Dr. Sindhu S presents information on photosynthesis including: - Photosynthesis has light dependent and light independent processes. The light dependent processes use energy from sunlight to produce ATP and NADPH, while the light independent processes use these products to fix carbon and produce sugars. - Photosynthetic pigments like chlorophyll a and b and carotenoids absorb light energy which is transferred to the reaction centers of the two photosystems. - The light independent reactions, also called the Calvin cycle or dark reactions, use ATP and NADPH from the light reactions to fix carbon and produce glucose which is then converted to starch.

1. Dr. Sindhu.S
Assistant Professor
Department of Biochemistry
St. Mary’s College, Thrissur
Photosynthesis—light and dark
reactions

2. Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

3. Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

4. Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

5. Photosynthesis
• The basic equation for photosynthesis is :
6 CO2
+ 6 H2
O + light energy
 C6
H12
O6
+ 6O2
Divided into 2 processes
I. Light dependent reactions
II. Light independent reactions
(Dark reaction)
Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

6. Light dependent Light independent
thylakoid
stroma
Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

7. Photosynthetic Pigments
• Chlorophyll a - found in all photosynthetic eukaryotes and
cyanobacteria
• Chlorophyll b - found in vascular plants, bryophytes, green algae -it
is an accessory pigment
•Chlorophyll was first isolated by Joseph Caventou and Pierre Joseph
Pelletier in 1817.Chloro=green, phylla=leaf -greek word
•Chlorophyll absorbs light most strongly in the blue portion of the
spectrum, followed by the red portion.
Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

8. • Carotenoids - red, orange or yellow fat-soluble accessory
pigments found in all chloroplasts and cyanobacteria .
• Caroteniods are embedded in thylakoids as chlorophylls - two
types - carotenes and xanthophyll.
Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

9. Photosynthetic pigments
Chlorophyll a
Chlorophyll b
Accessory pigments:
Carotenoids
- C20 H 39 tail, head with Mg
and porphyrin ring
the “main” photosynthetic pigment
Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

10. Now we will see how
• Photosynthetic pigments (chl a, chl b ,carotenoids etc)
• Photo systems ( PS700, PS 680 )
photosynthesis
carry out
Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

11. Photo systems
• Photosynthetic pigments are found in packs called
photosystems
• LHC –as light harvesting complexes
• Each photosystem has 250 to 400 pigment molecules in it.
• All pigments absorb photons of light.
• Transfer to the reaction center which can actually use the energy in
a photochemical reaction.
• Accessory pigments gather light and transfer that energy to the
photochemically active pigments in centre.
Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

12. The Photo systems
• There are two sets of photosystems –
• Photosystem II –
has chlorophyll a - optimum absorption peak - 680 nm of
light - this chlorophyll a is called P680
• Photosystem I - has special chlorophyll -active at 700
nanometers – P700 chlorophyll and
•In general the two photosystems PS I & PS II
work together to carry out photosynthesis.
Named as per sequence of discovery
Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

13. LHC
Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

14. P680
P700
Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

15. PETC
Photon
Photosystem
Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

16. Light independent reaction
( Calvin cycle / C3 cycle /Dark reaction )
Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

17. Glucose
Calvin cycle /C3 Cycle
2
2
2
Stroma
Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

18. The role of pigments
• A pigment is any substance that absorbs visible light
- most absorb only certain wavelengths and reflect or
transmit the wavelengths they don't absorb
• Chlorophyll absorbs light primarily in the violet, blue
and red wavelengths and reflects green wavelengths,
and thus appears green
• Absorption spectrum - the light absorption pattern of
a pigment
Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

19. • Action spectrum - the relative effectiveness of different
wavelengths for a specific light-requiring process eg
.photosynthesis
• Chlorophyll is implicated as the principle pigment in
photosynthesis because its absorption spectrum is the
same as the action spectrum for photosynthesis
Contd…
Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

20. Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

21. Light Independent Reactions
( calvin cycle -“dark reactions”)
• Happen in sunlight, and in the
dark.
– Hence “independent of light”
1. ATP generated by sunlight drives
the Calvin Cycle.
2. Monosaccarides (eg. glucose) are
manufactured in the cycle.
3. Monosaccarides are used to “build”
polysaccharides (eg. Starch).Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

22. Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

23. Far Red Drop
Robert Emerson in1930s found that in some green
algae, at different wavelength ,the rate of photosynthesis
differ very much .
No photosynthesis proceeds when the wavelength of
light absorbed is greater than 700 nm.
This is Far Red Drop.
Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

24. Overview
• Carbon dioxide and water are taken in by plants
Plants absorb light energy and convert it to a
usable form.
– ATP & NADPH
• Energy is used to “fix” carbon dioxide into sugar
molecules
Thus light energy - Chemical energy
• Sugar is converted to starch and stored for use by
the plant, and by animals when they eat plants.
Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College

25. Thylakkoids
Stroma
Photosynthesis—light and dark reactions,Photosynthesis—light and dark reactions, Sindhu.S,Sindhu.S, St. Mary’s CollegeSt. Mary’s College