Photosynthesis is important to living organisms because it is the number one source of oxygen in the atmosphere. Almost all the oxygen in the atmosphere is due to the process of photosynthesis. If photosynthesis ceased, there would soon be little food or other organic matter on Earth, most organisms would disappear, and Earth’s atmosphere would eventually become nearly devoid of gaseous oxygen.
2. PHOTOSYNTHESIS
Introduction: Photosynthesis is important to living organisms because it is the number one source
of oxygen in the atmosphere. Almost all the oxygen in the atmosphere is due to the process of
photosynthesis. If photosynthesis ceased, there would soon be little food or other organic matter on Earth,
most organisms would disappear, and Earth’s atmosphere would eventually become nearly devoid of
gaseous oxygen.
In all the different types of life processes, photosynthesis is the only process which can fix the sunlight
energy and conserve it in the form of carbon compounds. Therefore, photosynthesis in green plants, is the
most important physico-chemical process on which the existence of life on planet earth depends.
The word “photosynthesis” is derived from the Greek words phōs (pronounced: “fos”) and σύνθεσις
(pronounced: “synthesis“)
Phōs means “light” and σύνθεσις means, “combining together.” This means “combining together with the
help of light.”
3. PHOTOSYNTHESIS
Photosynthesis, the process by which green plants and certain other organisms transform light energy
into chemical energy. During photosynthesis in green plants, light energy is captured and used to
convert water, carbon dioxide, and minerals into oxygen and energy-rich organic compounds.
• Definition: Photosynthesis is a physico-chemical process in which solar energy (sunlight energy) is
converted into chemical energy.
Photosynthesis is the process by which plants, algae and some bacteria use sunlight, carbon dioxide
and water to prepare their food. The ability to photosynthesize is found in
both eukaryotic and prokaryotic organisms. The most well-known eukaryotes are plants. Photosynthesis
also applies to other organisms besides green plants. These include several prokaryotes such as
cyanobacteria, purple bacteria and green sulfur bacteria. These organisms exhibit photosynthesis just like
green plants.
Photosynthesis is the ability of the green plants to fix and utilize the sunlight energy for the synthesis of
carbon containing organic substances from inorganic raw materials, available in the environment.
5. Photosynthetic Apparatus
Photosynthetic Apparatus: A proper organization is required to carry out all the reactions in the photosynthesis.
Such specific structures are found embedded in the cytoplasm. In a simple, prokaryotic photosynthesizing cell,
such structures are present in the form of free, isolated photosynthetic lamellae called chromatophores.
• But in bryophytes and all vascular plants, belonging to higher groups, relatively evolved, well developed and
complicated photosynthetic apparatus is present known as Chloroplastid or Chloroplast.
Structure of Chloroplast:
• Under electron microscope, the chloroplast shows four major structural regions:
1. Chloroplast Envelope – It is double layered. Each membrane is about 5.0 to 7.5 nm thick & are separated by
about 10nm intramembrane space. Out of these two membranes, the inner is selectively permeable and it regulates
the exchange of metabolites between the chloroplast and the surrounding cytoplasm. The envelope encloses the
stroma or matrix.
2. Stroma or Matrix—It is mainly a protein solution with many other substances required for photosynthetic
reactions.This region contains all enzymes required for photosynthetic carbon reduction i.e. synthesis of
carbohydrates from atmospheric CO2.
6. Photosynthetic Apparatus
1. Thylakoids – In stroma region, along with the chemical substances, a complex
system of membranes, called lamellar system is embedded. This system is
composed of individual pairs of parallel membranes which appear to be joined at
the ends. The sectional view of the chloroplastid shows that these double layered
structures are flattened sac like and therefore, they are called thylakoids, appear
as it arranged one above the other forming stacks of tires ( like stack of coins).
Such stack like regions in the thylakoid system is called grana/granum. Majority
of the chloroplasts exhibit presence of grana regions known as granar chloroplast
and some are without grana regions known as agranar. Some thylakoids extend
beyond grana regions into stroma region and are called stroma thylakoids.
7. Photosynthetic Apparatus
1. Intrathylakoid space i.e. Lumen: The interior space of thylakoid is known as lumen. The lumen is
the site of photolysis of water i.e. splitting of water molecule and evolution of photosynthetic O2.
The main function of lumen is to store protons (H+) which are pumped across the thylakoid
membrane during electron transport. The protons are used to drive ATP synthesis during
photosynthetic light reaction.
