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Within each eukaryotic cell are a number of distinct, membrane-bounded structures, generically called organelles, including the nucleus, mitochondria, the endoplasmic reticulum, and chloroplasts (only found in plants, algae and some protists).
Each organelle is a specialized structure that performs a specific function for the cell as a whole. The rest of the cell, excluding the organelles, cell wall, and plasma membranes, is called the cytosol: the fluid mass that surrounds and provides a home for the organelles.
The cytosol is organized around a framework of fibrous molecules and protein filaments that constitute the cytoskeleton. Although the cytosol consists mostly of water, it contains many chemicals that control cell metabolism, including signal transmission and reception, cellular respiration, and protein transcription factors.
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The cytosol makes up more than 40 percent of the plant cell volume and contains thousands of different kinds of molecules that are involved in cellular biosynthesis. Because cytosol has so much material dissolved in it, it has a gelatinous consistency.
Function
The cytosol provides locations in the cell where chemical activities and energy transformations responsible for growth, repair, and reproduction can occur. Through diffusion and active transport, cytosol collects many essential nutrients from its surroundings, including carbon, hydrogen, oxygen, potassium, nitrogen, phosphorus, and a variety of micronutrients.
The constant motion of the cytosol provides a mechanism for moving and supplying these vital nutrients by ionic transport to the organelles so that they can perform their specific jobs.
Cytosol plays a key role in the vital processes of protein production, sorting, and transportation. All plant proteins are synthesized by ribosomes in the cytosol. Cytosol provides the medium that assists in the transporting of messenger ribonucleic acid (mRNA) to ribosomes, where they synthesize proteins.
The first portion of the amino acid sequence of a protein contains a signal sequence that is checked by proteins in the cytosol. Signal sequences are like street addresses that tell where the growing protein is to be transported.
Some signal sequences direct a ribosome to the endoplasmic reticulum (ER), where the protein sequence is completed, is stored in the ER’s lumen (the space inside the ER), and is eventually transported in a vesicle to another organelle or is exported through the cytosol out of the cell. Proteins lacking a signal sequence are completed by the ribosome and released into the cytosol.