Prokaryotic Cell : Structure Functions, Feature

A cell  is a structural and functional elementary unit of structure and life activity of all organisms. It has its own metabolism and is capable of self-reproduction. There are two types of cells:  Prokaryotes (pre-nuclear) are single-celled organisms: archaea, bacteria and blue-green algae. They do not have a formed nucleus and membrane organelles. They are more ancient and simpler in structure, and Eukaryotes (nuclear) – cells that have a nucleus: protozoa, fungi, plants, animals. Today we are going to discuss about prokaryotic Cell in detail. So lets Start,

Prokaryotes are single-celled organisms that are the oldest and most primitive life forms on Earth. As organized in the Three Domain System, prokaryotes include bacteria and archaea. Some prokaryotes, like cyanobacteria, are photosynthetic organisms and are capable of photosynthesis.

Many prokaryotes are extremophiles and can live and thrive in various types of extreme environments, including hydrothermal vents, hot springs, swamps, wetlands, and the guts of humans and animals (Helicobacter pylori). Prokaryotic bacteria can be found almost anywhere and are part of the human microbiota. They live on your skin, on your body, and on the everyday objects in your environment.

Bacteria (as representatives of the prokaryotes) are evolutionarily less developed organisms. The bacterial cell cytosol is surrounded by a membrane, cell wall, and mucus capsule. The absence of the nucleus helps to increase the rate of transcription and translation. binary rate of cell division is also increased: a colony of bacteria can double their numbers every 20 minutes.

What is a Prokaryotic Cell?

Prokaryotic cells are single celled living organisms, belonging to the Kingdom Monera. These cells are characterized by not having a cell nucleus, but rather having their genetic material dispersed in the cytoplasm, just gathered in an area called the nucleoid. Prokaryotic organisms are evolutionarily earlier than eukaryotes, that is, those with a nucleus. The simplest forms of life were and still are prokaryotes, such as bacteria and archaea.

Prokaryotes were the first life forms on earth, so it stands to reason that the prokaryotic cell was the first type of organized cell to exist. The complexity of life is difficult to trace back to its beginnings. Exactly how it was possible to pass from inanimate matter to the first forms of life proper, capable of nourishing, growing, and reproducing, is unknown.

Prokaryotic cells lack organelles found in eukaryotic cells, such as mitochondria, endoplasmic reticulums, and Golgi complexes. According to the Endosymbiotic Theory, eukaryotic organelles are thought to have evolved from prokaryotic cells that live in endosymbiotic relationships with each other. Like plant cells, bacteria have a cell wall. Some bacteria also have a layer of polysaccharide capsule that surrounds the cell wall.

In this layer that bacteria produce biofilms, a slimy substance that helps colonies of bacteria adhere to surfaces and each other for protection against antibiotics, chemicals, and other dangerous substances. Like plants and algae, some prokaryotes also have photosynthetic pigments. These light-absorbing pigments allow photosynthetic bacteria to obtain nutrition from light.

A prokaryotic cell is primarily made up of a plasma membrane, cell wall, cytoplasm, genetic material in the nucleoid, and the ribosome. They are unicellular and are much smaller compared to eukaryotic cells. They exist in different shapes, including spherical, rod, flat, coconut, spirochete, and some also have shapes, and they do not have a consistent shape.

Only some of them can move, swim, twist or turn with the help of a helically shaped membrane called flagella. The ways prokaryotes receive nutrients include synthesizing their own food by using energy from the light of the atmosphere, preparing their own food through the process of chemosynthesis, or depending on other substances for nutrition when they cannot synthesize their own food.

Prokaryotic Cell Structure

The prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma, and PPLO (Pleuropneumonia-Like Organisms). They are generally smaller and multiply more rapidly than eukaryotic cells. They may vary greatly in shape and size. The four basic shapes of bacteria are bacillus (rod-like), coccus (spherical), vibrio (comma-shaped), and spirillum (spiral). The organisation of the prokaryotic cell is fundamentally similar, even though prokaryotes exhibit a wide variety of shapes and functions.

