Question

Describe in short the various steps that occur during the photochemical phase of photosynthesis.

Answer 1

Hint: The process of photosynthesis completes in two steps –
(i) Light Reaction and (ii) Dark Reaction
- Light Reaction is known as photochemical phase
- It occurs in Thylakoid
- Products produced in photochemical phase \[ATP,{\text{ }}NADPH\] are utilised by the dark phase

Complete answer:
(a) Light Reaction or photochemical phase begins as soon as sunlight falls on green plants.
(b) Pigment molecules present in the photosystem\[I\]and photosystem\[II\]absorb light of specific wavelength.
(c) Accessory pigments absorb light and transfer energy to their reaction centres
(d) \[PS{\text{ I}}\] and \[PS{\text{ }}II\] are connected with each other by electron transport chain
(e) The electrons move in \[PS{\text{ }}I\] and \[PS{\text{ }}II\] through two pathways
(i) Non – cyclic and
(ii) Cyclic.
Non-cyclic photophosphorylation – it involve both \[PS{\text{ }}I\] and \[PS{\text{ }}II\]\[ATP\], \[NADPH\]$ + {H^ + }$ are synthesised by this electron flow. Firstly PSII, having ${P_{680}}$ molecules absorbs light, electrons gain energy from light and become excited and transfer energy to electron transport system cytochromes. This movement is downhill.
(f) The electrons then transport to the PSI $\left( {{P_{700}}} \right)$. These electrons then moved downhill again to a molecule of $NAD{P^ + }$. The addition of these electrons reduces the $NAD{P^ + }$ to $NADPH + {H^ + }$.
Cyclic photophosphorylation: In this process electrons released by the excited photo centre are returned to it after passing through a series of electron carriers. It involves only PSI. The excited electrons do not pass on to $NAD{P^ + }$ and are cyclic back to PSI.
Here only ATP synthesis takes place
It takes in stroma lamellae membranes.
The membrane or lamella of grana have both PSI and \[PS{\text{ }}II\]. The stroma lamellae membrane lacks \[PS{\text{ }}II\] and $NADP$ reductase enzyme.
Water splitting complex is associated with PSII. Water splits into ${H^ + },\left[ O \right]$ and electrons. The protons and oxygen formed by splitting of water are released into the lumen of Thylakoid.
The oxygen produced is released as one of the net product of photosynthesis
$2{H_2}O \to 4{H^ + } + {O_2} + 4{e^ - }$

Note:Photochemical phase generates assimilatory power in the form of \[ATP\] and $NADPH$, which is used in carbohydrate synthesis in dark reaction.