In granar chloroplastids, photosynthetic pigments are present in the grana regions, while in
agranar chloroplastids they are present in tube like thylakoids. Granum thylakoid membrane is also
double layered. This double layered envelope shows some distinct particles on the inner surface of the
outer membrane and on the outer surface of the inner membrane of the thylakoid. The particles on the
outer surface are loosely arranged, while on the inner surface are compactly arranged. These
compactly arranged particles on the outer surface of the inner membrane of thylakoid envelope are
called ‘quantasomes’
8. Photosynthetic Apparatus
These quantasomes are considered as sites of photochemical reactions i.e. in these quantosomes photosynthetic
pigments are present and they carry out reactions essential for fixation of light energy and its conservation in
ATP molecules. Each quantasome shows presence of 4 subunits, known as quantasomal subunits. Each
quantasomal subunit, shows presence of many molecules of photosynthetically active pigments. In
quantasomes these pigment molecules are arranged in the form of definite systems called ‘pigment system’ or
‘photosystems’. As per the nature of pigments, the photosystems are of two types , photosystem I (PSI) and
photosystem II(PSII).
Since lumen is on one side and pigments are on another side, the synthesis of assimilatory power
(ATP+NADPH2) takes place on the surface of inner membrane of the thylakoid in the granum region of the
chloroplastid. The grana thylakoids complete the photochemical act and synthesize assimilatory power
molecules. The stroma region completes all the reactions required for CO2 assimilation, utilizing assimilatory
power during carbohydrate synthesis. As both the major steps, light reaction and CO2 assimilation are
completed inside the chloroplast, it is the proper organization for photosynthesis in a cell and considered as
photosynthetic apparatus.
9. Photosynthetic Pigments
During the process of photosynthesis, energy from sunlight rays is trapped and it is converted into
chemical energy. This process is called ‘photochemical act’ or ‘photochemical reaction’. There are many
photosynthetic pigments present in the chloroplastids or photosynthetic apparatus of different plants.
Pigments are chemical compounds which reflect only certain wavelengths of visible light. This makes them
appear "colorful". Flowers, corals, and even animal skin contain pigments which give them their colors.
More important than their reflection of light is the ability of pigments to absorb certain wavelengths.
Because they interact with light to absorb only certain wavelengths, pigments are useful to plants and
other autotrophs --organisms which make their own food using photosynthesis. In plants, algae,
and cyanobacteria, pigments are the means by which the energy of sunlight is captured for photosynthesis.
However, since each pigment reacts with only a narrow range of the spectrum, there is usually a need to
produce several kinds of pigments, each of a different color, to capture more of the sun's energy.
Pigments are light-absorbing colored molecules. Different pigments absorb different wavelengths of light.
They absorb energy from violet-blue light and reflect green light, giving plants their green color.
10. Photosynthetic Pigments
There are major 3 types of photosynthetic pigments, namely; 1.chlorophylls, 2.carotenoids and 3.phycobilins.
1. Chlorophylls: The photosynthetic plants have a primary light-absorbing pigment known as chlorophylls.
Chlorophylls are
greenish pigments which contain a porphyrin ring. Chlorophyll is a water-insoluble magnesium porphyrin
compound. It shows two distinct regions- a head region and tail region. The head region shows tetrapyrol ring, with
atom of Magnesium(Mg) in the form of nucleus in the centre of the ring and the tail region shows long phytol ring.
There are several kinds of chlorophylls like chlo.a, chlo. b, chlo.c, chlo.d, chlo.e, bacteriochlorophyll a,
bacteriochlorophyll b and chlorobium chlorophyll. Though there are different classes of chlorophylls, major of them
are chlorophyll-a (chl-a) and chlorophyll-b (chl-b). In plants, the ratio of chl-a to chl-b is about three to one (3:1).
i. Chlorophyll ‘a’: Chlorophyll "a“ is most important pigment because it is the only pigment which takes direct part
in the photochemical act and makes photosynthesis possible. Only chl-a is a constituent of the photosynthetic
reaction centers. It is a bluish green colored pigment with molecular formula C55H72O5N4Mg. In reflected light
chl-a shows blood red color while in transmitted light, it shows blue green light. It is an universal pigment present in
all plants, algae, and cyanobacteria which photosynthesize except bacteria.
