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OBJECTIVE
Define photosynthesis
Write overall chemical
equation for photosynthesis
Explain light absorption
spectrum
Name the photosynthetic
pigments
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Photosynthesis
The process whereby light energy is
converted to chemical energy that is stored
in glucose or other organic compounds.
In the presence of light, green plants produce
organic compounds and oxygen from carbon
dioxide and water.
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EQUATION FOR PHOTOSYNTHESIS
6 CO2 + 12 H2O + 18 ATP + 12 NADPH
C6H12O6 + 6O2 + 6H2O +18 ADP + 12NADP+ + 18 Pi
NADP : nicotinamide adenine dinucleotide phosphate
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PHOTOSYNTHETIC PIGMENTS
Photosynthesis occurs in thechloroplasts
Chlorophylls are the most
important pigments.
In the centre of the chlorophyll
ring is a magnesium atom.
At the peripheral location of the
ring is a long hydrocarbon tail
that can be associated with thehydrophobic region of the
thylakoid membrane.
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Chlorophylls absorb lights
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Chlorophylls
absorbblue and red lights
Accessory pigments
absorb light between theblue and
the red wavelengths
transfer the energy to the
chlorophylls.
Eg : Carotenoids such as betacarotene which absorbs light
in theblue and the
blue-green regions.
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ASORPTION SPECTRUM
A graph of a pigments light absorption
versus wavelength is called an absorption
spectrum.
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Chromatography technique to separate chlorophylls and
accessory
to separate mixtures into their components
For photosynthesis, a paper chromatography
is commonly used.
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Steps :
1. The chlorophyll mixture is dissolved in a suitable solvent.
2. Drops of the resultant solution are repeatedly placed on
top of each other to form a small concentrated spot near onend of a paper strip.
3. A line is drawn across the paper to mark the position of the
spot.
4. When the solvent front moves up the paper and about
1cmfrom the end, a line is drawn to mark the position of the
solvent front.
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Rf VALUE
The position of various pigments are
marked.
The Rf value of a solute / pigment iscalculated using the formula
Rf = distance moved by solute
distance moved by solvent front
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Structure of chloroplast
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Chloroplasts
Any green part of a plant has chloroplasts.
Leaves are the major site of photosynthesis
The color of a leaf comes from chlorophyll, the
green pigment in the chloroplasts.
Chlorophyll plays an important role in the absorptionof light energy during photosynthesis.
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Chloroplasts are found mainly in mesophyllcells
O2 exits and CO2 enters the leaf through
microscopic pores, stomata, in the leaf.Veins deliver water
from the roots and
carry off sugar from
mesophyll cells to
other plant areas.
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Each chloroplast has two membranes around a
central aqueous space, the stroma.
In the stroma are
membranous sacs,
the thylakoids.These have an internalaqueous space, thethylakoid lumen or
thylakoid space.Thylakoids may be stackedinto columns called grana.
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STAGES OF PHOTOSYNTHESIS
2 stages:
Lightdependent
reactions
Lightindependent
reactions
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THE LIGHT
DEPENDENTREACTION
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LIGHT REACTIONS
Light reactions
convert solar energy to
chemical energy
Light energy absorbed by
chlorophyll in the thylakoids
drives the transfer of electronsand hydrogen from water to
NADP+ (nicotinamide adenine
dinucleotide phosphate), forming
NADPH.
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The thylakoids convert light energyinto the chemical energy of ATP and
NADPH.
The light reaction also generates
ATP by photophosphorylation for
the Calvin cycle
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Wavelength
Light, like other formof electromagneticenergy, travels in
rhythmic waves.
The distance betweencrests of
electromagnetic wavesis called thewavelength
wavelength
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The entire range of electromagnetic radiation
is the electromagnetic spectrum.
The most important segment for life is a
narrow band between 380 to 750 nm, visible
light.
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While light travels as a wave, many of itsproperties are those of a discrete particle, the
photon.
Photon light particle
The amount of energy packaged in a photon is
inversely related to its wavelength.
While the sun radiates a full electromagneticspectrum, the atmosphere selectively screens
out most wavelengths, permitting only visible
light to pass in significant quantities.
