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Cell Biology IV: Photosynthesis
Cell Biology IV:
Photosynthesis
Cell Biology IV: Photosynthesis
Why is it important to study photosynthesis?
Because increasing photosynthesis efficiency by just 1%
would solve food supply problems forever!
Cell Biology IV: Photosynthesis
Photosynthesis is kind of like
cellular respiration but
backwards
Spongebob
Cell Biology IV: Photosynthesis
Organisms that can perform
photosynthesis are called
photoautotrophs.
Cell Biology IV: Photosynthesis
Where is the photosynthesis located in plants?
Cell Biology IV: Photosynthesis
Where is the photosynthesis
located in plants?
Photosynthesis takes place on inner
membranes and spaces of chloroplasts
located in mesophyll cells (green tissues).
Cell Biology IV: Photosynthesis
Using «heavy» oxygen isotope 18O,
it has been found that oxygen released
during photosynthesis comes only
from water.
16
18
O
O
O
O
O
Cell Biology IV: Photosynthesis
Photosynthesis consists of
«light reactions» and «dark reactions» = Calvin cycle.
Light reactions are
necessary to energize
the Calvin cycle.
They convert solar energy
to the chemical energy of
ATP and NADPH.
Calvin cycle uses ATP and
NADPH to reduce CO2
(to synthesize sugars).
Oxygen gas is a side
product of light reactions.
Cell Biology IV: Photosynthesis
The nature of sunlight is electromagnetic waves of various lengths
Cell Biology IV: Photosynthesis
The nature of sunlight is electromagnetic waves of various lengths
A typical human eye will
respond to wavelengths from
about 390 nm to 700 nm
(visible light spectrum).
Cell Biology IV: Photosynthesis
Green color of leaves is caused by the reflection of light from chloroplasts
Chloroplasts contain two types of
green pigment called chlorophyll.
Chlorophyll structure is similar to the structure of heme in the blood hemoglobin.
Cell Biology IV: Photosynthesis
Green light is reflected and transmitted by chlorophyll, because it is not absorbed.
Blue light, in contrast, is strongly absorbed by this pigment.
Cell Biology IV: Photosynthesis
Which wavelenghts of light are most effective in driving photosynthesis?
Blue light and red light are most
important for the
photosynthesis.
Green light is not used.
Cell Biology IV: Photosynthesis
Information about the action spectrum of photosynthesis was first obtained
long time ago due to an elegant experiment performed with filamentous alga
called Spirogyra.
In that experiment, aerobic bacteria were attracted to alga portions illuminated with
blue and red light, becuase only those parts were producing oxygen (photosynthesis).
Cell Biology IV: Photosynthesis
Information about the action spectrum of photosynthesis was first obtained
long time ago due to an elegant experiment performed with filamentous alga
called Spirogyra.
In that experiment, aerobic bacteria were attracted to alga portions illuminated with
blue and red light, becuase only those parts were producing oxygen (photosynthesis).
Cell Biology IV: Photosynthesis
Energy of blue and red light converts purified chlorophyll to a state in which its electrons
have higher potential energy. This energy is gradually released as heat (far red light, > 700 nm)
and fluorescence (red light, 650-700 nm ).
In chloroplasts, such energy is not lost, but is captured to power photosynthesis.
Cell Biology IV: Photosynthesis
A group of proteins and pigments
that are involved in harvesting
light energy is called
photosystem.
A photosystem functions similar
to a solar cell.
Cell Biology IV: Photosynthesis
In modern plants, there are two types of photosystems:
the very ancient photosystem I (PS I) and
the evolutionary young one, photosystem II (PS II).
Powered by the light energy, linear electron flow through
photosystems I and II generates ATP and NADPH.
Pq - plastoquinone
Pc - plastocyanin
Fd - ferredoxin
Cell Biology IV: Photosynthesis
A mechanical analogy for linear electron flow during the light reactions.
Cell Biology IV: Photosynthesis
Cyclic electron flow involves only PS I.
There is ATP synthesis, but no NADPH is generated.
In some photosynthetic microbes, cyclic electron flow is the only way to do
photosynthesis (they have no PS II). In higher plants, cyclic electron flow
functions at very high light intensity to prevent damage (photoprotection).
Cell Biology IV: Photosynthesis
Comparison of chemiosmosis in mitochondria and chloroplasts
The difference is mainly in
direction of proton transport.
Chemiosmosis is the
mechanism that links electron
transport to ATP synthesis (via
proton translocation).
Cell Biology IV: Photosynthesis
The light reactions and chemiosmosis:
the organization of the thylakoid membrane.
Note that protons remaining after oxidation of water
contribute to the proton gradient.
Cell Biology IV: Photosynthesis
The Calvin cycle is responsible for fixation
and reduction of atmospheric CO2 using
energy from the light reactions.
Rubisco (ribulose bisphosphate carboxylase)
is the key enzyme for the life on our planet.
Cell Biology IV: Photosynthesis
Photorespiration
The full name of RubisCO is ribulose bisphosphate carboxylase-oxidase, because at low
CO2 concentartions it can bind oxygen instead of CO2.
Photorespiration occurs in hot and dry weather when stomata are closed and CO2 cannot
enter the leaf. Photorespiration leads to the loss of the fixed CO2, thus strongly decreasing
the efficiency of photosynthesis.
Cell Biology IV: Photosynthesis
C4 photosynthesis
Plants in dry and hot climates developed a modification of CO2 fixation which results in very
low level of photorespiration (very high photosynthesis effciency) even when stomata are
closed. Because the first product of CO2 fixation in such plants is a compound with four carbon
atoms, this type of photosynthesis is called C4 photosynthesis, while plants with normal
photosynthesis are called C3 plants (phosphoglycerate = 3 carbon atoms).
C4 plant examples
• maize (Zea mays),
• sugarcane (Saccharum officinarum)
• sorghum (Sorghum bicolor)
• millet (Panicum miliaceum)
• switchgrass (Panicum virganum)
and many strong weeds
No light reactions
→ no O2 release
Cell Biology IV: Photosynthesis
The principle difference between C3 and C4 plants is that, in C4 plants,
mesophyll cells function as a pump to increase CO2 concentration near RubisCO
Cell Biology IV: Photosynthesis
CAM photosynthesis (Crassulacean Acid Metabolism)
In CAM plants, CO2 fixation and Calvin cycle are separated in time (day/night)
Cell Biology IV: Photosynthesis
PHOTOSYNTHETIC
HUMANS
By year 3015???
Thank you for your
attention!
Some of the modern animals receive part of their energy from the sunlight because
of the symbiosis with algae. According to the Japanese computer-based scientific
forecast, in about 1000 years from now humans will also perform photosynthesis,
due to the progress in biotechnology.