11. Photosynthetic Pigments
ii. Chlorophyll ‘b’: The light energy absorbed by chl-b can be transferred very efficiently to chl-
a. In this way chl-b enhances the plant’s efficiency for utilizing sunlight energy. It is a yellowish
green colored pigment with molecular formula C55H70O6N4Mg.It appears dull brown in
reflected light and yellowish green color in transmitted light. Chlorophyll "b", occurs only in
“green algae” and in the higher plants. It is absent in brown algae, red algae, diatoms, etc.
2.carotenoids: Carotenoids are usually red, orange, or yellow pigments They are secondary light-
absorbing pigments or accessory pigments occurring in the thylakoid membranes. These are
chemically terpenoids.. Carotenoids cannot transfer sunlight energy directly to the photosynthetic
pathway, but must pass their absorbed energy to chlorophyll. For this reason, they are
called accessory pigments. The carotenoid pigments absorb light at wavelengths which are not
absorbed by the chlorophylls so they are supplementary light receptors. These accessory pigments
protect the chlorophyll against photo-oxidation and then transfer light energy to chlorophylls. It
also helps in preventing photodynamic damage.
12. Photosynthetic Pigments
There are 2 types of carotenoids; carotenes and xanthophylls. Carotene is in orange color while
xanthophylls are yellow in color.
a. Carotenes: Chemically, carotenes are polyunsaturated hydrocarbons containing 40 carbon atoms per
molecule. The term carotene (also carotin, from the Latin carota, "carrot") is used for many
related unsaturated hydrocarbon substances having the formula C40Hx. Carotene is responsible for the
orange color of carrots and the colors of many other fruits and vegetables and even some animals.
Carotenes contain no oxygen atoms. They absorb ultraviolet, violet, and blue light and scatter orange or
red light, and (in low concentrations) yellow light. Carotenes are important for photosynthesis as they
contribute to photosynthesis by transmitting the light energy they absorb to chlorophyll. They also protect
plant tissues by helping to absorb the energy from singlet oxygen an excited form of the oxygen molecule
O2 which is formed during photosynthesis. In algae and higher plants β-carotene is the major carotenoid. In
some plants α-carotene and in bacteria Ƴ- carotene is present. These three carotenes shows vitamin ‘A’
activity for humans and some other mammals.
13. Photosynthetic Pigments
b. Xanthophylls: Xanthophylls (originally phylloxanthins) are yellow pigments that occur widely
in nature. The name is from Greek xanthos (means "yellow") and phyllon (means, "leaf"), due to
their formation of the yellow band seen in early chromatography of leaf pigments. Like other
carotenoids, xanthophylls are found in highest quantity in the leaves of most green plants, where
they act to modulate light energy and perhaps serve as a non-photochemical quenching agent to
deal with triplet chlorophyll (an excited form of chlorophyll), which is overproduced at high light
levels in photosynthesis. The group of xanthophylls includes (among many other
compounds),lutein, zeaxanthin, neoxanthin, violaxanthin, flavoxanthin and α- and β-
cryptoxanthin.The characteristic color of egg yolk indicates the presence of a xanthophyll
pigment, and is the typical color of lutein or zeaxanthin of the xanthophylls. The xanthophylls
found in the bodies of animals including humans, and in dietary animal products, are ultimately
derived from plant sources in the diet. For example, the yellow color of chicken egg yolks.
14. PHOTOSYNTHESIS
The process of photosynthesis can be defined as, ‘a process in green plants by which
carbohydrates are synthesized using CO2 and water with the help of chlorophyll, in presence of
sunlight producing water and oxygen as by products.
6CO2 + 12H2O C6H12O6 + 6H2O + 6O2
Above reaction shows, that the process of photosynthesis is an oxidation reduction process, in
which water is oxidized (Removal of H2) and CO2 is reduced (Addition of H2). For the reduction
of CO2, the essential H2 is obtained by oxidation (splitting) of water molecules and in this process
O2 is evolved as a by-product.
It is the biological process of converting light energy into chemical energy. In this process, light
energy is captured and used for converting carbon dioxide and water into glucose and oxygen gas.
The complete process of photosynthesis is carried out into two processes: Light Reactions (Light
dependent phase) and Dark Reactions (Light independent phase).
