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I found this among some notes from undergrad that I was cleaning up. It's an answer from a test I wrote back in 2nd-year microbiology in 2001, and I think quite relevant as a primer on some biological concepts.
This essay explains:
- Why there can exist a size difference between eukaryotes and prokaryotes
- The minimal compliment of organelles required in a eukaryote
- How prokaryotes carry out life without those organelles
- How the endosymbiotic theory of eukaryotic evolution relates
- The role of the cell wall in prokaryotes to water balance in the cell
- The difference in the roles of the cell membrane between the two groups
Eukaryotes vs. Prokaryotes
In a eukaryotic organism, the membrane-bound organelles that are its trademark effectively raise the surface area-to-volume ratio, the traditional limit to cell size. In addition, they provide a high level of segregation and organisation for chemical reactions within the cell, arguably increasing efficiency and definitely increasing possible size. If environments are controlled by vacuoles and other organelles, reactions between reagents that would be too rare in the cytoplasm to react can be much faster, simply because the effective volume is smaller. Prokaryotes lack this ability, so in order for reactions involving low-concentration metabolites to occur, the cell must remain fairly small.
Eukaryotes require size for their diverse collection of structures. At the very least, a eukaryotic cell contains a nucleus, a membrane transport system including the endoplasmic reticulum and golgi, mitochondria, and the ever-present (and often transitory) vacuoles and vesicles. This arsenal requires quite a lot of space. Without so much cellular furniture to deal with, a prokaryote can afford to be the "bachelor's suite of life". As a case study, a single mitochondrion is roughly the size of a prokaryotic cell, and eukaryotes can have dozens of these. Mitochondria are, in fact, postulated as symbiotic prokaryotes adapted to life inside larger eukaryotic cells.
Their lack of dining set, chesterfield, and a china cabinet does not mean prokaryotes are less equipped to deal with survival, any less than a bachelor's suite is less condusive to survival than a three-bedroom apartment; in fact, they gain distinct advantages from their simplicity. The tiny prokaryote can do everything a eukaryote can do, and usually much faster. Their cellular membrane performs the ATP synthesis duties of a mitochondrion. DNA is constantly replicating in the bacterial cytoplasm, with neither the elegance nor the encumbrance of formal mitosis. The environment is small enough that reactions can occur in the hydrophilic cytoplasm or hydrophobic plasma membrane with satisfactory frequency. Some eukaryotic organisms, such as Paramecium, require a contractile vacuole to maintain water balance, while a bacterium achieves this passively with a semirigid cell wall that holds in hydrostatic pressure., keeping the cell from bursting. The eukaryotic membrane is little more than a transport region and a shield compared to the versatility of a prokaryote's, which produces ATP and acts as a hydrophobic environment as mentioned as well as anchoring the cell wall and chromosome and performing all the duties of a eukaryotic membrane.
Prokaryotic organisms are smaller and, in many ways, simpler than eukaryotes. This difference, however, could be likened to the difference between the terms "pen" and "ink-utilising cylindrical cone-tipped writing implement": one is certainly bigger, but size isn't everything.
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