Photosynthesis Photosynthesis is a photo-biochemical reaction which is mediated through the absorption of light energy . The light energy is absorbed photosynthetic pigments in plants . These pigments have capacity to absorb light of certain wavelength & reflect light of other wavelengths & imparting different colours to different pigments . The colour of the specific pigment indicates the wavelength reflected by the pigment . Crassulacean Acid Metabolism This dark CO2 fixation pathway is proposed by Ting (1971). It is operational in Succulent Xerophytes like Cactus , Sedum , Agave , Pineapple , Bryophyllum etc. The stomata of succulent plants remain closed during day & open during night to avoid water loss (i.e. Scotactive stomata ). They store CO2 during night is used in Calvin cycle during day time . Succulents refix CO2 during respiration & use it during photosynthesis . Since this diurnal change in acidity was first discovered in Crassulacean plants e.g. Bryophyllum it is called as Crassulacean Acid Metabolism. The formation of Malic acid during dark is called Acidification/ phase I Release of CO2 for actual photosynthesis during day is called Deacidification/ Phase II

1. LIGHT HARVESTING MECHANISM IN BACTERIA

2. Photosynthesis
• Photosynthesis is a photo-biochemical reaction
which is mediated through the absorption of light
energy .
• The light energy is absorbed photosynthetic
pigments in plants .
• These pigments have capacity to absorb light of
certain wavelength & reflect light of other
wavelengths & imparting different colours to
different pigments .
• The colour of the specific pigment indicates the
wavelength reflected by the pigment .

3. Types of Photosynthetic pigments
• There are three main types of photosynthetic pigments :
Chlorophylls
Carotenoids
Phycobilins

4. Chlorophylls
• The Chlorophylls are the most important &
abundant active pigments of photosynthesis .
• They are green in colour , are insoluble in water but
soluble in organic solvent .
• Chlorophyll-a is present in all photosynthetic
organisms ( except photosynthetic bacteria ) .
• Chlorophyll-b is also predominant & found in green
algae , bryophytes & all vascular plants .
• Chlorophyll-a & b show maximum absorption in
blue-violet & red regions of visible light .

5. Carotenoids
• Carotenoids are widely distributed in Chloroplasts &
Chromoplasts .
• They show wide range in colour , from yellow , orange
to red & insoluble in water but soluble in organic
solvents .
• They mainly absorb blue-violet region of visible light .
• There are two main types of carotenoids viz. carotenes
& xanthophylls.
Xanthophylls
• These are oxygenated hydrocarbons .
• Lutein is the major xanthophyll present in plants .

6. Phycobilins
• These are present only in cyanobacteria ( blue green algae ) & red
algae .
• There are two types viz. Phycocyanin (blue) & phycoerythrin (red) .
• In higher plants , there are Chlorophyll-a , Chlorophyll-b , Carotene &
Xanthophyll , as main photosynthetic pigments .
• (Anthocyanin , purple coloured pigment present in flower is not
photosynthetic ) .

7. Light Harvesting Complex (LHC)
• A light-harvesting complex consists of a number
chromophores which are complex subunit proteins
that may be part of a larger super complex of a
photosystem , the functional unit in photosynthesis .
• It is used by plants and photosynthetic bacteria to
collect more incoming light than would be captured
by the photosynthetic reaction center alone .
• Light harvesting complexes consists of proteins &
photosynthetic pigments & surround a
photosynthetic reaction center to focus energy ,
attained from photons absorbed by the pigment ,
toward the reaction center using Forster resonance
energy transfer .

8. Function
• Absorption of a photon by a molecule takes place when
pigment protein complexes harvest sunlight leading to
electronic excitation delivered to the reaction centre
where the process of charge separation can take place.
• Electronic excitation is The fate of such excitation can be a
return to the ground state or another electronic state of
the same molecule.
• Before an excited photon can transition back to ground
state, the energy needs to be harvested.
• This excitation is transferred among chromophores where
it is delivered to the reaction centre.
• Light-harvesting complexes have their pigments
specifically positioned to optimize these rates.

9. In purple bacteria
• Purple bacteria is a type of photosynthetic organism
with a light harvesting complex consisting of two
pigment protein complexes referred to as LH1 and
LH2.
• Within the photosynthetic membrane, these two
complexes differ in terms of their arrangement.
• The LH1 complexes surrounds the reaction centre,
while the LH2 complexes are arranged around the
LH1 complexes and the reaction centre in a
peripheral fashion.
• Purple bacteria use bacteriochlorophyll and
carotenoids to gather light energy.
• These proteins are arranged in a ring-like fashion
creating a cylinder that spans the membrane.

10. In green bacteria
• The main light harvesting complex in Green
bacteria is known as the chlorosome.
• The chlorosome is equipped with rod-like BChl c
aggregates with protein embedded lipids
surrounding it.
• Chlorosome are found outside of the membrane
which covers the reaction centre.
• Green Sulphur bacteria and some Chloroflexia
use ellipsoidal complexes known as the
chlorosome to capture light.
• Their form of bacteriochlorophyll is green.

