PHOTOSYNTHESIS
Bio 11 Mr. Allen
Simple Photosynthesis Overview
Simplified Chemical summary:
6CO2 + 6H2O + energy (sun) Æ C6H12O6 + 6O2
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Properties of Light
• Electromagnetic Radiation and the Visible
Light Spectrum
• Engleman’s experiment showing which
wavelength of visible light is best for
photosynthesis
Structure of a Leaf
• Look at the various cells
in the cross section of the
leaf. In which cells does
photosynthesis take
place?
• Take this test...
‘Palisade’ means to surround
with a wall in order to fortify
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Stoma
• This structure allows for the plant to exchange
gasses with its environment. What gasses??
Stoma
Guard cells
Chloroplast structure
• http://Animation: Show first 20 sec for chloroplast anatomy (link #2)
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Micrograph of Chloroplast
Label your diagram!
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2
take a quiz!
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http://indycc1.agri.huji.ac.il/~zacha/chloroplast.jpg
Photosynthesis: An Overview of the
Light and ‘Dark’ Reactions
• Occurs in Photoautotrophs (organisms
that can make their own using energy
from the sun).
• Photosynthesis takes place in the
chloroplasts.
• Photosynthesis includes two processes…
http://simple animation
• LIGHT REACTIONS
• ‘DARK’ REACTIONS
(a misnomer…aka Calvin cycle)
• Requires sunlight
• Doesn’t require sunlight (happens
24/7).
• Occurs in the granna of
• Occurs in the stroma of
chloroplasts
chloroplasts
• Produces ATP and NADPH (used • Produces PGAL (which can later
to power the Calvin cycle)
be used to make glucose)
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Photosystems
• Photosystems are arrangements
pigment-protein complexes. They
contain chlorophyll and other
accessory pigments packed into
thylakoids.
• Many prokaryotes have only one
photosystem, Photosystem I.
Eukaryotes have Photosystem I plus
Photosystem II.
• Photosystem I was the first to evolve.
ƒ Photosystem I uses chlorophyll a,
a, in the form referred to as P700.
P700.
It absorbs light up to 700 nm. Photosystem II uses a form of
chlorophyll a known as P680.
P680. It absorbs light up to 680 nm.
Graphic: http://kvhs.nbed.nb.ca/gallant/biology/photosystem.jpg
…Photosystems
• The accessory pigments (chlorophyll b, carotenoids , and
xanthophylls) play an indirect role in the formation of glucose
through photosynthesis. These pigments provide chlorophyll a
with the energy that they have captured from the sun. These
pigments capture varying wavelengths of light and thus allow the
plant to receive sun energy across a greater spectrum. Accessory
pigments absorb energy that chlorophyll a does not absorb.
• Some carotenoids play a role in energy absorption rather than in
photosynthesis. They absorb light to prevent damage to
chlorophyll. The energy is lost as heat.
• Why do leaves of deciduous trees turn pretty colors in autumn?
Image: http://www.thrivingnow.com/for/photos/image_med/82/
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A Closer Look at Photosystems…
The Chlorophyll Molecule
How does the chlorophyll molecule stay in the correct
orientation when embedded in the thylakoid membrane?
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Light Absorption by Various Pigments
•
http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect10.htm
• Why do most photosynthetic organisms look green?
……more
detail
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Phosphorylation
• Phosphorylation: The chemical addition of a
phosphate group (phosphorous and oxygen) to
a compound. i.e. adding Pi to ADP to get ATP
• Photophosphorylation is addition of a
phosphate using the sun’s energy!
• There are two types of photophosphorylation;
cyclic and non-cyclic.
Cyclic Photophosphorylation
• Cyclic photophosphorylation
probably occurs in plants when there
is too little NADP+ available (more on
this later).
• Cyclic photophosphorylation is also
seen in certain photosynthetic
bacteria. Note that the bacteria have
no chloroplasts. All structures are
embedded in the membrane. The
proton gradient is created between the
cell membrane and the capsule.
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Cyclic Photophosphorylation
• A single photosystem is involved.
• A photon of light strikes a pigment
molecule in the P700 antenna system.
• The energy eventually reaches a
molecule of P700 (specialized
chlorophyll a - the ‘reaction centre’).
This electron is ejected from the
photosystem.
• The energized electron leaves P700
and is passed to an acceptor
molecule; Ferrodoxin (fd).
• The electron is then passed through
the cytochrome b6f complex. This
complex pumps protons (H+) into the
space between bacterium’s cell
membrane and capsule (or in the case
of plants, inside the thylakoid). This
creates a proton gradient.
