PHOTOSYNTHESIS IN HIGHER PLANTS

 

Cyclic photophosphorylation : 

• Only PS-I works, the electron circulates within the photosystem.
• It happens in the stroma lamellae (possible location) because in this region PS-II and NADP reductase enzyme are absent.
• Hence only ATP molecules are synthesized.

Chemiosmotic Hypothesis : 

• Chemiosmotic hypothesis explain the mechanism of ATP synthesis in chloroplast.
• In photosynthesis, ATP synthesis is linked to development of a proton gradient across a membrane.
• The protons that are produced by the splitting of water are accumulated inside of membrane of thylakoids (in lumen).
• As the electron moves through the photosystem, protons are transported across the membrane.
• NADP reductase enzyme is located on the stroma side of the membrane, along with electrons from the acceptor it removes H+ from the stroma during reduction of NADPH + H⁺.
• This creates proton gradients across the thylakoid membrane as well as a measurable decrease in pH in the lumen.
• ATPase has a channel that allows diffusion of protons back to stroma across the membrane. 
• This releases energy to activate ATPase enzyme that catalyses the formation of ATP.

Biosynthetic phase in C3 plants :

• ATP  and  NADH,  the  products  of  light  reaction  are  used  in  synthesis  of food. The first CO₂ fixation product in C3 plant is 3-phosphoglyceric acid or PGA.
• In some other plants the first stable product is an organic acid called oxaloacetic acid a 4-C compound hence is called C4 plants.

The Calvin cycle :

• The CO₂ acceptor molecule is RuBP (Ribulose bisphosphate).
• The cyclic path of sugar formation is called Calvin cycle on the name of Melvin Calvin, the discoverer of this pathway. Calvin cycle proceeds in three stages:


• Carboxylation : 
  • Carboxylation is the fixation of CO₂ into a stable organic intermediate.
  • CO₂ combines with Ribulose 1, 5 bisphosphate to form 3 PGA in the presence of RuBisCo enzyme.
  
  
• Reduction : 
  • These are a series of reactions that lead to the formation of glucose.
  • 2 molecules of ATP for phosphorylation and two of NADPH for reduction per CO₂ molecule fixed.
  • The fixation of six molecules of CO₂ and 6 turns of the cycle are required for the formation of one molecule of glucose.
  
  
• Regeneration :
  • Regeneration of the CO₂ acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted.
  • Regeneration steps required one ATP for phosphorylation to form RuBP.
  
  
• Hence for every CO₂ molecule entering the Calvin cycle, 3 molecules of ATP and 2 molecules of NADPH are required.

The C₄ pathway :

• Plants that are adapted to dry tropical regions have the C₄ pathway.
• C₄ oxaloacetic acid is the first CO₂ fixation product.
• These plants have special type of leaf anatomy, they tolerate higher temperatures.
• The leaf has two types of cells: mesophyll cells and Bundle sheath cells (Kranz anatomy).
• Initially CO₂ is taken up by phosphoenol pyruvate (PEP) in mesophyll cells and changed to oxaloacetic acid (OAA) in the presence of PEP carboxylase.
• Oxaloacetate is reduced to malate/asparate that reaches into bundle sheath cells.
• In the bundle sheath cells these C₄ acids are broken down to release CO₂ and a 3-carbon molecule i.e. pyruvic acid.
• The CO₂ released in the bundle sheath cell enters the C3 cycle because these cells are rich in enzyme Ribulose bisphosphate carboxylase-oxygenase (RuBisCO).
• The pyruvate formed in the bundle sheath cell transported back to the mesophyll cell, get phosphorylated to form phosphoenol pyruvate.

 

Photorespiration:

• The light induced respiration (evolution of CO₂) in green plants is called photorespiration. 
• Active site of RuBisCO has active site for both O₂ and CO₂.
• In  C3  plants  some  O₂  binds  with  RuBisCo  and  hence  CO₂ fixation  is  decreased. 
• In  this  process  RuBP  instead  of  being  converted  to  2 molecules  of  PGA,  binds  with  O₂   to  form  one  molecule  of  PGA  and phosphoglycolate.
• In the photorespiratory pathway there is neither synthesis of sugar, nor of ATP. Rather it results in the release of CO₂ with utilization of ATP.
• In the photorespiratory pathway there is no synthesis of ATP or NADPH.
• Therefore photorespiration is a wasteful process.
• In C₄ plant photorespiration does not occur:
  • RuBisCO enzyme is present in the bundle sheath cells.
  • Primary carboxylation is takes place in the mesophyll cell by PEP carboxylase.
  • CO₂ supplied to bundle sheath cell by C₄ acid intermediate.
  • Hence C₄ plants are photosynthetically more efficient than C3 plant.

Law of Limiting Factors :

• If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value. It is the factor which directly affects the process if its quantity is changed.

CBSE Biology (Chapter Wise) Class XI ( By Mr. Hare Krushna Giri ) 
Email Id : [email protected]