Maharashtra BoardEnglish MediumSTD 11 ScienceBiologyPhotosynthesis5 Marks
Question
Give schematic representation of $\text{HSK}$ pathway.
✓
Answer
$1. \ \text{M. D.}$ Hatch and $\text{C. R.}$ Slack while working on sugarcane found four$-$carbon compounds $($dicarboxylic acid$)$ as the first stable product of photosynthesis.
$2.$ It occurs in tropical and sub$-$tropical grasses and some dicotyledons.
$3.$ The first product of this cycle is a $4-$carbon compound oxaloacetic acid. Hence it is also called as $C_4$ pathway and plants are called $C_4$ plants.
Mechanism:
$1. \ CO_2$ taken from atmosphere is accepted by a $3-$carbon compound, phosphoenolpyruvic acid in the chloroplasts of mesophyll cells, leading to the formation of $4-C$ compound, oxaloacetic acid with the help of enzyme phosphoenolpyruvate carboxylase.
$2.$It is converted to another $4-C$ compound, malic acid. It is transported to the chloroplasts of bundle sheath cells.
$3.$ Malic acid $(4-C)$ is converted to pyruvic acid $(3-C)$ with the release of $CO_2$ in the cytoplasm. Thus, concentration of $CO_2$ increases in the bundle sheath cells.
$4.$ Chloroplasts of these cells contain enzymes of Calvin cycle.
$5.$ Because of high concentration of $CO_2, RuBP$ carboxylase participates in Calvin cycle and not photorespiration.
$6.$ Sugar formed in Calvin cycle is transported into the phloem.
$7.$ Pyruvic acid generated in the bundle sheath cells re$-$enter mesophyll cells and regenerates
phosphoenolpyruvic acid by consuming one $\text{ATP.}$
$8.$ Since this conversion results in the formation of $\text{AMP} ($not $\text {ADP}),$ two $\text{ATP}$ are required to regenerate $\text{ATP}$ from $\text{AMP.}$ Thus, $C_4$ pathway needs $12$ additional $\text{ATP.}$
$9.$ The $C_3$ pathway requires $18 \ \text{ATP}$ for the synthesis of one glucose molecule, whereas $C_4$ pathway requires $30 \text{ ATP.}$
$10.$ Thus, $C_4$ plants are better photosynthesizers as compared to $C_3$ plants as there is no photorespiration in these plants.
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