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.