The route from the 3-carbon PGA to the 5- 

 carbon ribulose phosphate is very indirect, 

 however. It involves the formation of a num- 

 ber of interesting intermediaries, including 

 4-carbon (tetrose) and 7-carbon (heptose) 

 sugars, as combinations and recombinations 

 occur in the metabolic vortex. It is a cycle in 

 the sense that the same intermediaries keep 

 appearing over and over again. But only part 

 of the PGA is cycled back to replenish the 

 supplies of ribulose diphosphate. The rest 

 goes into the building of glucose, amino 

 acids, and other molecules essential to the 

 life of the cell — as is shown in Figure 9-6. 



As will be explained later (p. 181), a few 

 bacteria can synthesize glucose from COo and 

 H L ,0 at the expense of energy, derived not 

 from light, but from the oxidation of certain 

 inorganic compounds. This process of chemo- 

 synthesis is of very limited occurrence, how- 

 ever. Almost all organic matter, which sup- 

 ports the life of this planet, comes into being 

 as a result of the photosynthetic powers of 

 the green plants. Considerably more than a 

 billion tons a day of assorted carbohydrates, 

 proteins, lipids, and other organic com- 



Nutrition of Green Plant Cells - 1 67 



pounds are produced by green plants, al- 

 though about 90 percent of this huge pro- 

 duction comes from primitive, mainly uni- 

 cellular algae in the sea. 



OTHER ASPECTS OF PLANT NUTRITION 



Nutrition among green plants is much sim- 

 plified by the fact that a plant does not need 

 to take in any preformed organic foods. All 

 the foods are simple crystalloidal substances, 

 present practically everywhere, and conse- 

 quently they can be absorbed directly from 

 the environment. Thus plants generally do 

 not possess any digestive system, and the 

 holophytic mode of nutrition does not in- 

 clude any processes ingestion, digestion, and 

 egestion. Accordingly, nutrition in typical 

 plants begins with the process of absorption. 



Absorption in Unicellular and Colonial 

 Plants. Virtually all simple green plants that 

 lack roots, stems, and leaves are designated 

 collectively as the algae. Many of the algae, 

 such as Closterium and Chlamydomonas 

 (Fig. 9-7), are unicellular; but some, such as 

 Spirogyra (Fig. 9-8), are colonial, and others, 



Fig. 9-7. Unicellular green algae; A, 

 Closterium; B, Chlamydomonas. n, nu- 

 cleus; cy, cytoplasm; ch, chloroplast; p, 

 pyrenoid; s, starch grains; v, vacuole; 

 cw, cell wall; f, flagellum; e, red pig- 

 mented "eye spot." 



