Eukarya

By | July 24, 2021

Eukaryotic cells are cells that contain a nucleus and organelles, and are enclosed by a plasma membrane. Organisms that have eukaryotic cells include protozoa, fungi, plants and animals. These organisms are grouped into the biological domain Eukaryote.

Eukaryotic cells are larger and more complex than prokaryotic cells, which are found in Archaea and Bacteria, the other two domains of life. Eukaryote consists of organisms that have a true nucleus and structures called organelles that are surrounded with by membranes.

The presence of nucleus which contains genetic material and is enclosed by a nuclear membrane sets them apart from prokaryotic cells. All complex organisms are eukaryotic and they reproduce by mitosis or meiosis.

Eukaryotic cells are typically much larger than those of prokaryotes. Eukaryotic DNA is divided into several linear bundles called chromosomes, which are separated by a microtubule spindle during nuclear division.

The kingdoms associated with Eukarya are Plantae, Animalia and Fungi kingdoms,

Eukaryotes represent a tiny minority of all living things , but due to their much larger size , eukaryotes collective world wide biomass is estimated at about equal to that of prokaryotes. Eukaryotes are thought to have evolved between about 1.7 billion and 1.9 billion years ago.

The earliest known microfossils resembling eukaryotic organisms date to approximately 1.8 billion years ago. What would it be like to live in a one-room cabin? Well, things would probably be pretty simple. You would eat, sleep, work, and relax in a single room which might be a bit cramped, but would certainly make cleaning the house a snap!

Just as a large family home is split into many rooms with different purposes (bedrooms, bathrooms, kitchen, living room, etc.), so eukaryotic cells contain a variety of different compartments with specialized functions, neatly separated from one another by layers of membrane. This organization lets each compartment maintain its own conditions, the ones it needs to carry out its job.

For instance, compartments called lysosomes, which act as recycling centers for the cell, must maintain an acidic pH in order to dispose of cellular waste. Similarly, structures called peroxisomes carry out chemical reactions called oxidation

reactions and produce hydrogen peroxide, both of which would damage the cell if they weren’t safely stored away in their own “room.”

The ability to maintain different environments inside a single cell allows eukaryotic cells to carry out complex metabolic reactions that prokaryotes cannot. In fact, it’s a big part of the reason why eukaryotic cells can grow to be many times larger than prokaryotic ones.

Characteristics of Eukaryotic Cells:

Eukaryotic cells contain a variety of structures called organelles, which perform various functions within the cell. Examples of organelles are ribosomes, which make proteins, the endoplasmic reticulum, which sorts and packages the proteins, and mitochondria, which produce the energy molecule adenosine triphosphate (ATP). They also have a true nucleus, which contains the genetic material DNA and is surrounded by a nuclear envelope.

All of the organelles are stabilized and given physical support through the cytoskeleton, which is also involved in sending signals from one part of the cell to the other. In eukaryotic cells, the cytoskeleton is composed mainly of three types of filaments: microtubules, microfilaments, and intermediate filaments. The gel-like substance that surrounds all the organelles in the cell is called cytosol.

The known diversity of morphological characters in eukaryotes is simply staggering and can be attributed to the vast multitude of possible solutions to basic biological problems, such as nutrition/feeding, locomotion, defense, refuge, mate selection and reproduction. Eukaryotes are built from one or more internally differentiated cells comprised of intricate subcellular systems.

Several single-celled lineages, for instance, have reached the utmost degree of morphological complexity within the confines of a single enveloping cell membrane (e.g. parabasalids, ciliates, dinoflagellates), while others have reached the lower limits of morphological complexity by becoming extremely streamlined (e.g. picophytoeukaryotes, yeasts).  Moreover, some multicellular

eukaryotes have struck the upper physical limits of overall body size (e.g. dinosaurs, elephants, and whales), while others are miniaturized to the point of being smaller than single-celled counterparts in the same ecosystem (e.g. gastrotrichs, tardigrades, rotifers and nematodes). Regardless of major differences in body size and morphological peculiarities, eukaryotes share many characteristics in common. Many of these characteristics are homologous for the entire group, whether comparing a blue whale to an amoeba or a human to a giant redwood tree.

The figure below shows the structure of a eukaryotic cell; this is an animal cell. The nucleus and other organelles are shown. The cytosol is the blue substance surrounding all of the organelles. Together, the cytosol with all organelles besides the nucleus are grouped as cytoplasm.

Below is a list of important features that are likely to have been present in the common ancestor of eukaryotes. Some of these features are still universally found in all eukaryotic diversity, while others have been lost or drastically transformed in some lineages, but are nevertheless ancestral to those groups.

Cytoskeleton consisting of tubulin-based microtubules and actin-based microfilaments, and ancestrally including motile cell extensions called ‘flagella’ or ‘cilia’ that contain an axoneme of 9 peripheral microtubular doublets and 2 central microtubules.

An endomembrane system that consists of endoplasmic reticulum, Golgi bodies, vacuoles, lysosomes, peroxisomes, and the nuclear envelope.

Primary genome of each cell consisting of multiple linear chromosomes contained within a membrane-bound nucleus.  Following replication of the genome the chromosomes are segregated by the process of mitosis.  Cells in many species can have more than one nucleus.

Mitochondria – organelles with diverse functions, usually including aerobic respiration, iron sulfur cluster assembly, and synthesis and breakdown of small molecules such as lipids and amino acids. Mitochondria are bounded by two membranes, and usually contain a small genome. They are the descendents of an alpha-proteobacterial endosymbiont.

Translation machinery in the form of 80S ribosomes, each consisting of four molecules of RNA complexed with many proteins, and partitioned in a small (40S) and a large (60S) subunit.

A number of other characteristics are common to many eukaryotes and not to prokaryotes, but these are not ancestral to all eukaryotes, and many have evolved several times independently.

  • Multicellularity and tissue formation (e.g. green algae, land plants, red algae, brown algae, animals and fungi).
  • Secreted hard parts (e.g. mollusk shells, plant cell walls, ecdysozoan cuticles, coccoliths, vertebrate endoskeletons, chrysophyte scales, polychaete tubes, diatom frustules, brachiopod shells, cnidarian corallites, euglenophyte loricas, poriferan spicules, echinoderm ossicles, foraminiferan and radiozoan tests).
  • Extrusive organelles that function in defense, prey capture or parasitic invasion (e.g. ejectisomes of cryptomonads; trichocysts of alveolates; polar tubes of microsporidian fungi, gun cells of oomycetes; nematocysts of cnidarians, myxozoans and some dinoflagellates).
  • Plastids, including chloroplasts and their homologues. Referring to plastids as homoplasies is a qualified statement, since the vast majority of plastids do ultimately stem from a common primary endosymbiosis with a cyanobacterium (the one possible exception being the ‘chromophore’ of the euglyphid amoeba Paulinella), but their subsequent spread via secondary and tertiary endosymbioses has led to a complicated distribution on the tree of eukaryotes (see Symbiosis section below).

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