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- Here, 2 ATP molecules for phosphorylation and two
NADPH for reduction per CO2 molecule are used.
Atmosphere
�COz+HzO
RuBP
- It then forms other 4-carbon acids like malic acid or
aspartic acid. They are transported to bundle sheath cells.
Mesophyll
cell
Atmospheric CO,
tm,t0A�m1;;
xation
• acid
nsport
I
3-PGA
• acid
+
rans
Fixation by
l Calvin cycle
I
ATP
Triose
phosphate
I
,----'---,'CO
+
P,+NADP•
Sucrose, starch
3. Regeneration of Ru BP
- It requires one ATP for phosphorylation to form RuBP.
To make 1 glucose molecule, 6 turns of the cycle are needed.
In
Out
What does go in and
1 glucose
6C02
come out of the Calvin
18ATP
18ADP
cycle?
12 NADPH
12 NADP
C4 PATHWAY (HATCH & SLACK PATHWAY)
- It is present in plants adapted to dry tropical regions.
- The large cells around the vascular bundles of theC4 plants
are called bundle sheath cells. Such anatomy is called
'Kranz' anatomy.
Steps of Hatch and Slack Pathway
- Primary CO2 acceptor is phosphoenol pyruvate (PEP) - a
3-carbon molecule seen in mesophyll cells. The enzyme for
this fixation is PEP carboxylase (PEPcase).
- The mesophyll cells lack RuBisCO enzyme.
- TheC4 acid OAA is formed in the mesophyll cells.
- In the bundle sheath cells, C4 acids are broken down to
release CO2 and aC3 molecule.
- The CJ molecule is transported back to mesophyll where it
is converted to PEP again.
- The releasedCO2 enters theC3 pathway.
- Bundle sheath cells are rich in RuBisCO, but lack PEPcase.
ThusC3 pathway is common to C3 & C4 plants.
PHOTORESPIRATION
- InCalvin pathway, RuBP combines with CO2.
Rubisco
RuBP+ CO2------+ 2 x 3PGA
- RuBisCO has a greater affinity forCO2 than for 02.
- InC3 plants, some 02 bind to RuBisCO. Hence CO2 fixation is
decreased.RuBP binds with 02 to form phosphoglycerate &
phosphoglycolate. This is called photorespiration.
- In this, there is no synthesis of sugars, ATP and NADPH.
Hence photorespiration is a wasteful process.
- In (4 plants, photorespiration does not occur because
they can increase CO2 concentration at the enzyme site. So
C4 plants have better productivity.
Differences between (3 and (4 plants
(3 lants
1. Photos nthesis occurs in meso h II cells.
2. Kranz ana
3. RuBP is th
4. 3-PGA,
roduct.
5. Chloro
6. Photo
(4 )ants
In meso h II and bundle sheath cells.
Present.
PEP is the ri
8. E..Rice, wheat, bean, otato.
E..Maize, su arcane, amaranth, sor hum.
hi h.
FACTORS AFFECTING PHOTOSYNTHESIS
- Internal (plant) factors: Number, size, age & orientation
of leaves, mesophyll cells and chloroplasts, internal CO2
concentration and amount of chlorophyll.
- External
factors:
Sunlight,
temperature, CO2
concentration and water.
- Blackman's Law of Limiting Factors: "If a biochemical
process is affected by more than one factor, its rate is
determined by the factor nearest to its minimal value: it is
the factor which directly affects the process if its quantity is
changed."
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