• Protons can only cross back through
the membrane via ATP synthase.
ATP synthase uses the energy flow of
protons (proton motive force) to
make ATP (Phosphorylaion).
Animation 1:
Development of
Proton Motive
Force (proton
Animation 3:
gradient) via
ATP synthase
chemiosmosis
Animation 2: Formation of
ATP from Proton motive
force
…Cyclic Photophosphorylation
• The electron is then passed through
the plastocyanin (pC).
• The electron is passed back to the
reaction centre.
• The electron’s energy is gradually
lost during this process.
• The de-energized electron returns to
the chlorophyll a molecule to be
energized again.
• We call this process cyclic
photophosphorylation because
electrons return to the photosystem
and are then again energized. The
process is a cycle!
• The energy released during this
electron transport generates a proton
gradient which is used to produce
ATP.
•
Animation: (non) cyclic
photophosphorylation animation
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Light Reactions and Non-Cyclic Photophosphorylation
Hmmmm…
Non-cyclic
oto
ph phosphorylation
Try to
interpret this
diagram in
laymen’s
terms.
…Light Reactions and Non-Cyclic Photophosphorylation
•
•
•
Happens in PLANTS. Two photosystems are involved.
A photon hits Photosystem II (PS II or P680). This energy is relayed to the reaction centre
via accessory pigments. A high energy electron is emitted.
…meanwhile, an enzyme in PS II (enzyme Z) splits water. The oxygen is released as a
byproduct. Electrons from water are used to replace those lost by PS II.
• The electron
excited in PS II
then travels to
plastoquinone
(Q), then to the
b6f complex
(proton pump).
Proton pump
Q
PC
Fd
NADP
Reductase
Animation: (non) cyclic
photophosphorylation
animation
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…Light Reactions and Non-Cyclic Photophosphorylation
• The proton pump uses this energy to pump protons across the thylakoid
membrane, from the stroma into the thylakoid space. These protons can only exit
the thylakoid via ATP synthase. The flow of protons (proton motive force)
through ATP synthase is used to make ATP. ATP production in this manner is
called Chemiosmosis.
Proton pump
PC
Q
Fd
NADP
Reductase
Animation: (non) cyclic
photophosphorylation
animation
..Non-Cyclic Photophosphorylation
• The electron then goes to plastocyanin (PC) and then to PS I.
• Remember, the electron has lost energy because…the proton pump used it up! It’s
now de-energized!
• …A photon hits PS I (P 700). Energy is passed from accessory pigments to reaction
centre which ejects a high energy electron.
ƒ The dede-energized
electron replaces
the electron lost
from PS I.
Proton pump
Q
PC
Fd
NADP
Reductase
Animation: (non) cyclic
photophosphorylation
animation
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…Non-Cyclic Photophosphorylation
ƒ The electron is then passed to ferrodoxin (Fd) and then to NADP reductase, which
uses the newly energized electron to reduce NADP to NADPH.
ƒ The ATP and NADPH produced during nonnon-cyclic photophosphorylation go to the
Calvin cycle to provide energy and raw materials to make SUGAR!
Proton pump
Q
PC
Fd
NADP
Reductase
Animation: (non) cyclic
photophosphorylation
animation
NON-cyclic photo-phosphorylation…
Non-cyclic
photophosphorylation
Does this make
sense now?
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Watch the animation, then answer this question:
Where do the protons come from that go through ATP synthase?
http://www.cas.muohio.edu/~huertaaj/LIGHTRXBIG.gif
Cyclic vs. non-cyclic photophosphorylation in plants.
• Cyclic photophosphorylation occurs less commonly
in plants than noncyclic photophosphorylation does.
Examine the two diagrams below. What are the similarities
and differences?
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Action spectrum
• A graph that
depicts the relative
effectiveness of
different
wavelengths of
radiation in
driving a
photosynthesis.
Examine the formula that summarizes photosynthesis…
sunlight
CO2 + H2O
C6H12O6 + O2
You should know…
• Where the O2 byproduct comes from…
Infer…
• Where the carbon in glucose comes from…
• Where the hydrogen in glucose comes from…
• Where the oxygen in glucose comes from…
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The Calvin Cycle
• In Photosynthesis, ATP and NADPH are produced in
photophosphorylation, aka the Light Reactions. This happens in the
thylakoid but notice that the products are actually produced in the
stroma. This sets up the next series of reactions, the Calvin cycle
which happens completely in the stroma. This is where sugars are
manufactured. Melvin Calvin discovered this cycle in 1940.
