PHOTOSYNTHESIS
An anabolic process in which carbohydrates are synthesized from CO2 and H2O by green plants using radiant energy of the Sun, oxygen being a byproduct.
6CO2+12H2O → C6H12O6 +6O2 +6H2O
• Autotrophs, Heterotrophs• Raw materials• Products• Energy source• Site of photosynthesis - Chloroplast
Each mesophyll cell has 20 – 100 chloroplasts.
3 – 10 micro meter in length, 1 – 5 micro meter diameter.
Each chloroplast – 40 – 60 grana
Each granum – 50 or more thylakoids.
• Outer surface of thylakoid membrane is in contact with stroma .Inner surface is in contact with thylakoid lumen.
- photolysis of water, electron carrier involved in light reactions, light absorbing pigments, enzymes, etc.• Stroma contains molecules
involved in dark reaction.
Photosynthetic pigments:
• Primary pigments- Chlorophyll a
C55H72O5N4Mg – red and blue light.
has methyl group – CH3
• Accessory pigments- Chlorophyll b
C55H70O6N4Mg – other wave lengths
has aldehyde group- CHO
Carotenoids – β carotene, Xanthophylls
Photosystems - R Emerson in 1957
• Consists of around 200 mol. of chlorophylls and about 50 mol. of Carotenoids.
• Consists of photochemical reaction center having chlorophyll a with specific protein.
• Other pigment molecules are light harvesting mol. or antenna mol.
• A single thylakoid possess hundreds of photosystems.
• Photosystem I [P700 ] made of chlorophyll a, b - absorbs red light of 700nm
• Photosystem II [P680 ] responds better to slightly shorter wavelength.
• Only reaction centers give up energized electrons.
• Water is the source of H to reduce carbon and molecular O2 is released during photosynthesis.
LIGHT DEPENDENT REACTIONS
• Photo excitation of chlorophyll
chlo. → chlo+ (oxidized) + e-
• Photolysis or Photo oxidation of water
2H2O → 4H+ + 4e- + O2 ↑
• Photophosphorylation and Electron transport.
The Stroma/ Carbon/ dark reactions:
• Melvin Calvin (Nobel laureate 1961), J A Bassham, A A Benson
• Independent of light• Requires 18 ATPs, 12 NADPH
from light reactions, CO2 from surroundings, Ribulose1,5-biphosphate, enzymes found in stroma.
Fixation of CO2 by Ru 1,5-BP:• Ru 1,5- BP acts as an acceptor molecule
for CO2 from air. • Carboxylation is catalyzed by RuBP
Carboxylase Rubisco- most abundant protein on earth.
• The 5 C sugar Ru1,5BP becomes an unstable 6 C compound 2-Carboxy-3-Ketoarabinitol-1,5 bisphosphate.
Splitting
• Unstable 6 C compound hydrolyzed into 2 molecules of 3-phosphoglycerate-first stable product of dark rxn, hence called C3 cycle and the plants, C3 plants.
Second Phosphorylation :
• 3PGA receives a phosphate group from ATP and becomes 1,3- Bisphosphoglycerate.
Reduction :
• 1,3- Bisphosphoglycerate accepts Hydrogen from NADPH and gets reduced into energy rich 3C sugar Glyceraldehyde-3- Phosphate or Phosphoglyceraldehyde (PGAL).
* Reduction of 6CO2 forms12 PGAL in the stroma.
Regeneration of Ru1,5BP :
• Out of 12 PGAL , 10 molecules are used in regenerating Ru1,5BP. Uses 6ATPs and formation of intermediate compounds in the form of 3,4,5,6 and 7 C sugars.
* 6 revolutions of the cycle are required to produce the 6C carbohydrate.
Carbohydrate Synthesis
• It occurs in the cytosol
• Two molecules of PGAL is transported from stroma to the cytosol by a phosphate translocator – an antiport protein.
• It exchanges two PGAL for two molecules of Pi across the chloroplast membrane.
• Most Glucose is converted into transport sugar sucrose or storage sugar starch.
C4 Photosynthesis ( Hatch – Slack Pathway)
• MD Hatch and CR Slack – 1966, Mainly in monocots with the first product 4C molecule oxaloacetate.
• C4 plants exhibit a unique leaf anatomy called Kranz (= ahalo), where there are two rings of cells around a vascular bundle, the inner bundle sheath cells and outer mesophyll cells.
• The bundle sheath cells have rudimentary chloroplasts and mesophyll cells well developed chloroplasts – dimorphic.
• C4 pathway starts in the mesophyll cells where carbon dioxide condenses with 3C compound phosphoenol pyruvate (PEP). To produce 4C oxaloacetate, which is reduced to malate.
• Malate enters the bundle sheath cells and is decarboxylated into pyruvate and CO2.
• The pyruvate returns to mesophyll cells to form PEP again and repeat the process.
Advantages
• C4 plants have higher affinity for CO2 and hence the plants can carry out photosynthesis even at low CO2
concentrations.
• Carry out rapid photosynthesis at higher temperatures.
• Avoid photorespiration in which CO2 is released without ATP synthesis.
Crassulacean Acid Metabolism (CAM)
• Adaptation seen in succulents • First discovered in the family
Crassulaceae.• In these plants the stomata is scotactive –
closed during day and open at night. • Water loss is prevented and
photorespiration cannot occur. • In the night when stomata opens at cooler
temperatures CO2 is fixed by C4 pathway.