Assessments Statements
PHOTOSYNTHESIS
IB Biology HL
A. Allen
CORE
3.8.1 State that photosynthesis involves the conversion of light energy into chemical energy.
3.8.2 State that light from the Sun is composed of a range of wavelengths (colours).
3.8.3 State that chlorophyll is the main photosynthetic pigment
3.8.4 Outline the differences in absorption of red, blue and green light by chlorophyll.
3.8.5 State that light energy is used to produce ATP, and to split water molecules (photolysis) to form oxygen and hydrogen.
3.8.6 State that ATP and hydrogen (derived from the photolysis of water) are used to fix carbon dioxide to make organic
molecules. 3.8.7 Explain that the rate of photosynthesis can be measured directly by the production of oxygen or the
uptake of carbon dioxide, or indirectly by an increase in biomass.
3.8.8 Outline the effects of temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis.
AHL
8.2.1 Draw and label a diagram showing the structure of a chloroplast as seen in electron micrographs.
8.2.2 State that photosynthesis consists of light-dependent and light independent reactions.
8.2.3 Explain the light-dependent reactions.
8.2.4 Explain photophosphorylation in terms of chemiosmosis.
8.2.5 Explain the light-independent reactions.
8.2.6 Explain the relationship between the structure of the chloroplast and its function.
8.2.7 Explain the relationship between the action spectrum and the absorption spectrum of photosynthetic pigments in green
plants.
8.2.8 Explain the concept of limiting factors in photosynthesis, with reference to light intensity, temperature and concentration
of carbon dioxide.
Simple Photosynthesis Overview
Properties of Light
• Electromagnetic Radiation and the Visible
Light Spectrum
• Engleman’s experiment showing which
wavelength of visible light is best for
photosynthesis
Simplified Chemical summary:
6CO2 + 6H2O + energy (sun) C6H12O6 + 6O2
Visible Light is only part of the sun’s electromagnetic radiation
3.8.2 State that light from the Sun is composed of a range of wavelengths (colours).
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Chloroplast structure
X 33 300 Open University S Hurry (1965) Murray
•
http://Animation: Show first 20 sec for chloroplast anatomy (link #2)
X 22 000 Open University S Hurry (1965) Murray
Micrograph of Chloroplast
X 80 000 Open University S Hurry (1965) Murray
Photosynthesis: An Overview of the
Light and ‘Dark’ Reactions
Label your diagram!
• Occurs in Photoautotrophs (organisms
that can make their own using energy
from the sun).
• Photosynthesis takes place in the
chloroplasts.
• Photosynthesis includes two processes…
1
2
take a quiz!
3
4
http://simple animation
• LIGHT REACTIONS
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• Requires sunlight
• Occurs in the granna of
chloroplasts
• Produces ATP and NADPH (used
to power the Calvin cycle)
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• ‘DARK’ REACTIONS
(a misnomer…aka Calvin cycle)
• Doesn’t require sunlight (happens
24/7).
• Occurs in the stroma of
chloroplasts
• Produces PGAL (which can later
be used to make glucose)
http://indycc1.agri.huji.ac.il/~zacha/chloroplast.jpg
…Photosystems
Photosystems
• Photosystems are arrangements
pigment-protein complexes. They
contain (mainly) 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
and the first to be discovered.
A photosystem has a reaction centre a protein complex that
contains two chlorophyll a molecules and a primary electron
acceptor..
acceptor
Both photosystems use chlorophyll a in their reaction centres.
The reaction centre in photosystem I is referred to as P700
P700.. It
absorbs light up to 700 nm. The reactoin centre in photosystem II
is known as P680
P680.. It absorbs light up to 680 nm.
• 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?
3.8.3 State that chlorophyll is the main photosynthetic pigment
3.8.3 State that chlorophyll is the main photosynthetic pigment
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A Closer Look a Photosystems…
The Chlorophyll Molecule
How does the chlorophyll molecule stay in the correct
orientation when embedded in the thylakoid membrane?
Light Absorption by Various Pigments
……more detail
• Why do most photosynthetic organisms look green?
•
http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect10.htm
3.8.4 Outline the differences in absorption of red, blue and green light by chlorophyll.
Absorption Spectrum vs Action Spectrum
extracted chlorophyll fluoresces
8.2.7 Explain the relationship between the action spectrum and the
absorption spectrum of photosynthetic pigments in green plants.
3
Phosphorylation
Light Reactions and Non-Cyclic Photophosphorylation
Hmmmm…
• 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.
Try to
interpret this
diagram in
laymen’s
terms.
…Light Reactions and Non-Cyclic Photophosphorylation
•
•
•
•
Two photosystems are involved.
A photon hits Photosystem II (PS II). This energy is relayed to the reaction centre (P 680)
via accessory pigments. A high energy electron is emitted.
…meanwhile, an enzyme in PS II (enzyme Z) splits water. This is called photolysis. The
oxygen is released as a byproduct. Electrons from water are used to replace those lost by PS
II.
..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.
The electron
excited in PS II
then travels to a
mobile carrier,
plastoquinone
(pq), then to the
b6f complex
(proton pump).
Animation: (non) cyclic photophosphorylation animation
•
•
…Light Reactions and Non-Cyclic Photophosphorylation
The electron then goes to plastocyanin (Pc) and then to PS I.
