Photosynthesis in Higher pLants 3

Question 1

what is dark phase

Answer 1

ATP and NADPH are used to
drive the processes leading to the synthesis of food, more accurately, sugars.
This is the biosynthetic phase of photosynthesis. This process does not
directly depend on the presence of light but is dependent on the products
of the light reaction, i.e., ATP and NADPH, besides CO2 and H2O. You may
wonder how this could be verified; it is simple: immediately after light
becomes unavailable, the biosynthetic process continues for some time,
and then stops. If then, light is made available, the synthesis starts again. This occurs in the stroma.

Question 2

what did melvin calvin do

Answer 2

Calvin and Benson based on algal photosynthesis using radiactive c-14 isotopes, came to a conclusion on the Calvin cycle/ C3 cycle. Calvin
pathway occurs in all photosynthetic plants

Question 3

what are the steps of calvin cycle

Answer 3

• Carboxylation
  -Reduction
• Regeneration

Question 4

describe carboxylation

Answer 4

Carboxylation is the fixation of CO2
into a stable organic
intermediate. Carboxylation is the most crucial step of the Calvin cycle
where CO2
is utilised for the carboxylation of RuBP( Ribulose 1,5 Bisphosphate) . This reaction is
catalysed by the enzyme RuBP carboxylase which results in the formation
of two molecules of 3-PGA.( 3 Phosphoglyceric Acid/3-phosphoglycerate) Since this enzyme also has an oxygenation
activity it would be more correct to call it RuBP carboxylase-oxygenase
or RuBisCO.

3-Phosphoglyceirc acid is the 1st stable product of calvin cycle so it is also called as C3 cycle

Question 5

what happens in reduction phase

Answer 5

These are a series of reactions that lead to the formation
of glucose. The steps involve utilisation of 2 molecules of ATP for
phosphorylation and two of NADPH for reduction per CO2
molecule
fixed. It resulsts in the formation of Glyceraldehyde3-Phosphate or Triose Phasphate.

The fixation of six molecules of CO2
and 6 turns of the cycle are
required for the formation of one molecule of glucose from the pathway.

Question 6

what happens in regeneratinon

Answer 6

Regeneration of the CO2
acceptor molecule RuBP is
crucial if the cycle is to continue uninterrupted. The regeneration
steps require one ATP for phosphorylation to form RuBP from triose phosphate.

Question 7

why does cyclic photophosphorylation occur

Answer 7

Hence for every CO2
molecule entering the Calvin cycle, 3 molecules
of ATP and 2 of NADPH are required. It is probably to meet this difference
in number of ATP and NADPH used in the dark reaction that the cyclic
phosphorylation takes place.

Question 8

How much
(i) ATP
(ii) NADPH
is used for making 1 glucose molecule

Answer 8

To make one molecule of glucose 6 turns of the cycle are required.
(i) 18 ATP
(ii) 12 NADPH

Question 9

how does this conversion of triose phosphate to glucose and to rubp take place?

Answer 9

in 6 cycles, 12 triose phosphate molecules are produced. 2 of them are used for making Glucose and remaining 10 are used for making 6 molecules of RuBP.

Question 10

what kind of enzyme is rubisco?

Answer 10

RuBisCO that is the most abundant enzyme in the world (Do you
wonder why?) is characterised by the fact that its active site can bind to
both CO2
and O2
– hence the name. Can you think how this could be
possible? RuBisCO has a much greater affinity for CO2
when the CO2
: O2
is nearly equal. Imagine what would happen if this were not so! This
binding is competitive. It is the relative concentration of O2
and CO2
that
determines which of the two will bind to the enzyme.

Question 11

when does photorespiration occur

Answer 11

-when temperature is high
-when oxygen levels are high

Question 12

what happens in photorespiration

Answer 12

In C3
plants some O2
does bind to RuBisCO, and hence CO2
fixation is
decreased. Here the RuBP instead of being converted to 2 molecules of
PGA binds with O2
to form one molecule of phosphoglycerate and
phosphoglycolate (2 Carbon) in a pathway called photorespiration.

phosphoglycolate through a series of changes enters peroxisomes, where it is converted to glycine. Glycine enters the mitochondria, where it is decaboxylated to get converted to serine. Serine moves back to peroxisomes where using reducing power of NADPH2 serine forms glycerate. Glycerate uses ATP to regenerate PGA

Question 13

what is the photorespiratory pathway

Answer 13

n
the photorespiratory pathway, there is neither synthesis of sugars, nor of
ATP. Rather it results in the release of CO2
with the utilisation of ATP. In
the photorespiratory pathway there is no synthesis of ATP or NADPH.

Question 14

significance of photorespirartin

Answer 14

It is considered to be wasteful because:
-> involves the loss photosynthetically fixed carbon
-> doesnt yield atp/nadph but uses it
The biological function of photorespiration is not known yet.

Question 15

why does photorespiration not occur in C4 plants

Answer 15

In C4
plants photorespiration does not occur. This is because they
have a mechanism that increases the concentration of CO2
at the enzyme
site. This takes place when the C4
acid from the mesophyll is broken
down in the bundle sheath cells to release CO2
– this results in increasing
the intracellular concentration of CO2
. In turn, this ensures that the
RuBisCO functions as a carboxylase minimising the oxygenase activity.