Key difference - prokaryotic vs. eukaryotic cells
All living organisms can be divided into prokaryotes and eukaryotes. Bacteria and archaea belong to the prokaryotes. Eukaryotes are the organisms in the Kingdom of Protista, Fungi, Plantae, and Animalia. Prokaryotic cells have no nucleus, no nuclear membranes or nucleoli. But eukaryotic cells consist of a real nucleus surrounded by two membranes. Thus, the main difference between prokaryotic and eukaryotic cells is that prokaryotic cells lack membrane-bound organelles including a nucleus, while eukaryotic cells are composed of membrane-bound organelles including a nucleus.
This article is about,
1. What are prokaryotic cells - structure and properties 2. What are eukaryotic cells - structure and properties 3. What is the difference between prokaryotic and eukaryotic cells?
What are prokaryotic cells?
Protozoa that do not have membrane-bound organelles such as the nucleus and mitochondria are called 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 have separate compartments, but rather microcompartments function as primitive organelles that are formed by protein shells.
Most prokaryotes are 0.2 to 2 µm in size. Four types of shapes can be identified in bacteria: spherical (cocci), rod-shaped (bacilli), spiral-shaped (spirochaetes) and comma-shaped (vibrio). Cyanobacteria also form large colonies and myxobacteria, which contain multicellular stages in the life cycle. Bacteria consist of a primitive cytoskeleton and a cell wall made of peptidoglycan: a polymer made of linked carbohydrates and small proteins. The cell wall protects the cell, maintains its shape and prevents dehydration. Some bacteria have an outermost layer called a capsule, which is sticky and helps the cell to attach itself to surfaces. Flagella , a smudge-like structure, help move by acting as a rotor motor. On the other hand, fimbriae, a numerous hair-like structure, also help in attachment.
Some bacteria are made up of a glycocalyx that covers the area around the cell membrane. Bacterial cytoplasm is a gel-like substance that dissolves a large number of organic molecules along with cellular components. Small 70S ribosomes are available for protein synthesis. Genomic DNA is found in a region in the cytoplasm called a nucleoid . Bacteria are made up of a single circular chromosome. Some pieces of DNA 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 show asexual reproduction by binary fission. Bacterial gene transfer occurs in three ways: bacteriophage-mediated transduction, plasmid-mediated conjugation, and natural transformation. This is known as horizontal gene transfer. Rod-like structures, so-called pili, enable genetic transfer. Archaeal gene transfer occurs via cytoplasmic bridges.
Since prokaryotes are very diverse, they gain energy not only from photosynthesis and organic compounds but also from inorganic compounds such as hydrogen sulfide. They can also live in harsh conditions such as snow surfaces in Antarctica, hot springs, and underwater hydrothermal vents. Archaea are extremophiles who live in extreme pH, temperature and radiation values. It is believed that eukaryotes evolved from prokaryotes.
What are eukaryotic cells?
Eukaryotes are unicellular or multicellular organisms that have membrane-covered organelles such as, in particular, the cell nucleus, mitochondria, Golgi apparatus and chloroplasts in plants. Multicellular eukaryotes contain specialized tissues made up of different cell types. Eukaryotes can be identified under four kingdoms: Kingdom of Protista, Kingdom of Plantae, Kingdom of Fungi, and Kingdom of Animalia.
A eukaryotic cell is larger (10 to 100 µm) than prokaryotes. Different cell types such as animal cells, plant cells and fungal cells can be identified in eukaryotes. The eukaryotic cytoskeleton consists of microfilaments, microtubules and intermediate filaments. It plays a vital role in cellular organization and its shape. Plants and fungi have a cell wall that is made up of cellulose, hemicellulose, pectin and chitin.
Eukaryotic cells consist of a large number of membrane-bound organelles. The core is surrounded by two membranes called the nuclear envelope . The nuclear membrane forms the endoplasmic reticulum (ER), which is involved in the maturation and transport of proteins. Ribosomes are large, 80S in size, and attached to the ER. Ribosome-bound ER is called a rough ER. Vesicles are present for the conversion of various molecules within the cell such as Golgi bodies, lysosomes, and peroxizomes. Mitochondria are also surrounded by two phospholipid bilayers. They convert sugars into ATPs for use as energy. Plant cells contain chloroplasts for photosynthesis. The structure of a typical plant cell is shown in Figure 2.
