cell, the structural unit of living organisms, the smallest unit capable of independent existence. yeast and bacteria are examples of single cells while one grapevine has billions of cells. Each plant cell consists of a protoplast surrounded by a cell wall, but it can be differentiated into a host of forms. A cell in the flesh of a ripe grape berry, for example, can be 0.5 mm/0.02 in in length with a thin, wavy cell wall lined by an equally thin cytoplasm surrounding the vacuole, a ‘sea’ of water with dissolved sugars, acids, and hundreds of other solutes, otherwise known as grape juice. Other cells within the berry can be entirely different. A phloem element aligns end to end with others to form a tube through which elaborated sap moves in a network throughout the plant. Adjacent fibre cells in the xylem are long and slim with thick walls hardened by the wood polymer, lignin. The complex functioning of the leaf provides other examples of cell forms: stomatal cells function to allow carbon dioxide in and oxygen and water vapour out; others function as the ‘carbohydrate factory’; and cells in its dense network of veins are designed to move water and minerals up from the roots (in the xylem) and to move sugars and other elaborated organic nutrients out to the rest of the plant (in the phloem).
