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In organisms big and small, the cell is the basic unit of life, encompassing all the machinery needed to sustain life. Cytoplasm is the highly viscous, colorless, gel-like material enclosed within the cell membrane in which all the functional units of the cell are suspended. Cytoplasm can be observed under the microscope in larger microorganisms like paramecium at 600X magnification and higher. Cell biologists sometimes refer to all the contents of the cell, barring the nucleus, as the cytoplasm. This includes the cytosol, cytoskeleton, organelles and other cytoplasmic intrusions. Keep in mind, the cytoplasm isn’t just a floating bubble of liquid! Although it is mainly composed of water, it also contains a distinct combination of salts, lipids, and proteins which facilitate the complex events taking place inside it. In eukaryotes, all cellular organelles are either suspended or firmly attached to the cytoplasm in order to ensure they stay in place. On the other hand, Prokaryotic cells, which lack organelles, typically contain membrane-less, distinct compartments within the cytoplasm which contain the machinery for specific functions. Why do Cells Have Cytoplasm? Suspension of important machinery inside a liquid chamber is a common theme in living organisms. The brain, the spinal cord, and the lungs are also encompassed within their own fluids and held in place by connective tissues. When an object or force moves through another substance such as air or water, it encounters a resistance in movement called ‘friction’. However, depending on the substance being moved through, the level of friction can vary. Have you ever tried walking inside a pool? Your legs feel heavier than usual and you have to expend twice the energy to walk at the same speed as on ground. This is because, fluids like water exhibit greater resistance to movement. Inside the body, when a force is being transmitted across the cell or an organ, the liquid encompassing them act as ‘cushions’ in reducing the strength of the blow. In this way, the cytoplasm protects all its components from injury and damage. However, the cytoplasm shouldn’t be thought of us a functionless liquid encompassing the organelles. It is a functional ecosystem, regulating, facilitating and monitoring all the activities of the cell. Fun Cytoplasm Experiment Fill two plastic containers with water and place an egg inside them. Try Dropping one container from a height of 2-3 feet. What happens to the egg? Now dissolve some salt into the water until you see the egg begin to float in the water. The egg now resembles organelles inside the cytoplasm. Drop the container from the same height. Does the egg break? https://www.youtube.com/watch?v=ZenpNr5GtTQ&t=181s Origins of Cytoplasm The cytoplasm is older than other cellular components; even the most primitive cells had it. However, evolutionists are not very sure about how the cytoplasm first developed. A very popular theory proposed by astrophysicists speculates that a billion years after the earth was formed, the first cells evolved inside hydrothermal vents, deep in the sea, where light, heat and inorganic materials came together to form the organic molecules of life. Biologists propose that this organic fluid became enclosed within a primitive membrane to form the first cell. These primitive structures are thought to have been very simplistic, with molecules merely being diffused across the liquid compartment without any specialized mechanisms. However, how this primitive unit became capable of self-division and growth, thus leading to the emergence of life remains a mystery. Following the development of the first prokaryotic cell, random mutations, rapid division and natural selection is thought to have aided in the formation of the complex cytoplasm seen in cells today. In 1857, the Swiss anatomist, Rudolf von Kölliker, a pioneer of innovate specimen staining and sectioning methods, first studied the cytoplasm. While examining striated muscle cells, he noted the presence of distinct granules suspended in place in a liquid. In 1882, the biologist Eduard Strasburger first coined the term ‘cytoplasm’ to distinguish the contents of the cell from the nucleus. Initially, prokaryotic bacterial cells were thought to be sacs of cytoplasm, with nucleic acids, lipids, proteins and ions floating around. However, it is now known that the cytoplasm in prokaryotes is a highly organized unit containing distinct regions of DNA compaction, cytoskeleton, ribosomes and organized protein networks, with highly regulated movement of molecules between these zones. Did You Know? Cells do not maintain the same volume throughout their life. The volume can change as a product of both environmental conditions and time. Generally, the cytoplasm can constitute about 60-70% of the cell’s volume. However, when a cell divides, the cytoplasm rapidly replicates to about twice its size. In contrast, when cells are placed in a concentrated solution, the osmotic pressure causes the them to shrink rapidly in size. Here is a typical root cell isolated from an onion. Based on the stage of the cell division, the volume of cytoplasm in each cell varies. Cytoplasm Under a Microscope The cytoplasm can be easily