Although diverse in their organization, all eukaryotic cells have in common structures that perform unique functions. Eukaryote cells are larger and have more specific parts in compartments divided by membranes called organelles. The prokaryote cells do not have compartments and they do not possess a demarcated nucleus as the eukaryotes do. When seen under a microscope, three major components of the cell become evident the nucleus, the cytoplasm, and the cell membrane in the eukaryote cells (Fig. 4.1).
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 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 phospholipids 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 fat cells, triglycerides can comprise 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 (approximately 1%) are found in the cell, and these serve primarily as a rapid source of energy. Potassium, magnesium, phosphate, sulfate, and bicarbonate ions are the major intracellular electrolytes. Small quantities of sodium, chloride, and calcium ions are also present in the cell. These electrolytes participate in reactions that are necessary for the cell’s metabolism, and they help in the generation and transmission of electrochemical impulses in nerve and muscle cells.