Remember, the electron has lost energy due to the previous redox reactions! Every
time an electron is passed from one molecule to the next, its energy state lowers.
…A photon hits PS I Energy is passed from accessory pigments to its reaction centre
(P 700) which ejects a high energy electron.
Animation: (non) cyclic
photophosphorylation
animation
…Non-Cyclic Photophosphorylation
The electron is then passed to ferrodoxin (Fd) and then to NADP reductase, which
uses the newly energized electron to reduce NADP to NADPH.
The ATP and NADPH produced during nonnon-cyclic photophosphorylation go to the
Calvin cycle to provide energy and raw materials to make SUGAR!
The dede-energized
electron replaces
the electron lost
from PS I.
Proton pump
Q
Animation: (non) cyclic
photophosphorylation
animation
PC
Fd
Animation: (non) cyclic
photophosphorylation
animation
4
NON-cyclic photo-phosphorylation…
Watch the animation, then answer this question:
Where do the protons come from that go through ATP synthase?
Does this make
sense now?
Cyclic Photophosphorylation
• Cyclic photophosphorylation
probably occurs in plants when there
is too little NADP+ available.
• 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.
…Cyclic Photophosphorylation
•
•
•
•
•
•
•
The electron is then passed to
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.
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 (chemiosmosis) 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
gradient) via
chemiosmosis
Animation 3:
ATP Synthase
ATP synthase is thought to revolve at more than 100Hz (revolutions/sec.) in
human mitochondria.
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?
Animation: (non) cyclic
photophosphorylation animation (link #1)
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Examine the formula that summarizes photosynthesis…
Stop & Think
sunlight
• Explain why a lack of NADP+ availability
will result in some cyclic electron flow?
• What is produced from cyclic electron
flow? What is not produced?
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…
… The Calvin Cycle
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 end product of photosysnthesis isn’t really
glucose; it’s triose phosphate (TP). TP can be used
to manufacture glucose, or other sugars, fatty acids
or amino acids.
•
•
•
•
The Calvin Cycle
3 x CO2
1
3 x RuBP
(5-C)
Rubisco
1st Phase: Carbon
Fixation
1. Three fivefive-carbon sugar
molecules called ribulose
bisphosphate,, or RuBP
bisphosphate
RuBP,,
are the acceptors that bind
2
3 CO2 molecules
6 x glycerate 3-phosphate (3 C)
(dissolved in the stroma).
stroma).
This reaction is catalyzed
by the enzyme rubisco
(AKA RuBP
carboxylase)).
carboxylase
Phosphate
carbon
Animation: Calvin cycle
2. Three unstable 66- C
molecules are produced
(not shown) which quickly
break down to give six
molecules of the three
three-carbon
glycerate 3-phoglycerate
(GP)..
(GP)
The Calvin Cycle has three phases:
1st phase: Carbon Fixation
2nd phase: Reduction
3rd phase: Regeneration of the Carbon acceptor
molecule (RuBP)
2nd Phase: Reduction
3. The six glycerate 3-phosphate
The Calvin Cycle
molecules are phosphorylated
to six 1,3
bisphosphoglygerates (1,3,
BPG) as each they each accept
a high energy Pi from ATP.
3 x CO2
1
2
6 x glycerate 3-phosphate (3 C)
3 x RuBP
(5-C)
6 x ATP
Rubisco
6 x ADP
6 x 1,3 BPG
3
1,3 BPG is reduced to triose
phosphate (TP), a threethree-carbon
sugar. NADPH provides the
energy to split off a phosphate
and replace it with hydrogen
(reduction).
6 x NADPH
6 x triose phosphate
(3-C)
6 x NADP
6 x Pi
4
Animation: Calvin cycle
1 x triose phosphate(3-C)
phosphate
4. Six molecules of triose
phosphate are produced.
However, only one of the six
molecules exits the cycle as an
output (to make sugar, etc.)
while...
NOTE: IN Bio 11 triose
phosphate was called
PGAL or G3P
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3rd Phase: Regeneration
of the Carbon acceptor
molecule (RuBP
(RuBP))
5. ...the remaining five
3 x CO2
enter a complex process
that regenerates more
1
2
RuBP to continue the
6 x glycerate 3-phosphate (3 C)
cycle....
6.
In
this process, ATP is
3 x RuBP Rubisco
6 x ATP
(5-C)
used to convert the five
6 x ADP
3
triose phosphates to
3 x ADP
three RuBP’s
RuBP’s..
3 x ATP
6 x 1,3 BPG
7. Summary...
6
9 ATP used
6 x NADPH
6 NADPH used
6 x NADP
1
TP produced
5 x triose phosphate 6 x triose phosphate 6 x Pi
RuBP regenerated
(3 C)
(3-C)
5
The Calvin Cycle
Photosynthetic Rate
• Photosynthetic rate is often measured as
the rate of CO2 absorption per unit area of
the leaf. (mmolCO2/m2/s)
4
Animation: Calvin cycle
1 x triose phosphate(3-C)
phosphate
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.
How does Irradiance affects Rate of Photosynthesis?
•
•
•
•
•
http://www.marietta.edu/~spilatrs/biol103/photolab/compexpl.html
•
How Temperature affects Rate of
Photosynthesis
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 PRODUCES 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.
Interpret the graph!
• 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.
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