Nutrition of Green Plant Cells - 169 



cell. Also, many plant cells carry on a cyclic 

 streaming (cydosis, p. 198) of the protoplasm, 

 and this assures a more rapid equilibration 

 of absorbed substances. 



The problem of absorption and distribu- 

 tion in the larger land plants is somewhat 

 more difficult. In such forms, the root system 

 accomplishes practically all absorption, since 

 the roots are the only parts that lie in con- 

 tract with the soil water. The water and salts 

 absorbed by the roots are then carried in 

 special vessels throughout the stem and leaf 

 systems, which in some plants reach more 

 than 1 00 feet above ground. 



Constructive Metabolism in Green Plants. 

 Green plant cells use glucose, after it is 

 synthesized, as a source of energy and also as 

 a source of matter for the synthesis of other 

 organic compounds (proteins, carbohydrates, 

 fats, etc.) that are needed for maintenance 

 and growth. Frequently, however, the inter- 

 mediaries of such constructive metabolism, 

 particularly certain amino acids, glycerol, 

 and fatty acids, are synthesized more directly 

 as is shown in Figure 9-6. 



Protein Synthesis. The formation of proteins 

 in plant cells depends upon a preliminary 

 synthesis of the various amino acids. All 

 amino acids can be synthesized by plant cells, 

 provided that glucose (or precursors of glu- 

 cose) and inorganic salts are available. Glu- 

 cose or its precursors (Fig. 9-6) provides the 

 plant cell not only with energy for amino 

 acid synthesis, but also with matter, to form 

 the carbon-hydrogen parts of the molecules. 

 The plant derives nitrogen, sulfur, and other 

 constituents of the amino acids from the in- 

 organic salts. 



As a source of nitrogen, for the amino 

 ( — NH ; ) portions of the amino acids, typical 

 plant cells cannot draw upon the free nitro- 

 gen (No) of the atmosphere. But the plant 

 does possess the proper catalytic equipment 

 for using nitrogen in the form of nitrate salts 

 (NaNO s , KN0 3 , etc.). Similarly the plant 

 utilizes sulfur (for the synthesis of some of 

 the amino acids) chiefly in the form of the 

 sulfate ( — SQ 4 ) salts. 



The reducing potential necessary for the 

 conversion of the nitrate ( — NO,,) to the 

 amino ( — NH 2 ) form of nitrogen and for the 

 conversion of the sulfate ( — S0 4 ) to the sul- 

 hydryl ( — SH) form of sulfur is provided by 

 TPN • H, and other reduced primary ac- 

 ceptors generated by photosynthesis. Precisely 

 how the reduced nitrogen and sulfur radicals 

 become affiliated with the carbon frameworks 

 of the various precursors is not fully known, 

 although it is plain that a number of amino 

 acids begin to appear in the green plant cell 

 within a few minutes subsequent to illumina- 

 tion. But given nitrates as a source of nitro- 

 gen and sulfates as a source of sulfur, green 

 plants enjoy a full measure of growth. The 

 nitrate content of the soil and of fertilizers is 

 a prime consideration. If the nitrate supply 

 is limited, the formation of amino acids 

 within the plant is reduced; and since new 

 proteins are formed solely by dehydration 

 synthesis from previously formed amino 

 acids the growth of the plant is likewise 

 limited. 



Carbohydrate Synthesis. During periods of 

 abundant light, the plant cell produces glu- 

 cose in excess of its current needs. This excess 

 glucose is converted mainy into starch (Fig. 

 9-6). In many plant cells the enzymes re- 

 sponsible for starch synthesis are localized in 

 small visible bodies, the pyrenoids (Figs. 9-7 

 and 9-8); and consequently starch grains first 

 appear near this part of the cell. Starch repre- 

 sents a reserve of glucose; during periods of 

 darkness starch may be hydrolyzed, yielding 

 an immediate supply of glucose. Cellulose, 

 for the cell walls and other structural parts, 

 is also derived by dehydration synthesis from 

 photosynthesized glucose. 



Other Syntheses. Some complex proteins 

 contain sulfur, phosphorus, and iron, which 

 are derived from the inorganic salts of these 

 elements absorbed by the plant. Like other 

 cells, plant cells can form fats and other 

 lipids from carbohydrates, and many of the 

 intermediary stages of these transforma- 

 tions now are known (see Figs. 8-5 and 

 9-6). 



