Dark reaction. Kalvin cycle(light independent reaction)

  DARK REACTIONS, CALVIN CYCLE) (Light independent reactions)

Light independent reactions do not require direct energy of sunlight it may occur during day time but are called dark reactions so as to differentiate them from the light reactions. The sequence of dark reactions in photosynthesis was investigated by Melvin Calvin and his colleagues in 1950. They occur in a series of reactions in the stroma of chloroplast and taking the course of a cycle known as Calvin-Benson cycle.

The Calvin cycle is completed in three stages.

i. Carbon fixation

This is the first step of dark reaction 

The cycle starts when ribulose bisphosphate, a 5-carbon sugar,already present in stroma reacts with CO, of air to form a 6-Carbon compound. This compound is unstable and soon splits up into two molecules of 3-carbon compound called Phosphoglycerate (PGA). This process is accelerated by an enzyme known as Rubisco (Ribolose biphosphate carboxylase). This is regarded as the most common protein in nature. The carbon that was part of CQmolecule is now a part of an organic molecule. This is called carbon fixation. PGA is regarded as the first product of photosynthesis to be identified.

RuBP + CO2 -->short lived carbon compound 

6-C compound--->2PGA

B. ii. Reduction

PGA formed in the previous step is reduced into phosphoglyceraldehyde (PGAL) in this stage. The products of light reaction i.e. NADPH and ATP are used in the process. Each molecule of phosphoglyceric acid (PGA) receives energy from ATP and hydrogen from NADPH of light reaction, forming phosphoglyceraldehyde (PGAL) and water. ADP and NADP return back to light reaction where ADP is converted into ATP and NADP is reduced into NADPH. In reduction process fixed carbon is reduced to a 3-carbon sugar molecule of PGAL

H20 PGA+ATP + NADPH

PGAL+ ADP + P + NADP

. iii. Regeneration of RuBP

 In this stage RuBP molecules are regenerated so as to continue the cycle. The PGAL molecules formed in the reduction stage have many alternatives. Out of every six molecules of PGAL formed, only one

molecule leaves the cycle to be used by the plant for making glucose and other organic compounds. The other five PGAL molecules are recycled to regenerate 3 molecules of five carbons RuBP by means of several

intermediates including 3-C, 4-C, 6-C, 7-C etc. This process also uses some ATP produced in light reaction. Ribulose bisphosphate (RuBP) is then available to accept Coand restarts the cycle. With the regeneration of RuBP

the Calvin cycle or dark reactions complete.

 Respiration

Respiration is defined as oxidation-reduction processes which occur inside the living cells during which organic food is broken down and energy is released.Respiration is of two types i.e. aerobic and anaerobic

Aerobic respiration (Cellular Respiration)

Aerobic respiration needs free 0. In aerobic respiration organic food is completely broken down into CO, and HỌ and the stored energy is released. The overall equation of aerobic respiration for glucose breakdown

can be written as follows:

CH 20.+ 60---> 6C0,+ 6HQ + 36 ATP

Glucose and oxygen are used and carbon dioxide and water are produced. Energy is released which is used in the synthesis of ATP molecules. This is just the opposite of photosynthesis where glucose and oxygen are produced and carbon dioxide and water are used as raw materials.

The overall equation of aerobic respiration gives a perception that oxygen combines with glucose molecule which is broken down into water and carbon dioxide and stored energy is released. But in fact this does not

happen. Complete breakdown of glucose molecule, in aerobic respiration, occurs in three different steps i.e. glycolysis, Kreb's cycle and electron transport chain. Glycolysis occurs in cytosol (cytoplasm) while the latter two stages occur in mitochondria. Organic food molecules are used by the living organisms as building materials and source of energy. Among the food molecules carbohydrates are the primary source of energy broken down by the living cells for the synthesis of ATP molecules, ATP are energy rich molecules also called energy currency of the cells.

The purpose of respiration is to release energy stored in organic food molecules and ATP molecules are produced. Why living cells do not acquire direct energy from the breakdown of food molecules? Why do they

synthesize ATP? This is because if whole amount of energy of glucose is released it will be too great for individual reactions. This will result in heating up of the cells and also a large amount of energy will be wasted.

