170 - The Cell 



In general, the synthetic powers in plants 

 are greater and more varied than in animals. 

 Besides synthesizing carbohydrates, fats, and 

 all the various amino acids, plants can also 

 synthesize a wide variety of other compounds 

 that animals cannot. Such compounds in- 

 clude not only chlorophyll and other special 

 pigments, but also all the vitamins (p. 346), 

 and many other useful substances, such as 

 drugs, perfumes, etc. Moreover, in producing 

 this wide variety ol organic compounds, green 

 plants utilize nothing but inorganic sub- 

 stances that are absorbed from the environ- 

 ment. 



Destructive Metabolism. As to destructive 

 metabolism, plant and animal cells are es- 

 sentially alike (see Fig. 8-5). But the metab- 

 olism of plants is simpler because plant cells 

 depend almost solely upon glucose as a source 

 of energy when light is not available. The 

 supply of synthesized glucose is adequate to 

 balance the energy expenditures of the 

 plant, and consequently other fuels are sel- 

 dom utilized. In particular, amino acids and 

 proteins are not sacrificed for energy, except 

 under conditions of starvation — as, for ex- 

 ample, when a plant is deprived of light for 

 a long period. The plant limits the synthesis 

 of amino acids to its constructive needs, and 

 thus no excesses are available for energy, and 

 deamination seldom occurs. 



The energy expended by a plant in de- 

 structive metabolism goes mainly to foster 

 constructive metabolism. The plant does not 

 have to move about in search of food and 

 thus expends less total energy than the ani- 

 mal. In using a lesser proportion of its ma- 

 terials for destructive metabolism, the plant 

 has more available for constructive purposes. 

 Consequently plants in general grow more 

 rapidly than animals. 



Respiration in Plants. All plant cells con- 

 stantly use oxygen and produce carbon diox- 

 ide, as a result of oxidative metabolism. But 

 during periods when light is available, the 

 green plant cell produces 15 to 30 times more 

 oxygen than it uses, and consumes much more 

 carbon dioxide than it produces. Conse- 



quently in the daytime, respiration (taking 

 in On and giving off CO;,) is overbalanced by 

 the gas exchange of photosynthesis (taking 

 in C0 2 and giving off O.,), which is just the 

 opposite. But in darkness, when photosyn- 

 thesis is suspended, all plant cells carry on a 

 small but measurable respiration. 



Gas exchanges in a simple submerged plant 

 such as Closterium (Fig. 9-7) proceed on a 

 purely diffusional basis. While carbon diox- 

 ide is being used up faster than it is being 

 produced, the CO.. concentration in the cell 

 remains lower than in the surrounding water; 

 and while oxygen is being formed faster than 

 it is consumed, the 2 concentration in the 

 cell remains relatively high. Consequently, 

 during periods when light is available the 

 gas exchange of photosynthesis predominates. 

 But when photosynthesis ceases, the concen- 

 tration of oxygen inside the cell sinks below 

 the level of dissolved oxygen in the outside 

 water, and the quantity of carbon dioxide 

 rises above that of the environment. Under 

 these circumstances, therefore, respiration 

 holds sway. 



Excretion in Plants. Very little excretion is 

 necessary in green plants. Plants produce 

 scarcely any metabolic wastes other than car- 

 bon dioxide and water — and these end prod- 

 ucts are reused during photosynthesis. Inso- 

 far as small quantities of other wastes are 

 produced, these likewise may be consumed 

 again in constructive metabolism. To a re- 

 markable extent, therefore, it mav be said 

 that plant cells are able to "burn their own 

 smoke." In a few cases, certain plant cells 

 tend to accumulate special waste products, 

 depositing them as insoluble crystals in the 

 protoplasm or vacuoles. 



When photosynthesis is in abeyance, car- 

 bon dioxide and water tend to accumulate. 

 The carbon dioxide passes off in respiration, 

 leaving water as the sole excretory waste pro- 

 duced in any abundance. In aquatic forms 

 such as Closterium , this excess water also 

 passes out into the environment. Despite the 

 fact that the surrounding fresh water is very 

 hypotonic, the high turgor of the cell estab- 



