The Ultimate Enzymes

Kanwal asked:


Here is an explanation of the stages of photosynthesis. I just want to know whether it is right or wrong. I know that the dark reactions are very much shortened but these were the only three steps that were given in my text book. Also, the equations are from my text book. Plz tell me that the whole answer is right or wrong. I wanted to keep it short and in simple language.

STAGES OF PHOTOSYNTHESIS:
Photosynthesis occurs in two stages. In the first stage, light-dependent reactions called the Light Reactions capture the energy of light and use it to make high-energy molecules. During the second stage, the light-independent reactions called Dark Reactions or Calvin-Benson Cycle, use the high-energy molecules to capture and chemically reduce carbon dioxide (CO2) (also called carbon fixation) to make the precursors of carbohydrates.

(1)LIGHT REACTIONS:
(i) Transfer of Electrons:-
In the light reactions, one molecule of the pigment chlorophyll absorbs one photon and loses one electron. This electron is passed to a modified form of chlorophyll called Phaelophytin. It is the first electron carrier intermediate in the electron transfer pathway of photosystem II in plants and the photosynthetic reaction center RC P870 found in purple bacteria. Phaelophytin passes the electron to a plastoquinone molecule (QA) in RC P870 or plastoquinone in PS II., allowing the start of a flow of electrons down an electron transport chain. that leads to the ultimate reduction of NADP to NADPH. The electron transport chain couples a chemical reaction between an electron donor (such as NADH) and an electron acceptor (such as O2) to the transfer of H+ ions across a membrane, through a set of mediating biochemical reactions. These H+ ions are used to produce adenosine triphosphate (ATP).

(ii) Formation of ATP (Photophosphorylation):-
In chloroplasts, light drives the conversion of water to oxygen and NADP+ to NADPH and a transfer of H+ ions. In addition, light energy is used to pump protons across the chloroplast membrane. This produces a proton-motive force and this drives the ATP synthase,for the concomitant synthesis of ATP. The chlorophyll molecule regains the lost electron from a water molecule through a process called photolysis, photolysis is a chemical reaction in which a chemical compound is broken down by photons this releases a dioxygen (O2) molecule.
(2) DARK REACTIONS (CALVIN BENSON CYCLE):- After the light dependent reactions, the ATP and the NADPH produced in the light reactions goes to the stroma of the chloroplast for the Calvin cycle. The Calvin cycle fixes carbon dioxide and adds energy and hydrogen ions to the resulting molecules to yield a three carbon phosphate sugar called PGAL or G3P. Surplus molecules of this sugar are harvested by the plant for the manufacture of glucose and other carbohydrates.
(i) Carbon fixation: 3 Carbon dioxide molecules combine with three 5 carbon starting molecules called RuBP (Ribulose biphosphate) producing G3P (glycerate 3-phosphate). The reaction takes place in the presence of a catalysing enzyme called rubisco.
3CO2 + 3RuBP ——–? 6 G3P (in the presence of rubisco)
(ii) Reduction: Here, energy from ATP (Adenosine Triphosphate) & NADPH, produced during light reaction along with G3P (glycerate 3-phosphate), is used to make 6 high-energy molecules of PGAL (phosphoglyceraldehyde).
6G3P + 6ATP + 6NADPH ———-? 6GA3P + 6ADP + 6NADP + 6Pi
(iii) Regeneration: The remaining molecules of G3P are rearranged and phoshorylated using 3ATP’s to produce 3 new 5 carbon molecules of RuBP.
5GA3P + 3ATP ———? 3RuBP + 3ADP + 2Pi