Prokaryotic and Eukaryotic Cells: An Overview

All cellular life on Earth is organised according to one of two fundamentally different structural plans -- the prokaryotic plan and the eukaryotic plan -- and this distinction is the single most important organising idea in all of cell biology. The word "prokaryote" comes from the Greek pro (before) and karyon (nucleus), while "eukaryote" combines eu (true) with the same root -- literally, cells "before a true nucleus" and cells "with a true nucleus." Bacteria, cyanobacteria (blue-green algae) and mycoplasma are all prokaryotes; protists, fungi, plants and animals are all eukaryotes.

Despite this deep difference, prokaryotic and eukaryotic cells share several basic features that mark them out as cells in the first place. Both types are bounded by a plasma membrane that separates the living contents from the surrounding environment. Both contain cytoplasm, sometimes called protoplasm, the semi-fluid material within which all of a cell's metabolic reactions take place. And both contain genetic material in the form of DNA (and RNA) that carries the hereditary instructions the cell uses to build its proteins and regulate its activities.

The differences, however, are profound and touch nearly every aspect of cell organisation. In a prokaryotic cell, the genetic material lies directly in contact with the cytoplasm, condensed into an irregularly shaped region called the nucleoid, with no membrane separating it from the rest of the cell. In a eukaryotic cell, by contrast, the DNA is enclosed within a true nucleus, bound by a double-layered nuclear envelope that keeps the genetic material physically separate from the surrounding cytoplasm. This single difference has enormous knock-on consequences. Because a prokaryotic cell has no internal membranes dividing up its cytoplasm, it entirely lacks the membrane-bound organelles -- mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, plastids -- that are characteristic of eukaryotic cells and that compartmentalise the eukaryotic cell's many chemical processes into separate specialised regions.

Ribosomes, the particles responsible for protein synthesis, differ in size between the two types of cell, described by their sedimentation coefficient (a measure of how fast they settle in a centrifuge, given in Svedberg units, S). Prokaryotic cells possess smaller 70S ribosomes, built from a large 50S subunit and a small 30S subunit. The ribosomes found free in the cytoplasm of a eukaryotic cell, and those studding the rough endoplasmic reticulum, are larger 80S ribosomes, built from a 60S and a 40S subunit. Curiously, the mitochondria and plastids inside a eukaryotic cell contain their own 70S ribosomes, resembling those of prokaryotes rather than the 80S ribosomes of the surrounding eukaryotic cytoplasm -- a clue, discussed later in the chapter, to the evolutionary origin of these two organelles. …