Difference Between Prokaryotic and Eukaryotic Cells

Main Difference – Prokaryotic vs Eukaryotic Cells

All living organisms can be divided into prokaryotes and eukaryotes. Bacteria and archaea belong to prokaryotes. Eukaryotes are the organisms in kingdom protista, fungi, plantae and animalia. Prokaryotic cells don’t have possess nucleus, nuclear membranes and nucleoli. But eukaryotic cells consist of a true nucleus enclosed by two membranes. Thus, the key difference between prokaryotic and eukaryotic cells is that prokaryotic cells are lacking membrane-bound organelles including nucleus while eukaryotic cells consist of membrane-bound organelles including a nucleus.

This article looks at,

1. What are Prokaryotic Cells
      – Structure and Characteristics
2. What are Eukaryotic Cells
      – Structure and Characteristics
3. What is the difference between Prokaryotic and Eukaryotic Cells

What are Prokaryotic Cells

Unicellular organisms which don’t have membrane-bound organelles like nucleus and mitochondria are referred to as prokaryotic cells. These organisms are divided into two groups depending on the components of the cell wall: Bacteria and Archaea. In prokaryotes, the cell membrane encloses water-soluble proteins, DNA and metabolites in the cytoplasm. They do not possess separate compartments, but microcompartments act as primitive organelles which are created by protein shells. 

Most prokaryotes are 0.2 to 2 µm in size. Four types of shapes can be identified in bacteria: spherical-shaped (cocci), rod-shaped (bacilli), spiral-shaped (spirochaete) and comma-shaped (vibrio). Cyanobacteria also form large colonies and myxobacteria containing multicellular stages in the life cycle. Bacteria consist of a primitive cytoskeleton and a cell wall made up of peptidoglycan: a polymer of linked carbohydrates and small proteins. Cell wall provides protection to the cell, maintains the shape and prevents dehydration. Some bacteria possess an outermost layer called capsule which is sticky, helping the cell to attach to surfaces. Flagella, a wipe-like structure help to move by acting as a rotor motor. On the other hand, fimbriae, a numerous hair-like structure also help the attachment.

Some bacteria consist of a glycocalyx which covers the cell membrane surroundings. Bacterial cytoplasm is a gel-like substance which dissolves a variety of organic molecules together with cellular components. Small 70S ribosomes are present for the protein synthesis. Genomic DNA is found in a region called nucleoid in the cytoplasm. Bacteria consist of a single circular chromosome. Some DNA pieces are found in the cytoplasm as circular plasmids. The structure of a prokaryotic cell is shown in figure 1.

Figure 1: Prokaryotic Cell Structure

Both bacteria and archaea exhibit asexual reproduction via binary fission. Bacterial gene transfer occurs in three methods: transduction mediated by bacteriophages, conjugation mediated by plasmids, and natural transformation. This is called horizontal gene transfer. Rod-like structures called pili allow the genetic transfer. Archaeal gene transfer occurs through cytoplasmic bridges.

Since prokaryotes consist of great diversity, they obtain energy from inorganic compounds like hydrogen sulfide in addition to the photosynthesis and organic compounds. They can also be alive in harsh conditions such as Antarctica snow surfaces, hot springs and undersea hydrothermal vents. Archaea are extremophiles, living in extremes of pH, temperature and radiation. Eukaryotes are thought to be evolved from prokaryotes.

What are Eukaryotic Cells

Eukaryotes are unicellular or multicellular organisms, which have membrane-enclosed organelles such as specially nucleus, mitochondria, golgi apparatus and chloroplasts in plants. Multicellular eukaryotes contain specialized tissues made by different types of cells. Eukaryotes can be identified under four kingdoms: Kingdom Protista, Kingdom Plantae, Kingdom Fungi, and Kingdom Animalia.

A eukaryotic cell is larger in size (10 to 100 µm) compared to prokaryotes. In eukaryotes, various cell types such as animal cells, plant cells and fungal cells can be identified. Eukaryotic cytoskeleton is composed of microfilaments, microtubules and intermediate filaments. It plays a vital role in cellular organization and its shape. Plants and fungi possess a cell wall made up of cellulose, hemicellulose, pectin and chitin respectively.

Eukaryotic cells are composed of a variety of membrane-bound organelles. The nucleus is enclosed by two membranes called nuclear envelop. Nuclear membrane forms the endoplasmic reticulum (ER) which is involved in protein maturation and transportation. Ribosomes are large, 80S in size and they are bound to the ER. Ribosome-bound ER is referred to as rough ER. Vesicles are present for the transformation of various molecules within the cell such as golgi bodies, lysosomes and peroxizomes. Mitochondria is also surrounded by two phospholipid bilayers. They covert sugar into ATPs to utilize as energy. Plant cells contain chloroplasts for the photosynthesis. The structure of a typical plant cell is shown in figure 2.  

