Non-living inclusions of the cytoplasm of a cell are materials secreted by the cytoplasm either as reserve substances or as by-products of metabolism. Fats and oils occur in some animal cells as globules and are usually compounds of oleic, palmitic or stearic acids. They may also be found as ethereal oils in many plant cell vacuoles. Glycogen is a common animal carbohydrate inclusion and occurs as granules or may be in solution in the cytoplasm. It may sometimes occur in fungal cells but otherwise is rare in plants.
In mucus-secreting cells of animals, a substance called mucinogen may appear as clear globules. These concentrate over a period and are then passed externally, i.e. secreted as mucus, often leaving the cell with a characteristic goblet shape after its discharge. Yolk is another cell inclusion peculiar to animals. This is held as a reserve substance by the ova of many animals and is a mixture of protein, fats, lecithin and cholestrin. Generally speaking, the solid non-living inclusions of the cytoplasm in plant cells are more variable and the following may be numbered among them.
The food product starch is probably the commonest and occurs as grains in storage cells of cortex, pith and phloem of many plants. Inulin, another carbohydrate food, is stored by some compositae either in solution or as sphaero-crystals. Fruits of the cereals, and many seeds, include special cells where protein is stored in the form of aleurone grains. Each consists of an amorphous protein mass in which are embedded two kinds of bodies. One is an angular crystalloid structure composed of protein, the other being globose that is composed of protein combined with a double phosphate of calcium and magnesium. Crystals of various substances are commonly found in plant cells also. They appear to be by-products of metabolism and include calcium oxalate in bunches of needle-like crystals or raphides, or globose masses of needle crystals forming druses or sphaeroraphides. Calcium carbonate is sometimes found deposited on ingrowths of the cell wall forming the characteristic cystoliths.
Other crystalline materials of varying shapes and sizes have been identified as one or other of silica, gypsum, or organic crystals of carotin, berberin, saponin and nitrogenous substances such as asparagines. An almost universal part of plant cells is the cell sap. This is a watery solution of very varied constituents including several of the following: organic acids, e.g. citric, malic, tartaric, oxalic and so on. The constituents also include alkaloids, amides, glucosides, glycogen, inulin, mucilage and pigments such as anthocyanins(red), anthoxanthins or flavones(yellow), sugars, tannins, proteins, mineral salts and enzymes.