is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can create their own food. In plants, algae, and cyanobacteria, photosynthesis uses carbon dioxide and water, releasing oxygen as a waste product. Photosynthesis is vital for all aerobic life on Earth. In addition to maintaining normal levels of oxygen in the atmosphere, photosynthesis is the source of energy for nearly all life on earth, either directly, through primary production, or indirectly, as the ultimate source of the energy in their food, the exceptions being chemoautotrophs that live in rocks or around deep sea hydrothermal vents. The rate of energy capture by photosynthesis is immense, approximately 100 terawatts, which is about six times larger than the power consumption of human civilization.As well as energy, photosynthesis is also the source of the carbon in all the organic compounds within organisms' bodies. In all, photosynthetic organisms convert around 100–115 petagrams of carbon into biomass per year.
|Overall Reaction of Photosynthesis|
|Overall 3D structure of Plants important during Photosynthesis|
|3D -Microscopic Structure of Leaf|
|Microscopic Picture of Leaf and its different Structure|
|Microscopic picture of Leaf|
The light dependent reactions of photosynthesis take place at specialised cell membranes,such as Thylakoid membrane
inside chloroplasts.That membrane is folded into the system pf stacked disks, called grana, and flatten channels.The interior spaces of the disks achannels are open to one another, so the membrane system actually forms a single compartment.The compartment is a reservoir for hydrogen ions, and it is tapped for ATP formation.The synthesis of
phosphorylated sugars and starch occurs in the stroma, a semifluid matrix that surrounds the thylakoid compartment.
The internal organisation of a chloroplast might seem to be less than memorable, until you remember that we are taking about a very small space. If you clould lined up 2000 chloroplasts, one after another, the lineup would be no wider than your thumbnail.Imagine all the chloroplasts in one lettuce leaf, each a tiny factory of producing sugars and starch
and you begin to get a sense of the magnitude of metabolic events required to feed you and all other organism on earth.
1. outer membrane 2. intermembrane space 3. inner membrane (1+2+3: envelope) 4. stroma (aqueous fluid) 5. thylakoid lumen (inside of thylakoid) 6. thylakoid membrane 7. granum (stack of thylakoids) 8. thylakoid (lamella) 9. starch 10. ribosome 11. plastidial DNA
12. plastoglobule (drop of lipids)
1) LIGHT DEPENDENT REACTIONS IN PHOTOSYSNTHESIS:
A) Light Absorption:
1) In chloroplasts, light is absorbed by two types of photosystems (clusters of photosynthetic pigments embedded in the
2) Light absorption causes the transfer of electrons from photosystem 1 or 2 to an acceptor molecule,which will donate them toa transport system in the membrane.
B) Noncyclic pathway:
1) Exiting land plants rely mainly on noncyclic photophosphorylation,which yields ATP and NADPH.
2) In the non cyclic patheway, there is a one-way flow of electrons from photosystem 2 , through a trasport system, to
photosystem 1, and on through a second trasport system.Electrons released from water molecules replace the electrons
being expelled from photosystem 2.
3) During electron transfers, hydrogen ions picked up from the stroma are released inside the thylakoid compartment.
Hydrogen ions derived from water molecules also accumulate here.Both activities establish concentration and electric
gradients across the membrane.
4) Hydrogen ions flow down the gradients (through channel proteins that span the membrane), and the flow drives the
joining of inorganic phosphate and ADP to from ATP.
5) At the end of the second transport system,electrons are donated NADP+, which combines the H+ to form NADPH. The
Electrons and hydrogen can be used directly in assembling organic compounds.
C) Cyclic pathway:
1) In cyclic photophosphorylation, excited electrons flow from photosystem 1, through a transport system, then back to
photosystem. This pathway yields ATP only.
2) Operation of electron transport systems causes hydrogen ions to accumulate inside the thylakoid compartment.
3) Energy inherent in the resulting concentration and electric gradients between the compartment and the stroma is
tapped to from ATP, just like it is in the non-cyclic pathway.
LIGHT DEPENDENT REACTION:
12 H2O -----------> ELECTRON & HYDROGEN IONS ------------> USED IN FORMING -------------> 18 ATP, 12 NADPH, 12H+
2) LIGHT INDEPENDENT REACTION OF PHOTOSYNTHESIS:
1) Carbon-di-oxide is fixed to RuBP, making an unstable intemediate that is broken apart into two three carbon PGA
2) PGA is Phosphorylated (made more reactive) by ATP,and it receives H+ and electrons from NADPH.The result is
3) Through complex reactions, PGAL is rearranged into new RuBP molecules and into sugar phosphate.
4) It takes 6 turns of the Calvin-Benson cycle to produce one sugar phosphate(Only one very six PGAL molecules produced
in the cycle is funneled into carbohydrate synthesis).
5) The suger phosphates are intemediates in the light-indipendent reactions; they can enter pathways by which many
different carbohydrate end products form.
LIGHT INDEPENDENT REACTION:
18 ATP, 12 NADPH + 12H+ ----------------------------------> C6H12O6 ---> P-----> Typical sugar phosphate------>Carbohydrate
6H2O end product
(Water as (Sucrose,starch)
3D- Animation of Photosynthesis Process: