4.3
Prokaryotic Cells
Prokaryotes are small unicellular organisms that include the domains—Archaea and Bacteria. Bacteria include many common organisms, such as Salmonella…
Prokaryotes include archaea and bacteria, which are simple, unicellular organisms that lack membrane-bound organelles.
These cells are surrounded by a selectively permeable plasma membrane that encloses the cellular contents within a gel-like cytoplasm.
An additional protective layer called the cell wall surrounds the plasma membrane. In bacteria that possess a cell wall, it is made of peptidoglycan, a polymer of amino acids and sugars, and helps maintain cell shape and internal osmotic balance. In archaea, the cell wall does not contain peptidoglycan and is composed of other materials, such as proteins or polysaccharides.
Some bacteria also have a capsule, which is a polysaccharide layer that promotes adhesion to surfaces and provides protection.
Inside the cell, typically a single circular DNA molecule is located in a region called the nucleoid. This DNA is not enclosed within a membrane.
Many prokaryotes also contain smaller circular DNA molecules called plasmids, which replicate independently and can be transferred between cells. These plasmids can also carry genes that provide adaptive advantages, such as antibiotic resistance.
Although prokaryotes lack membrane-bound organelles, some species contain specialized internal structures. For example, cyanobacteria have carboxysomes and thylakoids that enable photosynthesis.
Prokaryotic cells have limited internal compartmentalization and can survive in diverse and sometimes extreme environments.
View the full transcript and gain access to JoVE Core videos
Q1: What are the main structural differences between prokaryotic and eukaryotic cells?
Prokaryotes lack a nucleus and membrane-bound organelles, with DNA floating freely in the nucleoid region. Eukaryotic cells compartmentalize these structures within membrane-bound compartments. Both cell types have plasma membranes, cytoplasm, and ribosomes, but prokaryotes are simpler and more compact, allowing them to survive in diverse environments.
Q2: What is the function of the cell wall in prokaryotes?
The prokaryotic cell wall, made of peptidoglycan in bacteria or other materials in archaea, physically protects the cell and maintains osmotic pressure in varying environments. This protective layer helps the cell retain its shape and internal balance, enabling survival in diverse conditions.
Q3: How do plasmids contribute to bacterial survival and adaptation?
Plasmids are small, circular DNA molecules that replicate independently and carry genes providing adaptive advantages, such as antibiotic resistance. These self-replicating elements can be transferred between prokaryotic cells, spreading beneficial traits throughout bacterial populations and enhancing survival in challenging environments.
Q4: What specialized structures do some prokaryotes use for photosynthesis?
Cyanobacteria contain infoldings of the plasma membrane called thylakoids and carboxysomes that enable photosynthesis. These specialized internal structures allow prokaryotes to perform complex functions despite lacking membrane-bound organelles, demonstrating that prokaryotic simplicity does not limit metabolic capability.
Q5: What role does the capsule play in prokaryotic cells?
The capsule is a polysaccharide layer covering the cell wall that promotes adhesion to surfaces and other cells while providing additional protection. This sticky outer layer helps prokaryotes attach to substrates and each other, enhancing their ability to colonize environments and resist external threats.
Q6: Where is genetic material located in prokaryotic cells?
Prokaryotic DNA is typically a single circular molecule located in the nucleoid region, an area not enclosed by a membrane. Unlike eukaryotes, prokaryotic genetic material floats freely within the cytoplasm alongside other cellular components, allowing rapid access for gene expression and replication.
Q7: Why can prokaryotes survive in extreme environments despite their simple structure?
Prokaryotes lack internal compartmentalization, allowing their cellular components to respond quickly to environmental changes. Their compact design, protective cell wall, and adaptive plasmids enable survival in extreme conditions like volcanic springs. This simplicity paradoxically provides flexibility and resilience in harsh habitats.