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Difference between Mesophyll Chloroplast and Bundle Sheath Chloroplast

Photosynthesis is the anabolic reaction carried out by green plants responsible for life on this planet. It is the process by which suns light energy is converted to chemical energy which is the source of energy for all other living organisms.

Photosynthesis involves 2 reactions: Light reaction and Dark reaction
  • Light reaction: takes place in the grana of the chloroplast where light energy from the sun is converted to chemical energy in the form of ATP and NADPH
  • Dark reaction: takes place in the stroma of the chloroplast. The ATP and NADPH produced in the light reaction are used to fix CO2 to carbohydrates.
Certain plants such as sugarcane, sorghum, Amaranthus etc possess an alternate pathway of CO2 fixation in addition to C3 cycle. The first stable compound produced in this pathway is a 4-carbon compound, oxaloacetic acid, hence called as C4 cycle. In C4 cycle, chloroplast in both mesophyll and bundle sheath cells are involved in CO2 fixation. In C4 plants, initial carbon fixation takes place in the mesophyll cells and Calvin Benson cycle or C3 cycle occurs in the bundle sheath cells. This alteration helps C4 plants to prevent photorespiration and also providing optimum condition for the enzyme RuBP for CO2 fixation.
Mesophyll chloroplast vs Bundle sheath chloroplast in C4 Plants
Mesophyll chloroplast
1. Large well developed numerous grana are present.
2. The enzyme Ribulose bi phosphate carboxylase (RuBP carboxylase) is absent. Therefore no CO2 fixation or C3 cycle occurs. In C3 plants, RuBP carboxylase is abundantly present in mesophyll chloroplast where CO2 fixation occurs
3. The activity of photosystem II is high making plenty of ATP, NADPH2 and O2
4.  Starch grains are absent in mesophyll cells
5. The key enzymes of starch synthesis are absent in mesophyll cells.

Mesophyll Chloroplast and Bundle Sheath Chloroplast - Kranz anatomy

Bundle sheath chloroplast
1. Grana few or absent, if present very small and poorly developed.
2.High concentration of RuBP carboxylase. Therefore CO2 fixation or C3 cycle occurs.
3. Low activity of photosystem II , hence few ATP, NADPH2 and O2 generated
4. Abundant large starch grains are present in bundle sheath chloroplast
5. The key enzymes of starch synthesis (ADP-glucose pyrophosphorylase, starch synthase, and branching enzyme) are located primarily in the bundle sheath cells

Reference: Roles of the bundle sheath cells in leaves of C3plants Richard C. Leegood* J. Exp. Bot. (2008) 59 (7): 1663-1673.

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