stained using a dye called - Eosin. Eosin is an acidic substance which binds to the basic proteins suspended in the cytoplasm to stain the cytoplasm with a bright pink color. Biologists who wish to differentiate between the cytoplasm and the nucleus will typically use a combination of Eosin and Haematoxylin (H&E). In contrast, a pap stain, which consists of a combination of Haematoxylin (nuclear dye) and polychromatic dyes (cytoplasmic dye) which produces a darkly stained nucleus against a transparent cytoplasmic background. Typically, a pap stain is used to study nuclear structures. Composition of Cytoplasm The cytoplasm is a highly crowded liquid, consisting of thousands of macromolecules that are constantly in motion. Without regulation, macromolecules would be simply colliding into each other. However, the cell ensures order inside the cytoplasm by creating distinct compartments, partitioning entry from one compartment to another and making movement of a molecule energetically ‘costly’, meaning a molecule must spend energy to move from point A to point B. The major macromolecules contained in the cytoplasm include Water molecules (80-85%)Proteins (10-15%)Lipids (2-4%)Polysaccharides (1%)Nucleic Acids (1%) https://www.youtube.com/watch?v=AgFpsVS3xXg In prokaryotic cells, the cytoplasm can be organized into three distinct zones: The Nucleoid Zone - which contains a primitive nuclear structure consisting of nucleic acids and proteins. The Structural Zone – which much like the body’s skeleton is made up of rigid proteins arranged into a distinct framework. This structure called the cytoskeleton underlies the entire surface of the cell. The Metabolic Zone – as the name implies, is the part of the cell where cellular functions take place. For this purpose, the metabolic zone typically consists of specialised macromolecules. In eukaryotic cells, the cytosol is divided into two parts: The endoplasm is the innermost layer of cytoplasm, surrounding the nucleus and endoplasmic reticulum. Due to its location right next to the control center of the cell, this portion of the cytosol contains numerous macromolecules, moving in and out of the nucleus. Therefore, the endoplasm is denser than the ectoplasm. The ectoplasm is the outer layer of the cytoplasm, flanking the plasma membrane or the cell wall. The ectoplasm does not contain aggregates of molecules (granules) like the endoplasm and is therefore, less dense. However, this portion of the cytosol is more concentrated in cytoskeletal structures. Cytoplasmic Structures The cytoplasm in cells can be divided into distinct structures, including the cytosol, cytoskeleton and cytoplasmic intrusions. The liquid phase of the cytoplasm is called the cytosol. All other structures and molecules in the cytoplasm are suspended in the cytosol. The cytosol is neither acidic or basic, with an aggregation of charged macromolecules. The function of the cytosol is to facilitate transport of molecules across the cell. Signal transduction refers to the process by which cells communicate with the outside environment and neighboring cells. Signal transduction functions like a relay race where one runner passes on the baton to another. Similarly, a set of cascading protein movements from the plasma membrane, across the cytosol, delivers the message to the nucleus. The cytoplasmic intrusions in eukaryotic cells include organelles such as the Golgi apparatus, endoplasmic reticulum, endosomes, lysosomes, mitochondria and vacuoles. However, some plant and animal cells also contain specialized structures. For example, fat cells which are specialized for the storage of lipid molecules contain spherical droplets within the cytoplasm. In the hydrophilic cytosol, the lipids and proteins are arranged in a distinct manner with proteins facing outside and lipids facing inside. Fat cell with lipid droplets Did You Know? Prokaryotic cells which do not contain organelles, still manage to create tiny micro-compartments inside the cytoplasm. Bacterial microcompartments were only discovered in the 1950’s, with many of its characteristics still remaining unknown. Bacterial cells contain tiny enclosures made up of proteins which serve as a semi-permeable membrane that regulates what molecules from the cytoplasm can enter and leave. Bacterial Microcompartment Cytoskeleton The cytoskeleton is the protein framework which functions as a scaffold to the cytoplasm, thus maintaining cell structure and shape. The criss-cross framework also renders the cell with resilience to bear stress from the environment. The three main structures composing the cytoskeleton include microtubules, microfilaments and intermediate filaments. The cytoskeleton is a highly developed complex structure which will elongate or shrink to facilitate movement, much the same way the muscles in our body help us move. Movement of the cytoskeleton requires energy in the form of GTP (Guanosine Triphosphate), which is an energy currency of the cell, like ATP (Adenosine Triphosphate). The Cytoskeleton Interestingly both prokaryotes and eukaryotes have cytoskeletons, but with varying complexity. The cytoskeleton is one of the most widely studied structures in cell biology today, which many of its functions remaining elusive. Did You Know? Protein