3. What is
Photosynthesis
It is a process in which energy from sunlight is
transformed into chemical energy that is stored in
carbohydrates and other organic molecules.
Two phase of photosynthesis.
1) Light reaction or light dependent phase.
- The reaction occurs in thylakoids.
- The products are ATP and NADPH.
2) Dark reaction or light independent phase
- The reaction occur in stroma.
- Product are Trioses, Tetroses, Pentoses, Hexoses &
4. Structure of chloroplast
It is Double membrane contains thylakoids sacs.
Chlorophyll, the green pigment within chloroplast
gives Green color of leaves.
Chloroplast also contains stroma, a dense interior
fluid.
Ultrastructure of chloroplast
Electron microscopic studies reveals that chloroplast
is composed of following two parts:-
1) Limiting membrane
2) Stroma
6. Photosynthetic pigment
These are the coloured complex of organic molecules.
Absorb solar energy in visible range.
Pigments absorb light due to possession of
conjugated double bonds.
Types:-
1) Chlorophylls
2) Carotenoids
3) Phycobilins
7. Light or Hill
reaction
The light dependent reactions use photosynthetic
pigments (organised into photosystems) to convert light
energy into chemical energy (specifically ATP and NADPH)
These reactions occur within specialised membrane discs
within the chloroplast called thylakoids and involve three
steps:
Excitation of photosystems by light energy
Production of ATP via an electron transport chain
Reduction of NADP+ and the photolysis of water
8. Excitation of Photosystems by
Light Energy
Photosystems are groups of photosynthetic pigments
(including chlorophyll) embedded within the thylakoid
membrane.
Photosystems are classed according to their maximal
absorption wavelengths (PS I = 700 nm ; PS II = 680 nm).
When a photosystem absorbs light energy, delocalised
electrons within the pigments become energised or
‘excited'.
These excited electrons are transferred to carrier
molecules within the thylakoid membrane.
9. Production of ATP via an Electron
Transport Chain
Excited electrons from Photosystem II (P680) are transferred to an
electron transport chain within the thylakoid membrane.
As the electrons are passed through the chain they lose energy, which
is used to translocate H+ ions into the thylakoid.
This build up of protons within the thylakoid creates an electrochemical
gradient, or proton motive force.
The H+ ions return to the stroma (along the proton gradient) via the
transmembrane enzyme ATP synthase (chemiosmosis).
ATP synthase uses the passage of H+ ions to catalyse the synthesis of
ATP (from ADP + Pi).
This process is called photophosphorylation – as light provided the
initial energy source for ATP production.
10. Reduction of NADP+ and the
Photolysis of Water
Excited electrons from Photosystem I may be transferred
to a carrier molecule and used to reduce NADP+.
This forms NADPH – which is needed (in conjunction with
ATP) for the light independent reactions.
The electrons lost from Photosystem I are replaced by de-
energised electrons from Photosystem II.
The electrons lost from Photosystem II are replaced by
electrons released from water via photolysis.
Water is split by light energy into H+ ions (used in
chemiosmosis) and oxygen (released as a by-product).
11. Z-SCHEME
The energy changes (oxidation / reduction) that occur during
photosynthesis may be represented as a Z scheme:
*First vertical bar: Photosystem II electrons are energised by
light (electrons replaced by photolysis of water molecules)
*Diagonal bar: Electrons lose energy as they pass through an
electron transport chain (synthesising ATP)
*Second vertical bar: Photosystem I electrons are energised by
light (electrons used to reduce NADP+)
13. Photophosphorylation
The production of ATP by the light dependent reactions is
called photophosphorylation, as it uses light as an energy
source
Photophosphorylation may be either a cyclic process or a
non-cyclic process
Occurs in thylakoid membrane
14. Cyclic Phosphorylation
In cyclic Phosphorylation only photosystem 1 is
required.
Water is not required and oxygen is not evolved.
It is used to produce additional ATP.
NADPH is not formed.
15. Non-Cyclic Phosphorylation
In non-cyclic Phosphorylation only photosystem 1 and
photosystem 2 is required.
Water is required and oxygen is evolved.
NADPH is synthesis.
Products can be used for light independent reaction.
16. Photochemical reaction
Absorption of light is the first step in any chemical
reaction.
Absorption of a photon excites chlorophyll II molecules
and other(accessory) pigment, which funnel the energy
into reaction centre.
In the reaction centre, photoexcitation results in a
charge separation that produces a strong electron
donor and a strong electron acceptor.
17.
18. Conclusion of Light reaction
It is used for the production of NADPH and ATP. It takes
place in chloroplast and organic food material.
