Eukaryotic cells are distinguished first and foremost by the fact that they possess a
nucleus - a separate, walled-off structure inside the cell which houses the genetic material (
chromosomes). In this they are different from
prokaryotic cells, which do not have a nucleus and in which the (single) chromosome is free-floating inside the cell. These two types constitute all of the living cells on earth.
The reason for the division along the lines of possession of nucleus are historical rather than scientific - the nucleus is easily visible with a light microscope and so was the first thing scientists noticed that separated the two types of cells. There are, however, two (among many more) other striking differences between eukaryotic and prokaryotic cells:
- Prokaryotes have a rigid cell wall which protects the inside of the cell, and which eukaryotes lack - they have a pliable outer membrane and some have developed a "skeleton" to provide structure.
- Eukaryotes have organelles - small structures inside the cell which are separated from the rest of it by more membranes and which act independantly to provide the cell with various biochemical functions. All eukaryotic cells possess a mitochondrion, an organelle which is indispensible in providing the cell with energy, and plants also have a plastid, that part of the cell which uses sunlight for photosynthesis.
The way cell division works in eukaryotes and procaryotes is also very different, but too complicated for me to explain without pen and paper. Suffice it to say that the rigid cell wall in procaryotes is necessary for the way they devide. Although it works very well for them, it also limits the number of their chromosome to one (just trust me on this one), an thus limits the overall amount of genetic material in procaryotes (bacteria - which as we know are very small). Cells with a nucleus can have an almost unlimited amount of DNA in that nucleus, and were therefore able to evolve much more genomic complexity and variety. All complex organisms are eukaryotic, from tiny ocean shrimp and microscopic fungi to elephants.
The origins of the evolution of eukaryotes is a very important question, and there are many theories out there that all of course violently disagree with each other. In their book The Origins of Life John Maynard Smith and Eörs Szathmáry put forward an interesting theory that shows in a way that so far we've been looking at the evolution of cells backwards. The crucial detail which led to the evolution of the eukaryotic cell with it nucleus and organelles, they claim, is not the evolution of the nucleus itself, but lies in a catastrophic event during which a large population (or several populations in several separate events) lost its rigid cell wall - for example, if another organism developed a poison which will dissolve the cell wall and free the nutrients inside for the organism to devour and utilize (bacteria can only "eat" by osmosis). At first this would lead to the near decimation of the population, but in time and with natural selection the lack of rigid structure can have certain benefits:
- With a flexible outer membrane, a cell can strech and engulf another organism, thus "eating" it in its entirety, instead of the bacterial method of releasing digestive enzimes into the environemnt and then absorbing the nutrients which are in their immediate area. A cell which is more efficient at eating can in turn afford to be bigger. In fact, the evolution of the nucleus itself depended on this development, since it has been shown that the outer membrane of the nucleus and that of the cell as a whole have the same structure and are assembled by the same organs.
- Without a rigid cell wall, bacterial cell division would no longer work. The system which evolved to replace the prokaryotic way of reproduction no longer depends on being fixed in one locus to the cell wall, and therefore led to a bigger diversity in eukaryotic organisms.
- The evolution of mitochondria and plastids, so indispensable to eukaryotes today, has long puzzled biologists. In the 70's, Lynn Margulis collated a large amount of evidence and put forward a theory, since accepted by the entire scientific community, that these organelles were originaly independant prokaryotic cells which were engulfed by the early wall-less cells and not digested, developing instead a symbiotic relationship with their hosts. Since every cell in your body contains a mitochondrion, this means that you're the product of ancient symbiosis between tiny bacteria!
LadySun says: One thing -- only some eukaryotic cells lack cell walls. The cell walls of eukaryotes are thinner and more flexible when present, though.
HCow says: I'd like to point out that not all eukaryotic cells have mitochondria. Anaerobic eukaryotes have hydrogenosomes instead (like Trichomonas).