All prokaryotes have a cell wall surrounding the cell membrane, except in Mycoplasma. The semifluid matrix filling the cell is the cytoplasm. There is no well-defined nucleus. The genetic material is naked, not enveloped by a nuclear membrane. In addition to the genomic DNA (the single chromosome/circular DNA), many bacteria have small circular DNA outside the genomic DNA. These smaller DNAs are called plasmids. The plasmid DNA confers certain unique phenotypic characters to such bacteria. One such characteristic is resistance to antibiotics.

Prokaryotic cells share a basic structure made up of:

Plasma membrane : Selective, permeable lipid barrier that distinguishes the inside of the cell from the outside. This membrane is also responsible for regulating the entry and exit of various substances found between the cytoplasm and the extracellular medium.

Cellular wall : A rigid and external barrier that provides support and toughness to the cell, although it hinders its growth. The cellular wall is a resistant layer, which in turn is rigid, since it is capable of withstanding the forces of growth of a living being. This is located outside the plasma membrane of the prokaryotic cells of bacteria and archaea. The cell wall is in charge of the internal protection of the cell, giving it, in turn, a certain rigidity.

Cytoplasm :  The wet interior of the cell, which is a kind of internal gel. It is a liquid that allows easy movement of the different structures. This is made up of: water, lipids, proteins, carbohydrates, and ions. This fluid is present in both eukaryotic and prokaryotic cells.

Nucleoid : A region of the cytoplasm in which the genetic material of the cell usually accumulates, and which serves as the nucleus, although it has a very irregular shape.

Ribosomes: In prokaryotes, ribosomes are associated with the plasma membrane of the cell. They are about 15 nm by 20 nm in size and are made of two subunits – 50S and 30S units which when present together form 70S prokaryotic ribosomes. Ribosomes are the site of protein synthesis. Several ribosomes may attach to a single mRNA and form a chain called polyribosomes or polysome. The ribosomes of a polysome translate the mRNA into proteins. Inclusion bodies: Reserve material in prokaryotic cells are stored in the cytoplasm in the form of inclusion bodies. These are not bound by any membrane system and lie free in the cytoplasm, e.g., phosphate granules, cyanophycean granules and glycogen granules. Gas vacuoles are found in blue green and purple and green photosynthetic bacteria.

The ribosomes in the prokaryotic cells possess 66% of messenger ribonucleic acid (rRNA) and are characterized subdivided into two: major subunit and minor subunit; the first with approximately 31 proteins, and the second with approximately 21 proteins.

Prokaryotic compartments : Segments of the cytoplasm that seem to be in charge of unique tasks in prokaryotic life, such as cytosomes, carboxysomes, magnetosomes, etc.

Additionally, certain prokaryotes may have:

Flagella : Cellular organs that allow the cell to move. Flagella are long, thin structures that are made up of a certain protein known as flagellin. These structures are responsible for the movement present on the surface of some bacteria. Bacterial flagella are structures where the filament maintains a rotation like a propeller, receiving impulses through a microscopic rotating motor.

Pilli : They are structures, also known as fimbriae, present on the surface of various bacteria, and that maintain the shape of hairs. These structures are responsible for transferring genetic information.

Periplasm. Or periplasmic space : a compartment that surrounds the cytoplasm of prokaryotic cells and that has a key role in energy metabolism (since prokaryotes lack mostly mitochondria).

Capsule or Glycocalyx : Depending on the type of prokaryote, they are external structures of the cell membrane that serve as a food deposit and defense against phagocytosis. The capsules are rigid and defined, while the glycocalyx is ​​diffuse as a mucous layer.

Mesosoma : Invaginations of the plasma membrane towards the cytoplasm are very frequent in prokaryotes and have been recognized as malformations, even though they were initially believed to have some type of function.

