Diversity of representatives and morphological classification
The superkingdom of prokaryotes includes a wide variety of microorganisms. Traditional main representatives include typical bacteria, as well as blue-green algae, which in modern nomenclature are called cyanobacteria. In addition, this group includes specific forms such as mycoplasmas, chlamydia, and actinomycetes. All these organisms, despite their tiny size and apparent simplicity, demonstrate a surprising diversity of forms.
Bacterial classification is largely based on morphological features: the shape of an individual cell and the pattern of their mutual arrangement after division. Three basic groups are distinguished:
- Rod-shaped bacteria. These are microorganisms elongated in length. If they are located as single rods, they are called bacilli. If the cells do not separate after division and form long threads, chains of rods are formed — streptobacilli.
- Convoluted and spiral bacteria. Cells of this group have characteristic curves. For example, vibrios feature a curved shape resembling a comma. Helicobacter also has a curved structure, and spirochetes are fully spiral-shaped forms.
Spherical bacteria (Cocci)
Spherical bacteria — cocci — deserve special attention. Depending on the plane of division and how the cells remain connected to each other, several morphological types are distinguished:
- Diplococci: spherical cells located in pairs. Pneumococci are a typical example. A separate variant is gonococci — these are also diplococci, but their cells have a specific bean-like shape.
- Streptococci: cocci lined up in long chains.
- Tetrads: cells grouped in fours.
- Staphylococci: clusters of spherical cells that visually resemble bunches of grapes.
- Sarcinae: geometrically regular clusters forming peculiar cell «packets».
External structures of the prokaryotic cell
The structure of a bacterial cell is strictly ordered and includes a number of envelopes that protect the internal environment and ensure interaction with the outside world.
Outwardly, the cell may be covered with a capsule — a special outer mucous layer performing protective functions. Beneath the capsule lies the cell wall, which gives the bacterium its constant shape (whether rod, coccus, or spiral) and provides reliable mechanical protection. Directly beneath the cell wall is the plasma membrane, which serves as a boundary separating and limiting the cytoplasm from the external environment.
In addition to envelopes, specialized structures may be located on the surface of prokaryotes. The flagellum acts as a locomotory organ, allowing the bacterium to move. Pili (also known as fimbriae) are fine villi on the surface; they are necessary for the cell to attach to a substrate or to carry out the process of conjugation.
Internal structure (Cytoplasm)
The internal space of the prokaryotic cell is filled with cytoplasm — a fluid internal environment in which all basic life processes take place and structural elements are located.
The cytoplasm contains ribosomes — non-membranous organelles responsible for the crucial process of protein synthesis. Also, various inclusions accumulate inside the cell, representing nutrient reserves necessary for survival under unfavorable conditions.
A special role is played by the mesosome — a specific invagination of the plasma membrane into the cell interior. It is believed that mesosomes take part in cell division processes and the energy metabolism of the bacterium. In some specialized groups, such as photosynthetic bacteria, photosynthetic membranes are present in the cytoplasm.
Finally, the most important feature of prokaryotes is their genetic apparatus. They lack a defined nucleus; instead, genetic information is represented by a nucleoid — a circular DNA molecule located freely right in the cytoplasm.
Briefly: answers to topical questions
What is the chemical structure of the cell wall in gram-positive and gram-negative bacteria?
The chemical structure of the bacterial cell wall is based on peptidoglycan (murein), but differs significantly between gram-positive and gram-negative species. Peptidoglycan consists of long polysaccharide chains cross-linked by short peptide bridges.
| Type of bacteria | Cell wall features |
|---|---|
| Gram-positive | Thick cell wall, consists almost exclusively of a robust peptidoglycan layer |
| Gram-negative | The peptidoglycan layer is thin. There is an additional outer membrane consisting of phospholipids, lipoproteins, and lipopolysaccharides. The outer membrane is connected to the peptidoglycan via lipoproteins |
How exactly does the process of bacterial conjugation involving pili occur?
The process of bacterial conjugation is a unidirectional direct transfer of genetic material between cells during their direct physical contact. The mechanism of DNA transfer involving pili includes the following features:
- Sex pili (F-pili) — formed by «male» donor cells carrying transmissible plasmids (with the F-factor).
- A small number of such conjugation pili (usually 1–3) are located on a single cell.
- The interaction between donor cells and recipient cells concludes with the mechanism of genetic material transfer through the formation of a cytoplasmic bridge.
The forms of spherical bacteria are easy to visualize: staphylococci (clusters), streptococci (chains), sarcinae (packets), and gonococci are bean-shaped diplococci.
Class and exam questions
What is the difference between diplococci and streptococci?
Diplococci are located in pairs (e.g., pneumococci), and streptococci form long chains of spherical cells after division.
How is the genetic apparatus organized in prokaryotes?
The genetic apparatus of prokaryotes is not surrounded by a nuclear envelope and is represented by a nucleoid — a circular DNA located directly in the cytoplasm.
Why do bacteria need pili (fimbriae)?
Pili are surface villi that the bacterial cell uses to attach to various surfaces or for the process of conjugation.
What is a mesosome and what is its purpose?
A mesosome is an invagination of the plasma membrane into the cytoplasm. It participates in the energy metabolism of the cell and its division processes.
What else to study on this topic
- Features of the conjugation process using pili
- Mechanisms of mesosome participation in energy metabolism
- Structure and function of the photosynthetic membrane in cyanobacteria
- Distinctive characteristics of mycoplasmas, chlamydia, and actinomycetes
Master the entire topic in the "Biology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on «Prokaryotes» and 3,793 other pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 129 video lessons on the subject
Other topics in the section "The Cell — The Elementary Unit of Life"
Free video lessons: BiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.