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When light meets matter, it may be reflected,
transmitted, or absorbed.
Different pigments absorb photons of different
wavelengths.
A leaf looks greenbecause chlorophyll,
the dominant pigment,
absorbs red and blue
light, while transmittingand reflecting green
light.
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LIGHT ABSORPTION
Chlorophyll a
participates directly in the
light reactions
Accessory photosynthetic
pigments
absorb light and transfer
energy to chlorophyll a.
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When a molecule absorbs aphoton,
one of that molecules electrons is elevated to an orbital with
more potential energy. Photons are absorbed by clusters of pigment molecules
in the thylakoid membranes.
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The energy of the photon is converted to the potential
energy
electron raised from its ground state to an excited state.
Excited electrons are unstable.
They drop to their ground state,
releasing heat energy.
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In the thylakoid membrane
chlorophyll is organized along with proteins and smaller
organic molecules into photosystems.
A photosystem acts like a light-gathering antenna
complex
consist of chlorophyll a, chlorophyll b,and carotenoidmolecules.
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When antenna molecule absorbs a photon,
photon is transmitted from molecule to molecule until it
reaches a particularchlorophyll a molecule (reaction center).
At the reaction center,
primary electron acceptor removes an excited electron from
the reaction center.
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PHOTOACTIVATION OF
PHOTOSYSTEM I
AND
PHOTOSYSTEM II
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2 types.
I. Photosystem I
has a reaction center chlorophyll (P700), that has an
absorption peak at700
nm.II. Photosystem II
has a reaction center chlorophyll (P680) with an absorption
peak at 680nm.
These two photosystems work together to use
light energy to generate ATP and NADPH
PHOTOSYSTEM
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2 possible routes for electron flow ;
Cyclic and Non cyclic.
Fd
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Photophosphorylation
Cyclic
and
Non cyclic
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Non cyclic photophosporylation
1. Both photosystem 1 and II are used.
2. Non cyclic electron flow, produces both ATP and NADPH.
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5. Electrons are transport along the electron transport chain.
6. Electron flow provides energy for chemiosmotic synthesis of
ATP
7. At the same time, photosystem I (P700) absorb photons
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Water photolysis
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Water photolysis
Photolysis is a process of splitting water molecules using light energywith the release of electrons, protons and oxygen.
The proton (H+) are used to reduced NADP+.
Oxygen is given off or used in respiration.
2H2O 4H+ +4e- + O2
The important of photolysis = To replace electron in photosystem II(noncyclic photophosphorylation)
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Cyclicphotophosphorylation
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1. In cyclic photophosphorylation, photosystem I acts as its
own, without photosystem II
Fd
Cyclic photophosphorylation
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2. The reaction centre of photosystem I (P700) absorbs a
photon and become energised
Fd
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3. The electrons are emitted and reduces an oxidising agent,
ferredoxin (Fd)
Fd
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4. Ferredoxin passes its electron to the cytochrome complex
Fd
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5. The electron continues down a redox chain, pumping
protons as it goes.
Fd
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6. The P700 gets back an electron from the last reducing
agent at the end of the chain.
Fd
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Summary
1. Cyclic electron flow takes photons through
chlorophyll molecules,
2. Passing excited electrons through a redox chain
to produce ATP and some free energy as heat.
3. Electron deficit chlorophyll is restored and the
process repeats
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LIG
HTINDEPENDENT
REACTION
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Three kinds of CO2 fixation pathways
exists :
Calvin Cycle for C3 plants
Hatch-SlackPathway for C4plants
Crassulacean Acid
Metabolisme (CAM)
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CALVIN CYCLE
CALVIN CYCLE
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CALVIN CYCLE
IN C3 PLANTS
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CALVIN CYCLE?
The stage of photosynthesis where theCO2 and H2O are converted into a
carbohydrate
The carbohydrate produced andreleased from the Calvin cycle is
Phosphoglyceraldehyde (PGAL - a 3
carbon compound) - not glucose
The ATP andNADPH from the light
reaction are used to supply electrons
and reducing power for this reduction
reaction
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Occurs in the stroma of the
chloroplast and each stage is
mediated by an enzyme
Consist of three stages :
i. CO2 fixation
ii. CO2 reduction
iii. RuBP regeneration
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Stage 1 : CO2 fixation
Each CO2 molecule is
attached to a 5C sugar,ribulose bisphosphate
(RuBP)
Catalyzed by RuBP
carboxylase orrubisco
The 6C intermediate splitsinto half to form two
molecules of 3-
phosphoglycerate per CO2.
