Eukaryotic cells are a type of cell more complex than their counterparts, prokaryotes. Prokaryotes include the simplistic bacteria and archaea, while eukaryotes make up all fungi, animals, plants, and protists such as amoeba. Together with viruses and other snippets of genetic material, prokaryotes and eukaryotes make up all known terrestrial life.
Eukaryotic cells are characterized by internal membranes and a strong cytoskeleton. A cytoskeleton is a framework of proteins, such as actin and keratin, which help hold a cell together and differentiate its organelles. Eukaryote means "true nut," referring to the fact that eukaryotic cells possess an internal nucleus whereas prokaryotes (meaning "before nut") do not. In prokaryotes, the genetic material floats freely in the cytoplasm (cellular blood), while in eukaryotes, it is protected in a special nucleus. Eukaryotic DNA is organized into chromosomes whereas prokaryotic DNA is not.
Eukaryotes are more recent in the history of life than prokaryotes, and the typical eukaryotic cell is larger than a typical prokaryotic cell. Whereas prokaryotic life emerged as long as 3.8 billion years ago, eukaryotes only evolved between 1.6 and 2.1 billion years ago. One of the first eukaryotic organisms was red algae, whose form has barely changed in 1.2 billion years.
Eukaryotes have organelles, or cellular organs, while prokaryotes basically don't. A characteristic organelle found in almost all eukaryotes is the mitochondria, known as the power station of the cell. It is thought that the mitochondria was once a free-moving prokaryote which cooperated so closely with the early eukaryotes that they became a part of the same organism in a process known as endosymbiotic attachment. Other organelles include the ribosome, vesicles, endoplasmic reticulum, Golgi apparatus, cytoskeleton, flagella, vacuoles, lysosomes, and centrioles. All organelles have special functions and are usually enclosed in their own lipid membranes.
Eukaryotic cells have a much greater diversity in their structure than prokaryotic cells. For instance, plant cells are very different from animal cells. Plant cells have a hard outer shell called a cell wall, while animal cells just have a flexible cell membrane. Plant cells also possess a special organelle, the chloroplast, which engages in photosynthesis.