Maharashtra BoardEnglish MediumSTD 11 ScienceBiologyPhotosynthesis5 Marks
Question
What are the two types of photosystems?
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Answer
$1.$ Photosystems:
$a.$ Two kinds of photosystems are present in thylakoid membranes of chloroplasts.
$b.$ Each has its own set of light harvesting chlorophyll and carotenoid molecules.
$c.$ Chlorophyll and accessory pigments help to capture light energy over large area and pass it on to the photocenters.
$d.$ Thus, a photon absorbed anywhere in the harvesting zone of $P-680$ center can pass its energy to the $P 680$ molecule.
$e.$ The cluster of pigments which transfer their energy to $P-680$ absorb at or below $680 \ nm.$
$f.$ Together with $P-680$ they form Photosystem $– II$.
$g.$ Likewise, $P-700$ forms Photosystem $– I$ along with pigment molecule which absorbs light at or below $700 \ nm.$
$2.$ Photosystem $II:$
$a.$ Photolysis of water and release of oxygen takes place in this system.
$b.$ In this process, when $PS-II$ absorbs light, electrons are released and chlorophyll molecule is oxidized.
$c.$ Electrons emitted by $P680 \ \ (PS-II)$ are ultimately trapped by $P700 \ \ (PS -1).$
$d.$ Oxygen is the byproduct by the photosynthesizing plants.
$e.$ Protons accumulate inside the thylakoid resulting in a Proton gradient.
$f.$ When the protons diffuse across the thylakoid membrane into stroma against the $H+$ gradient, energy, is released.
$g.$ This is used to produce $\text{ATP.}$
$3.$ Photosystem $1:$
$a.$ Upon absorption of light quanta by $PS-I \ \ (P700)$ reaction center emits energy rich electrons.
$b.$ These flow down a chain of electron carriers to $\text{NADP}$ along with the proton generated by splitting of water.
$c.$ This result in the formation of $\text{NADPH.}$
$d.$ Hydrogen attached to $\text{NADPH}$ is used for reduction of $CO_2$ in dark reaction also called as reducing power of the cell.
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