Re: Please explain why eukaryote cells are bigger than prokaryote cells

Eubacteria are ~ 1-10 micrometers in diameter.  Eukaryotes are roughly 10 
times bigger. Why? The answer to the question is evolution.

According to the currently accepted theories of evolution, from the 
primordial soup we got the first living cells, who "soon" (millions of 
years) became prokaryotic bacteria (appeared at least 3.5 billion years 
ago). At that time, it is thought the earth had a reduced atmosphere (i.e. 
no -- or very little -- oxygen, atmospheric composition: CO, CO2, N2, H2O, 
H2, CH4, NH3) and relatively harsh conditions of living (strong UV 
radiation and volcanic activity, possibly meteoric activity). Bacteria at 
that time are thought to have used RNA for replication and had no internal 
division of specialized organelles. 

From this original bacteria two branches evolved: the eubacteria (today's 
common prokaryotes) and a common ancestor of archaebacteria (love extreme 
environments: thermophilic, methanogenic and halophilic bacteria) and 
eukaryotes (protists, fungi, plants and animals). 

About 3.4 billion years ago, photosyntesis (producing O2 by using solar 
energy) started. Oxygen became a major component of the atmosphere. 
However, to anaerobic bacteria, oxygen is poisonous. Bacteria had to adapt 
or die. Purple bacteria evolved a system of reversing the electron flow of 
molecules through their carbon fixing pathways and modifying their 
electron transport chains. Cyanobacteria (thought to have evolved from 
purple bacteria) converted carbon dioxide (CO2) and water (H2O) into 
glucose, releasing O2 as a pathway biproduct.

Given the changing nature of the environment and multiple living organisms 
competing for resources, natural selection favored organisms that could 
perform an increasing number of metabolic ativities. The small size and 
simple internal structure of the original prokaryotic cell impose a number 
of limits on the number and type of metabolic activities that can be 
performed. Size allows for bigger genomes (can encode more enzymes for 
more metabolic activity) and possibility of engulfing smaller organisms 
(digesting more material, not just small molecules).

However, metabolic requirements also impose upper limits on the size of a 
cell. Increased volume decreases the surface area to volume ratio. Since 
the transport of molecules across the plasma membrane stays constant, the 
volume fed by the same surface area is greater. Hence, the cytoplasm is 
poorer in absorbed molecules per unit of volume, even though the surface 
area has been increased. 

Evolution dealt with the size to metabolic activity problem in 3 different 
ways:
1) becoming multicellular, where different cell type specialize in 
different activities (filaments of some cyanobacteria).
2) aggregating into communities, where each species benefits from the 
others' specialities
3) compartmentalizing of different functions within a single cell. 

The last solution is the evolutionary path the eukaryotes took. According 
to the endosymbiotic theory, it is thought that the prokaryotic ancestor 
of today's eukaryotes started invaginating its plasma membrane and 
creating structures we know today as the Golgi, the endoplasmic reticulum, 
the nuclear envelope... It also engulfed smaller prokaryotes, the purple 
bacteria and cyanobacteria, which became the mitochondria and the 
chloroplasts, respectively. Supporting this evidence, is the mode of 
replication of mitochondria and chloroplasts (similar to binary fission), 
as well as RNA analogy to prokaryotic RNA.

The eukaryotic evolution allowed not only for more complex metabolic 
activity, but also for better methods of replication, required for 
handling an increased genome size (mitosis and meiosis to reproduce large 
genomes, DNA compacted into strands associated with histones etc.). The 
larger genome was required in order to synthetize all the enzymes needed 
for the extra metabolic activities to be performed.

Thus, to summarize, the prokaryotes are generally thought to be the 
original life organisms. Due to evolutionary pressure, bigger and more 
sophisticated bacteria developed, and then further evolved into today's 
eukaryotes. 

References: Campbell's Biology, '96.