The document discusses the evolutionary transition of plants from C3 to C4 photosynthesis, outlining the adaptations that allow C4 plants to thrive in hot, dry environments. C3 plants primarily fix carbon dioxide through a 3-carbon compound but suffer from inefficiencies due to photorespiration, whereas C4 plants utilize a 4-carbon compound and specialized leaf anatomy to optimize carbon fixation and minimize water loss. The advantages of C4 plants include higher photosynthetic rates, reduced water loss, and increased adaptability to extreme temperatures, making them more efficient in certain climatic conditions.

1. Evolutionary
Transitions from C3 to
C4 plantsSUBMITTED BY: HAFSA ARSHAD
ROLL NUMBER: BBOF17M006

2. Submitted to: SIR ZAFAR IQBAL
2
DEPARTMENT OF BOTANY
UNIVERSITY OF SARGODHA

3. Plant Evolutionary
History
Life began in water
The earliest plants were aquatic
• Since plants evolved to live without
water, they had a hard time dealing
with dehydration
3

4. Plants
adaptations
 Most adaptations involve in some
sort of trade off.
 For plants the trade off with
photosynthesis is that they lose
water to the environment through
transpiration.
4

5. 5
 The openings on leaves is called
stomata.
 This is where 𝐶𝑂2 and 𝑂2 can enter
and exit the plant.
 Water is also lost through stomata.
 Plants often close stomata in very hot
days, and also 𝐶𝑂2 intake cuts off.
A little plant anatomy

6. 6
 Those plants whose first product of
photosynthesis is 3 carbon compound.
 About 85% of plant species are 𝑪 𝟑 plants.
 The term 𝑪 𝟑 photosynthesis is ancestral
pathway for carbon fixation.
 It occurs in all taxonomic plant groups.
 The term 𝑪 𝟑 photosynthesis is based on the
observations that the first product of
photosynthesis is a 3-carbon molecule.
 In this way such plants are known as
𝑪 𝟑 plants.
𝐶3 Plants

7. 7
In 𝑪 𝟑 plants, the bundle sheet cells do
not contain chloroplast
Carbon dioxide fixation takes place
only at one place.
𝐶3 Plants

8. 8
 𝑪 𝟑 photosynthesis is the major of
three pathways for carbon fixation by
plants.
 During hot days they close their
stomata part way and produce little
sugar.
𝐶3 Mechanism

9. 9
 And send it to the Calvin cycle
instead of 𝐶𝑂2
 Peroxisomes and mitochondria split
the new compound and release 𝐶𝑂2
 This is called photorespiration.
 s the 𝐶𝑂2 is used up by plant, rubisco
fix 𝑂2 instead

10. 10
 Photorespiration uses ATP but
don’t make ATP.
 Photorespiration makes no
sugar
 Photorespiration decreases
photosynthesis output.

11. 11
 Photorespiration may be an evolutionary leftover
 In early atmosphere, with little 𝑂2 it didn’t matter if
rubisco had an affinity for 𝑂2
 Today with so much 𝑂2 in the atmosphere, it is inevitable
that some 𝑂2 will be fixed in the environment.
So why would any plant do
this…

12. 12
o They include cereals, grains, wheat,
rice. Barley, oats, peanuts, cotton,
sugar beets, tobacco, spinach,
soybeans and most trees are 𝑪 𝟑
plants.
o Most lawn grasses rye and fescue are
𝑪 𝟑 plants.
𝐶3 Crop Plants

13. 13
 The plant whose first product of
photosynthesis is 4 carbon compound is
called C4 plant.
 Those plants which undergo 𝐶4
photosynthesis are known as 𝐶4 plants.
 𝐶4 photosynthesis occur in more advanced
plant taxa and specially common among
dicots(most trees and shrubs).
 𝐶4 plants evolved independently in a hot, dry
environment where their stomata must be
partially closed during the day.
𝐶4 Plants

