Van Helmont’s willow growth
experiment – early 1600’s
Joseph
Priestley
1771
Jan Ingenhousz - 1796
• CO2 + H2O + light energy =>
(CH2O) + O2 – he said Oxygen
came from splitting CO2
• His mechanism turned out to be
incorrect
C.B. Van Niel – 1930’s
• Observed photosynthesis in purple sulfur
bacteria
• CO2 + 2H2S + light energy => (CH2O) + H2O + 2S
• Van Niel then generalized this to the following
reaction for all photosynthetic activity
• CO2 + 2H2A + light energy => (CH2O) + H2O + 2A
The Most Important Equation in Biology
A Really Important Equation
Light and Dark Reactions
• Experiments by F.F. Blackman in 1905 demonstrated that
photosynthesis has two stages or steps - one is a lightdependent stage and the other is a light-independent stage
• Due to changes in the rate of the light-independent stage with
increases in temperature, Blackman concluded that this stage
was controlled by enzymes
• We shall see that the first, light-dependent stage of
photosynthesis uses light energy to form ATP from ADP and
to reduce electron carrier molecules, especially NADP+ to
NADPH – so here energy is captured
• In the light-independent reaction, the energy from the ATP and
NADPH is used to build organic carbon molecules - and this is
the process of carbon fixation
Light Spectrums
• Absorption spectrum - the light absorption
pattern of a pigment
• Action spectrum - the relative effectiveness of
different wavelengths for a specific lightrequiring process - such as photosynthesis,
flowering or phototropism
When pigments absorb light, electrons are
temporarily boosted to a higher energy level
One of three things may happen to that energy:
1. the energy may be dissipated as heat
2. the energy may be re-emitted almost instantly
as light of a longer wavelength - this is called
fluorescence
3. the energy may be captured by the formation
of a chemical bond - as in photosynthesis
The Photosynthetic Pigments
• Chlorophyll a - found in all photosynthetic
eukaryotes and cyanobacteria - essential for
photosynthesis in these organisms
• Chlorophyll b - found in vascular plants, bryophytes,
green algae and euglenoid algae - it is an accessory
pigment
• Carotenoids - red, orange or yellow fat-soluble
accessory pigments found in all chloroplasts and
cyanobacteria - caroteniods are embedded in
thylakoids along with chlorophylls
• Two types of carotenoids - carotenes and
xanthophylls
Overview
Of
Photosynthesis
Melvin Calvin 1940s
• Worked out the carbonfixation pathway – now
named for him
• Won Nobel Prize in
1961
Calvin Cycle Summary
• Each full turn of the Calvin cycle begins with
entry of a CO2 molecule and ends when RuBP
is regenerated - it takes 6 full turns of the
Calvin cycle to generate a 6 carbon sugar such
as glucose
• the equation to produce a molecule of glucose
is:
• 6CO2 + 12NADPH + 12H+ + 18ATP => 1 Glucose
+ 12NADP + 6O2 + 18ADP + 18 Pi + 6H2O
C4 Pathway
• In some plants the first carbon compound
produced through the light-independent
reactions is not the 3 carbon PGA, but rather is
a 4 carbon molecule oxaloacetate
• Leaves of C4 plants typically have very
orderly arrangement of mesophyll around a
layer of bundle sheath cells
Location of C4 Pathway
Why Use C4 Pathway?
• Fixation of CO2 has a higher energetic cost in C4 plants than in C3
plants – it takes 5 ATP to fix one molecule of CO2 in C4 but only 3
ATP in C3
• For all C3 plants photosynthesis is always accompanied by
photorespiration which consumes and releases CO2 in the presence
of light - it wastes carbon fixed by photosynthesis - up to 50% of
carbon fixed in photosynthesis may be used in photorespiration in
C3 plants as fixed carbon is reoxidized to CO2
• Photorespiration is nearly absent in C4 plants - this is because a high
CO2: low O2 concentration limits photorespiration - C4 plants
essentially pump CO2 into bundle sheath cells thus maintaining high
CO2 concentration in cells where Calvin cycle will occur
• Thus net photosynthetic rates for C4 plants (corn, sorgham,
sugarcane) are higher than in C3 relatives (wheat, rice, rye, oats)
CAM – Crassulacean Acid
Metabolism
• Crassulacean Acid Metabolism (CAM) has evolved
independently in many plant families including the stoneworts
(Crassulaceae) and cacti (Cactaceae)
• Plants which carry out CAM have ability to fix CO2 in the
dark (night)
• so CAM plants, like C4 plants, use both C4 and C3 pathways,
but CAM plants separate the cycles temporally and C4 plants
separate them spatially
• CAM plants typically open stomata at night and take in CO2
then, then close stomata during day and thus retard water loss