Compare and contrast eukaryotic and prokaryotic cells
It is thought that the mitochondria of a eukaryotic cell were originally foreign
prokaryotic cells. This explains the similarities between the mesosomes of a
prokaryotic cell and the cristae of a mitochondrion- both sites of aerobic
respiration. However, prokaryotic cells can be compared with the whole
eukaryotic cell, not just with one of its organelles.
Eukaryotic cells are much larger than prokaryotic cells – typically with a
diameter of around 10-100m, compared to a prokaryote’s 0.2-2m. The
prokaryotic cell’s greater surface area to volume ratio allow it to have a much
simpler internal structure, as molecules can easily and quickly be transported to
all parts of the cell by diffusion. This is not possible in a eukaryotic cell; instead,
it requires more complex organelles to produce and transport the molecules.
While both types of cell have a surface membrane, only eukaryotic cells have
membrane-bound organelles. In particular (leading to the name ‘eukaryote’),
they have a nucleus to contain the multiple strands of linear DNA. Prokaryotic
cells also have DNA as their genetic material, but it is circular and unbound
within the cell.
Free ribosome’s are found in both types of cell and are used in protein
production. However, the ribosomes are of different sizes; eukaryotes have 805
ribosomes while prokaryotes have 705. In addition, eukaryotes have
membrane-bound ribosomes on the rough endoplasmic reticulum – a feature
absent in prokaryotes.
While a cell wall is found in only some eukaryotes, it is necessary for all
prokaryotes. Animal cells (eukaryotes) have no cell wall, while plant and fungal
cells (both eukaryotes) have walls made of cellulose and chitin respectively.
Prokaryotic cells walls are made of peptidoglycan, so it is clear that the presence
and type of cell wall is not a factor in differentiating between prokaryotic and
eukaryotic cells.
The type of cell wall also goes some way toward explaining why antibiotics kill
bacteria, but have no effect on animal cells. Some antibiotics work by making the
peptidoglycan wall more porous, so the bacterial cell will die but the animal cell
with no wall will survive.
Another feature only found in some eukaryotic cells is the presence of
chloroplasts, allowing photosynthesis. Some prokaryotes can also
photosynthesise, but they do this using a photosynthetic membrane instead of in
chloroplasts. In the same way that mitochondria are thought to have originated
from prokaryotes, chloroplasts have a similar structure to photosynthesising
prokaryotes.
When eukaryotic cells divide, they use the process of mitosis to produce two
identical cells. Prokaryotes, however, divide by binary fission. They can also
change their own DNA by exchanging genetic material with another cell in the
process of conjugation. For eukaryotic cell to create cells with different DNA,
they use meiosis to produce gametes.
Gametes are the sex cells used in sexual reproduction of eukaryotic organisms.
The majority of eukaryotic cells are part of multi-cellular organisms. This is why
it is necessary for them to have a cell wall.
Clearly the two types of cell are very different, but on a basic level they are more
similar- they have the same genetic material, produce proteins in the same way,
and both respire to release energy. Structurally, prokaryotic cells are more
similar to particular eukaryotic organelles (mitochondria and chloroplasts) than
the eukaryotic cell itself, but stripped down to the bare essentials for life, they
function in the same way.