Page 275 - Biology_F5

Biology for Advanced Level Secondary Schools

ATP ADP

PGAL Rearrangement of carbon atoms RuP RuBP
Phosphoglyceraldehyde Ribulose phosphate Ribulose bishosphate
(3C) (5C) (5C)

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Note that in order to form a glucose consumed to synthesise twelve molecules
molecule, the cycle has to run six times of PGAL, whereby only two molecules
because each turn of the cycle adds only of PGAL are used to make carbohydrate
one carbon from the incoming carbon (glucose), while ten molecules are used
dioxide to form two molecules of PGAL, to regenerate 6 molecules of RuBP in the
that is, six molecules of carbon are regeneration phase (Figure 5.5).

6 molecules
CO 2
Stage 1
Carbon fixation

6 molecules 12 molecules of
RuBP 3-PGA
6ADP 12ATP
6ATP Calvin Cycle 12ADP + 12Pi
6 molecules RuP Stage 2
Stage 3 Reduction
Regeneration of RuBP +
10 molecules 3-PGAL 12NADPH+12H
12 molecules 12NADP +
2 molecules 3-PGAL 3-PGAL

Sugars example
Glucose (C H O )
2
12
6
and other products
Figure 5.5: The Calvin cycle
Exercises 5.2 C and C plants
3 4
1. Explain how the light and dark Task 5.7
reactions of photosynthesis are Search for information from the library
interdependent. and internet sources about C and C
3
2. Describe the steps involved in the plants. Note down their differences. 4
conversion of phosphoglyceric acid
into sugar.
3. What is the fate of PGAL formed Different plants use different pathways
during photosynthesis? to fix carbon during the process of
photosynthesis. Fixing carbon is the way
4. Naming Calvin cycle as a dark reaction plants take carbon from atmospheric
is somehow misleading. Substantiate.

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