Plasmids Or circular and extrachromosomal (non-coding) DNA : molecules that replicate independently of the cell cycle, and that contain various genetic information to conserve, such as antibiotic resistance in the case of certain bacteria.

Prokaryotic Cell Characteristics / Feature of Prokaryotes.

The characteristics of a prokaryotic cell are given below, and compared with those of a eukaryotic cell.

Prokaryotic Cell Nutrition

Prokaryotic cells present an enormous diversity of nutrition methods, depending on the type of organism and the habitat in which it evolves. This simplicity has allowed its great diversification, which translates into extremely diverse metabolisms (the same does not happen with eukaryotes) and enormous diversity in terms of environment, nutrition, or even structure.

Autotroph : Prokaryotic cells can be autotrophic (they make their own food).

Heterotroph : Heterotrophic (they feed on foreign organic matter), both aerobic (require oxygen) and anaerobic (do not require oxygen), which results in various nutritional mechanisms :

Photosynthesis : Like plants, some prokaryotes can synthesize chemical energy from sunlight, both in the presence and in the absence of oxygen.

Chemosynthesis :  Similar to photosynthesis, cells undertake the oxidation of inorganic matter as a mechanism to obtain their energy and obtain their own organic matter to grow.

Saprophytic Nutrition : Based on the decomposition of organic matter left by other living things, either when they die or as remains of their own diet.

Symbiotic nutrition : Some prokaryotic Organisms obtain their organic matter to exist from other living beings, generating a benefit and therefore collaborating to exist.

Parasitic nutrition : The opposite of symbiotics: the organism is nourished by the organic matter of another, which is harmed in the process even if it does not directly kill it.

Finally, the reproduction of prokaryotic cells is also usually very varied, both asexual (mitosis) and parasexual (conjugation, transduction, and transformation of DNA for adaptive purposes).

Types of Prokaryotic cell

Prokaryotic cells can be of four types, according to their morphology:

Coconut : A typical morphological type of bacteria, they have a more or less spherical and uniform shape.

Bacillus : Rod-shaped, they include a vast array of free-living bacteria and other saprophytic organisms.

Vibrio : A genus of proteobacteria responsible for most infectious diseases in man and higher animals, especially those typical of the digestive tract, such as cholera.

Spirals : They are helical or spiral-shaped, are usually very small, and range from pathogenic bacteria to autotrophs.

Pleomorphic : That is, in a changing way, mainly referring to the archaea.

Rectangular : An also typical form of archaea like the Haloquadratum.

Functions of a Prokaryotic Cell

• Prokaryotic cells store their DNA in the nucleoid region to control cell activities.
• These cells reproduce through binary fission without mitosis or meiosis.
• Prokaryotes absorb nutrients from the environment and perform metabolism.
• Photosynthetic prokaryotes like cyanobacteria generate energy from sunlight.
• Their cell wall provides protection and maintains shape.
• Prokaryotes produce toxins and antibiotics to enhance their survival ability.
• Some prokaryotes improve soil fertility by performing nitrogen fixation.
• With the help of flagella, these cells perform movement or locomotion.
• They can also be harmful pathogens that cause diseases.
• Prokaryotic cells can acquire new traits by horizontal gene transfer.

Reproduction of Prokaryotic Cells

Lacking a cell nucleus, prokaryotic cells divide in two ways only:

Asexual : By binary fission or bipartition, a fast and simple way in which the cell grows to divide into two new ones of identical genetic material.

Binary fission
Most prokaryotes reproduce asexually through a process called binary fission. During binary fission, the single DNA molecule replicates and the original cell divides into two identical cells.

Steps of binary fission

1. Binary fission begins with DNA replication of the single DNA molecule.
2. Both copies of DNA adhere to the cell membrane.
3. Next, the cell membrane begins to grow between the two DNA molecules.
4. Once the bacterium nearly doubles its original size, the cell membrane begins to pinch inward.
5. Then a cell wall is formed between the two DNA molecules that divide the original cell into two identical daughter cells.
6. Although E.coli and other bacteria reproduce more commonly by binary fission, this mode of reproduction does not produce genetic variation within the body.

