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Eukaryotic cells

Eukaryotic cells definition


(n., variant: eucaryotic cells)
Eukaryotic cells definition: cells of or derived from eukaryote characterized by having a distinct, membrane-bound nucleus

Eukaryotic Cells Definition

What is a eukaryotic cell? The term “cell” is a common word in biology, anatomy, and medical science. It is the basic unit of life. All living things are made up of at least one cell. In fact, they can be grouped according to the number of cells that make up an individual. Unicellular organisms are composed of a single cell whereas multicellular organisms consist of multiple cells organized into biological tissues, organs, and systems. Organisms may also be classified as prokaryotes or eukaryotes. Prokaryotes are organisms whose cell lacks a “true nucleus”. Examples of prokaryotes are domain Eubacteria (true bacteria) and domain Archaea. Their genetic material is located in a cytoplasmic region referred to as nucleoid. By contrast, eukaryotes are organisms made up of cells that contain a well-defined nucleus. They include protozoans, algae, fungi, plants, and animals. Thus, eukaryotic cells refer to the cells of or derived from eukaryote characterized by having a distinct, membrane-bound nucleus.

Etymology: The word eukaryote came from the Greek “eu”, meaning “good”, “well”, “true” and “karuon”, meaning nut.
Variant: eucaryotic cells.
Compare: prokaryotic cell.

Eukaryotic Cells Structure and Functions

The presence of a nucleus is one of the distinctive characteristics of eukaryotic cells. The nucleus is a large organelle that is often the most prominent structure inside the eukaryotic cell. See the eukaryotic cell diagram picture below. Not all cells of a eukaryote contain a nucleus. For example, mammalian red blood cells lose their nucleus at maturity to increase their affinity for respiratory gases. Eukaryotic cells that lack nucleus are described as “anucleate” as opposed to the nucleated cells that have a nucleus. In eukaryotic cells, the nucleus acts as the command center. (Ref.1) It contains the chromosomes that bear most of the genes (nuclear DNA) of a eukaryote. The genetic code in the nuclear DNA is used by the cell for regulating its growth, differentiation, homeostasis, reproduction, heredity, and death.

The genetic material plus the other components of the nucleus are enclosed by a nuclear envelope. This envelope is a double lipid layer that separates the nuclear contents from the cytoplasm. Nevertheless, it is perforated with holes so that certain molecules have a way to move to and fro. mRNA, for instance, leaves the nucleus after copying the genetic code from the nuclear DNA and brings it to the ribosome attached to the endoplasmic reticulum for protein translation. The ribosomes are not exclusive to eukaryotic cells. Prokaryotic cells also have them. However, the ribosomes in eukaryotic cells, i.e. 80S, are larger than those in prokaryotes, i.e. 70S. (Ref.2)

Apart from the nucleus, other sub-cellular components present in eukaryotic cells are as follows: (click each link for detailed description, specific function, and other essential facts)

The endoplasmic reticulum is associated with biomolecular synthesis, carbohydrate metabolism, and drug detoxification. The Golgi apparatus is involved in glycosylation, packaging of molecules for secretion, transporting of lipids within the cell, and giving rise to lysosomes. (Ref.3) The mitochondrion (plural: mitochondria) supplies the cell with biochemical energy (ATP). The plastids are involved in the manufacturing and storing of food. Chloroplasts are an example of a plastid. They contain chlorophylls (green pigments) that are involved in photosynthesis. The lysosomes digest, remove, and recycle worn-out subcellular components. The cytoskeleton is responsible for maintaining cell shape. The vacuoles are for intracellular secretion, excretion, storage, and digestion. Endosomes, in turn, are involved in the endocytotic membrane transport pathway. The cytosol, which is the aqueous portion of the cytoplasm, consists of water, organic molecules (proteins, carbohydrates, lipids, etc.), and ions (primarily, potassium, sodium, chloride, bicarbonate, etc.).

Both mitochondria and chloroplasts are semi-autonomous organelles. They contain DNA called mitochondrial DNA (mtDNA) and chloroplast DNA (cpDNA), respectively. They carry the code for certain proteins required for their metabolic activities. All eukaryotic cells possess mitochondria however not all of them have chloroplasts. Only photosynthetic organisms, such as plants and algae, have chloroplasts. They obtain energy from the sun to manufacture their own food. See the eukaryotic cell diagram (B) for plant cell features. Also, they have a cell wall next to their plasma membrane. Plant and algal cells require it for cell rigidity and protection. Animal cells, in contrast, lack cell walls. Only the plasma membrane confers protection to the cell and its contents from the potential stressors outside. Nevertheless, the animal cells are more flexible than plant cells simply because they lack the cell wall that makes the cell rigid.

Figure: Eukaryotic cell diagram of a typical animal cell (A) and a plant cell (B) with labeled parts. Cytoplasmic structures that are bound by a double layer of biological membranes such as nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, and plastids are found only in eukaryotic cells.

(A)

(B)

Types of Eukaryotic Cells

Eukaryotic cells may be classified into two groups based on the number of cells that make an individual organism: (1) unicellular eukaryotic cells and (2) multicellular eukaryotic cells. Unicellular eukaryotes include the protists. Multicellular eukaryotes include a variety of plant, fungal, and animal species.

Examples of Eukaryotic Cells

All animals, plants, fungi, and protists are eukaryotic cells. Their cells are organized into compartmentalized structures such as nuclei, mitochondria, chloroplasts, and Golgi bodies.

Evolutionary origin

According to the Endosymbiotic theory, the eukaryotes are conjectured to arise from the early eukaryotic cells that formed a symbiotic relationship with the early prokaryotic cells. They may have descended from a common microorganism ancestor that harbored a primitive prokaryotic cell inside. Over the years, the endosymbiosis of the two cells persisted for so long that eventually, the prokaryotic cell evolved into a subcellular component (organelle) of the eukaryotic cell. Some of the prokaryotes evolved into the modern-day mitochondria whereas those that are photosynthetic turned into chloroplasts. (Ref.4) Soon, these primitive eukaryotic cells diverged to different taxonomic kingdoms (e.g. Animalia, Plantae, Protista, and Fungi), each with distinctive characteristics that define one from the other.

See also

References

  1. Genetics Home Reference. (2020). What is a cell? Genetics Home Reference. https://ghr.nlm.nih.gov/primer/basics/cell
  2. Biology Online Editors. (2019, October 7). Ribosome Definition and Examples – Biology Online Dictionary. Biology Articles, Tutorials & Dictionary Online. https://www.biologyonline.com/dictionary/ribosome
  3. Biology Online Editors. (2019, October 7). Cell Definition and Examples – Biology Online Dictionary. Biology Articles, Tutorials & Dictionary Online. https://www.biologyonline.com/dictionary/cell
  4. Evidence for endosymbiosis. (2020). Berkeley.Edu. https://evolution.berkeley.edu/evolibrary/article/_0_0/endosymbiosis_04

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