Storage Products

Many cells exist in an environment in which resources alternate between abundance and scarcity. To survive times of scarcity, cells must accumulate and store extra nutrients. Most often, the reserves consist of sugars that have been polymerized into starch in amyloplasts or converted into lipids and stored as large oil droplets (spherosomes or lipid bodies) in oily material like peanuts and sunflower seeds (Fig. 1).  

FIGURE 1: Avocado contains very large numbers of oil bodies; this view was prepared merely by smearing a small amount of ripe avocado fruit on a microscope slide. Virtually every visible thing is an oil body; other organelles are much less abundant (X 200). 

But with many other storage products, the function and advantages are not so obvious. Many plants store crystals of calcium oxalate or calcium carbonate (Fig. 2); others accumulate large amounts of silica, tannins (Fig. 3), or phenols. Because plants have no excretory mechanism, numerous waste products must be stored within the cells.

FIGURE 2: Calcium oxalate crystals. When calcium is part of a crystal, it is physiologically inactive and cannot affect membranes or the tertiary structure of proteins. These are cubic crystals in wood (X 1000). 

FIGURE 3: (a) This type of preparation is called a leaf clearing: A leaf was treated to make its tissues transparent; then it was stained to reveal certain contents. The dark red, irregularly shaped vacuolar contents are tannins, which can denature proteins. They have a bitter taste and damage the mouth and stomach proteins of insects that try to eat them. The band running vertically through the micrograph is a leaf vein (X 200). (b) The bark of a cactus. As its cells aged, they accumulated irregular masses of tannin that have stained red (X 80).