Parasexual : A DNA exchange method that allows obtaining genetic variability between prokaryotic cells, and that can be done in three ways:

1. Conjugation. When a prokaryotic cell donates another part of its genetic material and the recipient incorporates it into its own.

2. Transduction. When a virus or other microscopic mechanism carries DNA fragments from one prokaryotic cell to another, it “infect” it with new genetic material.

3. Transformation. When prokaryotic cells incorporate free genetic material (plasmids) that come from other degraded cells or from their own cytoplasm.

Importance of the Prokaryotic Cell

Prokaryotic cells are the first to have formed, that is, they were the first forms of life. They allow us today to imagine what life on our planet would have been like in its earliest appearances, before more complex forms of life emerged.

In fact, the most accepted theory proposes that there was a process of serial endosymbiosis, that is, the beneficial coexistence of some prokaryotes within others of a larger size. After a slow evolutionary process, the first eukaryotic cells emerged from this coexistence, endowed with cytoplasmic organelles.

Examples of Prokaryotes

1. Escherichia Coli Bacterium (E. coli)

It is a rod-shaped bacterium commonly found in the lower intestine of warm-blooded organisms. Most strains of E. coli are harmless, but some can cause food poisoning and are sometimes responsible for food recalls. The harmless E. coli can be beneficial in producing vitamin K2 and preventing the intestine from colonizing with pathogenic bacteria.

2. Streptococcus Bacterium
This prokaryote is responsible for strep throat. It is an infection of the back of the throat that includes the tonsils. Symptoms include fever, red tonsils, sore throat, and enlarged lymph nodes in the neck. Cell division in this bacterium occurs along a single axis and they grow in chains or pairs.

3. Streptomyces Soil Bacteria
More than 500 of these types of bacteria have been described. They are found predominantly in the soil and in decomposing vegetation, with most of the producing spores. They have a distinctive earthy odor that results from the production of a volatile metabolite, geosmin.

Archaea

A completely special group of living organisms, archaea has non-nucleaus. In size and structure, archaea cells are very similar to bacterial cells, but differ greatly in biochemical and molecular biological characteristics. For example, some archaea have membranes that are completely different from the membranes of all other organisms – they do not consist of phospholipids, but of simple ethers of polyisoprenoid alcohols. The cell wall of archaea consists of either pseudomurein, which resembles murein, or proteins, which is also not found in other organisms. Archaea, unlike other bacteria, never form spores

Initially, all archaea were thought to be extremophiles, meaning they live in extreme conditions. Archaea live in salt-saturated lakes, such as the Dead Sea. They also live in hot springs, where the temperature can exceed 100°C. However, archaea were later discovered in other habitats, including soil, oceans, swamps, and the human colon. Archaea are numerous among oceanic plankton. Archaea play an important role in the carbon and nitrogen cycles. None of the known archaea are parasites or pathogens, but they often exhibit symbiosis or commensalism. Methanogenic archaea live in the digestive tract of humans and ruminants, where they are numerous and participate in digestion processes. Methanogenic archaea are used in the production of biogas and the purification of sewage wastewater. Enzymes of extremophilic archaea that remain active at high temperatures and in contact with organic solvents find application in biotechnology.

Difference Between Prokaryotic and Eukaryotic Cell

Eukaryotic cells are distinguished from prokaryotes in that they have a defined nucleus in their cytoplasm, where all of the cell’s DNA is contained. This difference seems subtle but it justifies a gigantic change in reproduction and other vital processes that led to a higher level of cellular complexity, without which multicellular beings could not have been born.