ATP contains the right amount of energy available to the cell for its functions when it is broken down into ADP and inorganic phosphate. All living cells therefore use ATP molecules for energy requirement.

 a. Glycolysis

Glycolysis is the breakdown of glucose, a 6-C molecule, in two molecules of pyruvate (3-C molecule) and a net gain of two ATP molecules. It takes place in cytosol (cytoplasm) and is common in both aerobic and anaerobic respirations. Glycolysis does not need free oxygen. Glycolysis completes in two phases i.e. preparatory phase and

oxidative phase.

i. Preparatory phase

Preparatory phase is phosphorylation of glucose by two ATP molecules.

Glycolysis starts when glucose reacts with ATP molecule. ATP transfers energy and phosphate to glucose forming

glucose 6-Phosphate and itself converts to ADP. a Glucose -6 Phosphate is isomerised into Fructose-6 Phosphate.

Fructose-6 Phsophate reacts with another ATP molecule forming fructose-1-6 bisphosphate. Glucose +ATP Glucose-6-Phsophate+ADP. Glucose-6 Phosphate Fructose-6 Phosphate. Fructose-6 Phosphate+ ATP-Fructose-1-6 bishosphate+ ADP.

Fructose-1-6-bishosphate splits into 3-carbon

Phosphosglyceraldehyde (PGAL) and dihydroxy acetone phosphate (DAP).

Dihydroxy acetone phosphate changes to

phosphoglyceryldehyde.

ii. Oxidative Phase

In oxidative phase PGAL is oxidized to PGA (phosphoglycerate). Hydrogen is removed from PGAL and the energy of oxidation results in the formation of high energy phosphate bonds and the generation of ATP

molecules. The process begins when two hydrogen atoms are removed from

3-Phosphoglyceraldehyde (PGAL) and transferred to a molecule of NAD, a

coenzyme. Thus PGAL is oxidized to PGA and NAD is reduced to NADH. This step and the subsequent steps occur twice because two PGAL are produced at the end of preparatory phase. Altogether two NADH arc

produced in the process. This reaction is accompanied by the addition of phosphate groups. The resultant molecules are 1-3-bisphosphoglycerate.

2PGAL + 2 Pi

2BPGA +2H

2NAD + 2H

2NADH

Each molecule of 1-3-biphosphoglycerate transfers high energy phosphate to ADP forming ATP molecule and itself changes to 3-Phosphoglycerate. 

1-3 bisphosphoglyceracate + ADP3-Phosphoglycerate + ATP

In next step 3-Phosphhoglycerate converts into 2-Phosphoenol pyruvate

(PEP) with the elimination of one water molecule.

2PGA → PEP + HO

In last step phosphoenol pyruvate reacts with ADP forming an ATP and

Pyruvic acid. In this step phosphoenol pyruvate gives up a phosphate group

to ADP generating ATP and itself oxidizes to pyruvic acid.

PEP+ADP

Pyruvic Acid + ATP.

Two molecules of pyruvate are the end product of glycolysis. Since two molecules of ATP are utilized to start the process of glycolysis and four molecules of ATP are produced in the metabolic pathway, therefore there is a net gain of two molecules of ATP. The generation of ATP in the process of glycolysis is called substrate

level phosphorylation because high energy phosphate bonds are transferred

from substrate to ADP. In aerobic respiration also called cellular respiration further steps occur in mitochondria. Pyruvate from glycolysis completely oxidized through

linked reactions, Krebs Cycle, and electron transport chain to carbon dioxide and water. During the pathways mostly hydrogen atoms are removed from organic compounds. These hydrogen atoms are picked up by oxidized NAD and FAD and are reduced to NADH and FADH. These reduced coenzymes

transfer electrons to oxygen through an electron transport system with generation of ATP. Oxygen is the final electron acceptor and water is formed.

 b. Conversion of pyruvate to acetyl-CoA(Linked reaction)

Pyruvate does not enter the Krebs cycle directly. Before entering the Krebs cycle pyruvate is oxidized to a carbon dioxide and a two carbon molecule called acetyl group. This molecule attaches to coenzyme A (COA)

forming a group called acetyl CoA. Coenzyme A consists of a nucleotide

and a portion of one of the B vitamins. During the process hydrogen is removed from the pyruvate which is taken by NAD. By getting hydrogen NAD is reduced to NADH,. Acetyl-CoA enters the Krebs cycle. This process is called linked reaction because it links Glycolysis to the Krebs cycle.

Pyruvic Acid

Acetyl group +CO,+2H

Acetyl group + COA Acetyl CoA

NAD + 2H

NADH,

Further oxidation of acetyl-CoA takes place in a cyclic manner. This cycle is called Krebs cycle.