5.2 Prokaryotic Cell Structure

A prokaryotic cell has no membrane-bound nucleus. Its main chromosome occupies a nucleoid, a region of cytoplasm rather than an organelle enclosed by a nuclear envelope. The chromosome is usually represented in IB comparisons as one circular DNA molecule. Many bacteria also carry plasmids: smaller DNA circles that replicate independently and may contain accessory genes.

Plasmids are not automatically present and are not miniature nuclei. They can move between cells and spread traits such as antibiotic resistance. Loss of one plasmid may leave the cell viable in ordinary conditions, whereas loss of the main chromosome prevents normal survival and reproduction. The distinction is functional as well as geometric.

The cytoplasm contains enzymes, metabolites and 70S ribosomes. Ribosomes synthesize polypeptides and are not membrane-bound in any cell. Because transcription and translation are not separated by a nuclear envelope, bacterial ribosomes can begin translating an mRNA while RNA polymerase is still transcribing it.

The plasma membrane is a selectively permeable boundary and a surface for energy-conserving processes. Prokaryotes have no mitochondria, so aerobic electron transport and ATP synthesis occur at the plasma membrane. Outside it, a bacterial cell wall containing peptidoglycan resists osmotic expansion and helps maintain shape.

Some bacteria have an additional capsule or slime layer that aids adhesion, reduces drying or interferes with host defences. These extracellular coatings are distinct from the plasma membrane and peptidoglycan wall. A diagram question may omit them because they are not universal.

Pili are protein projections. Short pili contribute to attachment; a conjugation pilus can connect cells and facilitate DNA transfer. Prokaryotic flagella rotate and are structurally different from eukaryotic flagella, which bend through microtubule movement. Similar function therefore does not imply identical construction.

Prokaryotes possess cytoskeletal proteins related in structure or function to eukaryotic actin and tubulin. These proteins help maintain shape, organize cell-wall synthesis and position the division site. Their discovery corrected the older oversimplification that bacterial cytoplasm is an unstructured solution.

During binary fission, the chromosome is replicated, the copies separate as the cell elongates, and membrane plus wall grow inward to divide the cell. Binary fission is not mitosis: there is no nucleus, no mitotic spindle and usually no multiple linear chromosomes requiring metaphase alignment.

Bacteria are commonly smaller than eukaryotic cells, which increases surface-area-to-volume ratio and shortens diffusion distances. Small size can support rapid exchange and growth, but bacterial capabilities are not simple. Regulatory networks, membrane domains, cytoskeletal systems and communal biofilms organize complex behaviour without membrane-bound organelles.