this presentation describes the basics of photosynthesis. it includes Significance of photosynthesis, Photosynthetic apparatus, Absorption & action spectra, Absorption & action spectra, Factors affecting photosynthesis, Photosynthetic apparatus, Position of photosynthetic pigments, Photosynthetic pigments, Functions of carotenoids, Phycobilins, Principle /Blackman’s law of limiting factors.

1. Photosynthesis : Introduction
BY: Dr. Sunita Sangwan
Assistant Professor, Botany
Dept. of Higher Education, Haryana

2. Contents
• Photosynthesis?
1.
• Significance of photosynthesis
2.
• Photosynthetic apparatus
3.
• Absorption & action spectra
4.
• Factors affecting photosynthesis
5.

3. What is Photosynthesis ?
Photo – light synthesis = to join
 is the single most important process on earth on which depends
the existence of human beings and almost all other living
organisms.
 Photosynthesis process by which photosynthetic organisms
convert light energy into chemical energy in the form of reducing
power (NADPH)and ATP and use these chemicals to drive
carbon dioxide fixation and reduction to produce sugars.
 The overall reaction of oxygen in photosynthesis can be
represented as -

4. Significance of
photosynthesis
Food
synthesis
Body
building &
energy
Energy for
lifestyle
By products
Purification
of
environment
Nature’s
Balance

5. CO2
• Through stomata by
respiration
Hydrogen
Donor
• By photolysis of
water
Light
• Radient energy consist of
discrete energy particles or
units called photon.
• Energy contained in one
photon is called quantum
and measured in Einstein.
RAW Materials for
Photosynthesis

6. •Out of total radiation output from sun
(1360Wm-1/sqm) only 900Wm-1/sqm reaches
to the earth surface.
•Plant receive about 400-500Wm-1/sqm PAR
(photosynthetically Active Radiations) from
sun.
•out of which 80% is absorbed by leaves but
only 10% is used for photosynthesis.
•Rest of them are changed in heat, used in
transpiration or re-emitted as fluorescence.

7. Photosynthetic apparatus

8. Position of photosynthetic pigments
Plant physiology, Taiz & zeigler

9. Photosynthetic
pigments
Chlorophylls
Chlorophyll a,b,c,d &
e
Bacteriochlorophylls
Bacteriochlorophylls
a,b,c ,d & e.
Baterioviridin
Carotenoids
Carotenes,
Xanthophylls
Phycobillins
Phycoerytrins,
Phycocyanins,
Allophycolyanin

10. Four pyrrole rings joined together by methane
bridges with magnesium at its nucleus.
Side chains are attached to the pyrrole rings
which are variable in different chlorophylls.
Phytol tail consist of an alcohol phytol of 20 C
atoms bounded to the 4th pyrrole ring by ester
linkage.
In chl b formyl (-CHO) on ring II instead of CH3
In chl c phytol chain is absent
In chl d (-O-CHO) group replaces (-CH=CH2) on
ring I

11. Carotenoids
Carotenes
α- carotenes
Found in all higher plants
β- carotenes
Found in algae & many higher
plants
γ- carotenes
Found in green photosynthetic
bacteria
Xanthophylls
Lutein, zeaxanthin,
cryptoxanthin, flavoxanthin,
violoxanthin
 Carotenes are unsaturated hydrocarbons, build up of isoprene units
with general formula C40H56.
 Carotenes consist of an open chain of conjugated double bond
system ending on both sides with an ‘ionone ring’.

12. Functions of carotenoids
 Prevent chlorophyll molecule from photo oxidation.
 Absorption spectra is from 425 nm – 490 nm which I s different
from chlorophyll. Hence these are known as supplementary light
harvesters.
 Absorb & transfer light to chlorophylls
 β- carotenes are the precursor of vitamin A in animals
 Make flower & fruit attractive by providing red & orange colour
to them.

13. Phycobilins
 Mainly found in blue green algae, red algae.
 Absorb short wavelength of light (below
500nm), due to short wavelength they penetrates
deep in to the sea where red algae grow.
 It consist of open conjugated system of 4-pyrrole
ring.
 It lacks Mg++ & phytol chain.
 Three types of phycobilins are:
i. Phycocyanin- blue colored
ii. Allophycocyanin- blue colored
iii. Phycoerythrin- red colored
Phytochrome is also an example of
biliproteins

14. Absorption Spectra
 The set of wavelengths absorbed by a pigment is its absorption
spectrum. The absorption spectrum of chlorophylls includes
wavelengths of blue and orange-red light, as is indicated by their
peaks around 450-475 nm and around 650-675 nm.
 As a note, chlorophyll a absorbs slightly different wavelengths
than chlorophyll b.
 Chlorophyll a, gives absorption peaks at 430 nm and 662 nm,
while Chlorophyll b gives peaks at 453 nm and 642 nm. The
different side groups in the two chlorophylls ‘tune’ the absorption
spectrum to slightly different wavelengths.

15. Absorption spectra
Curve1- Bacteriochlorophyll a
Curve2 – chlorophyll a
Curve3 – chlorophyll b
Curve4 – phycoerytrobilin
Curve5- beta- carotene

16. Action Spectra
 An action spectrum is a graph of the rate
of biological effectiveness plotted against wavelength of light. ...
For example, chlorophyll is much more efficient at using the red
and blue regions than the green region of the light spectrum to
carry out photosynthesis.

