Mechanism of Photosynthesis: The Dark Reaction

Mechanism of Photosynthesis: The Dark Reaction

The dark rxn: (Backman’s Reaction)

•      This second reaction of photosynthesis is independent of light, hence called dark reaction.

•      The dark rxn utilizes ATP & NADPH₂  molecules formed during light rxn for carbon fixation.

•      It occurs in the stroma region of chloroplast.

•      The CO₂ absorbed by the plants from the environment combines with certain compounds to form intermediate compounds and ultimately leads to the formation of sugar and starch.

•      Depending upon the initial product after CO₂  fixation, three photosynthetic pathways were recognised in plants:

1. Calvin Cycle(C₃ cycle )
2. Hatch and slack Cycle (C₄  cycle)
3. CAM (CAM plants)

Ø  In C₃  plants the first stable product after CO₂  fixation is 3- C compound  (Phosphoglyceric acid) and in C₄  plants, 4- C compound (oxaloacetic acid) is the first stable product . In CAM cycle, the first stable product is malic acid.

Ø  On the basis of different pathways for Dark rxn., plants are classified as;

            1. C₃  plants= follows C₃  cycle i.e. Calvin Cycle during photosynthesis.                             Examples: Most of dicots , rice , wheat, barley,

            2. C₄ Plants = follows Hatch & Slack cycle during photosynthesis.

                                    Examples: Most of monocots, maize , sugarcane.

            3. CAM Plants = follows CAM pathway during photosynthesis.

                                    Examples: Xerophytes plants.

1. Calvin  Cycle or C₃  cycle:

Ø  There are various steps involved in dark rxn of Calvin cycle. These are discussed on several headings as follows:

1. Carboxylation:

•      CO₂  is first accepted by ribulose-1,5 diphosphate (RuDP) & forms an unstable 6-C compound.

•      This 6-C compound immediately splits to form two molecules of phosphoglyceric acid (PGA).

•      The rxn takes place in the presence of enzyme RuDP Carboxylase (Rubisco).

RuDP Carboxylase

            RuDP + CO₂  + H₂O             =========>                                      2PGA

2. Hatch and slack Cycle (C₄  cycle):

•      M.D. Hatch and C.R. Slack (1967) gave a new process ofCO₂    fixation.

•      This cycle takes place mostly in monocots plants (Sugarcane, maize, Cyprus) and in some dicots (Amaranthus).

•      Plant in which CO₂ fixation takes place by Calvin cycle only are called C₃ plants (because first products of CO₂fixation is a 3 carbon containing compound, Phosphoglyceric acid). But in Hatch –Slack Pathway , first product of CO₂ fixation is a 4 carbon containing compound, Oxaloacetic acid , hence these plants are called C₄ –Plants and the cycle of photosynthesis is known as C₄  - Cycle.

Anatomical Features of Leaves of C₄ –Plants:

•      Anatomically, the C₄ –Plants are characterized by the presence of bundle sheath cells around the vascular bundles.

•      This type of arrangement is called Kranz Anatomy.

•      In the mesophyll of leaves of these plants, pallisade tissue is absent.

•      Typically C₄ –Plants possess two types of chloroplasts. One type in kranz-sheath, which is large and lack grana but contains enzymes of calvin cycle(C₃ –Cycle). Another type of chloroplast is found in mesophyll cells, which are smaller with well developed grana and contains enzymes for C₄ – Cycle.

3. CAM Cycle (CAM plants):

•      Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions.

•      In a plant using full CAM, the stomata in the leaves remain shut during the day to reduce evapotranspiration, but open at night to collect carbondioxide(CO₂).

•      The CO₂ is stored as the four-carbon acid malate, and then used during photosynthesis during the day.

•      The pre-collected CO₂ is concentrated around the enzyme RuBisCO, increasing photosynthetic efficiency.

Ø  The CAM cycle can be divided into two parts:

      1. CO₂ fixation during night & Synthesis of malate

            2. Utilization of Malate during day and release of CO₂

Ø  The CO₂ thus produced in either of the above two ways is then consumed in normal photosynthetic reaction to yield carbohydrates.

Ø  Pyruvate and PEP are also utilized for carbohydrates synthesis during the day. Pyruvate is first converted into PEP in the presence of enzyme pyruvate orthophosphate dikinase.

Ø  PEP is then converted into 3PGA by reverse reaction of glycolysis. Thereafter 3PGA is utilized in the calvin cycle.