to Prokaryotic Cells
Prokaryotes are single-celled organisms, such as bacteria and archaea, that lack a membrane-bound nucleus and other specialized internal compartments. Most prokaryotes possess both a cell wall and a cell membrane. These structures are crucial for their survival, providing protection and regulating what enters and exits the cell, enabling them to thrive in diverse habitats.
The Prokaryotic Cell Wall
The prokaryotic cell wall serves multiple functions, primarily providing structural support and maintaining the cell’s specific shape. This rigid outer layer also protects the cell from osmotic lysis, which is the bursting of the cell when too much water enters. It offers defense against external physical stresses and some harmful substances. The composition of the cell wall differs between the two main groups of prokaryotes, bacteria and archaea.
In bacteria, the cell wall is primarily composed of peptidoglycan, a unique polymer not found in eukaryotic cells. Peptidoglycan forms a mesh-like layer made of sugar derivatives linked by short chains of amino acids, creating a strong, protective barrier. The thickness and organization of this peptidoglycan layer are key features used to classify bacteria into two major groups: Gram-positive and Gram-negative. These classifications are based on their differential staining properties in the Gram stain procedure.
Gram-positive bacteria possess a thick layer of peptidoglycan, which readily absorbs and retains the crystal violet stain used in the Gram staining process. This thick wall also often contains teichoic acids, which are polymers of glycerol or ribitol phosphate that contribute to the wall’s rigidity. The extensive peptidoglycan network provides robust mechanical strength and protection from external factors.
In contrast, Gram-negative bacteria have a much thinner peptidoglycan layer, which is sandwiched between an inner cell membrane and an outer membrane. This outer membrane is a distinct feature of Gram-negative bacteria, composed of lipopolysaccharides (LPS), phospholipids, and proteins. The presence of this outer membrane makes Gram-negative bacteria less susceptible to certain antibiotics and lysozyme, an enzyme that breaks down peptidoglycan. The thin peptidoglycan layer does not retain the crystal violet stain, appearing pink or red after counterstaining.
The Prokaryotic Cell Membrane
The prokaryotic cell membrane, also known as the plasma membrane, is a universal structure found in all prokaryotic cells, including both bacteria and archaea. This membrane is positioned just inside the cell wall, or as the outermost boundary in species lacking a cell wall. It is fundamentally a phospholipid bilayer, meaning it consists of two layers of lipid molecules with their hydrophilic (water-attracting) heads facing outwards and their hydrophobic (water-repelling) tails pointing inwards, forming a barrier. Various proteins are embedded within or associated with this lipid bilayer.
One of the main functions of the cell membrane is selective permeability, which means it controls what substances enter and exit the cell. It acts as a gatekeeper, allowing necessary nutrients to pass through while preventing the leakage of important internal molecules and blocking the entry of harmful substances. This selective transport is facilitated by specific transport proteins embedded in the membrane. Without this precise regulation, the internal environment of the cell could not be maintained.
Beyond its role in transport, the prokaryotic cell membrane is also the site of several metabolic processes. For many prokaryotes, it houses the electron transport chain, involved in cellular respiration and ATP synthesis. In photosynthetic prokaryotes, like cyanobacteria, the cell membrane or its extensions also house machinery for light-dependent reactions, converting light energy into chemical energy.
Important Distinctions and Variations
While most prokaryotes have a cell wall and membrane, their compositions vary significantly. Bacterial cell walls contain peptidoglycan, a unique feature absent in archaea or eukaryotic cells. Most bacterial membranes also lack sterols, like cholesterol, which are common in eukaryotic cell membranes.
Archaea, though prokaryotic, differ significantly from bacteria in cell wall and membrane compositions. Their cell walls never contain peptidoglycan; instead, they may be composed of pseudopeptidoglycan (pseudomurein), glycoproteins, or other protein-based structures.
The cell membranes of archaea also possess unique characteristics. Unlike bacteria and eukaryotes, which have fatty acids linked to glycerol via ester bonds, archaeal lipids feature branched hydrocarbon chains attached to glycerol by ether linkages. These ether linkages provide greater stability in extreme environments, allowing many archaea to thrive in conditions of high temperature or acidity. The branched nature of their lipids also contributes to the membrane’s structural integrity.
A notable exception among bacteria regarding the cell wall is the genus Mycoplasma. These bacteria are unique because they completely lack a cell wall. This absence makes them pleomorphic, meaning they can adopt various shapes, unlike most bacteria with rigid cell walls. Without a protective cell wall, Mycoplasma species are more susceptible to osmotic stress and rely on isotonic environments for survival. Their cell membranes often incorporate sterols from their host, a rare feature among bacteria, which helps stabilize their membrane in the absence of a cell wall.