The dragon blood tree is native to the island of Socotra in Yemen. It is named for
its red sap.
Dracaena cinnabari the dragon blood tree, it is a monocot.
Photosynthesis
Definition of photosynthesis: the conversion of CO2
into organic compounds using the energy of
sunlight.
To study which wavelengths of light are used in photosynthesis, T. W. Engelmann
placed a green alga together with bacteria that are attracted to O2 on a slide
and illuminated them with light from a prism.
a crystal prism breaks up a beam of
light into a spectrum of colors
A thermodynamics definition: photosynthesis is the
conversion of light energy into chemical energy
(The first law of thermodynamics: energy can
change form but it cannot be created or destroyed.)
The biochemical definition
Photosynthesis: the chemical formula
6CO2 + 6H2O + energy --> C6H12O6 + 6O2
T. W. Engelmann (1843-1909) German Physiologist, he worked at the
University of Utrecht and the University of Berlin. He was also one of the
first to record an electrocariogram.
This is how ATP is made
during the light reactions.
1
Photosynthesis has two main parts: the light reactions and the carbon reactions.
The light reactions occur on the thylakoid membrane. The carbon reactions occur
in the stroma.
The function of the light reactions is to generate ATP and NADPH
thylakoid
ADP + Pi -> ATP
NADP+ + H+ + 2e- -> NADPH
The function of the carbon reactions (used to be called the dark reactions)
is to fix CO2 and regenerate ribulose 1,5 bisphosphate (RuBP).
This requires ATP and NADPH.
granum
stroma
oxygen released
granum
stroma
sunlight
energy
photosystem
II
e−
H+
H2O is split
lumen
(H+ reservoir)
H+
H+
NADP+
H+
electron
transport
system
H+
Stroma
Fig. 10-2b, p. 151
Lightdependent
reactions
photosystem
I
e−
ADP + Pi
electron
transport
system
sugar
phosphate
Lightindependent
reactions
carbon
dioxide
used
water
released
carbohydrate end product (e.g. sucrose, starch, cellulose)
In chloroplasts, the light reactions drive acidification of the
thylakoid lumen via electron transport reactions.
The proton gradient is used by ATP synthase to generate ATP
within the stroma.
Light also drives the reduction of NADP+ to NADPH in the stroma.
Other points of interest:
Water is the electron donor. Oxygen evolved in photosynthesis
comes from the oxidation of water. NADP+ is the terminal electron
acceptor.
Figure 10.16: Diagram of membrane with electron transport.
2
Intro to redox reactions
Reduction/oxidation (redox) reactions involve the transfer
of electrons or hydrogen atoms (hydrogen atoms are 1 proton (H+) and
1 electron (e-))
Oxidation reactions are written in the following form
using the element, Calcium, as an example
Ca -> Ca2+ + 2eReduction reactions are written the same way:
O + 2e- -> O2The first key reaction of photosynthesis:
2H2O -> 4e- + 4H+ + O2
Figure 10.03a: Diagram of NAD and NADH.
It takes 4e- to reduce each carbon from +4 in CO2 to 0 in carbohydrate
light
light harvesting complex
Action spectrum vs.
chlorophyll absorption
spectrum
3
Z scheme
Figure 10.13: Photosystem I.
Figure 10.14: Both photosystems, working together.
Products of the light reactions:
- O2
- NADPH, used in the Calvin cycle to reduce CO2
- ATP
The ultimate electron donor for the light reactions is H2O and
the ultimate electron acceptor is NADP+.
Net carbon reaction
Van Helmont (1580-1644) Belgian
6 CO2 + 11 H2O + 12 NADPH + 18 ATP --> fructose-6-phosphate +
Planted a willow branch that
weighed 5 lbs in 200 lbs of soil.
In 5 years, after only adding water, the
willow grew to 169 lbs but the soil weight
remained the same.
12 NADP+ + 6 H+ + 18 ADP + 17 Pi
He presumed that the tree’s weight had
come from the water.
Is that correct?
http://www.learner.org/resources/series26.html?
4
born in St. Paul, Minnesota
(CO2 from the air)
C3 photosynthesis
carbon reactions =
Calvin cycle.
Ph.D. degree in Chemistry from the
University of Minnesota in 1935
stroma
Carbon dioxide
fixation
rubisco
H 2O
(RuBP)
2
ADP
academic career at the University
of California at Berkeley
(intermediates)
Nobel Prize in chemistry in 1961 “for his
research on the carbon dioxide assimilation
in plants"
(PGA)
Calvin
cycle
ADP
Pi
NADP+
cyclic production of intermediate
sugar phosphates
(PGAL)
(PGAL)
Pi
sugar phosphate synthesis
sugar phosphate
typically used at once to form
carbohydrates (mainly
sucrose, starch, cellulose)
Calvin cycle
Melvin Calvin (1911 - 1997)
The enzyme that fixes carbon is called ribulose 1,5 bisphosphate
carboxylase/oxygenase also known as Rubisco (the most abundant
protein).
The carboxylation of 1 molecule of RuBP (C5) leads to synthesis of
two molecules of 3-phosphoglycerate (3-PGA). C3 photosynthesis
is named for the 3 carbon phosphoglycerate.
Carbon Reactions
Calvin Cycle: three stages carboxylation, reduction, regeneration
Carbon Reactions:
CO2 is fixed by Rubisco located in the stroma. The molecule that is
carboxylated is RuBP.
RuBP has 5 carbons and is regenerated in the Calvin cycle. In the Calvin
cycle, carbon is conserved, ATP is used and NADPH is used.
The result is that some extra triose phosphate is generated that can be exported
to the cytosol or used in the chloroplast stroma to make starch.
cytosol
stroma
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