"In cell biology, the cytoplasm is the material within a living cell, excluding the cell nucleus. It comprises cytosol (the gel-like substance enclosed within the cell membrane) and the organelles – the cell's internal sub-structures. All of the contents of the cells of prokaryote organisms (such as bacteria, which lack a cell nucleus) are contained within the cytoplasm. Within the cells of eukaryote organisms the contents of the cell nucleus are separated from the cytoplasm, and are then called the nucleoplasm. The cytoplasm is about 80% water and usually colorless."
Cytoplasm has three basic functions within the cells of living organisms. Made of three basic components, cytoplasm is a medium of suspension for the organelles in the cell. The function of cytoplasm is also a means of transport for genetic material and the products of cellular respiration. As cytoplasm is a fluid, it acts as a buffer, protecting the cell's genetic material and organelles from damage due to movement or collision with other cells.
The three main parts of cytoplasm are cytosol, the organelles and cytoplasmic inclusions. Cytosol is the liquid that suspends the organelles; it is mainly water with some protein strands that help support the organelles. Organelles are specialized parts of the cell, each having its own function; major functions of the organelles include cellular respiration, creation of new proteins and destruction of waste material. Finally, cytoplasmic inclusions are non-soluble molecules floating within the cytoplasm; in many cells, these inclusions are stored fats and sugars ready for cellular respiration.
Cytoplasm's primary function is to act as a medium of suspension for a cell's organelles, keeping a cell's inner structure intact. As organelles are not neutrally buoyant in cytoplasm due to the changing concentrations of solutes, the protein strands described in the previous paragraph are necessary to keep organelles in place. The cytoplasm and proteins prevent gravity from grouping the organelles near the bottom of the cell, an event that would greatly impede their function.
Each cell is a genetic and molecular factory. A secondary, but no less important, function of cytoplasm is to act as a means of transport for materials the cell uses and produces. Everything from the building blocks of RNA to the energy storage units each cell needs to survive all travel through the cytoplasm. In this regard, the function of cytoplasm has an added benefit: molecular building blocks can simply float through the cytoplasm until needed. No extra storage unit is needed.
The final function of cytoplasm is to offer protection. All cells experience movement in one form or another. Contact with other cells or outside surfaces is all but certain. The cytoplasm acts as a buffer in these cases, protecting organelles from the shock of impact. This feature is especially important for the lysosome, an organelle. Lysosomes contain enzymes to break down a cell's waste. If cytoplasm was not present, the lysosome would likely rupture during impact, its released enzymes killing the entire cell.
The cytosol is a complex mixture of substances dissolved in water. Although water forms the large majority of the cytosol, its structure and properties within cells is not well understood. The concentrations of ions such as sodium and potassium are different in the cytosol than in the extracellular fluid; these differences in ion levels are important in processes such as osmoregulation, cell signaling, and the generation of action potentials in excitable cells such as endocrine, nerve and muscle cells. The cytosol also contains large amounts of macromolecules, which can alter how molecules behave, through macromolecular crowding.
Although it was once thought to be a simple solution of molecules, the cytosol has multiple levels of organization. These include concentration gradients of small molecules such as calcium, large complexes of enzymes that act together to carry out metabolic pathways, and protein complexes such as proteasomes and carboxysomes that enclose and separate parts of the cytosol.