Cell Theory, prokaryote structure and physiology
Cell theory states that all living things are made of cells, that cells are the basic functional units of life, and that all cells arise from pre-existing cells. This foundational idea took shape in the mid-nineteenth century, driven by improvements in microscopy and the work of scientists such as Matthias Schleiden, Theodor Schwann, and Rudolf Virchow.
Early observations go back to Robert Hooke, who in 1665 described small “cells” in a slice of cork. Schleiden and Schwann’s work in the 1830s brought these observations together into a formal proposal: both plants and animals are built from cells. Soon after, Virchow added a key principle - new cells come from existing ones - rejecting earlier ideas of spontaneous generation.
Over time, cell theory expanded to include discoveries about DNA as hereditary material. In this modern view, cells don’t just provide structure; they also manage the storage and transmission of genetic information through cell division. These ideas reshaped biology and helped launch fields such as cell biology, molecular genetics, and biotechnology.
Classification and structure of prokaryotic cells
Within the prokaryotic domain, many familiar species are eubacteria, while archaea are often found in extreme environments (e.g., high salt or temperature).
Bacterial shapes are commonly categorized as bacilli (rod-shaped), spirilli (spiral), or cocci (spherical).
Bacteria lack a membrane-enclosed nucleus, so their genetic material sits in a region called the nucleoid. Because they do not form a mitotic apparatus, they use their cytoskeleton to help separate replicated DNA. Unlike eukaryotes, they also lack typical organelles such as the Golgi and mitochondria.
The bacterial cell wall is made of peptidoglycan, which differs from the cellulose in plants and the chitin in fungi. Many bacteria use flagella made of flagellin. These rotate like a rotor to propel the cell, powered by a proton or sodium gradient - a mechanism that differs from ATP-powered eukaryotic flagella.
Growth and physiology of prokaryotic cells
Bacteria reproduce through binary fission, an asexual process in which the DNA is duplicated and the copies segregate as the cell elongates. This division does not require spindle fibers.
This simple division can produce rapid, exponential growth. Growth continues until resources become scarce, after which the growth rate slows and eventually plateaus.
Genetic adaptability
- A striking feature of bacteria is their high genetic adaptability, seen clearly in the spread of antibiotic resistance.
Transposons (also present in eukaryotic cells), sometimes called “jumping genes,” are mobile DNA elements that can move within the genome. By relocating, they can increase genetic diversity and change gene expression. They move through cut-and-paste or copy-and-paste mechanisms and can strongly influence evolution and genome structure.
Genetic diversity also comes from mutations and from horizontal gene transfer methods:
- Transformation (uptake of external genetic material)
- Transduction (transfer via lysogenic bacteriophages)
- Conjugation (exchange of DNA through a sex pilus)
Variations of aerobic and anaerobic metabolism
- Bacterial metabolism varies widely:
- Some species are obligate aerobes, thriving only in oxygen
- Obligate anaerobes perish when exposed to oxygen
- Facultative anaerobes can survive without oxygen but grow more efficiently with it.
Interbacterial relationships and transportation
- Bacteria often form symbiotic relationships that may be parasitic (harming the host), mutualistic (benefiting both parties), or commensalistic (affecting only one partner).
- Bacteria navigate their environment using chemotaxis, sensing chemical gradients to move toward favorable conditions and away from harmful ones.
Genetics of prokaryotic cells
Prokaryotic cells often carry plasmids - small, double-stranded DNA molecules that replicate independently of the bacterial chromosome, or sometimes integrate into it. Plasmids can move between bacteria through conjugation. In conjugation, a bacterium carrying a specialized plasmid (F⁺) forms a pilus and transfers genetic material to an F⁻ recipient, sometimes including portions of chromosomal DNA.
Another horizontal gene transfer mechanism is transformation, in which free DNA fragments released by lysed bacteria are taken up by other cells and integrated into their genome. This can add new traits, including antibiotic resistance.
Within prokaryotes, gene regulation mainly works by controlling transcription. Cells use activators, inhibitors, and operons (for example, inducers that increase gene expression or repressors that decrease it). In bacteria, transcription and translation are coupled: ribosomes can begin translating an mRNA before transcription is finished. This setup supports regulatory strategies such as attenuation.
For example, when tryptophan levels are high, ribosomes move quickly along the newly made mRNA, which can trigger early termination of transcription of the trp operon. When tryptophan is scarce, ribosomes slow down, allowing the full mRNA to be synthesized.
Prokaryotes do not perform the extensive RNA processing seen in eukaryotes; there is no intron removal or complex post-transcriptional modifications. Instead, transcription-level regulation provides a direct and effective way to coordinate cellular functions and respond quickly to environmental change.