17. Action Spectrum compared with an absorption spectrum.
The absorption spectrum is measured by plotting a response to light such as oxygen
evolution, as a function of wavelength.
If the pigment used to obtain the absorption spectrum is same as those that cause the
response, the absorption & action spectrum will match.
 In the example shown here, the action spectrum for oxygen evolution matches the
absorption spectrum of intact chloroplast quite well, indicating that light absorption by
chlorophylls mediates oxygen evolution.
Discrepancies are found in the region of carotenoid absorption, from 450 to 550 nm,
indicating that energy transfer from carotenoids to chlorophylls is not as effective as
energy transfer between chlorophylls.

18. External Factors affecting photosynthesis
External Factors
affecting
photosynthesis
Light
Carbon
dioxide
Temperature
Water
Oxygen
Minerals
Air Pollutants

19. Principle /Blackman’s law of limiting factors
 Optimum value of a factor is never constant. It depends upon the
magnitude of other factors
 In such cases it is found that a factor called limiting factor is
holding the balance. A limiting factor is defined as a factor which
is deficient to such an extent that increase in its magnitude
directly increases the rate of the process.
 The effect of limiting factors was studied by Blackman in 1905.
 He formulated the principle of limiting factors which states that
when a process is conditioned as to its rapidity by a number of
separate factors, the rate of the process is limited by the pace of
the slowest factor. In other words the rate of a physiological
process is limited at a given time by one and only one factor
which is deficient.

21. Light
Light intensity
High light
intensity
Low light
intensity
Optimum light
intensity
Light Quality
Light duration

22. Light Intensity
 At low light intensities, the rate of
photosynthesis is directly proportional to the
light intensity.
 Because as more light becomes available, more
chlorophyll molecules can absorb light so more
electrons are excited leading to photolysis and
photophosphorylation.
 More ATP and NADPH are produced so the
light-independent reactions can occur at a higher
rate so more product is produced.
 Eventually a maximum rate is reached and so
increasing light intensity has no effect so the
graph levels off.
 This can be because all available chlorophyll
molecules are absorbing light or some other
factor is now the limiting factor.

23. Temperature
 When light is not a limiting factor (i.e. high light
intensities), increasing the temperature increases
the rate of photosynthesis.
 Above the optimum temperature, any further
increase causes the rate to decrease rapidly.
 Because the Calvin Cycle is enzyme controlled,
when the temperature increases both enzymes and
substrates gain kinetic energy, so more collisions
occur, so more enzyme substrate complexes form,
so more product forms.
 When the temperature exceeds the optimum, the
enzymes will denature and the specific shape of the
active site will change and no longer be
complementary to the substrate so fewer enzyme-
substrate complexes can form.

24. CO2
 At low CO2 levels an increase in
concentration causes a directly
proportional increase in the rate of
photosynthesis.
 A maximum rate is eventually reached
and further increase has no effect and so
the graph levels off.
 This is because atmospheric CO2 levels
are lower than the optimum value so
when concentration is increased more
CO2 is absorbed so more product is
made.
 Eventually, there is no more RuBP
available to absorb anymore CO2 so

25. Oxygen
Oxygen has been shown to inhibit photosynthesis in C3 plants
while C4 plants show little effect. It is suggested that C4 plants
have photorespiration and high O2 stimulates it.
The rate of photosynthesis increases by 30-50% when the
concentration of oxygen in air is reduced from 20% to 0.5% and
CO2, light and temperature are not the limiting factors.
Oxygen is inhibitory to photosynthesis because it would favour a
more rapid respiratory rate utilizing common intermediates, thus
reducing photosynthesis.
Secondly, oxygen may compete with CO2 and hydrogen becomes
reduced in place of CO2.
Thirdly, O2 destroys the excited (triplet) state of chlorophyll and
thus inhibits photosynthesis.

26.  Water: Water is an essential raw material in carbon assimilation.
Less than 1% of the water absorbed by a plant is used in
photosynthesis. The decrease in water contents of the soil from field
capacity to the permanent wilting point results in the decreased
photosynthesis.
 Mineral elements: As discussed earlier, several minerals are
essential for plant growth. These include Mg, Fe, Cu, CI, Mn, P and are
closely associated with reactions of photosynthesis.
 Air Pollutants: Gaseous and metallic pollutants decrease
photosynthetic activity. These include ozone, SO2, oxidants, hydrogen
fluorides, etc.
 Chemical Compounds: Compounds like HCN, H2S, etc. when
present even in small quantities, depress the rate of photosynthesis by
inhibiting enzymes. In addition chloroform, ether etc., also stop

27. Internal factors affecting photosynthesis
Anatomy
Chlorophyll
Protoplasmic factor
Accumulation of end products
Age of leaf
Demand
Hormones

28. You tube video links for these topics
 https://www.youtube.com/watch?v=oAH6IfQfZh4
photosynthesis - Introduction
 https://youtu.be/CrDEallsToo Photosynthesis part-2 Absorption
& action spectra
 Please like and subscribe the channel so that it will be helpful
for maximum number of students

29. References
 Taiz, L. and Zeiger, E. (2010) Plant Physiology. 5th Edition,
Sinauer Associates, Inc., Sunderland.
 Hopkins, W.G. and Hüner, N.P. (2004) Introduction to Plant
Physiology. 3rd Edition, John Wiley & Sons, Inc, Hoboken.