” as it were, by themselves. They take over and utilise the cellular resources within either prokaryotic or eukaryotic cells to reproduce themselves. All lifeforms, whether prokaryotic or eukaryotic can engender “young” given appropriate independent, environmental circumstances. The modern, that is current, scientific system of taxonomy has three domains at its highest level.
These are Archaea, Eubacteria and Eukaryotes. Both Archaea and Eubacteria are single-cell prokaryotes, commonly referred to as bacteria, although the genetic differences between these two domains is quite extensive, which is why they are seperated. All eukaryotic organisms, whether single- or multi-celled, from protozoa, molds and amoebas to mushrooms, plants, worms, insects, arachnids, amphibians, birds, reptiles and mammals, come under the domain of Eukaryotes.
The eukaryotic multi-cellular lifeforms have specialized cells that diffentiate from each other. This is clear if we simply consider ourselves. We have cardiac muscle cells in our hearts that are shaped and function quite differently from the cells of our brains (neurons) or for that matter the other types of muscle cells within our bodies, the skeletal and smooth muscle cells, let alone those in our various organs, tissues and glands. However, even the variation between these specialized cells is insignificant in comparison to the difference between eukaryotic and prokaryotic cells.
Prokaryotic cells are vastly simpler than eukaryotic cells. It is almost certainly because of this that there are no multi-cellular prokaryotic lifeforms. Within the cell membrane of a prokaryotic cell, which is effectively the whole organism, there is a main coil of DNA (de-oxyribonucleic acid) and possibly some additional smaller coils called plasmids and varying numbers of ribsomes. DNA provides the blueprint of life for any organism, it gets copied into RNA (ribonucleic acid) molecules that are then processed by ribosomes to produce proteins. The manufactured proteins control metabolic functions or provide the foundation base for cellular structure.
These functions also occur within eukaryotic cells, but are expanded upon considerably. The DNA of eukaryotic cells are contained within the nucleus, effectively cordoned off from the rest of the cell by a protective membrane similar in construction to the cell membrane that encloses the entire cell. While some of an eukaryotic cell’s ribosomes are free-floating within the cell’s cytoplasm (liquid contents) between the wall of the nucleus and that of the cell, the majority are attached to the endoplasmic reticulum, a type of mini-organ within each cell, one of many organelles.
Within the body of the eukaryotic cell are a multitude of various types of organelles, cellular organs, that provide a multitude of services to maintain its function. It is this plethory of functions and capabilities, varying significantly depending on cell function, that make multi-cellular life possible. And this is the primary difference between prokaryotic and eukaryotic cells. Eukaryotic cells have numerous organelles, from the nucleolus within the wall of the nucleus, to the mitochondria that may have started as mutualistic bacteria within the cell body. The primary difference between prokaryotes and eukaryotes is the vast disparity in complexity between the two cell types.
