Photosynthesis In Higher Plants- The Dark Phase
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Photosynthesis In Higher Plants- The Dark Phase

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This Was Done As Part Of My Bio Seminar 2012-2013. And I'm From Gulf Asian English School, Sharjah, U.A.E.

And I Hope This Will Help You In One Or The Other Way.

Gokul Anish HB

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  • 2. And My Part Is……………The DarkReactions
  • 3. Where Are The ATP And NADPH Used.• Products of light reaction are ATP, NADPH, O2 . O2 is diffused out of chloroplast• Biosynthetic phase of photosynthesis: Phase where ATP and NADPH is used for synthesizing sugar.• Phase don’t depend on light but on the products of light reaction.• This can be verified: When light is made unavailable the phase continues but stops later. And when light is brought back phase continues.
  • 4. How ATP And NADPH Is Used In Biosynthetic Phase• We know that CO2 combine with H2O to form sugar or (CH2O)n.• Scientist tried to find the product formed when CO2 is fixed.• Melvin Calvin used radioactive 14C in the algal photosynthesis and discovered that the first product of CO2 fixation is a 3- carbon organic acid called 3-phosphoglyceric acid or PGA• The whole biosynthetic pathway was explained by Calvin hence it was named Calvin’s Cycle after him.• Further experiments proved that PGA is not the only product of CO2 fixation.• The newly found product was a 4-carbon organic acid called the oxaloacetic acid or OAA
  • 5. CO2 assimilation during photosynthesis was said to be of two main types:• Plants in which the • Plants in which the first product of CO2 first product of CO2 fixation is a C3 acid fixation is a C4 acid where said to have a where said to have a C3 biosynthetic C4 biosynthetic pathway. pathway.• Its other wise called • Called Hatch and the Calvin’s Cycle. Slack Pathway. Primary Acceptor Of CO2• Scientists had a major doubt that how many carbon atoms would a molecule have which after accepting (fixing) CO2• The studies showed that the accepter molecule was a 5- carbon ketose sugar – it was ribulose bisphosphate (RuBP).
  • 6. A Small Introduction To Calvin Cycle.• Calvin and his co-workers then worked out the whole pathway and showed that the pathway operated in a cyclic manner; the RuBP was regenerated.• The Calvin pathway occurs in all photosynthetic plants; it does not matter whether they have C3 or C4 .• Calvin Cycle occurs in three distinct phases;• Carboxylation , Reduction and Regeneration.
  • 7. Step I - Carboxylation • It’s the fixation CO2 to a single stable organic intermediate. • Its an important step since CO2 used for carboxylation of RuBP. • The enzyme called RuBP carboxylase acts as the catalyst which results in the formation of two molecules of 3-PGA. • Catalyst enzyme also has an oxygenation activity it is known as RuBP carboxylase- oxygenase or RuBisCO.
  • 8. Step II- Reduction.• Reduction is the series of reaction leading to glucose formation.• The steps involve utilisation of 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 removal of one molecule of glucose from the pathway.
  • 9. Step III- Regeneration• Regeneration of the CO2 acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted.• Hence for every CO2 molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required.
  • 10. Introduction To C4 Pathway• Plants adapted to dry tropical region have C4 path way.• They have OAA as First product but most of the use Calvin’s Cycle as biosynthetic pathway.• C4 plants have large cells around the vascular bundle called bundle sheath cells.• Leaves with such an anatomy is called Kranz- Anatomy.• Bundle Sheath cells may have many layers: 1. They have large no. of chloroplasts. 2. They have no intercellular space. 3. They have thick walls which acts as a barrier for gas exchange.
  • 11. Difference of C4 Plants From C3 Plants.• They have special type of leaf anatomy.• They tolerate high temperature.• Respond to high light intensities.• Lack of Photorespiration.• More production of biomass. Note: Photorespiration is a special process in which O2 is bind to RuBP instead of CO2 by the RuBisCO.
  • 12. Leaf Anatomy Of C4 And C3 Plants.In C4 Plants CO2 is In C3 Plants CO2 isfixed two time in the fixed in Bundle SheathBundle Sheath Cells the CellsMesophyll Cells.
  • 13. Hatch Slack Pathway • Mesophyll lacks RuBisCo but have PEP. • Bundle Sheath cell lack PEP but they contain RuBisCO.
  • 14. • CO2 which is present in the free space of the leaf is accepted by the PEP(phosphoenol pyruvate) which is present in the mesophyll cells.• Enzyme responsible for CO2 fixation is PEP carboxylase.• C4 plants lack RuBisCO in the mesophyll cells.• When CO2 is accepted by PEP a 4-Carbon compound called oxaloacetate acid (OAA).
  • 15. • When NADPH acts on OAA in the mesophyll cell it forms malate(C4 Acid) and gives out NADP+.• Malate is taken to the bundle sheath cells and it gets splited into CO2 and pyruvate(C3 Acid).• The pyruvate moves to mesophyll cell and ATP from the light reaction act on the C3 Acid to form the PEP and give ADP back to light reaction.• The CO2 in the bundle sheath cell proceeds to Calvin Cycle and there by forming Glucose.
  • 16. Photorespiration• Photorespiration is a special process in which O2 is bind to RuBP instead of CO2 by the RuBisCO.• Since RuBP bnds with O2 the product formed will be a molecule Phosphoglycerate and Phosphoglycolate.• This pathway doesn’t help in synthesis of sugars or ATP.• It produces CO2 by utilizing ATP.• No ATP or NADPH is synthesised. So we can call it as a wasteful process.• C4 plants lack Photorespratory pathway. WHY?
  • 17. Factors Affecting Photosynthesis.• The rate of photosynthesis is determined by several internal and external factors.• Internal Factors: 1. Age, size, number and orientation of leaves. 2. Mesophyll Cells, chloroplast. 3. Internal CO2 concentration.• External Factors: Avaliablilty of sunlight, CO2 concentration, water and temperature.• Since many factors simultaneously affects the rate of photosynthesis, usually only one factor is the major factor which determines the rate of photosynthesis.
  • 18. Law Of Limiting Factors.If a chemical process is affected by more than one factorwhich is nearest to its minimal value: it is the factor whichdirectly affects the process if its quantity is changed.
  • 19. CO2 Concentration. • CO2 concentration is very low in atmosphere 0.03% . But when the CO2 concentration is taken to 0.05% the rate of photosynthesis increases. • At Low intensity light C4 and C3 do not respond to high CO2 concentration. • At High intensity light C4 and C3 respond to high CO2 concentration. • C3 plants show response to higher CO2 concentration thus they have high rates of photosynthesis and high productivity.
  • 20. Temperature.• Dark reactions are enzymatic and thus temperature have major role to play.• C4 plants respond to high temperature and have high rate of photosynthesis.• C3 plants shows less response to high temperature.Water. • Water has direct effects on light reaction • Water stress causes stomata to open and close. • Water stress causes the wilting of leaves which reduce the surface of leaf which reduce the metabolic activity of leaf.