LIGHT INDEPENDENT REACTION// DARK REACTION// CARBON REACTION// SYNTHESIS OF FOOD// CO2 REDUCTION//CO2 ASSIMILATION// CO2 FIXATION//CALVIN CYCLE//HATCH-SLACK CYCLE//CAM CYCLE//CAM PATHWAY//RUBISCO ENZYME

DARK REACTION (SYNTHESIS OF FOOD)

Dark reaction is also known as Chemical Dark Reaction or Light Independent Reaction or Blackman Reaction or Photosynthetic Carbon Reduction cycle (PCR cycle).

Dark reaction takes place in Stroma of Chloroplast.

This reaction does not require the presence of light, so it is called a Light Independent Reaction or Chemical dark Reaction.

ATP and NADPH₂ synthesized during light reaction are utilized during this reaction.

In the dark reaction sugar molecule are formed from the carbon dioxide and water molecule.

The path of carbon that is carbon dioxide is arranged in a sequential manner from which compounds to forms which compounds, it is mainly occurs in three ways-

1.    1. Calvin – Benson cycle or Calvin Cycle: Occurs in C₃ plants.

2.    2. Hatch –Slack Cycle: Occurs in C₄ plants.

3.     3.Crassulacean Acid Metabolism Or CAM cycle: Occurs in CAM plants.

1.    1.Calvin – Benson cycle or Calvin Cycle:

Calvin cycle is also known as Calvin- Benson cycle or C₃ cycle. C₃ cycle occurs in stroma of chloroplast.

In 1954 Calvin, Benson and his coworkers used ¹⁴CO₂ and green alga Chlorella to study the path of Carbohydrate synthesis during Photosynthesis.

In this cycle the first CO₂ fixation product is 3- carbon organic acid which is 3- Phosphoglyceric acid (3-PGA).

 

The calvin cycle occurs in all photosynthetic plants it does not matter whether they have C₃ or C₄ or any other pathway.

The calvin cycle can be described under three stages:

A.   A.Carboxylation

B.   B.Reduction

C.   C..Regeneration

A. Carboxylation:

Carboxlation is the process of fixation of CO₂ and CO₂ is accepted by a 5 carbon molecule which is Ribulose-5- phosphate  in presence of Rubisco (Ribulose biphosphate carboxylase oxygenase) enzyme.

Rubisco: Rubisco works efficiently in the presence of Mg⁺⁺ ion. It performs two activity that is carboxylation and oxygenation.

Due to this formation of an unstable 6 carbon compound takes place which is breaks and from the first stable 3 carbon compound which is 3- Phosphoglycerate or 3- Phosphoglyceric acid (3-PGA).

B.  Reduction:

The 3-PGA molecules get reduced in presence of ATP and NADPH₂ (from light reaction) to  form 3-Phosphoglyceraldehyde (3-PGAL).

Few molecules of 3- PGAL are driven out from C₃ Cycle and used for synthesis of Glucose (Carbohydrates).

C. Regeneration:

In this stage regeneration of RuBP takes place  from 3-PGAL molecules in the presence of ATP Molecules (from light reaction).

Note:

·        6 Calvin cycles are required for synthesis of 1 glucose molecule .

·        18 ATP and 12 NADPH₂ are required in Calvin cycle (for every CO₂ molecule entering the Calvin cycle 3 molecule of ATP and 2 NADPH₂ are required).

 

IN	OUT
6 CO2	1 Glucose
18 ATP	18 ADP
12 NADPH2	12 NADP+

Calvin cycle (Simple presentation)

2. Hatch-Slack Cycle or C₄ Cycle:

Until recently it was universally accepted that Calvin Cycle was the only pathway of CO₂ fixation in Photosynthesis.

In 1966 , M .D. Hatch and C. R. Slack described the another pathway of  CO₂ fixation in Photosynthesis  called a Hatch and Slack Cycle or C₄ Cycle because the first stable product of CO₂ fixation was a 4- carbon compound in this pathway.

The plant which exhibits this cycle are called C₄ Plants.

C₄ Plants are the better Photo synthesizers.

The plants that perform C4 cycle are found in tropical (dry and hot) and sub- tropical regions.

It is mainly found in monocot plants like Sugarcane, Maize, Cyperus and also in dicot plant like Amaranthus.

These plants have a very special type of leaf anatomy called the Kranz (German term meaning halo or wreath) type.

In it the vascular elements (Phloem and xylem) in C₄ leaves, are surrounded by a layer of Bundle-Sheath cells that contain large number of chloroplasts. These are surrounded by one or more layers of mesophyll cell that contain normal types of chloroplasts.

The mesophyll cells, in C₄ plants perform C₄ cycle and the cells of bundle sheath perform C₃ cycle.

                               Cross Section of leaf showing Kranz anatomy 

Path of Carbon in C4 Cycle-

CO₂ of the atmosphere first comes in contact with mesophyll cells.

The primary acceptor of CO₂ in C4 cycle is 3-carbon compound Phosphoenol Pyruvic Acid (PEP) and formed first stable 4- carbon compound which is  Oxaloacetic Acid( OAA) in presence of enzyme Phosphoenol  Pyruvate Carboxylase (PEP Carboxylase).

OAA is then transported in chloroplasts of mesophyll cells and reduced to form Malic Acid or Aspartic Acid using NADPH₂ generated in Light reaction.

 Malic acid or Aspartic acid transported to bundle sheath cells and undergoes to decarboxylation and release Pyruvic acid and CO₂.

Produced CO₂ enters into the Calvin Cycle and combines with ribulose bisphosphate to form 3- phosphoglyceric acid (3-PGA).

The Pyruvic  acid, formed by decarboxylation, return to the mesophyll cells where it is converted into Phosphoenol Pyruvic Acid (PEP).

One Glucose molecule synthesize through C₄ cycle  required –

·        18 ATP in C₃ Cycle

·        12 ATP in C₄ Cycle

·        12NADPH₂  in C₃ Cycle

Note: So that total ATP requirement in C4 plants is 30 ATP for synthesis of one Glucose molecule.

                                                Hatch-Slack Cycle or C₄ cycle

KRANZ Anatomy: It is found C₄ plants. there are some features of CRANZ anatomy given below-

a. Presence of granal cloroplast that is dimorphic in C₄ plants.

b. Bundle sheath cells covers the vascular bundles.

c. A special type of cells are present called Bundle sheath cells.

Significance:

i . C4 pathway is more advanced than  C_{3  pathway.}

ii. This pathway is more useful for those plants which are grow in Xerophytic condition.

3. CRASSULACEAN ACID METABOLISM OR (CAM)

Some plants, like succulents which grow under semi-arid conditions, fix atmospheric CO₂ in dark.

This process was first time seen in plant belonging to family crassulaceae so it was termed Crassulacean Acid Metabolism or CAM.

In these plants stomata remain closed during day and open during the night. This help these plants to conserve their water content.

Crassulacean Acid Metabolism found in members of Crassulaceae family (include flowering succulents) examples, Succulent plants, Pineapple, Bryophyllum, some members of Euphorbiaceae.

At night when the stomata are open CO₂ enters into the leaf and fixed to Oxalo acetic acid (OAA) which is then converted to Malic Acid.

During day time, when the stomata are closed, the Malic acid is converted to Pyruvic acid and releases CO₂ . This CO₂ is used up by C₃ Cycle.

In genral stomata opens at day time and remain closed at night but in CAM stomata closed in day time and opens at night. This type of stomata is known as scotoactive stomata.

Significance:

These plants are better adopted for desret (high temperature) condition.

CAM PATHWAY