“Photosynthesis is a metabolic pathway that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight.”
Photosynthetic
organisms are called photoautotrophs.
- The light-dependent reactions, or light
reactions, are the First Stage of Photosynthesis.
- In this process light energy is converted into chemical
energy, in the form of the energy-carriers ATP and NADPH.
- In the light-independent reactions, the formed
NADPH and ATP drive the reduction of CO2 to more useful organic
compounds, such as glucose.
ROLE OF
THYLAKOID IN PHOTOSYNTHESIS
- The light-dependent reactions take place on
the thylakoid membrane inside a chloroplast
- . The thylakoid membrane contains some integral
membrane protein complexes which catalyze the light reactions. There are four
major protein complexes in the thylakoid membrane:
1.
Photosystem I (PSI),
2.
Photosystem II (PSII),
3.
Cytochrome b6f complex and
4.
ATP synthase
- These four complexes work together to ultimately
create the products ATP and NADPH.
PHOTOPHOSPHORYLATION
- Cytochrome b6f and ATP synthase are working together to create ATP. This process
is called Photophosphorylation, which occurs in two different ways;
- In Non-Cyclic Photophosphorylation,
cytochrome b6f uses the energy of electrons from PSII to pump protons from
the stroma to the lumen. The proton gradient across the thylakoid membrane
creates a proton-motive force, used by ATP synthase to form ATP.
- In Cyclic Photophosphorylation, cytochrome
b6f uses the energy of electrons from not only PSII, but also PSI to
create more ATP and to stop the production of NADPH.
- Cyclic Phosphorylation is important to create ATP and maintain NADPH in
the right proportion for the light-independent reactions.
PHOTOSYSTEMS
- Photosystems are arrangements of chlorophyll and
other pigments packed into thylakoids.
- Many Prokaryotes have only one
photosystem, Photosystem II (so numbered because, while it was most likely
the first to evolve, it was the second one discovered).
- Eukaryotes have Photosystem II plus Photosystem I.
- Photosystem I uses chlorophyll a, in the form referred to as P700.
- Photosystem II uses a form of chlorophyll
a known as P680.
- Both "active" forms of chlorophyll a
function in photosynthesis due to their association with proteins in
the thylakoid membrane.
THE REACTION
CENTER
- It is a dimer of two chlorophyll molecules.
This dimer is called a special pair because of its
fundamental role in photosynthesis.
- This special pair is slightly different in PSI
and PSII reaction center.
- In PSII it absorbs photons with a
wavelength of 680 nm and it is therefore called P680.
- In PSI it absorbs photons at 700 nm and it
is called P700.
- In Bacteria the special pair is called
P760, P840, P870 or P960.
PHOTO-INDUCED
CHARGE SEPERATION
- If a special pigment molecule in a so-called reaction
center absorbs a photon.
- An electron in this pigment attains the excited
state and then is transferred to another molecule in the
reaction center, it is called photoinduced charge separation.
- It is the start of the electron flow and is very
unique because it transforms light energy into chemical forms.
Z-SCHEME AND
NON-CYCLIC PHOTOPHOSPHORYLATION
CYCLIC
PHOTOPHOSPHORYLATION
CHEMIOSMOTIC
THEORY
- Peter Mitchell
proposed chemiosmotic theory in 1978.
The basic principle is that ion concentration difference and electric potential across membranes are utilized by the cell.
