Difference Between Prokaryotic Cells and Eukaryotic Cells

Introduction

All living organisms are composed of Prokaryotic and Eukaryotic cells. Microbiologists Roger Stanier and C. B. Van Niel distinguished these two cells in 1962.  The reason this classification came to be was that the blue-green algae could not be grouped with plants. Blue-green algae is a type of bacteria (modern-day cyanobacteria).

Prokaryotic Cells vs Eukaryotic Cells

Eukaryotic cells have true nuclei and other membrane-bound organelles (subunits within a cell with specific functions), whereas prokaryotic cells do not have a nucleus or most of the other membrane-bound organelles. Prokaryotes are single-celled organisms (only one prokaryotic cell is present); however, eukaryotes may be unicellular or multicellular (one or more eukaryotic cells are present).

Difference Between Prokaryotic Cells And Eukaryotic Cells In Tabular Form

Parameters Of Comparison	Prokaryotic Cells	Eukaryotic Cells
Biological Evolution	Prokaryotic cells evolved before eukaryotic cells.	Eukaryotic cells emerged from the Archaea domain of prokaryotic cells.
Cell type	Unicellular	Multicellular
Size	Prokaryotic cells are 0.2 – 2 mm in diameter.	Eukaryotic cells are 10 – 100 mm in diameter.
Nucleus	The nucleus is not present. Instead, they have nucleoids (an area within the cytoplasm).	All eukaryotic cells have a nucleus.
Reproduction	Asexual	Maybe asexual or sexual based on the species.
Mitochondria, endoplasmic reticulum, and lysosomes	None of them is present in prokaryotic cells.	All of them can be found in a eukaryotic cell.
Ribosomes	Small spherical-shaped ribosomes are present.	Large linear-shaped ribosomes are present.
Domains/lineage	Bacteria and Archaea	Animals, symbiomycotan fungi, land plants, and red, brown, and green algae.

What Are Prokaryotic Cells?

Prokaryotic cells are found in unicellular organisms. Previously, it was thought that prokaryotic cells lacked a cytoskeleton; however, later studies showed that their cytoplasm (semi-fluid substance outside the nuclear membrane and inside the cellular membrane) had a cytoskeleton (thread-like filaments). The prokaryotic cell structure consists of a cell membrane (regulates how substances move in and out of the cell), cell wall (a cell’s shape depends on this), cytoplasm, ribosomes (produces protein), and nucleoid. The nucleoid contains the prokaryote’s single DNA molecule.

Types Of Prokaryotic Cells

The prokaryotic cells are classified into two main domains: bacteria and archaea. However, prokaryotic cells with a nucleus are categorized as eukaryota.

Bacteria

Bacteria are present everywhere and were the first life forms to appear on Earth. Most bacteria are harmless, and some are beneficial; however, several species of bacteria are harmful, as they can cause diseases like cholera, tuberculosis, syphilis, bubonic plague, and so on. Bacteria are helpful in the fixation of nitrogen – a chemical process by which nitrogen is converted into nitrogenous compounds (essential for agriculture and fertilizer manufacturing).

Typically, Bacteria are 0.5 – 5 micrometers in length. However, some bacteria can grow up to 2 cm, and the smallest bacteria is 0.3 mm in size. The bacteria has four types of shapes. Spherical-shaped bacteria are called cocci, bacteria with cylindrical shapes are known as bacilli, spirilla are spiral-shaped bacteria, and comma-shaped bacteria are called Vibrio.

The bacterial cell structure consists of a cell membrane that encloses the cytoplasm. However, the cytoplasm lacks several components like the nucleus, mitochondria, and chloroplasts that are commonly found in eukaryotic cells. Moreover, bacterial cells have cell walls made of peptidoglycan, unlike the cell walls of plants and fungi, which are made of cellulose and chitin. Bacteria cannot live without the cell wall; therefore, to eliminate them the anti-biotic penicillin is used – it interferes with the production of peptidoglycan, which results in the damage of bacteria’s cell walls.

