1. PENTOSE PHOSPHATE CYCLE
75
Later, it was shown that rust-affected safflower and bean tissues yield
only trace amounts of radioactive ribose (formed from radioactive
glucose) in the neutral compounds fraction (493).
Moisture conditions also govern the oxidation of glucose (494). Thus
the C-6:C-l ratio is about 0.5 when the root tips of corn seedlings are
grown under limiting moisture conditions. When the growth conditions
are quite wet, this C0 2 ratio approaches unity (485).
Even the medium in which respirometry was measured can affect the
outcome of the results. Thus when etiolated seedlings of Sorghum are
grown in distilled water and then the respiratory C0 2 is measured in
phosphate medium, the glucose catabolized via the shunt is markedly
reduced whereas when the measurements were performed in an inorganic nutrient medium, a much greater proportion of the glucose was
metabolized via this pathway (487).
B. THE REDUCTIVE PENTOSE PHOSPHATE CYCLE
Respecting carbohydrate metabolism, the photosynthetic fixation of
carbon dioxide is one that belongs uniquely to plants and a few bacteria.
This process gives rise to a typical pattern (referred to as the labeling
or carbon dioxide fixation pattern, cf. Section XIV) of incorporation of
C
14
from C
14 0 2 into intermediates of the cycle. The pattern is characterized by the presence of radioactivity in phosphorylated intermediates,
particularly, phosphoglycerate, sugar monophosphates and diphosphates,
after brief photosynthesis. In some cases, however, owing to the presence
of some highly active phosphatases, only the corresponding dephosphorylated compounds are found in the pattern. Another distinctive
feature is the C
14 distribution in the hexose molecule. The radioactivity
is concentrated predominantly in the C-3 and the C-4 positions (10),
the C
14 being slightly less in the former position than in the latter (HO).
Furthermore, the radioactivity in positions 1 and 2 is generally greater
than that in positions 5 and 6 (110).
Practically all varieties of higher plant leaves exhibit the so-called
C0 2 fixation pattern of photosynthesis when they are exposed to radioactive carbon dioxide in the light. Examples of these higher plants are
barley seedlings, mature sugar beet, alfalfa, soybean, Vicia faba, Kcdanchoe blossfeldiana, and Crassula arborescens (495). The yew, juniper,
squash, tomato, and ivy plants along with avocado seedlings also display
more or less the same fixation pattern (451). In the dark, the C0 2 fixation pattern in Bryophyllum suggested the presence of carboxydismutase (496).
Arnon and his associates, in a series of classic experiments, have
shown that the chloroplasts are capable of performing all the light and
75
Later, it was shown that rust-affected safflower and bean tissues yield
only trace amounts of radioactive ribose (formed from radioactive
glucose) in the neutral compounds fraction (493).
Moisture conditions also govern the oxidation of glucose (494). Thus
the C-6:C-l ratio is about 0.5 when the root tips of corn seedlings are
grown under limiting moisture conditions. When the growth conditions
are quite wet, this C0 2 ratio approaches unity (485).
Even the medium in which respirometry was measured can affect the
outcome of the results. Thus when etiolated seedlings of Sorghum are
grown in distilled water and then the respiratory C0 2 is measured in
phosphate medium, the glucose catabolized via the shunt is markedly
reduced whereas when the measurements were performed in an inorganic nutrient medium, a much greater proportion of the glucose was
metabolized via this pathway (487).
B. THE REDUCTIVE PENTOSE PHOSPHATE CYCLE
Respecting carbohydrate metabolism, the photosynthetic fixation of
carbon dioxide is one that belongs uniquely to plants and a few bacteria.
This process gives rise to a typical pattern (referred to as the labeling
or carbon dioxide fixation pattern, cf. Section XIV) of incorporation of
C
14
from C
14 0 2 into intermediates of the cycle. The pattern is characterized by the presence of radioactivity in phosphorylated intermediates,
particularly, phosphoglycerate, sugar monophosphates and diphosphates,
after brief photosynthesis. In some cases, however, owing to the presence
of some highly active phosphatases, only the corresponding dephosphorylated compounds are found in the pattern. Another distinctive
feature is the C
14 distribution in the hexose molecule. The radioactivity
is concentrated predominantly in the C-3 and the C-4 positions (10),
the C
14 being slightly less in the former position than in the latter (HO).
Furthermore, the radioactivity in positions 1 and 2 is generally greater
than that in positions 5 and 6 (110).
Practically all varieties of higher plant leaves exhibit the so-called
C0 2 fixation pattern of photosynthesis when they are exposed to radioactive carbon dioxide in the light. Examples of these higher plants are
barley seedlings, mature sugar beet, alfalfa, soybean, Vicia faba, Kcdanchoe blossfeldiana, and Crassula arborescens (495). The yew, juniper,
squash, tomato, and ivy plants along with avocado seedlings also display
more or less the same fixation pattern (451). In the dark, the C0 2 fixation pattern in Bryophyllum suggested the presence of carboxydismutase (496).
Arnon and his associates, in a series of classic experiments, have
shown that the chloroplasts are capable of performing all the light and