… The Calvin Cycle
• The end product of photosysnthesis isn’t really
glucose; it’s PGAL (phosphoglyceraldehyde).
PGAL (AKA G3P) can be used to manufacture
glucose, or other sugars, fatty acids or amino acids.
•
•
•
•
The Calvin Cycle has three phases:
1st phase: Carbon Fixation
2nd phase: Reduction
3rd phase: Regeneration of the Carbon acceptor
molecule (RuBP)
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1st Phase: Carbon
Fixation
1. Three fivefive-carbon sugar
molecules called ribulose
bisphosphate,
bisphosphate, or RuBP,
RuBP,
are the acceptors that
bind 3 CO2 molecules
(dissolved in the stroma).
stroma).
This reaction is catalyzed
by the enzyme rubisco.
rubisco.
The Calvin Cycle
3 x CO2
1
3 x RuBP
(5-C)
2
Rubisco
6 x PGA
(3-C)
Phosphate
Animation: Calvin cycle
carbon
The Calvin Cycle
3 x CO2
1
3 x RuBP
(5-C)
2
Rubisco
6 x PGA
(3-C)
6 x ATP
6 x ADP
6 x 1,3 BPG
6 x NADPH
6 x PGAL
(3-C)
6 x NADP
6 x Pi
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Animation: Calvin cycle
1 x PGAL (3-C)
2. Three unstable 66-C
molecules are produced
(not shown) which
quickly break down to
give six molecules of the
threethree-carbon
phosphoglyceric acid
(PGA)
PGA).
2nd Phase: Reduction
3. The six PGA molecules are
phosphorylated to six 1,3
BPG (1,3
bisphosp
isphosphog
hoglycerate)
lycerate) as
each PGA accepts a high
energy P from ATP. 1,3
BPG is reduced to PGAL
(phosphoglyceraldehyde)
phosphoglyceraldehyde),
three-carbon sugar. This
3 a threephosphate bond is then
broken and hydrogen is
added from NADPH.
4. Six molecules of PGAL are
produced. However, only
one of the six molecules
exits the cycle as an output
(to make sugar, etc.)
while...
NOTE: PGAL is also
referred to as G3P
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The Calvin Cycle
3 x CO2
1
2
6 x PGA
Rubisco (3-C)
3 x RuBP
(5-C)
6 x ATP
6 x ADP
3 x ADP
3 x ATP
6 x 1,3 BPG
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6 x NADPH
5
5 x PGAL
(3 C)
6 x PGAL
(3-C)
6 x NADP
6 x Pi
4
1 x PGAL
(3-C)
Animation: Calvin cycle
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3rd Phase: Regeneration
of the Carbon
acceptor molecule
(RuBP)
RuBP)
5. ...the remaining five
enter a complex
process that
regenerates more
RuBP to continue the
cycle....
6. In this process, ATP is
used to convert the
five PGAL’
PGAL’s to three
RuBP’
RuBP’s.
7. Summary...
9 ATP used
6 NADPH used
1 PGAL produced
RuBP regenerated
Photosynthetic Rate
• Photosynthetic rate is often measured as
the rate of CO2 absorption per unit area of
the leaf. (mmolCO2/m2/s)
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How does Irradiance Affect Rate of Photosynthesis?
• Light-compensation point: the point on a lightresponse curve at which
photosynthetic CO2 uptake = respiratory CO2 evolution
• Light saturation point: the irradiance level at which the
carbon fixation levels reach a maximum rate.
•
http://www.marietta.edu/~spilatrs/biol103/photolab/compexpl.html
How does Irradiance affects Rate of Photosynthesis?
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•
•
•
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How does irradiance initially affect rate of CO2 uptake?
As irradiance increases, CO2 uptake increases in a linear fashion.
Describe CO2 absorption in absence of light. Explain.
It is negative. Plant USES CO2 due to cell respiration.
What is the significance of the light saturation point?
What is the significance of
the light saturation point?
the maximum irradiance
that can be used by the
plant. Not enough enzymes
to take advantage of
increased light intensities.
Explain the significance of
the flat portion of the curve.
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How Temperature affects Rate of
Photosynthesis
• Temperature affects
enzyme efficacy.
Enzymes will work
within an optimal
temperature range.
They can become
denatured if the
temperature is outside
this range.
• How does temperature
affect photosynthetic
rate? Explain.
Interpret the graph!
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Overview of light dependent
reactions
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