11. Phycobilisome
• The light harvesting complex of cyanobacteria, and red
algae is known as the Phycobilisome which is composed of
linear tetrapyrrole pigments.
• Pigment-protein complexes referred to as R-phycoerythrin
are rod-like in shape and make up the rods and core of the
Phycobilisome.
• Little light reaches algae that reside at a depth of one meter
or more in seawater, as light is absorbed by seawater.
• A Phycobilisome is a light-harvesting protein complex
present in cyanobacteria, glaucocystophyta, and red algae
and is structured like a real antenna.
• The pigments, such as phycocyanobilin and
phycoerythrobilin, are the chromophores that bind through
a covalent thioether bond to their apoproteins at cysteine
residues.
• The apoprotein with its chromophore is called phycocyanin,
phycoerythrin, and allophycocyanin, respectively.

12. In cyanobacteria and plants
• Chlorophyll b is almost identical to chlorophyll a,
except it has a formyl group in place of a methyl
group.
• This small difference makes chlorophyll b absorb
light with wavelengths between 400 and 500 nm
more efficiently.
• Carotenoids are long linear organic molecules
that have alternating single and double bonds
along their length.
• These molecules also absorb light most efficiently
in the 400 – 500 nm range.
• on.
• Carotenoid molecules suppress damaging
photochemical reactions, in particular those
including oxygen, which exposure to sunlight can
cause.

13. REGULATION OF CAM THROUGH TRANSPORT OF METABOLITES

14. Crassulacean Acid Metabolism
• This dark CO2 fixation pathway is proposed by Ting (1971).
• It is operational in Succulent Xerophytes like Cactus , Sedum , Agave ,
Pineapple , Bryophyllum etc.
• The stomata of succulent plants remain closed during day & open during
night to avoid water loss (i.e. Scotactive stomata ).
• They store CO2 during night is used in Calvin cycle during day time .
• Succulents refix CO2 during respiration & use it during photosynthesis .
• Since this diurnal change in acidity was first discovered in Crassulacean
plants e.g. Bryophyllum it is called as Crassulacean Acid Metabolism.
• The formation of Malic acid during dark is called Acidification/ phase I
• Release of CO2 for actual photosynthesis during day is called
Deacidification/ Phase II

15. Overview of CAM : A two part cycle

17. Regulation of CAM through transport of
metabolite
1. Carboxylation
• PEPA (Phosphonelphosphate) receives CO2 & then PEPA is converted
into OAA ( Oxaloacetic acid) in presence of enzyme PEP carboxylase
2. Reduction
• OAA is then converted into Malic acid in the presence of enzyme
Malate dehydrogenase
• This malic acid is stored in the large vacuoles of Mesophyll cells
during the entire night

18. 3. Decarboxylation
• During the day time Malic acid undergoes decarboxylation & forms
Pyruvic acid in presence of enzyme Decarboxylase
• The CO2 which is released during decarboxylation enters Calvin cycle
& forms glucose
• During the entire day Pyruvic acid is stored in the chloroplast
4. Phosphorylation
• During the night time Pyruvic acid undergoes phosphorylation to
form PEPA in presence of enzyme Pyruvate kinase

19. Let’s ask Mr. Cactus to tell us , how he
survives in the hot desert everyday ?
Hi There !

20. • It is so hot there in the desert ! But I don’t want my water
to evaporate…..
So what do I do now?
AHA ! I will use my Crassulacean Acid Metabolism !
Here’s how I do it !

21. How CAM pathway is going to help Mr. Cactus
• I have to keep my stomata closed during the day time so that not
much water will evaporate out of me .
• The problem? Photosynthesis can only occur in the day time because
of Light dependent reaction! I also need photons! Right?
• ………But our kind has found a way to solve this. Yippee!
• We have decided that we will do carbon fixation during Night!

22. So what I do every night is ………
• Open my stomata
• Take in CO2
• Do carbon fixation & make a four carbon acid malate by using an
enzyme called PEP Carboxylase because it can only react to carbon .
• Since I can’t do photosynthesis yet , I just store the malate in my Big
Vacuoles .

23. When the Sun is now up ……..
• I close my stomata
• Start doing photosynthesis
• Since I can’t take in CO2 now because my stomata are closed . I use
the malate that has been stored in my vacuoles .
• In the Calvin cycle , the RuBisCo will not be able to waste energy since
it cannot react to oxygen anymore . The enzymes can only react to
carbon .
• I can now produce sugar in a very efficient way .

24. REFERENCE’S
1. https://en.wikipedia.org/wiki/Crassulacean_acid_metabolism
2. https://en.wikipedia.org/wiki/Light-harvesting_complex#In_purple_bacteria
3. www.biologydiscussion.com
4. Biology XII Text-Book
5. NCERT

26. THANK YOU