15. LIGHT REACTIONS
The Light dependent phase is the first phase in the process of photosynthesis and it
is called Light reaction or Hill reaction. The light reaction takes place in the grana
of the chloroplast. Here, light energy gets converted to chemical energy as ATP and
NADPH. In this very light reaction, the addition of phosphate in the presence of
light or the synthesizing of ATP by cells is known as photophosphorylation.
• Photophosphorylation: Photophosphorylation is the process of utilizing light
energy from photosynthesis to convert ADP to ATP. It is the process of synthesizing
energy-rich ATP molecules by transferring the phosphate group into ADP molecule
in the presence of light.
16. LIGHT REACTIONS - Photophosphorylation
Photophosphorylation is of two types: Cyclic Photophosphorylation and Non-cyclic
Photophosphorylation
• Cyclic Photophosphorylation: The photophosphorylation process which results in the
movement of the electrons in a cyclic manner for synthesizing ATP molecules is called
cyclic photophosphorylation.
• In this process, plant cells just accomplish the ADP to ATP for immediate energy for the
cells. This process usually takes place in the thylakoid membrane and uses Photosystem I
and the chlorophyll P700.
• During cyclic photophosphorylation, the electrons are transferred back to P700 instead
of moving into the NADP from the electron acceptor. This downward movement of
electrons from an acceptor to P700 results in the formation of ATP molecules.
18. LIGHT REACTIONS - Photophosphorylation
• Non-cyclic Photophosphorylation: The photophosphorylation process which results in the
movement of the electrons in a non-cyclic manner for synthesizing ATP molecules using the
energy from excited electrons provided by photosystem II is called non-cyclic
photophosphorylation.
• This process is referred to as non- cyclic photophosphorylation because the lost electrons by
P680 of Photosystem II are occupied by P700 of Photosystem I and are not reverted to P680.
Here the complete movement of the electrons is in a unidirectional or in a non- cyclic manner.
• During non-cyclic photophosphorylation, the electrons released by P700 are carried by
primary acceptor and are finally passed on to NADP. Here, the electrons combine with the
protons – H+ which is produced by splitting up of the water molecule and reduces NADP to
NADPH2.
21. LIGHT REACTIONS – Cyclic and Non- cyclic Photophosphorylation
Difference between Cyclic and Non-Cyclic Photophosphorylation
Cyclic Photophosphorylation Non-Cyclic Photophosphorylation
Only Photosystem I is involved. Both Photosystem I and II are involved.
P700 is the active reaction center. P680 is the active reaction center.
Electrons travel in a cyclic manner. Electrons travel in a non – cyclic manner.
Electrons revert to Photosystem I Electrons from Photosystem I are accepted by NADP.
ATP molecules are produced. Both NADPH and ATP molecules are produced.
Photolysis of water is absent.
Water is not consumed.
Source of electron is P700.
Photolysis of water is present.
Water is consumed.
Source of electron is water.
NADPH is not synthesized. NADPH is synthesized.
Oxygen is not evolved as the by-product Oxygen is evolved as a by-product.
This process is predominant only in bacteria. This process is predominant in all green plants.
22. DARK REACTIONS
Light-independent reactions – It is also called the dark reaction or Calvin cycle or C3 cycle. This reaction occurs both in the
presence and absence of sunlight. It is a cyclic reaction occurring in the dark phase of photosynthesis. In this reaction, CO2 is
converted into sugars and hence it is a process of carbon fixation. The Calvin cycle was first observed by Melvin Calvin and his
colleagues in the 1950s. Calvin was awarded Nobel Prize for this work in 1961. In this cycle, the first stable compound in Calvin
cycle is a 3 carbon compound (3-phosphoglyceric acid), the cycle is also called as C3 cycle or PCR (Photosynthetic Carbon
Reduction). About 85% of the plant species on the planet are C3 plants, including rice, wheat, soybeans and all trees.
The reactions of Calvin’s cycle occur in three distinct phases. These phases are :-
1. Carboxylation phase
2. Reductive phase
3. Regeneration phase
23. DARK REACTIONS- C3 pathway
• C3 plants are known as cool-season or temperate plants. They grow best at an optimum temperature
between 65 to 75°F with the soil temperature suited at 40- 45°F. These types of plants show less efficiency
at high temperature.
• The primary product of C3 plants is 3-carbon acid or 3-phosphoglyceric acid (PGA). This is
considered as the first product during carbon dioxide fixation. The C3 pathway completes in three steps:
carboxylation, reduction, and regeneration.
