Prokaryotes and Eukaryotes
Prokaryotic cell
Prokaryotes include all the various types of bacteria,
it is divided into two categories
1)
Archaebacteria
2) Eubacteria
Archaebacteria thrive in
extreme environments that may have been common on early Earth, but are rare
today. For example, thermoacidophiles live in hot sulfur springs with
temperatures as high as 90°C and pH values as low as 2.
Eubacteria are a
diverse group of organisms that inhabit various environments, such as soil,
water, and other organisms. They include the common forms of present-day
bacteria (e.g., human pathogens residing in the gut).
Bacterial cells can be of various shapes like
spherical shape, rod shape, comma shape, spiral-shape etc with diameters of
around 1 to 10 picometers. Their DNA contents range from about 6 lakh to
50 lakh base pairs, which can encode about 5000 different types of
proteins.
Cyanobacteria are the largest and most complex
prokaryotes and are responsible for the evolution of photosynthesis in
bacteria.
E. coli
(Escherichia coli)
It is rod-shaped, about 1 picometer in diameter and
about 2 picometers long. E. coli, like other prokaryotes, has a cell wall
composed of polysaccharides and peptides. Towards the inner side of the cell
wall plasma membrane is present, which is a phospholipid bilayer and
associated proteins.
The plasma membrane provides the functional separation
between the inside of the cell and its external environment, while the cell
wall is porous and can be easily penetrated by various molecules. The DNA of E.
coli is a single circular molecule located in the nucleoid region, which is not
separated from the cytoplasm by a membrane, unlike the nucleus of
eukaryotes.The granular appearance of the cytoplasm is due to approximately
30,000 ribosomes, the sites of protein synthesis.
Eukaryotic cell
It is also surrounded by a plasma membrane and
contains ribosomes as in prokaryotes. However, eukaryotic cells are
much more complex and contain a nucleus containing the genetic material, a
variety of cell organelles, and cytoskeleton which help in maintaining
shape and organization.
The nucleus is the largest organelle
of eukaryotic cells, with a diameter of approximately 5 picometers.
The nucleus contains the genetic material of the cell, which in eukaryotes
is organized in linear form (rather than circular DNA as in
prokaryotes).
DNA transcription takes place in the nucleus which
leads to the formation of RNA, whereas the translation process takes place in
the cytoplasm which leads to the formation of protein from synthesized
RNA.
Along with the nucleus, eukaryotic cells also contain
a variety of membrane-bound (single or double membrane) cell organelles within
their cytoplasm. These organelles provide separate compartments in which
different metabolic activities take place. Eukaryotic cells are normally
much larger than prokaryotic cells in size, therefore having cell volume many
times greater than prokaryotes. Due to the formation of compartment by these
cell organelles the eukaryotic cells function more efficiently. Two of the cell
organelles, mitochondria and chloroplasts, play important roles in energy
metabolism.
Mitochondria, which are
found in all eukaryotic cells, are the sites of oxidative metabolism which
leads to the formation of ATP by breaking down organic molecules.
Chloroplasts are the
sites of photosynthesis in the cells of plants and green algae and are
therefore found only in them. Lysosomes and peroxisomes have a specific
metabolic function which is involved in the digestion of macromolecules
they contain their own digestive enzymes required for this purpose.
Vacuoles are
present in most plant cells. It performs various functions like digestion of
macromolecules and also stores both digested waste material as well as
nutrients. It's size increases with increase in the amount of storage.
Various types of proteins are synthesized in
eukaryotic cell which need to transported to it's specific location for which
Two cell organelles, the endoplasmic reticulum and the Golgi apparatus are
present. Their function is sorting and transport of proteins destined for
location which can be either outside of the cell or incorporated into the
plasma membrane or incorporation into lysosomes.
The endoplasmic reticulum is a
network of intracellular membranes, extending from the nuclear membrane
starting from the nucleus to throughout the cytoplasm. It is involved not only
in the processing and transport of proteins but also involved in lipid
synthesis. Proteins are transported from the endoplasmic reticulum to the Golgi
apparatus in small membrane vesicles. At the Golgi apparatus, they
undergo further processing and sorting before being transported to their final
destinations. Additionally, the Golgi apparatus is responsible for lipid
synthesis and, in plant cells, some polysaccharide synthesis for the cell wall.
Eukaryotic cells also contain another cellular-level
organization: the cytoskeleton. The cytoskeleton, which is made of protein
filaments, determines cell shape and provides the structural framework of the
cell and the cytoplasm. In addition, the cytoskeleton is also responsible for
cell movement (e.g., the contraction of muscle cells), intracellular transport
and providing organelle positioning, it also helps in the movements of
chromosomes during cell division.
