This document discusses key concepts related to ATP synthesis in photosynthesis including: 1) Coupling factors and the chemiosmotic mechanism of ATP synthesis, where a proton gradient across the thylakoid membrane powers ATP synthase to form ATP from ADP and inorganic phosphate. 2) The concept of quantum yield, which is the number of oxygen molecules released per photon of light absorbed in photosynthesis. 3) The difference between light and dark reactions, cyclic and non-cyclic photophosphorylation, and how assimilatory powers like ATP and NADPH are generated during the light reaction.

1. COUPLING FACTORS AND MECHANISM OF ATP
SYNTHESIS AND CONCEPT OF QUANTUM YIELD
PRESENTED BY,
P S KIRAN
(Msc.plant physiology
student)

2. LIGHT REACTION V/S DARK REACTION
• What differentiate between light and dark reactions?
• Does dark reaction completely independent of light?
• What happens in light reaction?
• Cyclic and Non cyclic photophosphorylation
• What is the difference between NADPH and NADH2?
• How the assimilatory powers are formed in light reaction?
• Coupling factors and mechanism of ATP synthesis
• Concept of quantum yield

3. ACTION AND ABSORPTION SPECTRA

4. DIFFERENCE BETWEEN LIGHT AND DARK REACTION
LIGHT REACTION
• Light reaction only takes place in the
presence of light
• Takes place in grana
• Also called photo-chemical phase
• Oxidation of water and production of
“assimilatory powers”
DARK REACTION
• Takes place with the presence or
absence of sunlight
• Takes place in stroma
• Also called as biosynthetic phase
• Reduction of co2 to carbohydrate
using the “assimilatory powers”

5. DOES DARK REACTION IS COMPLETELY
INDEPENDENT OF LIGHT?
• This is a misnomer that light reaction occurs in morning(presence of sunlight) and
dark reaction occurs in the night(absence of sunlight)
• Light reaction and dark reaction occurs spontaneously i.e , one after the other
• Four enzymes in dark reaction are indirectly dependent (activated) on light
a)Ribulose 5-phosphate kinase
b)Fructose 1,6-bisphosphatase
c)Sedoheptulose 1,7-bisphosphatase
d)Glyceraldehyde 3-phosphate dehydrogenase

6. WHAT HAPPENS IN LIGHT REACTION?
• Light dependant phase of photosynthesis in which ATP and NADPH are formed in
the presence of light
• Consists of following steps:
a) light absorption
b) water splitting
c)oxygen release
d)formation of high energy chemical intermediates (ATP & NADPH)

7. LIGHT ABSORPTION
 Chl + h 𝜗 → Chl*
 Role of photosystems I and II (PSI and PSII)
 Light absorbed by PSII PSII* ETC (pheophytin,PQ,Cyt b6f
complex) PSI PSI*
 PSII-680nm & PSI-700nm
 PSI present in stroma lamellae and non appressed region of grana
whereas PSII present in appressed region of grana lamella
 Photosystem I produces a strong reductant, capable of
reducing NADP+, and a weak oxidant
Photosystem II produces a very strong oxidant, capable of oxidizing water,
and a weaker reductant than the one produced by photosystem I.

9. WATER SPLITTING
• Complex for splitting of water present in inner side of thylakoid membrane
attached to PSII
• H+ ions , e- and oxygen is liberated on photolysis of water
• PSII is essential for photolysis of water to occur
• H2O 2H+ + 2e- + O2

10. OXYGEN RELEASE
• Oxygen is liberated from splitting of water and not from carbon dioxide
• Oxygen produced as a by product in splitting of water is liberated in the lumen of
the thylakoid which then diffuses into the stroma

11. FORMATION OF ASSIMILATORY POWERS
• ATP and NADPH are called as the assimilatory powers of light reaction which is
used as reducing agent in carbon fixation in dark reaction
• ATP and NADPH is formed in non cyclic photophosphorylation whereas in cyclic
photophosphorylation only ATP is formed
• NADP reductase enzyme is present in the outer membrane of thylakoid
• H+ ions for reduction of NADP comes from stroma

12. CYCLIC AND NON CYCLIC
PHOTOPHOSPHORYLATION
CYCLIC PHOTOPHOSPHORYLATION
• PSI is functional
• Only ATP is formed
• Photolysis of water is absent
• Mostly occurs in stroma lamella
• Found dominant in photosynthetic
bacteria
NON CYCLIC
PHOTOPHOSPHORYLATION
• Both PSI and PSII are functional
• ATP and NADPH is formed
• Photolysis of water is present
• Occurs mostly in grana lamella
• Dominant in green plants

14. NADPH AND NADH2
NADPH
• Occurs as an assimilatory power in
photosynthetic light reaction
• Mostly involved in anabolic processes
• Acts as a strong reducing agent for
reducing carbon dioxide to glucose
NADH2
• Occurs as a proton carrier in
respiration
• Mostly involved in catabolic
processes
• It liberates ATP when required and
also act as a proton carrier in
mitochondrial ETC

15. PROTON TRANSPORT AND ATP
SYNTHESIS IN THE CHLOROPLAST
• In the preceding sections we learned how captured light energy is used to reduce
NADP+ to NADPH. Another fraction of the captured light energy is used for light-
dependent ATP synthesis, which is known as photophosphorylation
• It is now widely accepted that photo phosphorylation works via the chemiosmotic
mechanism, first proposed in the 1960s by Peter Mitchell.
• Differences in chemical potential of any molecular species whose concentrations are
not the same on opposite sides of a membrane provide such a source of energy.
• The direction of proton translocation is such that the stroma becomes more alkaline
(fewer H+ ions) and the lumen becomes more acidic (more H+ ions) as a result of
electron transport

17. COUPLING FACTORS AND MECHANISM OF ATP
SYNTHESIS
• Chemiosmotic hypothesis by Peter Mitchell
• ATP production driven by proton gradient difference
• High concentration of H+ in lumen and low concentration of H+ in stroma creates
a gradient and proton is transported from lumen to stroma through CF0-CF1
complex
• CF0 is an integral protein present in the outer thylakoid membrane which helps in
the facilitated diffusion of H+ from lumen to stroma
• CF1 is the ATP synthase complex where iP combines with ADP to form ATP in the
presence of high energy H+ ions

28. CONCEPT OF QUANTUM YIELD
• The number of photons/quanta required to release one molecule of oxygen in
photosynthesis is called as quantum requirement . On the other hand, number
of oxygen molecule released per photon of light in photosynthesis is called as
quantum yield . The quantum yield is always in fraction of one

29. RED DROP AND EMERSON ENHANCEMENT EFFECT
• Sudden fall in photosynthesis yield observed beyond red region of visible
spectrum is called Red drop/Emerson’s first effect
• The increase in photosynthetic activity by combined effect of short and long
wavelengths of light is called Emerson’s enhancement effect/second effect

30. THANK U