Cells share certain common features that define their basic structure and function. One of these fundamental components is the cytoplasm. Although often described simply as the “gel-like” interior of the cell, the cytoplasm is far more than a passive filler. It is a dynamic, semi-fluid matrix brimming with organelles, structural filaments, and dissolved molecules that together maintain cellular integrity, facilitate biochemical reactions, and enable cellular motions.
Cytoplasm: Key Points
- All cells (both prokaryotic and eukaryotic) contain cytoplasm.
- The cytoplasm includes include everything enclosed within the cell membrane besides the nucleus. It includes cytosol, organelles, cytoskeleton, and various inclusions.
- The cytoplasm is a dynamic sol-gel system, not merely a static “filler.”
- Major metabolic pathways, nutrient distribution, and intracellular movements depend on the cytoplasm’s structure and composition.
- Changes in cytoplasmic structure are involved in cell division, shape changes, and movement.
Cytoplasm Definition and Location
The cytoplasm is the region of a cell that lies between the cell membrane (plasma membrane) and the nucleus in eukaryotic cells. In prokaryotic cells, which lack a nucleus and other membrane-bound organelles, the cytoplasm encompasses virtually everything inside the plasma membrane.
The cytoplasm includes the cytosol (the fluid phase), the cytoskeleton (protein filaments providing support and shape), the organelles or cellular structures suspended within it, and insoluble particles called inclusions. The cytoplasm is the dense, nutrient-rich matrix that surrounds the genetic material and houses the ribosomes, various enzymes, and other components necessary for life.
History and Word Origin
The concept of cytoplasm emerged as scientists began examining cells under light microscopes in the 17th and 18th centuries. Early cell biologists observed a “cellular substance” distinct from the nucleus but did not understand its complexity. Swiss scientist Rudolf von Kölliker introduced the term term “cytoplasm” in 1863. The term derives from the Greek words kytos, meaning “hollow vessel” or “cell,” and plasma, meaning “formed or molded substance.” By the late 19th century, researchers like Eduard Strasburger and others helped refine the definition and distinguish it from the nucleus. In the 20th century, electron microscopy revealed the intricate network of filaments, organelles, and molecular machines that it contains.
Cytosol vs Cytoplasm
Sometimes the terms cytosol and cytoplasm are used interchangeably (especially in older or introductory texts). However, modern cell biology distinguishes between them. The cytoplasm includes all of the cell’s internal components outside the nucleus, namely the cytosol, organelles, and cytoskeletal elements. In contrast, the cytosol refers strictly to the semi-fluid, aqueous solution in which these organelles and structures are suspended. The cytosol is essentially the “soluble” portion of the cytoplasm, devoid of the organelles and large structural assemblies.
Physical Nature of the Cytoplasm
Far from a simple static fluid, the cytoplasm is best described as a dynamic “sol-gel” matrix. This term refers to its ability to shift between more fluid (sol) and more viscous (gel) states depending on cellular conditions. The cytoplasm is neither a pure liquid nor a solid. Instead, it behaves like a soft, viscoelastic material or even as a glass. It contains a high concentration of proteins, salts, and other molecules dissolved in water. It alters its fluidity as the cell’s needs change. For instance, areas of the cytoplasm become more gel-like to stabilize organelles or more fluid to allow vesicles and other components to move freely. This dynamic, responsive physical state is crucial for processes like cell division, intracellular transport, and shape changes.
Components of the Cytoplasm
The cytoplasm includes the cytosol, organelles (eukaryotes) or analogous structures (prokaryotes), ribosomes, cytoskeleton, and inclusions:
- Cytosol:
- A complex aqueous solution of water, ions, small molecules (sugars, amino acids, nucleotides), and large macromolecules (proteins, enzymes).
- The cytosol is the main site of many metabolic pathways, including glycolysis and portions of biosynthetic processes.
- Organelles (in Eukaryotes):
- Mitochondria: Sites of cellular respiration and energy (ATP) production.
- Endoplasmic Reticulum (ER): Involved in protein and lipid synthesis; Rough ER has ribosomes for protein production, while Smooth ER functions in lipid synthesis and detoxification.
- Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for transport.
- Lysosomes and Peroxisomes: Contain digestive enzymes or oxidation reactions to break down macromolecules and detoxify harmful compounds.
- Chloroplasts (in Plants and Algae): Sites of photosynthesis.
- Ribosomes:
- Present in both prokaryotes and eukaryotes.
- Molecular machines for protein synthesis.
- Found free in the cytosol or attached to the ER in eukaryotes.
- Cytoskeleton:
- A network of protein filaments (microtubules, actin filaments, and intermediate filaments in eukaryotes) that provide structural support, maintain cell shape, and facilitate internal transport and cell movement.
- In prokaryotes, structural filaments are less complex but still essential for maintaining shape and aiding in cell division.
- Inclusions and Other Particles:
- Non-membranous structures like glycogen granules, lipid droplets, pigments, mineral crystals, starch, and storage materials.
- Serve as energy reserves or other specialized functions depending on cell type.
Cytoplasm Functions
Given that the cytoplasm includes most of the cell’s components, it’s not surprising that it performs a lot of functions:
- Structural Support: The cytoskeleton and cytoplasmic matrix work together to maintain the cell’s shape, resisting deformation and providing mechanical strength.
- Biochemical Reactions: Many metabolic processes occur in the cytoplasm, including glycolysis and parts of lipid and amino acid synthesis. The cytoplasm thus acts as a stage for crucial enzymatic reactions.
- Nutrient Storage and Distribution: Dissolved nutrients and ions in the cytosol are readily available to organelles and molecular machines. Stored materials (e.g., glycogen granules) can be mobilized as needed.
- Intracellular Transport: Organelles, vesicles, and molecules move along cytoskeletal tracks, allowing for efficient distribution of molecules and communication between different parts of the cell.
- Cell Division: The cytoskeleton reorganizes during mitosis or binary fission, ensuring proper segregation of chromosomes and division of cellular contents.
- Response to Stimuli: The cytoplasm changes its consistency and rearranges its cytoskeletal components in response to environmental or internal signals. This facilitates movements such as amoeboid motion, changes in cell shape, and the formation of cell extensions.
References
- Guo, M.; et al. (2014). “Probing the stochastic, motor-driven properties of the cytoplasm using force spectrum microscopy”. Cell. 158 (4): 822–832. doi:10.1016/j.cell.2014.06.051
- Luby-Phelps, K. (1999). “Cytoarchitecture and physical properties of cytoplasm: volume, viscosity, diffusion, intracellular surface area”. International Review of Cytology. 192: 189–221. doi:10.1016/S0074-7696(08)60527-6. ISBN 9780123645968.
- Parry, B.R.; et al. (2014). “The bacterial cytoplasm has glass-like properties and is fluidized by metabolic activity”. Cell. 156 (1–2): 183–94. doi:10.1016/j.cell.2013.11.028
- Shepherd, V.A. (2006). “The cytomatrix as a cooperative system of macromolecular and water networks”. Current Topics in Developmental Biology. 75: 171–223. doi:10.1016/S0070-2153(06)75006-2. ISBN 9780121531751.