St 2 CO d ti
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Stage 2 : CO2 reduction
Occurs in two steps:i. Phosphorylation of 3-
phosphoglycerate by ATP toform a "bis- phosphate
ii. Reduction of1,3-
bisphosphoglycerate byNADPH to form triosephosphate, a simple 3Ccarbohydrate
The NADP+and ADP formedin this process return to thethylakoids to regenerate
NADPH and ATP in the lightreactions.
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If our goal was to produce one
glycerate-3-phosphate (G3P)
net, we would start with 3 CO2(3C) and 3 RuBP (15C)
After fixation and reduction we
would have six molecules of
G3P (18C)
One of these six G3P (3C) is a net
gain of carbohydrate.
This molecule can exit the cycle
to be used by the plant cell.
The other five (15C) must remain
in the cycle to regenerate three
RuBP.
St 3 R BP ti
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Stage 3 : RuBP regeneration
Regeneration of the CO2 acceptor
(RuBP)
Involves a series of reactions
convert G3P to the 5C intermediate
Ru5P (ribulose5-phosphate),
phosphorylation of Ru5P to
regenerate RuBP (ribulose-
bisphosphate).
Requires ATP formed in the light
reactions
Overall
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Overall,
for every 3 turns of the cycle 1 molecule of product (G3P) is formed
(3CO2:1G3P).
G3P formed in the Calvin cycle can remain in the chloroplast where it is
converted to starch
The remaining 15 carbon atoms (5G3P) re-enter the cycle to produce three
molecules of RuBP
Photosynthesis
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Photosynthesis Structure of chloroplast
Light absorption Chlorophyll a and b
Accessory photosynthetic pigments
Light dependent reaction Location
Photoactivation Photosystem I and Photosystem II
definition differences
Photophosphorylation Cyclic
Non cyclic
Light independent reaction
Calvin cycle Hatch-Slack pathway
CAM
Photorespiration
Limiting factor of photosynthesis
Location, Products, Types of plant involved
Differences between Calvin cycle and Hatch-Slack
The function
The flow, The products and its significant, Differences
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HATCH-SLACK PATHWAYIN C4 PLANTS
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HATCH-SLACK PATHWAY
IN C4 PLANTS
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PHOTORESPIRATION
Light-dependent reactions released O2
O2 competes with CO2 for the active site of theenzyme RuBP carboxylase
If O2 is bound to RuBP, a faulty reactionmechanism occurs, producing one 3C molecule,
3-phosphoglycerate, and one 2C molecule,phosphoglycollate, which is lost from the Calvincycle, removing both carbon and energy from thecycle
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Plants have evolved a mechanism to return the lost
carbon, but this is complicated and 1 of every 4
carbon molecules to enter this pathway is lost ascarbon dioxide.
The recovery pathway uses oxygen and producescarbon dioxide, and so is known as
photorespiration.
Plants which are photorespiring can lose up to40% of the carbohydrate that they would
otherwise have produced under those conditions.
HATCH SLACK PATHWAY
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HATCH-SLACK PATHWAY In hot climates with high light
intensities, the rate of
photosynthesis is high,
increasing the oxygen
concentration within cells
and
causing a high rate ofphotorespiration
A group of plants has evolved
by using an alternative methodof fixing carbon dioxide which
does not rely on ribulose
1,3-bisphosphate
carboxylase
In these plants carbon
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t ese p a ts ca bo
fixation isseparated
spatially from
photochemical reactions.
Carbon fixation by
RUBISCO takes place in
bundle sheath cells, whereas
the oxygen is produced inmesophyllcells rich in
chloroplasts.
Separation of the light
reactions (photochemicalreactions) from the dark
reactions,
decreases photorespiration
increases photosynthesis.