• C3 plants reduce into the CO2 directly in the chloroplast. With the help of ribulose biphosphate
carboxylase (RuBPcase), the two molecules of 3-carbon acid or 3-phosphoglyceric acid are produced.
This 3- phosphoglyceric justifies the name of the pathway as C3.
• In another step, NADPH and ATP phosphorylate to give 3-PGA and glucose. And then the cycle again
starts by regenerating the RuBP.
24. DARK REACTIONS- C3 pathway
• The C3 pathway is the single step process,
takes place in the chloroplast. This organelle
act as the storage of sunlight energy. Of the
total plant present on earth, 85 percent uses
this pathway for the production of energy.
• The C3 plants can be perennial or annual.
They are highly proteinaceous than the C4
plants. The examples of annual C3 plants are
wheat, oats, and rye and the perennial plants
include fescues, ryegrass, and orchardgrass.
C3 plants provide a higher amount of protein
than the C4 plants.
25. DARK REACTIONS: C3 Pathway
The conversion of CO2 to carbohydrate is
called Calvin Cycle or C3 cycle and is
named after Melvin Calvin who discovered
it. The plants that undergo the Calvin cycle
for carbon fixation are known as C3 plants.
Calvin Cycle requires the enzyme ribulose-
1,5-bisphosphate carboxylase/oxygenase
commonly called RuBisCO. It generates the
triose phosphates, 3-phosphoglycerate (3-
PGA), glyceraldehyde-3P (GAP), and
dihydroxyacetone phosphate (DHAP), all of
which are used to synthesize the hexose
phosphates fructose-1,6-bisphosphate and
fructose 6-phosphate.
26. DARK REACTIONS: C3 Pathway
The Calvin cycle (C3-cycle) or PCR-cycle can be divided into three stages:
(a) Car-boxylation, during which atmospheric CO2 combines with 5-C acceptor molecule ribulose 1, 5-bisphosphate (RuBP) and
converts it into 3-phosphoglyceric acid (3-PGA);
(b) Reduction, which consumes ATP + NADPH (produced during primary photochemical reaction) and converts 3-PGA into 3-
phosphoglyceraldehyde (3PGAld) or triose phosphate (TRI- OSE-P); and
(c) Formation of hexose sugar and regeneration of RuBP which consumes additional ATP, so that the cycle continues
(a) Carboxylation
(i) The CO2 is accepted by ribulose 1, 5-bisphosphate
(RuBP) already present in the cells and a 6-carbon
addition compound is formed which is unstable. It soon
gets hydrolysed into 2 molecules of 3-phosphoglyceric
acid (3PGA). Both these reactions take place in the
presence of ribulose bisphosphate carboxylase
(Rubisco). 3-Phosphoglyceric acid is the first stable
product of dark reaction of photosynthesis.
(b) Reduction
(ii) 3- Phosphoglyceric
acid is reduced to 3-
phosphoglyceraldehyde
by the assimilatory
power (generated in
light reaction) in the
presence of triose
phosphate
dehydrogenase.
(c) Formation of Hexose Sugar
and Regeneration of RuBP
(iii) Some of the molecules of 3-
phosphoglyceraldehyde isomerise
into dihydroxyaeetone phosphate,
both of which then unite in the
presence of the enzyme aldolase to
form fructose 1, 6-bisphophate.
27. DARK REACTIONS: C3 Pathway- It involves the following steps
i) Some of the molecules of 3-phosphoglyceraldehyde into dihydroxyacetone phosphate in the presence of enzyme triose phosphate
isomerase. Both 3-phospho glyceraldehyde and dihydroxy acetone phosphate then unite in the presence of the enzyme, aldolase to form
fructose 1,6- bisphosphate.
ii). Fructose 1,6-phosphate is converted into fructose 6-phosphate in the presence of phosphatase.
Some of the fructose 6-phosphate (hexose suger) is tapped off from the calvin cycle and is converted into glucose, sucrose, and starch.
