Category: 2. Eukaryotes and Prokaryotes

Prokaryotes include the domains, Eubacteria and Archaea. Examples of prokaryotes are bacteria, archaea, and cyanobacteria (blue-green algae).

Bacteria

Bacteria are microscopic, single-celled organisms that belong to Domain Eubacteria (true bacteria). Their cells lack lipid-bound subcellular compartments and their DNA is found in the cytoplasm (nucleoid region) rather than inside a nucleus. They reproduce by fission or by forming spores. They can inhabit all kinds of environments, such as in soil, acidic hot springs, radioactive waste, seawater, deep in the Earths crust, in the stratosphere, and even in the bodies of other organisms. Bacteria include the bacilli, the cocci, the spirochetes, and the vibrios.

Archaea

Archaea belong to the Domain Archaea. They are unicellular microorganisms that are genetically distinct from bacteria and eukaryotes. Similar to prokaryotes, they lack a nucleus. However, the genes of archaebacterial are more similar to eukaryotes. Both of them produce certain enzymes that are used in transcription, translation, and other metabolic pathways. Many archaebacterial are found thriving in extreme habitats. Archaebacteria include the halophiles (those inhabiting extremely salty environments), the methanogens (archaea species producing methane), and the thermophiles (those that can thrive in extremely hot habitats).

Cyanobacteria

Cyanobacteria, also called blue-green algae, are microorganisms that are formerly considered as protists for being single-celled and photosynthetic. However, they now belong to a group or phylum of photosynthetic bacteria that inhabit aquatic habitats and moist soils. Cyanobacteria are ecologically significant because a huge percentage of gaseous oxygen comes from their photosynthetic activity. They may lack a nucleus but they possess microcompartments (e.g. thylakoids and carboxysomes). They also have photosynthetic pigments (particularly, phycobiliproteins) that account for the bluish-green color of their cells. Cyanobacteria include Chroococcales, Pleurocapsales, Oscillatoriales, Nostocales, and
Stigonematales.

Eukaryotes are organisms in which at the cellular level most of their genetic material is located inside a double-membraned nucleus. Other genetic materials outside the nucleus are found in the mitochondria and the chloroplasts (if present). The chromosomes of eukaryotes are linear strands of DNA. Both eukaryotes and prokaryotes store their genetic information in their genes. The main source of metabolic energy is ATP.

In prokaryotes, the chromosome is mostly circular. Prokaryotes are smaller in size than eukaryotes. Therefore, prokaryotes have a large surface area to volume ratio. And because of this, they have a high metabolic rate and high growth rate.

Both eukaryotes and prokaryotes have ribosomes that serve as the site of protein synthesis. However, the composition of their ribosomes differs. The prokaryotic ribosome is 70S and it is made up of 50S (large subunit) and 30S (small subunit). The eukaryotic ribosome is 80S and it consists of 60S (large subunit) and 40S (small subunit). [N.B. the S units do not add up since they represent measures of sedimentation rate, not mass.]

Etymology

The term prokaryote (plural: prokaryotes) came from the Latin pro, meaning “in favour of” or “on behalf of” and káry(on), meaning “nut“, “kernel”. The term prokaryotic is a derived word and used to refer to a prokaryote. Compare: eukaryote. Variant: procaryote.

Prokaryotic Cell

Prokaryotes are unicellular organisms that lack a well-defined nucleus. They have instead a nucleoid region in their cytoplasm where their genetic material occurs in most instances as a single, circular molecule of DNA. They generally reproduce asexually, which is by binary fission or by budding. Most of them are unicellular, others are capable of forming stable aggregate communities. Conjugation, apparently, is the counterpart of sexual reproduction in eukaryotes where two cells exchange genetic materials via a conjugation tube.

Although prokaryotes lack the true organelles found in a eukaryotic cell, they possess certain cytoplasmic structures. For instance, they have a primitive cytoskeleton. Flagellin is the cytoskeletal protein that confers structural support to bacteria that is essential during chemotaxis. Other cytoplasmic structures are ribosomes, carboxysomes, chlorosomes, magnetosomes, and thylakoid systems. Some references regard them as prokaryotic organelles. However, they are not bounded by lipids; rather, they are proteinaceous. For example, carboxysomes are protein-shell compartments that are involved in carbon fixation in certain bacteria. Chlorosomes are light-harvesting complex in green sulfur bacteria. Magnetosomes are present in magnetotactic bacteria. Thylakoids are present in photosynthetic bacteria, such as cyanobacteria. Some prokaryotes have a cell wall that surrounds the cell membrane. Bacterial cell walls are composed chiefly of peptidoglycan. Its thickness can be used to determine if the bacterial cell is Gram-positive (thicker cell wall) or Gram-negative (thinner cell wall). As for the archaea, their cell wall is made up of glycoprotein S-layers, pseudopeptidoglycan, or polysaccharides rather than peptidoglycan (except for a group of methanogens).