Figure 2: Plant cell structure
Usually eukaryotes consist of more than one chromosome in the nucleus. These chromosomes are linear and usually exist in several copies, which are referred to as homologous. Eukaryotes reproduce either asexually by mitosis or sexually by meiosis, followed by the fusion of gametes.
Difference between prokaryotic and eukaryotic cells
definition
Prokaryotic cell: Prokaryotic cells have no nucleus and no membrane-bound organelles.
Eukaryotic cells: Eukaryotic cells have membrane-bound organelles including the cell nucleus.
Size of the cell
Prokaryotic cell: These cells are usually 0.2 to 2 µm in diameter.
Eukaryotic cell: These cells are usually 10 to 100 µm in diameter.
core
Prokaryotic cell: Prokaryotic cells have no real nucleus, no nuclear membranes or nucleoli.
Eukaryotic cell: Eukaryotic cells consist of a real nucleus with nuclear membranes and nucleoli.
DNA
Prokaryotic cell: Prokaryotic cells consist of a single, circular DNA molecule in the nucleoid. They don't have histones or exons.
Eukaryotic cells: Eukaryotic cells consist of several linear chromosomes in the cell nucleus. They contain histones and exons.
Membrane-bound organelles
Prokaryotic cell: Prokaryotic cells have no membrane-bound organelles.
Eukaryotic cell: membrane-bound organelles such as mitochondria, chloroplasts, ER and vesicles are present.
Scourges
Prokaryotic cell: Flagella consist of two proteins.
Eukaryotic cell: some cells without a cell wall contain flagella.
Glycocalyx
Prokaryotic cell: Glycocalyx acts like a capsule.
Eukaryotic cell: Some eukaryotic cells that have no cell walls have a glycocalyx.
Cell wall
Prokaryotic cells: Prokaryotic cells consist mainly of peptidoglycans. They are chemically complex.
Eukaryotic cells: Eukaryotic cells consist 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 serve as receptors on the plasma membrane.
cytoplasm
Prokaryotic cells: Prokaryotic cells are primitive cytoskeletons with no cytoplasmic flow.
Eukaryotic cells: Eukaryotic cells have a complex cytoskeleton with cytoplasmic flow.
Ribosomes
Prokaryotic Cell: Prokaryotic cells are small, 70S.
Eukaryotic cells: Eukaryotic cells are large, 80S. 70S ribosomes are present in organelles such as mitochondria and chloroplasts.
Cell division
Prokaryotic cell: cells divide by binary splitting.
Eukaryotic cell: Cell division occurs through mitosis.
Sexual reproduction
Prokaryotic cell: no sexual reproduction, horizontal gene transfer and recombination can be observed.
Eukaryotic cell: Sexual reproduction occurs through meiosis.
Examples
Prokaryotic cell: bacteria and archaea are examples
Eukaryotic cell: Protista, fungi, plants and animals are examples.
diploma
Prokaryotic cells transport their metabolites across the cytoplasm, but eukaryotic cells are made up of different types of vesicles to transport different metabolites. Protein synthesis in eukaryotic cells occurs in 80S ribosomes attached to the ER. Synthesized polynucleotide chains enter the ER. Protein folding and transport to different parts of the cell are maintained by the ER. Therefore, the main difference between prokaryotic and eukaryotic cells lies in the presence or absence of the nucleus and the membrane-bound organelles.
Reference: 1. “Prokaryote”. Wikipedia, the free encyclopedia, 2017. Accessed February 22, 2017 2. Prokaryotic Cells. KHANACEDAMY, 2017. Accessed February 22, 2017 3. “Eukaryote”. Wikipedia, the free encyclopedia, 2017. Accessed February 22, 2017 4. “Eukaryotic cells” . Learn Science at Scitable, Nature Education, 2017. Retrieved February 22, 2017
Image courtesy of: 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 - Homemade with Adobe Illustrator (Public Domain) via Commons Wikimedia
