Oil bodies are found in many plant seeds and represent an efficient storage form of energy for a germinating embryo. Other, non-oilseed, plants rely primarily on starch as a storage form of energy in their seeds. On a weight basis, oils contain more the twice the amount of energy as starches, because they contain more carbon and hydrogen at- oms and fewer oxygen atoms than starch.
Oil thus represents a more compact storage form of energy. Some high oil content species include castor beans, canola, safflower, peanuts, sunflower, macadamia nuts, and hazelnuts. All of these contain about 50 percent or more oil on a dry-weight basis.
Despite the ubiquitous presence of corn and soybean oil in the marketplace, these oils contain only 5 percent and 17 percent oil, respectively. Although oil bodies are found predominantly in seeds, they may also be present in fruits, such as olives and avocados.
Oil bodies are unique in structure. They are surrounded by single-layer phospholipid membrane, as opposed to the bilayer membrane found around most other plant organelles.
The single-layer membrane is derived from the endoplasmic reticulum (ER), from which oil bodies are believed to originate. Triacylglycerols are synthesized in the ER and accumulate within the two layers of its phospholipid membrane.
Eventually the two layers are separated as oil accumulates and pinches off to form the oil body. The hydrophobic (water-avoiding) tails of the resulting phospholipid monolayer are associated with the oily interior, and the polar heads face outward.
In addition, proteins called oleosins are present in the single-layer membrane and are thought to prevent fusion with other oil bodies. Oleosomes appear to be consistent in size, about 1 micron. Thus, a cell with a large amount of oil will have more oil bodies present.
When seeds imbibe water and germination is initiated, many metabolic activities are activated. In order for a germinating seed to utilize the energy in an oil body, the triacylglycerols must be converted to a form of sugar, typically sucrose, that can be used by the growing embryo.
The sequence of enzymatic events that makes this possible involves the close coordination of three organelles: the oleosome, the mitochondrion, and the highly specialized glyoxysome. It is in the glyoxysome where the many carbon atoms present in the fatty acids of the triacylglycerols are removed, two at a time, in a process called oxidation.
These reactions, together with those of the mitochondria, are often referred to as the glyoxylate cycle. The end products allow for reverse glycolysis, which produces sugars that can be metabolized by typical respiratory pathways in the growing seedling.