Sucrose is synthesized in cytosol while starch is synthesized in the chloroplast.
iii). Some of the molecules of 3-phosphoglyceraldehyde instead of forming hexose sugars are diverted to regenerate ribulose 1,5-
bisphosphate.
iv). 3-phosphoglyceraldehyde reacts with fructose 6-phosphate in the presence of enzyme transketolase to form erythrose 4-phosphate (4-C
atoms sugar) and xylulose 5-phosphate(5-C atoms sugar).
v). Erythrose 4-phosphate combines with dihydroxyacetone phosphate in the presence of the enzyme aldolase to form sedoheptulose 1, 7-
bisphosphate(7-C atoms sugar).
vi). Sedoheptulose 1, 7-bisphosphate loses one phosphate group in the presence of the enzyme phosphatase to form sedoheptulose 7-
phosphate.
vii). Sedoheptulose 7-phosphate reacts with 3-phosphoglyceraldehyde in the presence of transketolase to form xylulose 5-phosphate and
ribose 5- phosphate ( both 5-C atoms sugars).
viii). Xylulose 5-phosphate is converted into another 5-C atoms suger ribulose 5-phosphate in the presence of the enzyme phosphoketopentose
epimerase.
ix). Ribose 5-phosphate is also converted into ribulose 5-phosphate. The reaction is catalysed by the enzyme phosphopentose isomerase.
x). Ribulose 5-phosphate is finally converted into ribulose 1,5-bisphosphate in the presence of enzyme, phosphopentose kinase and ATP.
Thus completing Calvin cycle.
In the dark reaction, CO2 is fixed to carbohydrates and the CO2 acceptor ribulose diphosphate is regenerated. In Calvin cycle, 12 NADPH2
and 18 ATPs are required to fix 6 CO2 molecules into one hexose sugar molecule (fructose 6 phosphate).
The net reaction of the calvin cycle is:
6 CO2 + 18 ATP + 12 NADPH + 12 H+ + 12 H2O fructose 6 phosphate +18 ADP + 18Pi + 12NADP+
28. DARK REACTIONS: C3 Pathway
Products of C3 Cycle
• One molecule of carbon is fixed at each turn of the Calvin cycle.
• One molecule of glyceraldehyde-3 phosphate is created in three turns of
the Calvin cycle.
• Two molecules of glyceraldehyde-3 phosphate combine together to form
one glucose molecule.
• 3 ATP and 2 NADPH molecules are used during the reduction of 3-phosphoglyceric
acid to glyceraldehyde-3 phosphate and in the regeneration of RuBP.
• 18 ATP and 12 NADPH are consumed in the production of 1 glucose molecule.
Key Points on C3 Cycle
• C3 cycle refers to the dark reaction of photosynthesis.
• It is indirectly dependent on light and the essential energy carriers are products of light-dependent reactions.
• In the first stage of the Calvin cycle, the light-independent reactions are initiated and carbon dioxide is fixed.
• In the second stage of the C3 cycle, ATP and NADPH reduce 3PGA to G3P. ATP and NADPH are then converted into ATP and NADP+.
• In the last stage, RuBP is regenerated. This helps in more carbon dioxide fixation.
30. DARK REACTIONS: C4 Pathway
• Plants, especially in the tropical region, follow this pathway. Before
Calvin or C3 cycle, some plants follow the C4 or Hatch, Slack and
Kortschak pathway(HSK pathway) It is a twostep process where
Oxaloacetic acid (OAA) which is a 4-carbon compound is produced. It
occurs in mesophyll and bundle sheath cell present in a chloroplast.
• C4 plants are also known as warm-season or tropical plants. These can
be perennial or annual. The perfect temperature to grow for these plants
is 90-95°F. The C4 plants are much more efficient in utilizing nitrogen and
gathering carbon dioxide from the soil and atmosphere. The protein content
is low as compared to C3 plants.
• These plants got their name from the product called as oxaloacetate
which is 4 carbon acid. The examples of perennial C4 plants are Indian
grass, Bermudagrass, switchgrass, big bluestem and that of annual C4 plants
are Sudan grasses, corn, pearl millet.
31. DARK REACTIONS: C4 Pathway
The C4 process is also known as the Hatch-Slack-Kortschak pathway and is named for the 4-carbon
intermediate molecules that are produced, malic acid or aspartic acid. It wasn’t until the 1960s that
scientists discovered the C4 pathway while studying sugar cane. C4 has one step in the pathway before the
Calvin Cycle which reduces the amount of carbon that is lost in the overall process. The carbon dioxide
that is taken in by the plant is moved to bundle sheath cells by the malic acid or aspartic acid molecules (at
this point the molecules are called malate and aspartate). The oxygen content inside bundle sheath cells is
very low, so the RuBisCO enzymes are less likely to catalyze oxidation reactions and waste carbon
molecules. The malate and aspartate molecules release the carbon dioxide in the chloroplasts of the bundle
sheath cells and the Calvin Cycle begins. Bundle sheath cells are part of the Kranz leaf anatomy that is
characteristic of C4 plants.
