Prokaryotic Cells: Classification and Structure

Prokaryotic cells are the simplest form of cellular life — they lack a nucleus and any membrane-bound organelles. Their DNA floats freely in the cytoplasm, typically organized into a single circular chromosome. All bacteria are prokaryotes. This page covers how prokaryotes are classified and what each part of a prokaryotic cell does.

Key Takeaways

• Prokaryotic cells lack a nucleus and membrane-bound organelles; DNA sits in the cytoplasm, usually as a single circular chromosome.

• Bacteria and Archaea are both prokaryotic domains; Archaea favor extreme environments and have eukaryote-like membrane/gene-expression machinery, while Bacteria are more widespread.

• Bacteria are classified by shape (cocci, bacilli, spirilli) and by oxygen requirement (obligate aerobe, obligate anaerobe, facultative anaerobe, aerotolerant anaerobe).

• Gram-positive bacteria have a thick peptidoglycan wall and teichoic acids; Gram-negative bacteria have a thin peptidoglycan wall, an outer membrane, a periplasmic space, and LPS.

• Flagella (filament, hook, basal body) drive chemotaxis, powered by a proton gradient.

• Prokaryotic ribosomes (70S) differ in size from eukaryotic ribosomes (80S) — the basis for selective antibiotic targeting.

What Is a Prokaryotic Cell?

The defining feature of a prokaryotic cell is the absence of a nucleus: genetic material sits directly in the cytoplasm rather than inside a membrane-bound compartment. Prokaryotic cells also lack membrane-bound organelles — everything happens within the cytoplasm or at the cell membrane itself. Despite this structural simplicity, prokaryotes are highly efficient and remarkably diverse. (Eukaryotic cells, which do have a nucleus and membrane-bound organelles, are covered on their own dedicated page.)

The Three Domains of Life: Bacteria, Archaea, and Eukarya

All life is classified into three domains: Bacteria, Archaea, and Eukarya. Bacteria and Archaea are both prokaryotic domains — Eukarya is the only domain made of eukaryotic cells.

Bacteria vs. Archaea

Bacteria and Archaea share certain structural similarities: both lack a nucleus, and both reproduce through binary fission. But they differ in important ways:

• Archaea tend to live in extreme environments — hot springs, salt lakes, and deep-sea vents. Their cell membrane composition and gene-expression machinery more closely resemble eukaryotes than bacteria.

• Bacteria are far more widespread, including species that are harmless, beneficial, or pathogenic to humans.

The Human Microbiome

The relationship between bacteria and the human body is complex. While some bacteria are clearly pathogenic, many others play essential roles in digestion, vitamin production, and immune system regulation. The human microbiome — the trillions of microbes living in and on the human body — is mostly made up of bacterial species. Not all bacteria are harmful; in many cases, they’re vital to human health.

Classifying Bacteria

Scientists and pathologists classify bacteria using a shared vocabulary based on shape and oxygen requirement.

By Shape: Cocci, Bacilli, and Spirilli

There are three major bacterial shapes:

• Cocci — spherical bacteria

• Bacilli — rod-shaped bacteria

• Spirilli — spiral-shaped bacteria

By Oxygen Requirement

Bacteria are also classified by how they use oxygen:

MCAT Callout — Obligate vs. Aerotolerant Anaerobes: Obligate anaerobes and aerotolerant anaerobes both rely entirely on anaerobic metabolism — but obligate anaerobes cannot survive when oxygen is present, while aerotolerant anaerobes can survive in oxygen-containing environments without being harmed by it.

The Bacterial Cell Envelope

Starting from the outside, the cell envelope includes the cell wall and the plasma membrane, and in many bacteria, an additional outer capsule layer for extra protection.

The Cell Wall and Peptidoglycan

The cell wall provides structural support and helps the cell maintain its shape. In bacteria, it’s primarily made of peptidoglycan — a polymer of sugar chains cross-linked by short peptides, forming a rigid but flexible mesh. Together, the cell wall and plasma membrane form the bacterial envelope, which provides protection and structure while mediating exchange between the cell and its environment.

Gram-Positive vs. Gram-Negative Bacteria

There are two major categories of bacteria based on differences in the cell wall, distinguished using a technique called the Gram stain.

In Gram-positive bacteria, the thick peptidoglycan wall contains teichoic acids — including lipoteichoic acid (LTA) — which the immune system recognizes and which can trigger an immune response.

In Gram-negative bacteria, the peptidoglycan layer is much thinner, but these bacteria have an additional outer membrane outside the cell wall. Between the inner (plasma) membrane and the outer membrane is the periplasmic space, which contains enzymes and other proteins involved in nutrient processing and defense. The outer membrane also contains lipopolysaccharides (LPS) — large molecules that contribute to the structural integrity of the membrane and are recognized by the immune system. LPS is often what triggers the inflammatory response during Gram-negative bacterial infections.

Bacterial Flagella and Chemotaxis

Flagella are long, whip-like structures that many bacteria use for movement. Bacteria don’t move randomly — they sense chemicals in their environment and move toward or away from them, a behavior known as chemotaxis.

Each bacterial flagellum is made up of three major parts:

1. Filament — the long, helical tail, composed of a protein called flagellin. The filament connects to the basal body through a structure called the hook.

2. Hook — connects the filament to the basal body.

3. Basal body — acts like a tiny motor. It’s embedded in the cell membrane and rotates to spin the flagellum, propelling the bacterium forward. This rotation is powered by a flow of protons across the membrane, allowing the bacterium to swim toward nutrients or away from toxins.