Bacteria are classified as Gram-positive bacteria and Gram-negative bacteria based on their cell walls. Though both are made of peptidoglycan, the former’s cell wall is thick and has teichoic acids, while the latter’s cell wall is thin and has a lipid membrane. In many bacteria, an S-layer (part of the cell envelope) consisting of monomolecular layers of protein is present. This layer acts as a diffusion barrier that provides physical and chemical protection to the cell.

Flagella and fimbriae are the protein filaments found in bacterial cells. The former is essential for motility (the ability to move independently), and the latter helps the cells attach themselves to other cells or surfaces. Bacteria reproduce using binary fission (asexual reproduction); the parent cell divides into two parts that become separate entities resembling the original. That is, two clone daughters are ‘born’ as a result of cell division.

Phototrophs are a type of bacteria that derive energy from light. Chemotrophs produce energy by breaking down chemical compounds. They are further classified as lithotrophs and organotrophs. Bacteria that derive energy by breaking down inorganic compounds are referred to as lithotrophs, whereas those whose source of energy is organic compounds are called organotrophs. A species of bacteria known as predatory bacteria prey on other microorganisms (who knew even bacteria could be predators?).

Archaea

Initially classified as bacteria, the archaea were identified as a separate domain of prokaryotes. Their cells are similar in shape to bacterial cells; however, some species have flat, square cells. Archaea can be found in soil, marshland, and especially, oceans. Most archaea are commensals (they reap benefits from other organisms) or mutualists (a give-and-take relationship). Many believe that the eukaryotic cells evolved from a group that split off from the archaea cells.

The cell structure of archaea is similar to that of gram-positive bacteria but their cell walls lack peptidoglycan; instead, they are made of pseudo peptidoglycan that resembles it. The cell membrane of archaea is different from that of bacteria and eukaryotes; it is made of phospholipids.

The cells of archaea reproduce asexually through binary or multiple fission. Some cell types have thick cell walls that make the organisms resistant to osmosis shock. That is, the thick cell wall prevents the cell activities from being disrupted if the salt concentration in the water changes. Extremophile bacteria – those that can resist extreme heat, acidity, or salinity are excellent sources of beneficial enzymes (proteins that act as biological catalysts). They aid in producing low-lactose milk and whey.

What Are Eukaryotic Cells?

All animals, plants, and fungi have eukaryotic cells. Eukaryotic cells are defined as cells with a nucleus; this is the distinguishing feature that makes them distinct from prokaryotic cells. The nucleus stores the genetic information (DNA) of the organism in the form of chromosomes (long, thin DNA fibers). The chromosomes are coated in packaging proteins (the most significant of which are the histones). These packaging proteins along with the chaperone proteins (proteins that aid the packaging proteins to fold and wrap around the DNA) bind to the DNA molecules and compact them to help it seamlessly carry out its functions.

A double-walled membrane known as the nuclear envelope surrounds the nucleus present in eukaryotic cells. Approximately, a thousand nuclear pores (allows material/substance to move in and out) are found in a nuclear envelope. The endoplasmic reticulum is an extension of the nuclear membrane that facilitates protein transport. All eukaryotic cells (except one eukaryotic species) have mitochondria, commonly referred to as the powerhouse. It chemically breaks down sugar or fat to produce and store energy.

Plastids are present in plants and many types of algae. Perhaps, the most crucial plastid is the chloroplast which aids in photosynthesis; the other plastids help to store the food produced. All plant cells have a cell wall that surrounds the cell membrane; however, cell walls are absent in animal cells. The cell wall determines the cell’s shape and regulates what enters and exits the cell. Eukaryotic cells have flagella or cilia that aid in movement. These cells may reproduce either asexually or sexually.

Reproduction In Eukaryotic Cells

The asexual reproduction process is known as mitosis. It results in the division of the parent cell into two genetically identical daughter cells. During mitosis, the duplicated chromosomes are pulled to the opposite ends of the cell by the microtubular spindle (thread-like fibers). Thus, two identical cells are born. In animal cells, when mitosis takes place, the nuclear envelope breaks open before the chromosomes duplicate. However, in fungi cells, the nucleus remains intact.

Phases Of Mitosis

Preprophase, prophase, prometaphase, metaphase, anaphase, and telophase are the phases of mitosis.

