PHOTOSYNTHESIS
The development of ideas
Hales 1727
Plants took their nourishment in part from the
atmosphere
Light energy participated in this process.
Preistly 1771
Green plants “renewed” air was made “bad” by
breathing animals or burning candles
“Renewed” air = Oxygen
“Bad” air = Carbon Dioxide
© 2010 Paul Billiet ODWS
The composition of the
atmosphere
O2 Scheele 1773 and Preistly 1774
CO2 Black 1777
N2 Rutherford 1772
The composition of air Lavoisier 1774
© 2010 Paul Billiet ODWS
Ingenhousz 1779
Light is necessary for the production of O2 by
plants
Senebier 1782
Plants breathe like animals in the dark
De Saussure 1804
Quantitative measurements of photosynthesis.
Mass gain by plants through growth > Mass of CO2
absorbed – Mass of O2 released
The difference was attributed to water
© 2010 Paul Billiet ODWS
Sachs 1864
 Observed the growth of starch grains in
illuminated chloroplasts
 Classic experiment: Leaf exposed to light
with certain parts covered
 In the exposed parts starch is produced
In the covered parts starch is absent
© 2010 Paul Billiet ODWS
By the end of C19th
Sunlight
CO2 + H2O
© 2010 Paul Billiet ODWS
(CH2O) + O2 
Carbohydrate
Light, Chlorophyll and Photosynthesis
Engelmann 1894
Experiment 1 Using
Cladophora, a filamentous
alga, and motile oxygen
sensitive bacteria
Image Credit: Cladophora
© 2010 Paul Billiet ODWS
Red
Orange
Yellow
Visible
spectrum Green
Cladophora
cells
Chloroplasts
which
practically
fill each cell
Blue
Indigo
Violet
© 2010 Paul Billiet ODWS
Motile
bacteria
Observation
Motile bacteria sensitive to oxygen
accumulate in the areas illuminated by
red and blue light
Conclusion
Photosynthesis, which produces oxygen,
takes place in the parts illuminated by red
and blue light
This is the action spectrum of
photosynthesis
© 2010 Paul Billiet ODWS
Experiment 2 Using Spirogyra, a filamentous alga
+ motile oxygen sensitive bacteria
© 2010 Paul Billiet ODWS
W
H
I
T
E
L
I
G
H
T
© 2010 Paul Billiet ODWS
Oxygen
sensitive
bacteria
Chloroplast
Observation
The bacteria accumulate in the regions
nearest the chloroplasts
Conclusion
O2 is produced by the chloroplast
This is the site of photosynthesis
© 2010 Paul Billiet ODWS
Light Intensity v Temperature
Blackman 1920s
Experiment 1
Rate of
photosynthesis
T + 10°C
T°C
© 2010 Paul Billiet ODWS
Light intensity
At high light intensities – when
light is not limiting – temperature
has an effect upon
photosynthesis. The rate doubles
for every 10°C rise in
temperature
Therefore Q10 = 2
© 2010 Paul Billiet ODWS
Rate of photosynthesis
Observations
At low light intensities – when
light is a limiting factor –
temperature has no effect upon
photosynthesis.
Therefore Q10 = 1
T + 10°C
T°C
Light intensity
Conclusion
Photosynthesis consists of two reactions
I The Light Dependent Reaction
Photochemical and unaffected by the
temperature (Q10 = 1)
II The Light Independent Reaction
Enzyme controlled as it has a Q10  2
© 2010 Paul Billiet ODWS
Experiment 2:
Light Intensity and CO2 Concentration
0.13% CO2
Rate of
photosynthesis
0.03% CO2
Light intensity
© 2010 Paul Billiet ODWS
Carbon fixation is light independent
Observation
CO2 limits the rate of photosynthesis at high light
intensities i.e. when light is not a limiting factor
Conclusion
Carbon fixation takes place in the Light
Independent Reaction
It is an enzyme controlled process
© 2010 Paul Billiet ODWS