Definition of eukaryote
Eukaryote refers to any of the single-celled or multicellular organisms whose cell contains a distinct, membrane-bound nucleus. Organisms such as animals, plants, fungi, and protists are examples of eukaryotes because their cells are organized into compartmentalized structures called organelles, such as the nucleus. The presence of a distinct nucleus encased within membranes differentiates the eukaryotes from the prokaryotes. The eukaryotes are also known for having cytoplasmic organelles apart from nucleus, such as mitochondria, chloroplasts, and Golgi bodies. Eukaryotes often have unique flagella made of microtubules in a 9+2 arrangement.
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.
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. Compare: prokaryote.
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 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, macrophage, 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.
Eukaryote vs. prokaryote
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.]
All eukaryotes belong to Domain Eukaryota. Organisms belonging to this domain are animals, plants, fungi, and protists.
Animals are eukaryotes that distinct from the other groups of eukaryotes by being heterotrophic, motile, and multicellular, body organized into cells, tissues, organs, and systems, lacking cell walls and chloroplasts, and growing from a blastula during embryonic development.
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 them 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.
Similar to plants, fungi have cell walls. However, the cell walls are made up chiefly of chitin (a material in the exoskeleton of insects). Fungi lack the 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 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.
- BIOdotEDU. (2019). Retrieved from Cuny.edu website: http://www.brooklyn.cuny.edu/bc/ahp/LAD/C5/C5_Eukary.html
- More on Eukaryote Morphology. (2019). Retrieved from Berkeley.edu website: https://ucmp.berkeley.edu/alllife/eukaryotamm.html
- Prokaryotic vs. Eukaryotic. (2019). Retrieved from Nku.edu website: https://www.nku.edu/~whitsonma/Bio150LSite/Lab%205%20Cells/Bio150LRCellTypes.htm
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