Photosynthesis in Higher Plants

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2. Ç4 PÅTHWÅÝ (HÅTÇH & SLÅÇK PÅTHWÅÝ)

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It is present in plants that are

adapted to dry tropical regions.

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They also use C3 pathway as main

biosynthetic pathway. The large

cells around the vascular bundles

of the C4 plants are called bundle

sheath

cells.

Such

anatomy

is

called

‘Kranz’

anatomy

(‘Kranz’ = ‘wreath’).

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The bundle sheath cells may form

several layers around the vascular

bundles.

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They

have

large

number

of

chloroplasts,

thick

walls

impervious to gas exchange and no intercellular spaces.

STËPS ØF HÅTÇH ÅÑD SLÅÇK PÅTHWÅÝ

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Primary CO2 acceptor is phosphoenol pyruvate (PEP), a

3-carbon molecule seen in mesophyll cells. The enzyme for this

fixation is PEP carboxylase (PEPcase).

(NEET 2017)

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The mesophyll cells lack RuBisCO enzyme.

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The C4 acid OAA is formed in the mesophyll cells.

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OAA then forms other 4-carbon acids like malic acid or aspartic

acid. They are transported to bundle sheath cells. (AIPMT 2008)

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In the bundle sheath cells, C4 acids are broken down to release

CO2 and a C3 molecule.

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The C3 molecule is transported back to mesophyll where it is

converted to PEP again.

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The released CO2 enters the C3 pathway.

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Bundle sheath cells are rich in RuBisCO, but lack PEPcase.

(Karnataka NEET 2013)

Thus, C3 pathway is common to C3 & C4 plants. C4 plants are special

because:

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They have a special type of leaf anatomy (Kranz). (AIPMT 2010)

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They tolerate higher temperatures.

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They show a response to high light intensities.

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They lack photorespiration.

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They have greater productivity of biomass.

Atmospheric CO2

Mesophyll

cell

Plasma

membrane

Cell wall

Phosphoenol

pyruvate

C3 acid

C

C4 acid

4 ^acid

Plasmodesmata

Bundle

sheath cell

Fixation

by

calvin

cycle

Transport

Decarboxylation

Fixation

Transport

Regeneration

C acid

3

CO2

HCO3

–