PROTOPLASM

Biologists call the internal matrix of the cell protoplasm. Protoplasm is composed of water, proteins, lipids, carbo­hydrates, and electrolytes. Two distinct regions of proto­plasm exist in the cell: the cytoplasm, which lies outside the nucleus, and the karyoplasm or nucleoplasm, which lies inside the nucleus.

Water makes up 70% to 85% of the cell's protoplasm. The second most abundant constituents (10% to 20%) of protoplasm are the cell proteins, which form cell structures and the enzymes necessary for cellular reactions. Proteins can be bound to other compounds to form nucleoproteins, glycoproteins, and lipoproteins. Lipids comprise 2% to 3% of most cells. The most important lipids are the phospho-lipids and cholesterol, which are mainly insoluble in water; they combine with proteins to form the cell membrane and the membranous barriers that separate different cell compartments. Some cells also contain large quantities of triglycerides. In the fat cells, triglycerides can compose up to 95% of the total cell mass. This fat represents stored energy, which can be mobilized and used wherever it is needed in the body. Only a few carbohydrates are found in the cell, and these serve primarily as a rapid source of energy. Potassium, magnesium, phosphate, sulfate, and bicarbon­ate are the major intracellular electrolytes. Small quantities of sodium, chloride, and calcium ions are also present in the cell. These electrolytes help in the generation and trans­mission of electrochemical impulses in nerve and muscle cells. Intracellular electrolytes participate in reactions that are necessary for the cell's metabolism.