Prokaryote refers to any of the group of organisms primarily characterized by the lack of true nucleus and other membrane-bound cell compartments: such as mitochondria and chloroplasts, and by the possession of a single loop of stable chromosomal DNA in the nucleoid region and cytoplasmic structures, such as plasma membrane, vacuoles, primitive cytoskeleton, and ribosomes. Examples of prokaryotes are bacteria and archaea.

Prokaryote Definition

Schematic diagram of a prokaryotic cell.

A prokaryote is defined as any organism that is chiefly characterized by a cell devoid of a well-defined nucleus as opposed to a eukaryote that has a nucleus. Instead of a nucleus, the prokaryotes have a nucleoid region where the genetic materials are located.

All eukaryotes belong to Domain Eukaryota. Organisms belonging to this domain are animals, plants, fungi, and protists.

Animals

Animals are eukaryotes that distinct from the other groups of eukaryotes by being heterotrophic, motile, and multicellular, a body organized into cells, tissues, organs, and systems, lacking cell walls and chloroplasts, and growing from a blastula during embryonic development.

Plants

Plants are photosynthetic eukaryotes. They have chlorophyll and other pigments that help in photosynthesis. They have a cell wall comprised mainly of cellulose. It provides structural support. They are not as motile as the animals. Movements are limited but their growth is not. They are capable of unlimited growth through meristematic tissues. They lack the sense organs in animals. Nevertheless, they can sense certain stimuli and respond accordingly by tropisms.

Fungi

Similar to plants, fungi have cell walls. However, the cell walls are made up chiefly of chitin (material in the exoskeleton of insects). Fungi lack chlorophyll and therefore are heterotrophic. Many of them are multicellular, forming hyphae and mycelium. Few species are unicellular. Examples of fungi are yeasts, rusts, stinkhorns, puffballs, truffles, molds, mildews, and mushrooms.

Protists

Protists are unicellular eukaryotes. However, some species form filaments or colonies of the same species. They move around as they have locomotory organs, such as pseudopods, cilia, and flagella. Others lack these organs and therefore are non-motile. Protists include the following: (1) protozoa, the animal-like protists, (2) algae, the plant-like protists, and (3) slime molds and water molds, the fungus-like protists.

Prokaryotes are organisms characterized by lacking a nucleus and other membrane-bound cytoplasmic structures. They are considerably smaller than eukaryotes. They also have a greater surface area to volume ratio and therefore have greater metabolic rates. Examples of prokaryotes are eubacteria and archaea.

Eukaryotes have a nucleus that contains nuclear DNA. The nucleus has a lipid bilayer membrane that is perforated with nuclear pores. The DNAs inside the nucleus are complexed with histone proteins forming chromatin. In cell division, the chromatin condenses into a chromosome. The chromosomes are linear strands of DNA as opposed to the chromosomes of prokaryotes that are mostly circular.

Both eukaryotes and prokaryotes have genetic information stored in genes. Their main source of metabolic energy is ATP. Both of them also have ribosomes that assist during protein synthesis. However, the ribosomes of eukaryotes are 80S. In prokaryotes, the ribosomes are 70S. Both prokaryotic and eukaryotic ribosomes are made up of two ribosomal subunits. The prokaryotic ribosome (70S) is made up of 50S (large subunit) and 30S (small subunit). The eukaryotic ribosome (80S) consists of 60S (large subunit) and 40S (small subunit). [N.B. the S units do not add up since they represent measures of sedimentation rate, not mass.]

The cell of a eukaryote has several membrane-bound structures dispersed in the cytoplasm. They are called organelles. Organelles typically found inside a eukaryotic cell are the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondrion, and plastid. Other cytoplasmic structures are cytoskeleton, inclusions, and biomolecules. These subcellular structures have their distinct functions and involved in various metabolic activities that regulate homeostasis.

The cells of eukaryotes divide by mitosis and meiosis. While mitosis gives rise to two daughter cells meiosis gives rise to four daughter cells. The cells from meiosis will be haploid after two consecutive divisions. In males, the haploid cell will grow into a spermatozoon (sperm cell) whereas, in females, it could develop into an ovum (egg cell). These two gametes could come together in a union via fertilization and give rise to a diploid zygote. In multicellular eukaryotes, the zygote divides by series of mitoses to give rise to stem cells that can develop and differentiate later into specialized cells that carry out a particular function and assemble into tissues, organs, and biological systems. In humans, there are several cell types: myocytes, adipocytes, blood cells, neurons, hepatocytes, osteocytes, macrophages, etc.

Some eukaryotes are single-celled. The cell is an entire organism capable of performing all the fundamental functions (e.g. ingestion, respiration, excretion, osmoregulation, homeostasis, etc.) that different systems do in a multicellular organism. These single-celled organisms are exemplified by protists.

A eukaryote is defined as any organism that is chiefly characterized by a cell with one or more nuclei at least once in its lifetime as opposed to a prokaryote that has a cell lacking a well-defined nucleus and with a nucleoid only. Etymology: the term eukaryote (plural: eukaryotes) came from Greek ‘eu’, meaning “good”, “well”, “true” and ”káry(on)”, meaning “nut”, “kernel”. The term eukaryotic is a derived word and used to refer to eukaryote.