PHOTOSYNTHESIS LIGHT INDEPENDANT REACTION

LIGHT INDEPENDANT REACTION

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D
Light-independent reactions
3.3 Stage two of photosynthesis: the Calvin cycle
The light-independent stage of photosynthesis is the second and final set of reactions. It is named so because the
reactions involved do not need light to occur, and so technically can take place without light. However, the products (ATP
and reduced NADP) of the light-dependent reactions are required, and so in practice, these reactions will not continue for
very long without light, as the light-dependent stage will stop producing ATP and NADPH.
The light-independent reactions occur in the stroma of the chloroplast (fluid matrix surrounding the grana). It is also
called the Calvin cycle, named after Melvin Calvin et al. (1946-53). Whilst the products of the light-dependent stage (ATP
and NADPH) are needed for this stage, carbon dioxide is also required. This comes from the atmosphere. Carbon dioxide
diffuses through the stomata of leaves, and enters leaf cells and then the chloroplasts.
The Calvin cycle
The Calvin cycle cannot take place without carbon dioxide. When it enters a leaf, diffusing through stomata (almost
exclusively on the underside of the leaf), it diffuses through tiny air spaces in the spongy mesophyll layer, until it reaches
the palisade mesophyll layer. It then diffuses through a thin cellulose cell wall, the plasma membrane, the cytoplasm and
through the chloroplast envelope (double membrane), into the stroma.
CO2
RuBisCO
lipids
fatty acids
ribulose x2 glycerate
(5C) (3C) amino acids
biphosphate 3-phosphate
(RuBP) (2GP)
ADP + Pi NADPH
ATP ATP NADP
x2 triose ADP + Pi
(3C)
phosphate
(2TP)
hexose sugars lipids
glycerol
cellulose (e.g. glucose)
starch
polysaccharides
(e.g. glycogen)
1 The carbon dioxide combines with a molecule called ribulose biphosphate (RuBP), a five-carbon compound and a
carbon dioxide acceptor. This reaction is catalysed by RuBisCO (ribulose biphosphate carboxylase-oxygenase), an
enzyme which adds a carboxyl group to the RuBP molecule
2 This combination forms an unstable six-carbon compound, before splitting into the products of the reaction, which are
two molecules of glycerate 3-phosphate (GP), a three-carbon compound, and at this stage the carbon dioxide is fixed
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The GP molecules are reduced (using two hydrogen atoms donated by reduced NADP from the light-dependent stage)
and then phosphorylated (using an inorganic phosphate group from one molecule of ATP, also from the light-
dependent stage) to form another three-carbon compound called triose phosphate (either TP or GALP)
4 Five out of every six molecules of TP/GALP are recycled via phosphorylation (using another ATP molecule from the
light-dependent stage of photosynthesis) into three more molecules of RuBP (five-carbon compound), the remaining
molecule of TP produced…read more

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