4.2 Prokaryotic Cells

Prokaryotic Cell Structure and Function

Prokaryotic cells are the simplest and most ancient cell type on Earth. They lack a nucleus and membrane-bound organelles, yet they dominate nearly every habitat on the planet. Understanding how these cells are built helps you see why compartmentalization (or the lack of it) matters for how a cell operates.

Characteristics of Prokaryotic Organisms

Prokaryotes include all organisms in the domains Bacteria and Archaea. They share several defining features:

• No nucleus or membrane-bound organelles. Their genetic material sits in a region called the nucleoid, which is not enclosed by a membrane. Ribosomes (the molecular machines that build proteins) are the only "organelles" present.
• Mostly unicellular. While some prokaryotes form colonies or filaments, each cell functions independently.
• Cell walls in most species. Bacterial cell walls are made of peptidoglycan, a rigid mesh of sugars and amino acids that gives the cell its shape and protection. Archaeal cell walls differ in composition and may contain pseudopeptidoglycan or S-layer proteins instead.
• Small size. Most prokaryotic cells range from 0.1 to 5 μm in diameter, far smaller than typical eukaryotic cells (10–100 μm).
• Binary fission for reproduction. Rather than mitosis, prokaryotes copy their single circular chromosome and split into two identical daughter cells through binary fission.

Prokaryotic vs. Eukaryotic Cell Structure

This comparison comes up constantly on exams, so know it well.

The ribosome size difference (70S vs. 80S) is more than a trivia fact. Many antibiotics specifically target 70S ribosomes, which is why they can kill bacteria without harming your own cells.

Because prokaryotic cells lack internal compartments, all their chemical reactions happen in the cytoplasm or at the plasma membrane. Eukaryotic cells, by contrast, can separate different reactions into different organelles. Think of it this way: a prokaryotic cell is a studio apartment where everything happens in one room, while a eukaryotic cell is a house with specialized rooms.

Cell Size and Efficiency in Prokaryotes

Why are prokaryotic cells so small? It comes down to the surface area to volume ratio.

As a cell gets smaller, its surface area increases relative to its volume. That matters because nutrients enter and waste exits through the cell surface. A higher surface area to volume ratio means:

• Faster nutrient uptake and waste removal across the plasma membrane
• Quicker diffusion of molecules from one side of the cell to the other (no need for internal transport systems)
• Faster growth and reproduction, since the cell can fuel its metabolism more efficiently

The lack of membrane-bound organelles also plays a role here. Without internal membranes separating reactions, enzymes and substrates can interact more directly, which speeds up metabolic reactions. For a cell this small, compartmentalization isn't necessary for efficiency.

Prokaryotic Adaptations for Diverse Environments

Prokaryotes thrive in places that would kill most eukaryotes. These organisms called extremophiles have evolved specific adaptations:

• Thermophiles survive in high temperatures (above 60°C), such as hot springs and hydrothermal vents, by producing heat-stable enzymes that don't denature.
• Psychrophiles grow in near-freezing environments like polar ice and deep ocean floors, using cold-adapted enzymes and antifreeze proteins.
• Halophiles tolerate extreme salt concentrations in places like salt lakes by maintaining internal osmotic balance.

Beyond extremophiles, prokaryotes also show remarkable metabolic diversity:

• Chemolithotrophs extract energy from inorganic compounds like hydrogen sulfide or ammonia, allowing them to live in nutrient-poor environments where organic food sources are scarce.
• Phototrophs such as cyanobacteria capture light energy to drive photosynthesis, contributing significantly to oxygen production and primary productivity in aquatic ecosystems.

Prokaryotes can also form biofilms, which are structured communities of cells attached to surfaces and coated in a sticky matrix. Biofilms protect cells from environmental stress and antibiotics, and they allow cells to share nutrients and communicate chemically. The plaque on your teeth is a familiar example of a biofilm.

Antibiotic resistance is another critical adaptation. Through horizontal gene transfer, bacteria can pass resistance genes to other bacteria, even across different species. This is distinct from the vertical inheritance (parent to offspring) you see in eukaryotes, and it allows resistance to spread rapidly through a population.

Additional Prokaryotic Structures

A few more structural features are worth knowing:

• Gram-negative bacteria have an outer membrane in addition to their cell wall. This outer membrane contains lipopolysaccharide (LPS), which acts as a barrier against many antibiotics and contributes to the cell's structural integrity. Gram-positive bacteria lack this outer membrane but have a thicker peptidoglycan layer.
• Prokaryotes have a cytoskeleton made of protein filaments. It's simpler than the eukaryotic cytoskeleton, but it helps maintain cell shape and plays a role in cell division.
• Quorum sensing is a communication system where prokaryotes release and detect signaling molecules. When the concentration of these molecules reaches a threshold (indicating high population density), cells coordinate group behaviors like biofilm formation or toxin release.

Prokaryotic Diversity and Ecology

Domains and Morphology

Bacteria and Archaea are the two prokaryotic domains, and they differ in important ways despite their structural similarities.

• Bacteria are far more abundant and diverse. You'll find them in soil, water, air, and throughout the human body.
• Archaea were originally discovered in extreme environments, but they also live in moderate habitats like ocean water and soil. Their membrane lipids and cell wall chemistry are distinct from bacteria.

Prokaryotes come in several common shapes, and these shapes often reflect adaptations to their environment:

• Cocci (spherical)
• Bacilli (rod-shaped)
• Spirilla (spiral)
• Vibrios (comma-shaped)

Some species form characteristic groupings. Streptococci form chains, while staphylococci form grape-like clusters. These arrangements can help with identification in a lab setting.

Ecological Roles

Prokaryotes are not just background players in ecosystems. They drive several processes that other life depends on:

• Decomposition. Prokaryotic decomposers break down dead organic matter and recycle carbon, nitrogen, and phosphorus back into forms that other organisms can use.
• Nitrogen fixation. Certain bacteria (like Rhizobium) convert atmospheric nitrogen gas ($N_2$) into ammonia ($NH_3$), a form that plants can absorb. Without these bacteria, most ecosystems would be nitrogen-starved.
• Primary production. Cyanobacteria carry out photosynthesis in aquatic environments and are responsible for a significant portion of global oxygen production.

Symbiotic Relationships

Some of the most important prokaryotic roles involve close partnerships with other organisms:

• Rhizobia and legumes. Rhizobium bacteria colonize root nodules of legume plants (beans, peas, clover). The bacteria fix nitrogen for the plant, and the plant provides the bacteria with sugars. This mutualism is why farmers rotate crops with legumes to naturally replenish soil nitrogen.
• Gut microbiota. Trillions of bacteria live in animal digestive tracts, where they help break down food, synthesize vitamins (like vitamin K and certain B vitamins), and produce short-chain fatty acids that nourish intestinal cells. Your gut bacteria are so integral to your health that they're sometimes called a "forgotten organ."