14. 14
 In 𝐶4 plants the bundle sheet cells
contain chloroplasts.
 Carbon dioxide fixation takes place
twice (one in mesophyll cells, second in
bundle sheath cells).
 In 𝐶4 photosynthesis the initial
photosynthetic product is 4-carbon
molecule.
𝐶4 Plants

15. 15
Why plant use C4 mechanism ?
 Some plants which live in drought, at high temperature and
nitrogen and carbon dioxide limitation environment, they use C4
mechanism
 Because C4 plants fix more carbon dioxide as compared to C3
plants due to having KRANZ anatomy
 C4 plants lose 277 molecules of water to fix per molecule of CO2
as compared to c3 use 833 molecules of water approximately at
30°C
 They also avoid photorespiration process
 C4 enzyme fix carbon more efficiently as compared to C3 plants
who also fix oxygen during carbon fixation process

16. 16
Why plant use C4 mechanism ?
 Fixation of oxygen starts the process of photorespiration instead of
carboxylation of substrate, substrate oxidize and as a result loss
and start the process of photorespiration.
 This process occur due to the dual activity of rubisco present in C3
plants such as oxygenase and carboxylase activity.

17. 17
KRANZ ANATOMY
Their vascular bundles are surrounded by the two ring cells
 Inner ring
Contain bundle sheath cells (bundle sheath cells contain starch rich
chloroplast lacking grana)
 Outer ring
Contain mesophyll cells

18. 18
ADVANTAGES OF KRANZ ANATOMY
Main function of Kranz
anatomy is:
• To provide a site in which
CO2 concentrated around
rubisco
• And avoiding
photorespiration mechanism

19. 19
MECHANISM OF C4 PLANTS
FIRST STEP:
Conversion of (PEP) into oxalo-acetic acid
 In this step CO2 react with phosphoenol pyruvic acid to
form oxaloacetic acid
 This reaction takes place in the presence of enzyme
phosphoenol pyruvate carboxylase

20. 20
MECHANISM OF C4 PLANTS
2nd STEP:
Conversion of OAA into Malate
 In this step oxaloacetic acid is converted into Malate
 This reaction is catalyzed by malate dehydrogenase.

21. 21
MECHANISM OF C4 PLANTS
3rd STEP:
Conversion of Malate into pyruvic acid
 In this step malate is transported into bundle sheath cell.
In bundle sheath cells it is converted into pyruvic acid by
releasing CO2

22. 22
MECHANISM OF C4 PLANTS
4th STEP:
Conversion of pyruvic acid into PEP
 In this step pyruvic acidreacts with ATP and regenerate
phosphoenol pyruvate.
 This reaction takes place in the presence of pyruvate
orthophosphate dikinase.

23. 23
C4 MECHANISM

24. 24
 𝐶4 photosynthesis is an adaptation for
plants living in hot, arid climate
 Some plants that live in hot, dry climates
maintain low oxygen levels in their leaves
by keeping stomata closed to prevent
water loss.
 𝐶4 plants have special leaf anatomy with
prominent bundle sheath cells
surrounding the leaf veins.
𝐶4 Adaptations

25. 25
 𝐶4 plants have
adaptations that allow
them to minimize the
effect of photorespiration.
𝐶4 Adaptations

26. 26
 They have an alternate
means of fixing carbon.
 𝐶4 plants fix carbon out in
the cytoplasm before it
enters the Calvin Cycle

27. 27
 They have special type of
leaf anatomy
 They tolerate high
temperatures.
 They show response to
high light intensities.

28. 28
 The large cells present
around the vascular
bundle of the 𝐶4 pathway
plants are known as
bundle sheath cells.
 The leaves which have
such anatomy are said to
have KRANZ anatomy.
Bundle Sheath
Cells

29. 29
𝑪 𝟒 Plants undergo
𝑪 𝟒 cycle which is
also known as
Hatch And Slack
cycle.
Hatch and slack cycle

30. 30
 1st step is binding 𝐶𝑂2 to PEP (Phosphoenolpyruvate) by enzyme PEP
carboxylase to make a 4-C compound
 PEP has a high affinity of 𝐶𝑂2 and none for 𝑶 𝟐
 The 4-Carbon compound enters the photosynthetic cells.
 𝑪𝑶 𝟐 is released from PEP and Calvin cycle continues as normal
𝐶4 Plant Adaptation
s

31. 31
 They have greater productivity in biomass.
 Despite having Oxaloacetic acid as the first 𝐶𝑂2 fixation product
they use 𝑪 𝟑 pathway or Calvin cycle as the main biosynthetic
pathway
 𝐶4 plants have twice as the photosynthetic capacity as 𝐶3 plants
and can cope with higher temperature, less water and available
nitrogen.

