Page 274 - Biology_F5

Nutrition

(Ribulose biphosphate carboxylase) enzyme, Carboxylation is followed by a chain of
which is abundant in the stroma of the reactions which involve reduction of PGA
chloroplast, and it is usually abbreviated as Reduction of phosphoglyceric acid
RuBP carboxylase, or RuBisCo, or Rubisco, (PGA)
or RuBPCase, or RuBPco. The PGA are the The reduction reaction begins when energy
first stable product of photosynthesis and supplied by ATP and the reducing power of
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their formation marks the first major step NADPH are used to remove oxygen from
in carbon dioxide fixation in green plants the PGA. This reduction reaction produces
and photoautotrophic bacteria into energy a 3-phosphoglyceraldehyde (3-PGAL or
rich molecules. PGAL), which is also called glyceraldehyde

RuBP + CO + H O RuBP Carboxylase 2PGA phosphate (GP), a triose sugar (3-Carbon
2
2
(5C) (3C) sugar) containing phosphate group.
ATP ADP + Pi

PGA PGAL
Phosphoglyceric acid Phosphoglyceraldehyde
(3 carbon acid) NADP + H O (3 carbon sugar)
+
NADPH+H + 2
The difference between the PGA and the phase of the Calvin cycle to regenerate
PGAL is that, the former is a 3-carbon RuBP consumed in the first reaction of
acid (-COOH) while the latter is 3-carbon carbon dioxide fixation.
aldehyde (-CHO), therefore, PGAL is the Regeneration of RuBP
first carbohydrate in photosynthesis which RuBP is an important molecule utilised
is a sugar with more chemical energy than in the synthesis of sugars, therefore, it is
PGA. It should be noted that the ATP and necessary for plants to produce enough of
NADPH utilised in this phase of Calvin it for synthesis of more sugars and other
cycle are obtained from the light dependent vital molecules. RuBP is regenerated
reaction. The ADP and NADP return to from PGAL, which combines with
+
the thylakoids to be converted back to ATP Ribulose phosphate (RuP) molecules
and NADPH, respectively, by the light as summarised in the equation below.
reaction. One of the PGAL molecules is This process involves a complex series
set aside to be used as a building block of rearrangement of carbon atoms
of glucose and other molecules, such as between sugar phosphates to generate
sucrose, starch, cellulose, amino acids, 5-carbon sugar from 3 carbon-sugar. The
fatty acids and glycerol. These products formed 5-carbon sugar (RuP) is then
phosphorylated by the remaining ATP
are used for various biological functions, from light reaction to generate ribulose
including respiration, building plant cell bisphosphate (RuBP). The produced
wall, building the body, and component of RuBP can combine with the additional
cell membranes. The majority of PGAL carbon dioxide molecules and continue to
molecules will be forwarded to the third
the Calvin cycle reaction.

Form Five Student’s Book
267