What is a Cell?

Life on planet Earth is immensely diverse in size and form; from the microscopic bacterium, to the majestic blue whale. Life is also cellular. This means that in spite of the diversity that exists, all life is composed of cells. This is the primary reason why it is famously said; “the building block of life is the cell”. Although the term “cell” is mostly well known, there are a few reasonable gaps in the general understanding of this term. What exactly is a cell? Are all cells identical? What are the components of a cell?

The National Cancer Institute of the American National Institute of Health (NIH), defines a cell as “the smallest unit that can live on its own and that makes up all living organisms and the tissues of the body” [1]. There are billions of different types of cells, however, they are divided into two main groups: prokaryotes and eukaryotes - which then are further divided into multiple subgroups. The main difference between these two main groups is the presence of the nucleus.

Phylogenetic tree of different organisms [2].

Prokaryotes

The definition of “prokaryote” is inside the word itself: “pro” = before; “karyon” = nucleus. Prokaryotic are single-celled organisms (also called unicellular) that have their genetic material in the cytoplasm [3]. Even though they are one single cell, they are able to react to stimuli, reproduce, grow, adapt, and maintain homeostasis as any other multicellular organism (e.g., humans). They can be further classified into:

Bacteria

Bacteria are found everywhere, from the soil to the oceans, and to the arctic - some even live in other organisms like humans! [4]. They are very important for many processes, such as recycling nutrients in the environment, or even helping animals digest food. Although some bacteria can damage crops or cause diseases, most bacteria are harmless or even helpful. In fact, humans have 10 times more bacteria than human cells, and they are widely used to produce food, energy, clean water, and medicines [5-7].

Archaea

Archaea are very similar to bacteria but differ in their cell wall composition [8]. Archaea have been found in extreme environments such as height pressures, salt concentrations, or temperatures. There are fewer applications of these organisms, although they have been studied more recently because their enzymes are very stable [9]. The most recent used of archeas is for the degradation or removal of contaminants in soils, sediments, and waters [10].

Eukaryotes

“Eukaryotes” is also defined within the word: “eu” = true; “karyon” = nucleus. Therefore, contrasting to prokaryotes, eukaryotes enclose their genes in a nucleus. They are also more specialized and usually group together to form tissues and organs in multicellular organisms [3]. Eukaryotic cells can be divided into:

Animal cells

There are numerous types of animal cells, each designed to serve specific functions [11]. They are most commonly categorized as skin cells, muscle cells, blood cells, fat cells, nerve cells, sex cells, and steam cells. Together, these cells will form many different organisms from insects to humans.

Plant cells

Unlike animal cells, plant cells have a cell wall surrounding the cell membrane. Also, each cell allows the plant to perform different functions like transporting water and nutrients, providing structural support, exchanging gases, and reproducing [12]. Some examples are: meristematic cells, parenchyma cells, collenchyma cells, sclerenchyma cells, and reproductive cells.

Fungi

Fungi cells give rise to many organisms like yeast, rusts, smuts, mildews, molds, and mushrooms [13].

Protists

Like the rest of eukaryotic cells, protists have a nucleus. However, most of them are unicellular with just a few types being primitive multicellular organisms [14]. Protists can be divided into three subgroups, depending on whether the cellular structure is similar to animal, plant, or fungi cells.

As mentioned, there are billions of different types of cells that differ in their molecular composition and activities. Stay tuned for next week post where we delve deeper into the similarities and differences of the two main groups: prokaryotic and eukaryotic cells, and how their molecular structures carry out their functions.

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Written by: Renard

Edited by: María and Natasha

BioDecoded is a volunteer group committed to sharing accurate scientific information. If you have any questions about this topic, please comment or send them to our email.

References:

1. NCI Dictionary of Cancer Terms (2022). Available at: https://www.cancer.gov/publications/dictionaries/cancer-terms/def/cell (Accessed: 7 December 2022).

2. Whitman, W. (2009) “The Modern Concept of the Prokaryote”, Journal of Bacteriology, 191 (7), p. 2000-2005. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2655495/pdf/0962-08.pdf

3. Structure and types of cells (2022). Encyclopedia Britannica. Available at: https://www.britannica.com/summary/cell-biology (Accessed: 7 December 2022).

4. Bacteria | Cell, Evolution, & Classification (2022). Encyclopedia Britannica. Available at: https://www.britannica.com/science/bacteria (Accessed: 7 December 2022).

5. Bacteria - The importance of bacteria to humans (2022). Encyclopedia Britannica. Available at: https://www.britannica.com/science/bacteria/The-importance-of-bacteria-to-humans (Accessed: 7 December 2022).

6. Bacteria - Bacteria in industry (2022). Encyclopedia Britannica. Available at: https://www.britannica.com/science/bacteria/Bacteria-in-industry (Accessed: 7 December 2022).

7. Bacteria - Bacteria in medicine (2022). Encyclopedia Britannica. Available at: https://www.britannica.com/science/bacteria/Bacteria-in-medicine (Accessed: 7 December 2022).

8. Archaea | What is microbiology? (2022). Microbiology Society. Available at: https://microbiologysociety.org/why-microbiology-matters/what-is-microbiology/archaea.html (Accessed: 7 December 2022).

9. Pfeifer, K. et al. (2021) “Archaea Biotechnology”, Biotechnology Advances, 47, p. 107668. Available at: https://www.sciencedirect.com/science/article/pii/S0734975020301701#bb2045

10. Krzmarzick, M. et al. (2018) “Diversity and Niche of Archaea in Bioremediation”, Archaea, 2018, p. 17. Available at: https://www.hindawi.com/journals/archaea/2018/3194108/

11. Animal Cell (2022). Biology Online. Available at: https://www.biologyonline.com/dictionary/animal-cell (Accessed: 7 December 2022).

12. Moss | Definition, Characteristics, Species, Types, & Facts (2022). Available at: https://www.britannica.com/plant/moss-plant (Accessed: 7 December 2022).

13. Fungus | Definition, Characteristics, Types, & Facts (2022). Available at: https://www.britannica.com/science/fungus (Accessed: 7 December 2022).

14. Protists (2022) Biology Online. Available at: https://www.biologyonline.com/dictionary/protist (Accessed: 7 December 2022).