32. 32
 𝐶4 plants thrive in hot
climate where stomata
will be closed often
 Some important 𝐶4
plants are sugarcane,
corn, tropical grasses

33. 33
This adaptation allows 𝐶4 plants
to keep a high concentration of
𝐶𝑂2 in the photosynthetic cells,
preventing from building of 𝑶 𝟐
instead of 𝐶𝑂2

34. 34
They include black mustard seed,
watercress, lettuce, cabbage,
maize, sorghum, millets, switch-
grass, broccoli, cauliflower, turnip
etc.
𝐶4 Crop Plants

35. COMPARISON
𝑪 𝟑 Plants
 Photosynthesis occur in mesophyll
tissues
• The carbon dioxide accepter is
RuBisco
• KRANZ anatomy is absent
• The 1st stable compound formed is
3C compound called 3-
phosphoglyceric acid (PGA).
𝑪 𝟒 Plants
 Photosynthesis occurs both in
mesophyll and bundle sheath cells.
 The carbon dioxide acceptor is
PEP carboxylase.
 KRANZ anatomy is present
 The 1st stable compound is 4-
carbon Oxaloacetic acid (OAA)
35

36. COMPARISON
𝑪 𝟑 Plants
 The optimum temperature is 20 -
25°C.
 Photo-respiratory loss is high.
 Photosynthetic rate is low.
 They are more adapted to
environment with more carbon
dioxide.
𝑪 𝟒 Plants
 The optimum temperature is 35 –
44°C.
 Photorespiration does not takes
place.
 Photosynthetic rate is comparatively
high
 They are more adapted to
environment with more oxygen.
36

37. 𝑪 𝟑 Plants 𝑪 𝟒 Plants
Most plants Tropical grasses like
corn, sugarcane
Fix carbon in Calvin cycle –
attach 𝐶𝑂2 to RuBP
Fix carbon in cytoplasm
- attach 𝐶𝑂2 to PEP
Enzyme - Rubisco Enzyme – PEP-ase
Most energy efficient
method Less efficient
Lose water through
photorespiration Lose less water
Comparison
of 𝐶3 & 𝐶4 Plants
37

38. 38
CARBON FIXATION
 C4 pathway is certainly more efficient than C3 pathway in
sense of carbon fixation.
 The enzyme responsible for this step is rubisco
 In C4 plants the inner cells get only carbon dioxide in the form
of malate.
 This avoid the oxygenation process and hence makes the
pathway more efficient

39. 39
Why is C3 photosynthesis inefficient
 C3 plants have disadvantage that in hot dry conditions their
photosynthetic efficiency suffers because of processes
called photorespiration.
 When CO2 concentration in chloroplast drops below about
50ppm, the catalyst rubisco helps to fix carbon begin to fix
oxygen instead

40. 40
Leaf Anatomy Of 𝐶3 & 𝐶4 Plants

41. 41
𝐶3 & 𝐶4 cycles

42. 42

43. Significance of 𝑪 𝟒 Plants
43
 In 𝑪 𝟒 plants, it increase the
photosynthetic yield two to three
times more than 𝑪 𝟑 plants
 In 𝑪 𝟒 plants, it performs a high rate
of photosynthesis even when the
stomata are nearly closed.
 It increases the adaptability of 𝑪 𝟒
Plants to high temperature and
light intensities.
 They can grow very well in saline
soils because of presence of 𝑪 𝟒
organic acid.

44. THANK YOU!
Email
hafsaranjha.botanist@gmail.com