1. Preprophase: Preprophase is exclusive to plant cells and precedes prophase. In this stage, the nucleus moves to the center of the cell. After this migration, mitosis begins.
2. Prophase: The chromatin present in the nucleus form into distinct chromosomes and each chromosome has two chromatids. The microtubular spindle forms during this stage.
3. Prometaphase: In animal cells, the nuclear membrane breaks in this stage, and the microtubules enter the space within. However, in fungi, the microtubules enter the intact nuclear membrane. Next, the microtubules attach themselves to the chromosomes, and the mitotic spindle is formed. At this point, the molecular motor is activated, which provides the necessary force to separate the chromosomes.
4. Metaphase: At this stage, the chromosomes are arranged along the metaphase plate (midline of the cell) to ensure equal distribution.
5. Anaphase: The chromatids of a chromosome are divided and two identical daughter chromosomes are created.
6. Telophase: The cells elongate, and if the nuclear envelope had broken down (as in animal cells), a new nuclear envelope is formed around each cell using the parent cell’s broken one. Now, the daughter chromosomes relax and mitosis ends.

The sexual reproduction of eukaryotic cells takes place through meiosis. In this process, two rounds of cell divisions take place. In the end, four daughter cells are born with half the amount of chromosomes that were present in the parent cell.

Phases Of Meiosis

Meiosis I and meiosis II are the phases of Meiosis; each phase has sub-divisions similar to phases of mitosis.

Meiosis I

Meiosis I separates the homologous chromosomes (a homologous chromosome is a combination of one paternal chromosome and one maternal chromosome) and produce two haploid cells (each cell has a single set of the chromosome). It is also known as the reductional division. Prophase I, Metaphase I, Anaphase I, and Telophase I are the stages of meiosis I.

Meiosis II

In meiosis II, four haploid daughter cells are produced from the two cells that were born due to meiosis I. It is referred to as an equational division. Prophase II, Metaphase II, Anaphase II, and Telophase II are the stages of meiosis II.

Main Difference Between Prokaryotic Cells And Eukaryotic Cells (In Points)

• Eukaryotic cells are a minority compared to prokaryotic cells; however, as they are significantly larger, the global biomass (mass of living organisms in a given area, in this case – the world) of eukaryotes nearly equals that of prokaryotes.
• Plant cells (eukaryotic) have chloroplasts that aid in photosynthesis, whereas photosynthesis takes place in the cell membrane of prokaryotic cells.
• Prokaryotic cells have nucleoids, whereas eukaryotic cells have a nucleus.
• The cell wall of prokaryotic cells is made up of peptidoglycan or pseudo peptidoglycan, whereas the cell wall present in land plants (eukaryotes) are made up of cellulose, hemicellulose, and pectin.
• In prokaryotic cells, reproduction is done through binary fission (asexual reproduction), whereas cell reproduction takes place through nuclear division (asexual or sexual reproduction).
• Prokaryotic cells are smaller than eukaryotic cells; therefore, the time taken to generate a prokaryotic cell is shorter than the time required to generate a eukaryotic cell.
• Prokaryotic cells inhabited Earth before the evolution of eukaryotic cells. Some propose that the eukaryotic cells evolved from the cells of the Asgard species of archaea.

Conclusion

The distinction between prokaryotic and eukaryotic cells is hailed as the most significant distinction among organisms. The first cells are believed to have originated 3.8 billion years ago, and cellular evolution still baffles humans. The evolutionary origin of prokaryotes (bacteria and archaea) is still unclear and people still debate whether eukaryotic cells evolved from the cells of archaea, specifically Asgard archaea.

While some argue that increasing evidence points to eukaryotic cells having their roots in archaea cells, others argue that such claims are similar to saying birds are dinosaurs. The former’s claims hold weight because the histones found in eukaryotic cells are also present in several archaea groups; this points to homology (similarities due to shared ancestry). However, others posit that claiming eukaryotes are archaea is like saying birds are dinosaurs because they share a common ancestor. Whatever the true origin story may be, Cell Biology (the study of cells) is remarkable and exciting!