Figure 2: Plant cell structure

Usually, eukaryotes are composed of more than one chromosomes in the nucleus. These chromosomes are linear and most of the times they exist in multiple copies called homologous. Eukaryotes reproduce either asexually through mitosis or sexually through meiosis followed by the fusion of gametes.

Difference Between Prokaryotic And Eukaryotic Cells

Definition

Prokaryotic Cell: Prokaryotic cells don’t possess nucleus and membrane-bound organelles.

Eukaryotic Cell: Eukaryotic cells possess membrane-bound organelles including the nucleus.

Size of the cell

Prokaryotic Cell: These cells are normally 0.2 to 2 µm in diameter.

Eukaryotic Cell: These cells are normally 10 to 100 µm in diameter.

Nucleus

Prokaryotic Cell: Prokaryotic cells have no true nucleus, no nuclear membranes or nucleoli.

Eukaryotic Cell: Eukaryotic cells consist of a true nucleus with nuclear membranes and nucleoli.

DNA

Prokaryotic Cell: Prokaryotic cells consist of single, circular DNA molecule in the nucleoid, They don’t have histones or exons.

Eukaryotic Cell: Eukaryotic cells consist of multiple, linear chromosomes in the nucleus. They contain Histones, and exons.

Membrane-bound Organelles

Prokaryotic Cell: Prokaryotic cells do not have membrane-bound organelles.

Eukaryotic Cell: Membrane-bound organelles such as mitochondria, chloroplast, ER and vesicles are present.

Flagella

Prokaryotic Cell: Flagella are made up of two proteins.

Eukaryotic Cell: Some cells without cell wall contain flagella.

Glycocalyx

Prokaryotic Cell: Glycocalyx acts as a capsule.

Eukaryotic Cell: Some eukaryotic cells that don’t have cell walls possess a glycocalyx.

Cell Wall

Prokaryotic Cell: Prokaryotic cells are mostly made up of peptidoglycans. They are chemically complex.

Eukaryotic Cell: Eukaryotic cells are made up of cellulose, chitin and pectin. Prokaryotic cells are chemically simple.

Plasma Membrane

Prokaryotic Cell: Carbohydrates and sterols are not found in the plasma membrane.

Eukaryotic Cell: Carbohydrates and sterols serves as receptors on the plasma membrane.

Cytoplasm

Prokaryotic Cell: Prokaryotic cells are primitive cytoskeleton with no cytoplasmic streaming.

Eukaryotic Cell: Eukaryotic cells have complex cytoskeleton with cytoplasmic streaming.

Ribosomes

Prokaryotic Cell: Prokaryotic cells are small in size, 70S.

Eukaryotic Cell: Eukaryotic cells are large in size, 80S. 70S ribosomes are present in organelles like mitochondria and chloroplast.

Cell Division

Prokaryotic Cell: Cell division occurs through binary fission.

Eukaryotic Cell: Cell division takes place through mitosis.

Sexual Reproduction

Prokaryotic Cell: No sexual reproduction, horizontal gene transfer and recombination can be observed.

Eukaryotic Cell: Sexual reproduction happens through meiosis.

Examples

Prokaryotic Cell: Bacteria and archaea are examples

Eukaryotic Cell: Protista, fungi, plants and animals are examples.

Conclusion

Prokaryotic cells transport their metabolites through the cytoplasm, but eukaryotic cells consist of different kinds of vesicles to transport different metabolites. Protein synthesis in eukaryotic cells occurs in 80S ribosomes attached to the ER. Synthesised polynucleotide chains enter the ER. Protein folding and transportation into various parts of the cell are maintained by the ER. Therefore, the key difference between prokaryotic and eukaryotic cells stems from the presence or absence of the nucleus and the membrane-bound organelles. 

Reference:
1. “Prokaryote”. Wikipedia, the free encyclopedia, 2017. Accessed 22 Feb 2017
2. “Prokaryotic cells”. KHANACEDAMY, 2017. Accessed 22 Feb 2017
3. “Eukaryote”. Wikipedia, the free encyclopedia, 2017. Accessed 22 Feb 2017
4. “Eukaryotic cells”. Learn Science at Scitable, Nature Education, 2017. Accessed 22 Feb 2017

Image Courtesy:
1. “Average prokaryote cell- en.svg”. By Mariana Ruiz Villarreal, LadyofHats – Own work (Public Domain) via Commons Wikimedia
2. “Plant cell structure-en.svg”. By LadyofHats – Self-made using Adobe Illustrator  (Public Domain) via Commons Wikimedia