Cellular Structures of Microorganisms: Differences Between
Prokaryotes and Eukaryotes
Microorganisms can be broadly
classified into two categories based on their cellular structures: prokaryotes
and eukaryotes.
Overview
of Prokaryotic and Eukaryotic Cells
- Prokaryotes:
These are the simplest and most ancient forms of life, consisting of
unicellular organisms that lack a nucleus and other membrane-bound
organelles. Prokaryotes include eubacteria and archaeabacteria.
- Eukaryotes:
Eukaryotic cells are more complex, featuring a true nucleus enclosed by a
nuclear membrane, along with various membrane-bound organelles. Eukaryotic
microorganisms include fungi, protozoa, and algae, which can be
unicellular or multicellular.
Differences
Between Prokaryotes and Eukaryotes
- Cell Size
- Prokaryotes:
Generally smaller, ranging from 0.1 to 5 micrometers in diameter. The
small size allows for rapid nutrient uptake and distribution within the
cell.
- Eukaryotes:
Typically larger, ranging from 10 to 100 micrometers in diameter. The
larger size is associated with the presence of more complex internal
structures and compartmentalization.
- Cell Wall
- Prokaryotes:
Most prokaryotic cells have a rigid cell wall that provides structural
support and protection. In bacteria, the cell wall is primarily composed
of peptidoglycan (polymer of sugars and amino acids). Archaea have cell
walls made of pseudopeptidoglycan or other unique polymers, differing
from bacterial cell walls.
- Eukaryotes:
In eukaryotic microorganisms, the cell wall is present in some groups but
not in others. For example, fungi have cell walls made of chitin, while
algae have cell walls composed of cellulose. Protozoa typically lack a
cell wall, relying on other structures for shape and protection.
- Nucleus
- Prokaryotes:
Prokaryotic cells lack a true nucleus, their genetic material is located
in a region called the nucleoid, which is not enclosed by a membrane. The
DNA is circular.
- Eukaryotes:
Eukaryotic cells have a well-defined nucleus, surrounded by a membrane
called the nuclear envelope. The nucleus contains linear DNA molecules
associated with histone proteins, forming chromatin. The nucleus
functions as the control center of the cell, regulating gene expression
and cell division.
- Genetic Material
- Prokaryotes:
Prokaryotes have a single, circular chromosome, which contains all the
necessary genes for survival. In addition to the main chromosome,
prokaryotes often have small, circular DNA molecules called plasmids,
which can carry additional genes, such as those for antibiotic
resistance.
- Eukaryotes:
Eukaryotic cells have multiple, linear chromosomes located within the
nucleus. The number of chromosomes varies among species. Eukaryotes do
not typically carry plasmids, although some fungi and protozoa may have
extrachromosomal elements.
- Ribosomes
- Prokaryotes:
Prokaryotic ribosomes are smaller (70S) and consist of two subunits: 30S
and 50S. Ribosomes are scattered throughout the cytoplasm and are
responsible for protein synthesis.
- Eukaryotes: Eukaryotic ribosomes are larger (80S) and consist of two subunits: 40S and 60S. Eukaryotic ribosomes can be found in the cytoplasm (free ribosomes) or attached to the endoplasmic reticulum (bound ribosomes), playing a crucial role in protein synthesis
- Membrane-bound Organelles
- Prokaryotes:
Prokaryotic cells lack membrane-bound organelles. Their cellular
processes occur directly in the cytoplasm or are associated with the cell
membrane. For example, cell membrane is involved in energy production
through processes like respiration and photosynthesis.
- Eukaryotes:
Eukaryotic cells contain various membrane-bound organelles, each with
specialized functions:
- Nucleus:
Contains genetic material and regulates cellular activities.
- Mitochondria:
Powerhouse of the cell, responsible for energy production through
cellular respiration.
- Chloroplasts:
Found in photosynthetic eukaryotes like algae, responsible for
photosynthesis.
- Endoplasmic Reticulum (ER): Rough ER is involved in protein synthesis and
modification, while smooth ER is involved in lipid synthesis and
detoxification.
- Golgi Apparatus:
Modifies, sorts, and packages proteins and lipids for secretion or
delivery to other organelles.
- Lysosomes:
Contain digestive enzymes that break down macromolecules, cellular
debris, and foreign invaders.
- Vacuoles:
Storage and transport organelles, larger in plant cells and algae.
Eukaryotic cell (https://images.app.goo.gl/DQ1g55zqAorugUEt8)
- Cell Division
- Prokaryotes:
Prokaryotic cells reproduce asexually through binary fission, a simple
process where the cell divides into two identical daughter cells. The
process is rapid, allowing for quick population growth under favorable
conditions. Some bacteria
reproduce through budding, fragmentation, spore formation, etc also.
- Eukaryotes:
Eukaryotic cells undergo more complex processes of cell division. Mitosis
is used for somatic cell division, resulting in two genetically identical
daughter cells. Meiosis, on the other hand, is used in the production of
gametes (in sexually reproducing organisms), resulting in four
genetically diverse daughter cells.
- Flagella and Motility
- Prokaryotes:
Prokaryotic flagella are simpler in structure, composed of a protein
called flagellin, and rotate like a propeller to provide motility. The
arrangement and number of flagella in the cell can vary among different
prokaryotic species.
- Eukaryotes:
Eukaryotic flagella (and cilia) are more complex, consisting of a
"9+2" arrangement of microtubules. Eukaryotic flagella move in
a whip-like motion, and cilia beat in coordinated waves to provide
movement or move substances across the cell surface.
- Cellular Respiration
- Prokaryotes:
In prokaryotes, cellular respiration occurs in the cell membrane, where
the electron transport chain is located. Prokaryotes can carry out
aerobic or anaerobic respiration, depending on the availability of oxygen
and the organism's metabolic capabilities.
- Eukaryotes:
In eukaryotes, cellular respiration primarily occurs in the mitochondria,
where the electron transport chain is housed. Eukaryotic cells primarily
rely on aerobic respiration, but some can also undergo anaerobic
processes, such as fermentation, in the absence of oxygen.
- Genetic Recombination
- Prokaryotes:
Genetic recombination in prokaryotes occurs through processes like
conjugation (transfer of plasmids via direct cell-to-cell contact),
transformation (uptake of free DNA from the environment), and
transduction (transfer of DNA by viruses). These processes contribute to
genetic diversity and adaptability through horizontal gene transfer.
- Eukaryotes:
Eukaryotic cells undergo genetic recombination during sexual reproduction
through meiosis, where homologous chromosomes exchange genetic material
(crossing over). This process generates genetic diversity in offspring.
Evolutionary
Implications
The
structural differences between prokaryotic and eukaryotic cells reflect their
evolutionary histories. Prokaryotes are the earliest forms of life, and they have
simpler structures adapted to a wide range of environments. Eukaryotes evolved later and developed complex cellular organization, allowing for greater
specialization and the evolution of multicellular organisms.
The Endosymbiotic theory suggests that mitochondria and chlopoplasts in today's eukaryotic cells were once separate prokaryotic microbes. These organelles in eukaryotic cells is hypothesised to be originated from ancient prokaryotic cells that were engulfed by another cell, and instead of being digested or killed, the inner cell survived. Then both cells thrived and evolved together. This theory is supported by the fact that mitochondria and chloroplasts have their own DNA and resemble prokaryotic cells in structure and function.
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