About 3% or 7,600 species of plants use the C4 pathway, about 85% of which are angiosperms (flowering
plants). C4 plants include corn, sugar cane, millet, sorghum, pineapple, daisies and cabbage. C4 plants are
common in habitats that are hot, but are less abundant in areas that are cooler. In hot conditions, the
benefits of reduced photorespiration likely exceed the ATP cost of moving from the mesophyll cell to the
bundle-sheath cell.
32. DARK REACTIONS: C4 Pathway
On the basis of the chemical nature of organic acid synthesized in mesophyll cells & then transported to Bundle Sheath cells, two
groups of C4 plants are known -The first group synthesize malic acid by using atmospheric CO2 such plants are called malate
formers and the second group synthesize Aspartic acid called Aspartate formers.
Malate formers-e.g., Maize, sugarcane Aspartate formers- e.g., Panicum, Chloris
33. DARK REACTIONS: C4 Pathway
The C4 plants exhibit dimorphism in the chloroplastids. The chloroplastids in mesophyll cells
are granar chloroplastids & they are without the enzyme system for synthesis of starch. The
B.S chloroplastids are agraner & are without photosystem II (PSII) due to absence of psII,
B.S chloroplast cannot synthesize NADP.H2 required for PCR for NADP.H2 molecules the
B.S chloroplast have to depend on mesophyll chloroplastds. It means neither mesophylls cells
nor the B.S cells can complete all the reactions in the process of photosynthesis
independently. For this reason, in C4 plants for the completion of all the reaction in the
process of photosynthesis, involvement of both type of chlroplastids from mesophyll cells &
B.S cells are essential. Some reaction of CO2 fixation is completed in mesophyll cells & the
remaining reactions are completed by distinct co-operation between mesophyll cells & bundle
sheath cells this process is called co-operative photosynthesis.
34. DARK REACTIONS: C4 Pathway
The most important character of C4 cycle is the enzyme PEPcase (Phosphoenol Pyruvate
Carboxylase) which canalizes carboxylation reaction using PEP as initial CO2 acceptor in
mesophyll cells. The product of carboxylation by PEPcase is OAA but it is unstable. It is
quickly reduced to Malic acid (in malate former) or to Aspartic acid (in aspartic former) &
then transported to B.S. The transport of these acid from mesophyll cells to B.S cells takes
place through cytoplasmic connections-plasmodesmata.
In the B.S cells the organic acids undergo decarboxylation & produces CO2 for PCR
cycle. The acid is transported back from B. S. cell to mesophyll cell. In mesophyll cell
pyruvic acid is converted to PEP. In B.S cell chloroplastid from net gain PGAL molecules,
various carbohydrates including starch molecule are synthesized using specific enzyme
system.
35. DARK REACTIONS: C4 Pathway
The Malic acid synthesized in mesophyll chloroplast is then transported to adjacent bundle sheath
chloroplast through cytoplasmic connections between these cells. In bundle sheath cell chloroplast, malic
acid get decarboxylated and dehydrogenated in presence of NADP, to produce CO2, NADP.H2 and
pyruvic acid. In these products, CO2 and NADP.H2 are used for C3 cycle to produce starch and sugars
while pyruvic acid is transported back to mesophyll cells and utilized again for fixation of atmospheric
CO2
38. DARK REACTIONS: CAM Pathway
• The noteworthy remark which distinguishes this process from the above two is that in this
type of photosynthesis the organism absorbs the energy from the sunlight at the day time and
uses this energy at the night time for the assimilation of carbon dioxide.
• It is a kind of adaptation at the time of periodic drought. This process permits an exchange
of gases at the night time when the air temperature is cooler, and there is the loss of water
vapor.
• Around 10% of the vascular plants have adapted the CAM photosynthesis but mainly
found in plants grown in the arid region. The plants like cactus and euphorbias are the
examples. Even the orchids and bromeliads, adapted this pathway due to an irregular water
supply.
• In the day time, malate gets decarboxylated to provide CO2 for the fixation of the Benson-
Calvin cycle in closed stomata. The main feature of CAM plants is an assimilation of CO2 at
night into malic acid, stored in the vacuole. PEP carboxylase plays the main role in the
production of malate.