Part I (in mesophyll cells)
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Part - I (in mesophyll cells)
First CO2 Fixation:
CO2 first combines with 3Cphosphoenol pyruvate (PEP) to form4C OAA (oxaloacetic acid).
Catalyzed by phosphoenol pyruvatecarboxylase.
As OAA is a dicarboxylic acid, this isalso known as the dicarboxylic acidpathway.
OAA may be converted into malate(4C) or aspartate (4C) and transportedto bundle sheath cells.
Part II (in bundle sheath cells)
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Part - II (in bundle sheath cells)
malate (4C) undergoes
decarboxylation to formCO2 and pyruvate (3C).
Second CO2 fixation :
CO2 combines withRUBP (5C) to form 2molecules of 3-
phosphoglycerate (3C)as in the Calvin cycle.
Further conversion of 3-phosphoglycerate (3C)to sugars is the same as
in the Calvin cycle.
The pyruvate produced in decarboxylation of malate is
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The pyruvate produced in decarboxylation of malate is
transported back to the mesophyll cells.
It was converted into PEP and again made available for the
C4pathway.
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DIFFERENCES BETWEEN C3 AND C4PLANTS
C3 C4
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C3 C4CO2 fixation Occurs once Occurs twice, first in mesophyll cells,
then in bundle sheath cells
CO2 acceptor RuBP, a 5C compound Mesophyll cells
PEP, a 3C
compound
Bundle sheath
cells
RuBP
CO2 fixing enzyme RuBP carboxylase PEP carboxylase
which is very
efficient
RuBP
carboxylase
First product of
photosynthesis
A C3 acid, G3P A C4 acid, Oxaloacetate
Photorespiration Occurs ; therefore O2is an inhibitor of
photosynthesis
Is inhibited by high CO2concentration. Therefore atmospheric
O2 not an inhibitor of photosynthesisEfficiency Less efficient
photosynthesis than C4plants. Yields usually
much lower
More efficient photosynthesis than
C3plants. Yields usually much higher
Photosynthesis
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y Structure of chloroplast
Light absorption Chlorophyll a and b
Accessory photosynthetic pigments
Light dependent reaction Location
Photoactivation Photosystem I and Photosystem II
definition differences
Photophosphorylation Cyclic
Non cyclic
Light independent reaction
Calvin cycle Hatch-Slack pathway
CAM
Photorespiration
Limiting factor of photosynthesis
Location, Products, Types of plant involved
Differences between Calvin cycle and Hatch-Slack
The function
The flow, The products and its significant, Differences
CRASSULACEAN ACID
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CRASSULACEAN ACID
METABOLISM
CRASSULACEAN ACID
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CRASSULACEAN ACID
METABOLISM
A second strategy to minimize photorespiration is found in succulentplants, cacti, pineapples, and several other plant families.
These plants, known as CAM plants forcrassulacean acidmetabolism (CAM), open stomata during the night and
close them during the day.
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Temperatures aretypically lower atnight and humidity is
higher.
During the night, theseplants fix CO2 into avariety of organicacids in mesophyllcells.
During the day, the
light reactions supplyATP and NADPH tothe Calvin cycle andCO2 is released fromthe organic acids.
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Both C4
and CAM plants addCO2 into organic intermediates
before it enters the Calvincycle.
In C4plants, carbon fixation
and the Calvin cycle are
spatially separated.
In CAM plants, carbon fixationand the Calvin cycle aretemporally separated.
Both eventually use the Calvincycle to incorporate lightenergy into the production ofsugar.
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Less than 5% plants (e.g.cactus) have anotherbiochemical adaptation that
allows them to survive hot anddry environments CAMplants (crassulaceanacidmetabolism)
They utilize PEP carboxylase to
fix CO2, just like C4plants
Unlike C4plants, CAM plants
conduct the light dependentreactions and CO2 fixation at
different times of the day, ratherthan in different cells of the leaf
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CAM MECHANISM
During the night, stomata are
open
CO2 enters the leaf tissue
CO2 +PEP
Oxaloacetate(OAA) Malate
Malate is transported into the
vacuole
During the day, stomata are
closed
Malate is moved into the
chloroplasts
Malate CO2 +Pyruvate
CO2 enters Calvin cycle
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PHOTOSYNTHESIS IS THE BIOSPHERES
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PHOTOSYNTHESISIS THE BIOSPHERES
METABOLIC FOUNDATION
In photosynthesis, the energy that enters the chloroplasts as sunlightbecomes stored as chemical energy in organic compounds.
