User:Atcovi/Science/Prokaryotes vs. Eukaryotes

For the basics of cytology, there are two basic types of cell structures:
 * Prokaryotes are unicellular organisms with NO nucleus and NO membrane-bound organelles. Examples of prokaryotes are Bacteria (Archaebacteria and Eubacteria).
 * Eukaryotes are unicellular/multicellular (or) organisms that DO have a nucleus and membrane-bound organelles. Examples of eukaryotes are the kingdoms: Protista, Fungi, Plantae and Animalia.

Endosymbiotic Theory
This theory suggests that mitochondria and plastids in eukaryotic cells were once independent prokaryotic cells. Basically, this means that once upon a time, there were three prokaryotic cells: one that was capable of aerobic respiration and converting energy, one that was capable of photosynthesis and one that was incapable of doing either of these processes. The lazy cell that was incapable of doing respiration and photosynthesis engulfed - or ate - the other cells.

Cells living inside a host cell are called endosymbionts. Now, this may have been intentional or accidental. However, when the lazy cell engulfed the respiration cell, it was then able to make useful energy. When it engulfed the photosynthesis cell, it was then able to convert energy from the sun into stored chemical energy.

(SOURCE: http://study.com/academy/lesson/the-endosymbiosis-theory-evolution-of-cells.html)