1. The differences between prokaryotic and eukaryotic cells are not just the absence or presence of the cell nucleus where the DNA is contained. Other notable differences are:
2. The form of DNA. That in eukaryotes it is linear, while prokaryotes are circular.
3. The number of chromosomes. That in eukaryotes it is multiple and in prokaryotes one.
4. The size of the cell. That in eukaryotes it amounts to 10-100 µm, and in prokaryotes only between 0.2-2.0 µm.
5. The size of the genome. That in eukaryotes it covers between 2.2 and 150,000 million base pairs, and in prokaryotes only between 0.5 and 10.
6. The presence of organelles. Like the mitochondria, the nucleus, the lysosomes, the Golgi apparatus, etc. Present in eukaryotes and absent in prokaryotes.
7. The complexity of the cytoskeleton. Which is barely structural proteins in prokaryotic cells and complex (microtubules and microfilaments) in eukaryotes.

FAQs on Prokaryotic Cell

1. What are Prokaryotes?
Answer : They are unicellular microorganisms without a defined nucleus, which means that their genetic material is dispersed in the cytoplasm, but it is still gathered in an area called a nucleoid and they are contrary to eukaryotes that do have a defined nucleus.

2. Where do Prokaryotic animals live?
Answer : These cells lived on our planet more than 3 billion years ago, they grew in mounds in the shallowest parts of the ocean, they were concentrated in large numbers, being able to add enough oxygen through photosynthesis.

They created a more stable environment for animals that needed oxygen, but due to the variation in climatic conditions today they are almost non-existent, and current organisms are said to have derived from these cells.

3. What are the components of prokaryotes?
Answer : Prokaryotic cells are much smaller and less complex than eukaryotes, they lack endomembranes that are delimited by biological membranes, they have genetic material in the cytosol and they have an inner membrane system.

There are organisms such as preludes that surround the genetic material through an intracytoplasmic membrane and gemmate obscuriglobus giving it a double membrane.

It could be said that prokaryotes lack a cytoskeleton, thus supporting a complex metabolism, exclusive of certain taxa, including their versatility.

4. What are 2 examples of prokaryotic cells?
Answer : Examples of Prokaryotes:

• Escherichia Coli Bacterium (E. coli)
• Streptococcus Bacterium.
• Streptomyces Soil Bacteria.
• Archaea.

5. How do Prokaryotic cells reproduce?
Answer: Prokaryotic cells reproduce asexually through a process called binary fission. In this method, the cell replicates its DNA, and the cell membrane grows inward to divide the cytoplasm, forming two genetically identical daughter cells. This process is fast and efficient, allowing prokaryotes to multiply rapidly under favorable conditions.

6. Do Prokaryotic cells have organelles?
Answer: Prokaryotic cells do not have membrane-bound organelles like mitochondria or a nucleus. However, they do contain ribosomes, which are smaller than those found in eukaryotic cells and are essential for protein synthesis. Some prokaryotes may also have plasmids, which are small circular DNA molecules, and inclusion bodies for storing nutrients.

7. What is the difference between Bacteria and Archaea?
Answer: Both Bacteria and Archaea are prokaryotes, but they differ significantly in cell wall composition, genetic sequences, and metabolism. Bacteria often have peptidoglycan in their cell walls, while Archaea do not. Archaea are known to live in extreme environments like hot springs and salty lakes and have unique enzymes that help them survive in such conditions.

8. What is the function of the nucleoid in prokaryotic cells?
Answer: The nucleoid is the region in a prokaryotic cell where the DNA is concentrated. Unlike a nucleus in eukaryotes, the nucleoid is not enclosed by a membrane. It plays a central role in controlling cell activity, replication, and gene expression by housing the chromosomal DNA, which contains most of the genetic instructions needed for the cell’s functions.

9. Why are prokaryotes important in biotechnology and medicine?
Answer: Prokaryotes, especially bacteria, are widely used in biotechnology, genetic engineering, and medicine. They help in producing antibiotics, insulin, enzymes, and fermented products like yogurt. Their simple structure and rapid reproduction make them ideal tools for research, vaccine development, and bioremediation processes.