39. DARK REACTIONS: CAM Pathway
• Some plants that are adapted to dry environments, such as cacti and pineapples, use the crassulacean acid
metabolism (CAM) pathway to minimize photorespiration. This name comes from the family of plants, the
Crassulaceae, in which scientists first discovered the pathway.
• Instead of separating the light-dependent reactions and the use of CO2 in the Calvin cycle in space, CAM plants
separate these processes in time. At night, CAM plants open their stomata, allowing CO2 to diffuse into the leaves.
This CO2 is fixed into oxaloacetate by PEP carboxylase, then converted to malate or another type of organic acid.
• The organic acid is stored inside vacuoles until the next day. In the daylight, the CAM plants do not open their
stomata, but they can still photosynthesize. That's because the organic acids are transported out of the vacuole and
broken down to release CO2 which enters the Calvin cycle.
• The CAM pathway requires ATP at multiple steps, so like C4, photosynthesis, it is not an energetic However, plant
species that use CAM photosynthesis not only avoid photorespiration, but are also very water-efficient. Their
stomata only open at night, when humidity tends to be higher and temperatures are cooler, both factors that reduce
water loss from leaves. CAM plants are typically dominant in very hot, dry areas, like deserts.
42. DARK REACTIONS: CAM Pathway
The molecules of Glu. 6 phosphate
are returned back to the system.
They are utilized again for
acidification reactions during night.
During day time starch is
synthesized using Malic acid
molecules (stored in vacuoles). Due
to utilization of stored malic acid
molecules, the acid concentration of
the cells decreases during day, called
deacidification. As Glu.6 phosphate
molecules, initially utilized are
returned back to the system, the
starch and other carbohydrates added
in the system are “net gain”
molecules in the process.
43. Comparison Chart
BASIS FOR
COMPARISO
N
C3 PATHWAY C4 PATHWAY CAM
Definition Such plants whose first product after
the carbon assimilation from sunlight
is 3-carbon molecule or 3-
phosphoglyceric acid for the
production of energy is called C3
plants, and the pathway is called as
the C3 pathway. It is most commonly
used by plants.
Plants in the tropical area, convert
the sunlight energy into C4 carbon
molecule or oxaloacetice acid, which
takes place before the C3 cycle
and then it further convert into the
energy, is called C4 plants and
pathway is called as the C4 pathway.
This is more efficient than the C3
pathway.
The plants which store the energy
from the sun and then convert it
into energy during night follows
the CAM or crassulacean acid
metabolism.
Cells involved Mesophyll cells. Mesophyll cell, bundle sheath cells. Both C3 and C4 in same
mesophyll cells.
Example Sunflower, Spinach, Beans, Rice,
Cotton.
Sugarcane, Sorghum and Maize. Cacti, orchids.
Can be seen in All photosynthetic plants. In tropical plants Semi-arid condition.
44. Comparison Chart
Types of plants using
this cycle
Mesophytic, hydrophytic,
xerophytic.
Mesophytic. Xerophytic.
Photorespiration Present in high rate. Not easily detectable. Detectable in the afternoon.
For the production of
glucose
12 NADPH and 18 ATPs are
required.
12 NADPH and 30 ATPs are
required.
12 NADPH and 39 ATPs are
required.
First stable product 3-phosphoglycerate (3-PGA). Oxaloacetate (OAA). Oxaloacetate (OAA) at night, 3
PGA at daytime.
Calvin cycle
operative
Alone. Along with the Hatch and Slack
cycle.
C3 and Hatch and Slack cycle.
Optimum
temperature for
photosynthesis
15-25 °C 30-40 °C > 40 degrees °C
Carboxylating
Enzyme
RuBP carboxylase. In mesophyll: PEP carboxylase.
In bundle sheath: RuBP
carboxylase.
In the dark: PEP carboxylase.
In light: RUBP carboxylase.
45. Photosynthesis ------ Conclusion
• We all are aware of the fact that plants prepare their food, by the process of photosynthesis.
They convert atmospheric carbon dioxide into plant food or energy (glucose). But as the plants
grow in the different habitat, they have different atmospheric and climatic condition; they differ in
the process of gaining energy.
• Like in case C4 and CAM pathways are the two adaptations arose by natural selection, for the
survival of the plants of high temperature and arid region. So we can say that these are the three
distinct biochemical methods, of plants to obtain energy and C3 is the most common among them.
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