Page 273 - Biology_F5

Biology for Advanced Level Secondary Schools

Questions of reactions are described as dark reactions
1. Why was it important to destarch the does not imply that the reactions cannot take
leaf before starting the experiment? place in the presence of light. It simply means
2. What was the reason for covering the that light energy is not a vital factor in this
phase of photosynthesis process. The vital
leaf with a black paper? factor here is atmospheric carbondioxide,
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3. What was the reason for boiling the and the products of light reaction which
leaf in alcohol? are ATP and NADPH. Dark reaction is
4. Which colour was observed after therefore a synthetic stage of photosynthesis.
adding Iodine solution on a boiled leaf? However, such reactions, as cited earlier are
5. What was the implication of the colour energetically dependent on light reactions.
change after addition of iodine solution The dark reactions involve a chain of enzyme
to the leaf? controlled reactions described by Melvin
Calvin, Andrew Benson and James Bassham
Exercise 5.1 in 1946-1953. These reactions, involve a full
cycle of reactions which are also called the
1. Explain in detail how plants Calvin cycle or Calvin-Benson cycle; named
obtain light energy and convert it after Calvin and Benson. The Calvin cycle
to biochemical energy. (Figure 5.5) has three major phases which are

2. Briefly explain the importance of carbondioxide (CO ) fixation, reduction of
2
cyclic photophosphorylation in glycerate phosphate (GP) or phosphoglyceric
photosynthesis. acid (PGA), and regeneration of ribulose
biphosphate (RuBP).

Dark reaction (Light independent Carbon dioxide fixation
reactions) Carbondioxide (CO ) used in dark reaction
2
comes from the atmosphere. It diffuses
Task 5.6 into the plant tissue via stomata pores
Search from internet sources the located in the leaves or stems of plants.
simulations showing dark reaction, then The absorbed CO is attached to a 5-carbon
2
make short notes. compound called RuBP to form unstable
6-carbon compound by carboxylation
Dark reaction does not require light energy, reaction. The 6-carbon molecule quickly
and it takes place in the stroma of the splits into two 3-carbon energy rich
chloroplasts. The overall purpose of dark molecules called glycerate phosphate
reaction is to convert carbon dioxide from
the atmosphere into carbohydrates or sugars which, in principle are carboxylic acid,
which are used to power primary activities 3-phosphoglyceric acid (3-PGA or PGA).
in plants and build their structures. The
process of converting carbon dioxide into RuBP is therefore called the carbon dioxide
carbohydrates requires energy produced acceptor due to its ability to accept or combine
by ATP and the reducing power obtained with carbondioxide. The reaction is catalysed
from NADPH, both produced from the by Ribulose 1,5-biphosphate carboxylase
light reaction. The fact that these chains

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