Peering into a Eukaryotic cell

The exact components within a cell depend on whether the cell is a prokaryote or a eukaryote. Prokaryotic cells are relatively simple in comparison to their complex eukaryotic brethren. To learn more about the similarities and differences between them, click here. In this text, we are diving deeper into the structures of a eukaryotic cell.

Eukaryotes tend to have multiple structures known as organelles. Organelles are small structures within a cell that carries out specific cellular functions. One can compare organelles to the organs in a human body, with each organelle having a specialized function, and the collective action of all organelles allow a cell to survive. Some of the organelles in a eukaryotic cell are:

Mitochondria

Everyone has heard the saying “The mitochondria are the powerhouse of the cell”. This statement is correct because it generates the bulk of the cell's energy [1]. This organelle has the ability to create copious amounts of Adenosine triphosphate (ATP) - an energy molecule. Mitochondria carries its own genetic material (mtDNA) - separate from the cell’s DNA - which allows itself to replicate independently [2].

Figure 1. Mitochondria [3, 4]

Chloroplasts

They convert light energy into chemical energy due to the presence of chlorophyll - a molecule used by plants to perform photosynthesis by giving them a green colour, and allowing the absorption of light [5]. Therefore, chloroplasts are only found in organisms that perform photosynthesis: plants and some other marine eukaryotes (for example, algae or plankton). Like mitochondria, this organelle also has the ability to self replicate as it contains its own genetic material (cpDNA) [2].

Figure 2. Chloroplasts [6, 7].

Fun fact! It is hypothesized that mitochondria and chloroplasts were independent prokaryotes engulfed by an ancient eukaryote. Instead of getting digested by the eukaryote, they developed a partnership (symbiosis relationship). The engulfed prokaryote provided the cells an additional boost of energy, and the ancient eukaryotic cell provided protection and nutrients. Over time they became completely dependent on each other. This theory is based on the fact that chloroplasts are extremely similar to the free-living photosynthetic prokaryote cyanobacteria, and mitochondria are very similar to some bacterias [8-10].

The Endoplasmic Reticulum (ER)

This organelle is most massive in length and diverse in its functions. It consists of a continuous membrane system that surrounds the nucleus and branches out into tubules and flat sheets. Despite being continuous, ER has a “smooth” and “rough” section - where “rough” ER contains ribosomes* while “smooth” ER does not. Its functions include: creation of lipids and proteins, protein transport, carbohydrate metabolism, and calcium storage [11]. To transfer lipids, the ER makes contact with multiple other organelles within the cell.

Figure 3. Rough endoplasmic reticulum with ribosomes (outside) surrounding the smooth endoplasmic reticulum (centre) [11]. The right image showcases the differences between the ER [12].

The Golgi apparatus

Similar to the ER, the Golgi apparatus is composed of a series of internal membranes (endomembranes) that package and export the proteins and lipids that were produced in the ER.

In the case where a protein or lipid need to be modified further before it becomes functional, the Golgi will add chemical “tags” to them. These “tags” ensure a protein’s functionality and inform them where they are needed. This guarantees that proteins and lipids are exactly where they are required, whether it be in or out of the cell (e.g. hormones) [13]. Those without “tags” become part of the cell membrane.

Figure 4. Golgi apparatus [14, 15].

In this post, we did not cover the most important organelle, so stay tuned for next week’s post on the nucleus!

*Ribosomes - One of the components of cells, it is the structure responsible for producing proteins. It translates the code that an RNA molecule contains into a protein sequence.

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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 or would like to learn more, please comment below or send us your questions.

References:

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3. Anne Trafton, M. (2012) A new glow for electron microscopy, MIT News | Massachusetts Institute of Technology. Available at: https://news.mit.edu/2012/a-new-glow-for-electron-microscopy-1021 (Accessed: 22 December 2022).

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7. Chloroplast | Definition, Function, Structure, Location, & Diagram (2022). Encyclopedia Britannica. Available at: https://www.britannica.com/science/chloroplast (Accessed: 27 December 2022).

8. Origin of Mitochondria (2022). Scitable. Available at: https://www.nature.com/scitable/topicpage/the-origin-of-mitochondria-14232356/ (Accessed: 22 December 2022).

9. Gray, M. (2014). The Pre-Endosymbiont Hypothesis: A New Perspective on the Origin and Evolution of Mitochondria". Cold Spring Harbor Perspectives in Biology, 6(3), pp. 16097. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3949359/

10. Miyagishima S,Y (2005). Origin and evolution of the chloroplast division machinery. Journal of Plant Research. 118(5):295-306. Available at: https://pubmed.ncbi.nlm.nih.gov/16143878/#:~:text=Chloroplasts%20were%20originally%20established%20in,algae%20by%20previously%20nonphotosynthetic%20eukaryotes.

11. Schwarz, D. and Blower, M. (2015). The endoplasmic reticulum: structure, function and response to cellular signaling. Cellular and Molecular Life Sciences, 73(1), pp. 79-94. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4700099/

12. Sapkota, A. (2022) Rough ER vs Smooth ER. Microbe Notes. Available at: https://microbenotes.com/rough-vs-smooth-endoplasmic-reticulum/ (Accessed: 27 December 2022).

13. Golgi Body (2022). National Human Genome National Institute. Available at: https://www.genome.gov/genetics-glossary/golgi-body#:~:text=A%20Golgi%20body%2C%20also%20known,a%20series%20of%20stacked%20membranes (Accessed: 22 December 2022).

14. Golgi apparatus in a plant cell (2022). Science Photo Library. Available at: https://www.sciencephoto.fr/image/12362449-Golgi-apparatus-in-a-plant-cell-TEM (Accessed: 22 December 2022).

15. Golgi apparatus | Definition, Function, Location, & Facts (2022). Encyclopedia Britannica. Available at: https://www.britannica.com/science/Golgi-apparatus (Accessed: 27 December 2022).