Prokaryotic cells, which include Bacteria and Archaea, lack a membrane-bound nucleus and other internal compartments. Many complex functions relegated to organelles in eukaryotic cells must be carried out elsewhere. The cell membrane, also known as the plasma membrane, serves as the defining outer boundary of the cell’s cytoplasm and is the primary location for these activities.
Structurally, it is a fluid phospholipid bilayer, a sheet approximately 6 to 8 nanometers thick, composed of two layers of lipid molecules with hydrophobic tails facing inward and hydrophilic heads facing the aqueous environment. Situated just inside the rigid cell wall found in most prokaryotes, the membrane acts as the final physical barrier between the cell’s interior and the external world. A defining characteristic of most bacterial membranes is the absence of sterols, such as cholesterol, which are common in eukaryotic membranes and help regulate fluidity.
Selective Boundary and Nutrient Transport
The prokaryotic cell membrane serves as a selectively permeable barrier, controlling the movement of substances into and out of the cell. This permeability is accomplished by the phospholipid bilayer, which naturally restricts the passage of charged ions and large, hydrophilic molecules. The bilayer maintains necessary concentration gradients, ensuring the internal cellular environment remains stable, a state referred to as homeostasis.
Because the cell requires nutrients like sugars, amino acids, and various ions that cannot freely diffuse, the membrane is densely populated with specific transport proteins. Passive transport, such as facilitated diffusion, utilizes channel and carrier proteins to move molecules down their concentration gradients without expending cellular energy. However, the cell often needs to accumulate nutrients from a scarce external environment, requiring active transport systems.
Active transport uses metabolic energy to pump materials into the cell against a concentration gradient, often mediated by ATP-binding cassette (ABC) transporters. Another specialized mechanism is group translocation, which chemically modifies a substance during its transport across the membrane. For example, the phosphotransferase system phosphorylates glucose as it enters the cell, trapping the sugar inside and ensuring its immediate entry into metabolism.
Generating Cellular Energy
The prokaryotic cell membrane is the exclusive site for generating the cell’s primary energy currency, adenosine triphosphate (ATP), a function performed by mitochondria in complex cells. The membrane hosts the entire machinery of the electron transport chain (ETC) and ATP synthase. During cellular respiration, electrons are passed through membrane-embedded protein complexes.
The energy released by these transfers is used to actively pump protons (hydrogen ions) from the cytoplasm across the membrane. This establishes a high concentration of protons outside the cell, creating the proton motive force (PMF), an electrochemical gradient that stores energy.
The final step involves the ATP synthase enzyme. Protons flow back into the cytoplasm down their concentration gradient through the ATP synthase channel. This controlled influx drives the enzyme’s rotation, forcing a phosphate group onto adenosine diphosphate (ADP) to synthesize ATP. In photosynthetic prokaryotes, such as cyanobacteria, the membrane also contains pigments necessary to capture light energy and establish the PMF.
Anchoring the Cell Wall and Assisting Division
The cell membrane provides structural support and coordinates the mechanical process of cell duplication. It serves as an anchoring point for components that construct and maintain the cell wall, which lies immediately outside the membrane. For instance, in Gram-positive bacteria, lipoteichoic acids link the thick peptidoglycan layer of the cell wall directly to the underlying membrane lipids.
During cell replication, known as binary fission, the membrane actively divides the cell. After the single, circular chromosome is replicated, the two resulting DNA copies attach to separate points on the inner face of the membrane. The growth and elongation of the cell membrane between these attachment points physically separates the two chromosomes to opposite ends of the cell.
The final stage of division is orchestrated by the Z-ring, a structure composed of the protein FtsZ, a homolog of eukaryotic tubulin. FtsZ assembles into a ring-like filament network beneath the cytoplasmic membrane at the cell’s mid-point. Accessory proteins tether the Z-ring to the inner membrane surface. The Z-ring then constricts, pulling the membrane inward to form a septum and facilitating the synthesis of a new cell wall that ultimately pinches the mother cell into two identical daughter cells.
Environmental Sensing and Signaling
The prokaryotic cell membrane acts as the cell’s interface with the external environment, housing sophisticated receptors that enable the organism to sense and respond to stimuli. This function is accomplished through transmembrane proteins that detect chemical signals, temperature shifts, or light. A primary example of this sensory role is bacterial chemotaxis, the process by which a cell moves toward attractants or away from repellents.
Chemotaxis relies on Methyl-accepting Chemotaxis Proteins (MCPs), which are transmembrane chemoreceptors often clustered at the cell poles. When the external domain of an MCP binds a chemical signal, it causes a conformational change transmitted across the membrane to the internal domain. This shift regulates the activity of an associated protein complex, including the histidine protein kinase CheA.
The resulting signaling cascade involves the phosphorylation of the response regulator protein CheY, which ultimately controls the direction of the cell’s flagellar motor. By continuously sensing the concentration of external chemicals and adjusting flagellar rotation, these membrane-embedded receptors allow the prokaryote to execute a directed movement that maximizes its chances of survival.