Some plants use lipids as a fuel stored as minute globules in the seed

There are many hundred varieties of plants known to have oil-bearing seeds, but only a few are significant commercially. These are listed in Table 3.2 with their fatty acid compositions. Most are important sources of edible oils for human foods or animal feeds, but some are used for other industrial purposes such as paints, varnishes and lubricants. Although the seed is the most important organ for the storage of triacylglycerols, some species store large quantities of oil in the mesocarp or pericarp of the fruit surrounding the seed kernel. The avocado is a familiar example of a plant with an oily meso-carp and has been the subject of extensive studies of fatty acid biosynthesis. The mesocarp of the oil palm is a commercially important source of palm oil, used in soap and margarine making, which is quite distinct in chemical and physical properties from the seed oil of the same plant: palm kernel oil (Fig. 3.3 and Table 3.2). Droplets of triacylglycerols are also present in the seeds of plants that store predominantly carbohydrates (starch) as the primary source of fuel for seed germination. These include legumes such as peas or beans and cereals like wheat or barley.

Microscopic examination of a mature oil seed or one in the active phase of oil accumulation reveals a cytoplasm packed with spherical organelles that consist mainly of triacylglycerols, called oil bodies (Fig. 3.4). Unfortunately, the nomenclature has not

Oil Palm Pericarp
Fig. 3.3 Section through an oil palm fruit. Note the location of the two quite distinct types of oil, palm oil and palm kernel oil. Reproduced with kind permission of Dr L.H. Jones.

been standardized and you may also encounter terms such as lipid bodies, storage oil bodies, storage oil droplets, oleosomes, lipid-containing vesicles and reserve oil droplets, all of which are synonymous. Some authors have used the term spherosomes to refer to the same organelle, but it should strictly be used to describe another particle in plant cells that has a high content of phospholi-pids.

The core of the oil body is composed mainly of triacylglycerols, with minor quantities of other hydrophobic lipids such as sterols, hydrocarbons and carotenoids. The oil body is surrounded by a half-unit membrane of protein and phospholipids. The origin of the oil body continues to be controversial and Murphy's review (see Further Reading) refers to several early theories. A current view is illustrated in Fig. 3.5. TAG are synthesized in the endoplasmic reticulum as described in Section Accumulation of oil in the space between the membrane bilayers results in mem-

Table 3.2 The fatty acid composition of some vegetable oils* (g per 100 g of total fatty acids)

8:0 10:0 12:0 14:0 16:0 18:0 18:1 18:2 18:3 20:1+ total n-9 n-6 n-3 22:1

8:0 10:0 12:0 14:0 16:0 18:0 18:1 18:2 18:3 20:1+ total n-9 n-6 n-3 22:1

A. Major edible oil crops

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