The specific structure of the flagellum varies slightly between Gram-positive and Gram-negative bacteria. Archaea have their own distinct flagella variant.

Inside the Prokaryotic Cell

The Nucleoid and Plasmids

In prokaryotes, genetic material sits in a region called the nucleoid. Unlike eukaryotic cells, this DNA isn’t enclosed by a nuclear membrane — it’s usually organized as a single circular chromosome containing all the essential genes needed for basic cell function.

Many bacteria also carry plasmids — small, circular pieces of DNA that replicate independently of the main chromosome. Plasmids often carry genes that provide a survival advantage, such as antibiotic resistance, and they can be transferred between bacteria. (The mechanisms bacteria use to transfer plasmids and other genetic material between each other are covered in detail on the Genetics and Growth of Prokaryotic Cells page.)

Ribosomes: 70S vs. 80S

Even though prokaryotes lack membrane-bound organelles, they still synthesize proteins using ribosomes. Prokaryotic ribosomes are structurally different from eukaryotic ribosomes: they consist of two subunits — a 30S and a 50S — which together form a 70S ribosome. Eukaryotic ribosomes, by contrast, are made of 40S and 60S subunits, forming an 80S ribosome. This size difference matters clinically: it allows certain antibiotics to target bacterial ribosomes selectively, without affecting human ribosomes.

The Prokaryotic Cytoskeleton

Prokaryotes don’t have a complex cytoskeleton like eukaryotic cells, but they do have a more primitive version. It’s less developed, but it still helps with structural organization and cell division.

Why Prokaryotic Cell Structure Matters for the MCAT

Prokaryotic classification and structure connects directly to several other heavily tested MCAT topics:

• Antibiotic mechanisms frequently target structures covered here — cell wall synthesis (peptidoglycan) and the 70S ribosome are classic drug targets precisely because they’re absent or different in human cells.

• Gram-positive vs. Gram-negative distinction is a recurring passage theme, since LPS and teichoic acids trigger immune and inflammatory responses through different mechanisms.

• Structure-function reasoning, not memorization — knowing why the outer membrane and periplasmic space matter for Gram-negative bacteria (or why flagellar rotation requires a proton gradient) matters more than memorizing isolated terms.

Common MCAT Mistakes

• Confusing obligate anaerobes with aerotolerant anaerobes. Both rely entirely on anaerobic metabolism, but obligate anaerobes are harmed by oxygen and aerotolerant anaerobes simply tolerate it without using it.

• Assuming Gram-negative bacteria have a thicker cell wall because they have an extra membrane. It’s the opposite — Gram-negative bacteria have a thin peptidoglycan layer; the outer membrane and periplasmic space are separate structures outside that thin wall.

• Mixing up LPS and teichoic acids. LPS is the immune-activating molecule in the Gram-negative outer membrane; teichoic acids (including LTA) are the immune-activating molecules embedded in the Gram-positive peptidoglycan wall.

• Treating the 70S/80S ribosome distinction as trivia. It’s the structural basis for why certain antibiotics selectively target bacterial ribosomes without harming human (80S) ribosomes — a frequent bridge to pharmacology-flavored passages.

MCAT-Style Concept Check

Question: A bacterium is found to survive equally well in oxygen-rich and oxygen-free environments, but its metabolism is entirely anaerobic even when oxygen is present. How should this organism be classified, and what distinguishes it from an obligate anaerobe?

• A) Obligate aerobe — it can survive in oxygen, so it must require oxygen for metabolism

• B) Facultative anaerobe — it switches between aerobic and anaerobic metabolism depending on conditions

• C) Aerotolerant anaerobe — it survives in oxygen without being harmed by it, but never uses oxygen for metabolism, unlike an obligate anaerobe, which cannot survive when oxygen is present

• D) Obligate anaerobe — it relies entirely on anaerobic pathways and is unaffected by oxygen exposure

Answer: C

Explanation: The organism's anaerobic-only metabolism rules out choices A and B, since it never uses oxygen even when available. That leaves obligate anaerobe and aerotolerant anaerobe, both of which rely entirely on anaerobic pathways — the distinguishing factor is survival in oxygen. Obligate anaerobes cannot survive when oxygen is present, while aerotolerant anaerobes tolerate oxygen without being harmed by it. Since this organism survives equally well in both conditions, it's an aerotolerant anaerobe, making D incorrect.

FAQ

What’s the difference between a prokaryotic cell and a eukaryotic cell?

Prokaryotic cells lack a nucleus and membrane-bound organelles — their DNA sits directly in the cytoplasm, usually as a single circular chromosome. Eukaryotic cells enclose their DNA in a membrane-bound nucleus and contain membrane-bound organelles.

What’s the difference between Gram-positive and Gram-negative bacteria?

Gram-positive bacteria have a thick peptidoglycan cell wall containing teichoic acids and no outer membrane. Gram-negative bacteria have a thin peptidoglycan layer, plus an outer membrane, a periplasmic space, and lipopolysaccharide (LPS) — the molecule that often triggers inflammation during infection.

Why do prokaryotic ribosomes matter for antibiotics?

Prokaryotic ribosomes are 70S (made of 30S and 50S subunits), while eukaryotic ribosomes are 80S (made of 40S and 60S subunits). Because these structures differ, certain antibiotics can selectively target bacterial 70S ribosomes without affecting human 80S ribosomes.

What powers bacterial flagellar movement?

The basal body of a bacterial flagellum acts as a motor, rotating the flagellum to propel the cell. That rotation is powered by a flow of protons across the cell membrane, not directly by ATP.