Cytoplasm

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Schematic of typical animal cell, showing subcellular components. Organelles: (1) nucleolus (2) nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (ER) (6) Golgi apparatus (7) Cytoskeleton (8) smooth ER (9) mitochondria (10) vacuole (11) cytosole (12) lysosome (13) centrioles

Cytoplasm is all the "stuff" inside the enclosing membrane of a biological cell, except for the nucleus and nuclear membrane in the case of eukaryotes. "Cytoplasm" is sometimes used to refer to only the fluid that is not compartmentalized into organelles (membrane-bounded, distinct compartments), but it is used here in its broad sense as including the organelles.

In prokaryotes, the inside of the cell is filled with cytoplasm within which the genetic material and ribosomes float freely.

In eukaryotes, the cytoplasm is that portion of the cell that lies between the nuclear envelope (nuclear membrane) and the cell membrane. It comprises both the cytosol (fluid-filled space outside the organelles) and the cellular organelles floating in the cytosol (Alberts et al. 1989). The cytosol is made up of water, salts, organic molecules, and many enzymes that catalyze reactions. Organelles in the cytoplasm include mitochondria, chloroplasts (in plants), endoplasmic reticulum (ER) (rough and smooth ER), golgi apparatus, lysosomes, endosomes, and peroxisomes (Alberts et al. 1989). Each type of organelle has its own distinct function, enzymes, and other specialized molecules.

Although the inside of cells would seem to be chaotic, with various organelles scattered throughout the cytosol, in reality there is a complex organization. For example, there is an internal framework (cytoskeleton) that integrates the organelles and coordinates cell division, while protein are moved to their appropriate location based on transport signals molecular units attached to them. Each organelle provides a function for the cell (and thus other organelles), and in turn receives benefits from the cell (and other organelles).

Contents

Function

The cytoplasm holds all of the cellular organelles outside of the nucleus and also maintains the shape and consistency of the cell. It is also a storage place for chemical substances indispensable to life, which are involved in vital metabolic reactions, such as anaerobic glycolysis and protein synthesis.

The cytosol, which is that portion of the cytoplasm that occupies the intracellular space outside the membrane-bounded organelles, is the site of protein synthesis and most of the cell's intermediary metabolism (Alberts et al. 1989). It has thousands of enzymes involved in metabolism (Alberts et al. 1989). Many of the newly synthesized proteins remain in the cytosol if they lack a signal for transport—about half according to Alberts et al. (1989). The cytosol also plays an important role in a cell by serving as a "molecular chowder" in which the organelles are suspended and held together by a fatty membrane.

In bacteria, chemical reactions take place in the cytoplasm and all of the genetic material is suspended in the cytoplasm.

Components of the cytoplasm

Organelles; Cytoplasm labeled at center right

The cytoplasm is composed of ions and soluble macromolecules like enzymes, carbohydrates, different salts, and proteins, as well as a great proportion of RNA.

The cytoskeleton is the internal framework (or "scaffolding") in the cytoplasm and is made up of actin filaments (or microfilaments), microtubules, and intermediate filaments. The cytoskeleton helps maintain the shape of the cell, aids movement of the cell (using structures such as flagella and cilia), organizes the organelles, aids intra-cellular transport (movement of vesicles and organelles, for example), aids cellular division and chromosome movement, and helps in adhesion of cell to a surface.

The cytoplasm's watery component—the clear, structureless, fluid portion—is also known as hyaloplasm. Hyaloplasm is basically the cytosol without the microtubles and microfilaments. It can be more or less water-like or liquid depending on the milieu's conditions and the activity phases of the cell. In the case of being a viscous solid mass, the fluid outside the organelles may be referred to as cytogel, versus using cytosol in more liquid cases. In general, margin regions of the cell are water-like.

The organelles (such as the mitochondria, the chloroplast, lysosomes, peroxysomes, ribosomes, vacuoles, cytoskeletons, and complex cell membrane structures like the endoplasmic reticulums) in the cytoplasm are insoluble.

While all cells possess cytoplasm, cells from different biological domains can differ widely in the characteristics of their cytoplasms. In the animal kingdom, cytoplasm occupies nearly half the cell's volume, while in plant cells, the cytoplasm occupies much less space because of the presence of vacuoles (vacuoles are membrane-bounded compartments within some eukaryotic cells that can serve a variety of secretory, excretory, and storage functions, and are sometimes considered not part of the cytoplasm) (Esau 1965).

References

  • Alberts, B., D. Bray, J. Lewis, M. Raff, K. Roberts, and J. D. Watson. 1989. Molecular Biology of the Cell. New York: Garland Publishing. ISBN 0824036956
  • Esau, K. 1965. Plant Anatomy, 2nd ed. John Wiley & Sons.
  • Nanney, D. L. 1983. “The Ciliates and the Cytoplasm.” The Journal of Heredity 74(3): 163-170.
Organelles of the cell
Acrosome | Chloroplast | Cilium/Flagellum | Centriole | Endoplasmic reticulum | Golgi apparatus | Lysosome | Melanosome | Mitochondrion | Myofibril | Nucleus | Parenthesome | Peroxisome | Plastid | Ribosome | Vacuole | Vesicle

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