movement across the cytoskeleton happens much like crowd surfing where an individual is transported overhead from person to person. Smaller proteins and macromolecules can easily diffuse across the cytoplasm. However, larger molecules are carried through by the filaments and tubules underlying the cytoplasm. Cytoplasm and Specialized Cells Osteocytes The cells that make up the bone are called osteocytes. The cytoplasm of an osteocyte typically extends away from the nucleus, reaching toward neighboring osteocytes. These protrusions are called canaliculi. The cells communicate with each other inside the bone. Adipocytes Adipocytes or fat cells are specialized for the storage of lipids. In order to maximize storage, the cells only contain small droplets of cytoplasm that store hydrophilic proteins. The rest of the cell’s volume is made up of lipid stores. These lipid stores also protect the cell from external stress. This is why fat tissues are typically present as a protective layer over sensitive tissue. Giant Cells / Langshan's Cells These giant cells are formed by the fusion of multiple epitheloid cells in an infected site. The cytoplasm of individual cells fuses while the nucleus concentrates at the edge of the cell forming a mass-nuclear structure. The presence of these cells is also used to diagnose infections. Takeaways Cytoplasm is so much more than the often described “jelly-like” substance inside the cell. Cytoplasm is a complex, compartmentalized structure that facilitates the movements and transport of various macromolecules throughout the cell. The cytoplasm also supports the cytoskeleton which gives the cell its flexibility and iconic shape. I hope this article has given you a deeper understanding of cytoplasm and its overall importance to cells and the study of microbiology. References Baum, D. A., & Baum, B. (2014). An inside-out origin for the eukaryotic cell. BMC biology, 12, 76. https://doi.org/10.1186/s12915-014-0076-2Baumeister, W. (2002). Electron tomography: Towards visualizing the molecular organization of the cytoplasm. Current Opinion in Structural Biology, 12(5), 679–684. https://doi.org/10.1016/S0959-440X(02)00378-0Cooper, G. M. (2000). The Origin and Evolution of Cells. The Cell: A Molecular Approach. 2nd Edition. https://www.ncbi.nlm.nih.gov/books/NBK9841/Fulton, A. B. (1982). How crowded is the cytoplasm? Cell, 30(2), 345–347. https://doi.org/10.1016/0092-8674(82)90231-8J. Hu, S. Jafari, Y. Han, AJ. Grodzinsky, S. Cai, M. Guo#, Size and speed dependent mechanical behavior in living mammalian cytoplasm, Proceedings of the National Academy of Sciences. 114(36): 9529–9534 (2017).K Luby-Phelps, D L Taylor, F Lanni; Probing the structure of cytoplasm.. J Cell Biol 1 June 1986; 102 (6): 2015–2022. doi: https://doi.org/10.1083/jcb.102.6.2015Klionsky, D. Protein Transport from the Cytoplasm into the Vacuole . J. Membrane Biol. 157 , 105 –115 (1997). https://doi.org/10.1007/s002329900220Lalan, M., Bagchi, T., & Misra, A. (2011). The Cell. Challenges in Delivery of Therapeutic Genomics and Proteomics, 1–43. doi:10.1016/b978-0-12-384964-9.00001-3 Luby-Phelps K. The physical chemistry of cytoplasm and its influence on cell function: an update. MBoC. 2013;24(17):2593-2596. doi:10.1091/mbc.e12-08-0617Luby-Phelps, K., Lanni, F., & Taylor, D. L. (1988). The submicroscopic properties of cytoplasm as a determinant of cellular function. Annual Review of Biophysics and Biophysical Chemistry, 17, 369–396. https://doi.org/10.1146/annurev.bb.17.060188.002101Mak, Victor, Keith Jarvi, Martin Buckspan, Marc Freeman, Sloane Hechter, and Armand Zini. ‘Smoking Is Associated with the Retention of Cytoplasm by Human Spermatozoa’. Urology 56, no. 3 (1 September 2000): 463–66. https://doi.org/10.1016/S0090-4295(00)00700-7.Novoselov, A. A., Serrano, P., Pacheco, M. L. A. F., Chaffin, M. S., O’Malley-James, J. T., Moreno, S. C., & Ribeiro, F. B. (2013). From Cytoplasm to Environment: The Inorganic Ingredients for the Origin of Life. Astrobiology, 13(3), 294–302. doi:10.1089/ast.2012.0836Pollack, G. H. (2001). Cells, gels and the engines of life: A new, unifying approach to cell function. Ebner & Sons.Tools of the Cell Biologist. (2008). Medical Cell Biology, 1–26. doi:10.1016/b978-0-12-370458-0.50006-2 Transport Between the Cell Nucleus and the Cytoplasm. Dirk Görlich and and Ulrike Kutay. Annual Review of Cell and Developmental Biology 1999 15:1, 607-660Trevors J. T. (2011). The composition and organization of cytoplasm in prebiotic cells. International journal of molecular sciences, 12(3), 1650–1659. https://doi.org/10.3390/ijms12031650von Knebel Doeberitz, M., & Wentzensen, N. (2008). CHAPTER 1 - The Cell: Basic Structure and Function. In M. Bibbo & D. Wilbur (Eds.), Comprehensive Cytopathology (Third Edition) (pp. 3–22). W.B. Saunders. https://doi.org/10.1016/B978-141604208-2.10001-6Von Knebel Doeberitz, M., & Wentzensen, N. (2008). The Cell: Basic Structure and Function. Comprehensive Cytopathology, 3–22. doi:10.1016/b978-141604208-2.10001-6 Yoshihiro Yoneda, How Proteins Are Transported from Cytoplasm to the Nucleus, The Journal of Biochemistry, Volume 121, Issue 5, May 1997, Pages 811–817, https://doi.org/10.1093/oxfordjournals.jbchem.a021657

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