S
ugar made in the chloroplasts supplies the entire plant with chemicalenergy and carbon skeletons to synthesize all the major organicmolecules of cells.
About 50% of the organic material is consumed as fuel for cellular
respiration in plant mitochondria.
Carbohydrate in the form of the disaccharide sucrose travels via theveins to non photosynthetic cells.
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There, it provides fuel for respiration and
the raw materials for anabolic pathways includingsynthesis of proteins and lipids and building theextracellular polysaccharide cellulose.
Plants also store excess sugar by synthesizingstarch.
Some is stored as starch in chloroplasts or in
storage cells in roots, tubers, seeds, and fruits.
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Photosynthesis
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Structure of chloroplast
Light absorption Chlorophyll a and b
Accessory photosynthetic pigments
Light dependent reaction Location
Photoactivation Photosystem I and Photosystem II
definition differences
Photophosphorylation Cyclic
Non cyclic
Light independent reaction
Calvin cycle Hatch-Slack pathway
CAM
Photorespiration
Limiting factor of photosynthesis
Location, Products, Types of plant involved
Differences between Calvin cycle and Hatch-Slack
The function
The flow, The products and its significant, Differences
LIMITING FACTORS OF
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LIMITING FACTORS OF
PHOTOSYNTHESIS
WAVELENGHT
LIGHT INTENSITY
TEMPERATURE
CARBON DIOXIDE
WAVELENGHT
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WAVELENGHT
WAVELENGHT
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WAVELENGHT
A pigment is anysubstance that absorbslight.
The color of thepigment comes fromthe wavelengths oflight reflected (in
other words, those notabsorbed).
Chl h ll h i
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Chlorophyll, the green pigment
common to all photosynthetic
cells, absorbs all wavelengths of
visible light except green, which
it reflects to be detected by our
eyes.
Black pigments absorb all of thewavelengths that strike them.
White pigments/lighter colors
reflect all or almost all of theenergy striking them.
LIGHT INTENSITY
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LIGHT INTENSITY
Plants need light energy to
make the chemical energy
needed to create
carbohydrates.
The greater the intensity
of light, the plant receives
more light energy. The
plant can photosynthesize
faster as a result.
As the light intensity decreases,
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g y ,the amount of light the plantreceives is less and therefore therate of photosynthesis
decreases.
Light is a limiting factor at lowlight intensities.
There comes a point though thatany extra light energy will notincrease the rate of the reaction.
This is because the enzymes
controlling the reaction areworking as fast as possible.
At this point light is no longer alimiting factor.
TEMPERATURE
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TEMPERATURE
When the temperaturerises the rate ofphotosynthesis risesalso.
This is because theparticles in thereaction move quickerand collide more.
There is an optimumtemperature however.
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At this point the rate of
photosynthesis progressesas fast as it can, limitedonly by the other factors.
Beyond this temperaturethe enzymes controllingthe reaction becomedenatured and the
reaction quickly comes toa halt.
CARBON DIOXIDE
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CARBON DIOXIDE
When the concentration of
carbon dioxide is low the
rate of photosynthesis is
also low.
This is because the plant
has to spend a certain
amount of time doing
nothing, waiting for more
carbon dioxide to arrive.
Increasing the concentration of
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Increasing the concentration ofcarbon dioxide increases therate of photosynthesis.
There is a point at which furtheraddition of carbon dioxide willnot increase the rate of
photosynthesis.
The enzymes controlling thereaction are working as fast as
possible, so the excess carbon
dioxide cannot be utilized.
Carbon dioxide is not thelimiting factor at this point.
PhotosynthesisSt t f hl l t
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Structure of chloroplast
Light absorption Chlorophyll a and b
Accessory photosynthetic pigments Light dependent reaction
Location
Photoactivation Photosystem I and Photosystem II
definition differences
Photophosphorylation Cyclic
Non cyclic
Light independent reaction
Calvin cycle Hatch-Slack pathway
CAM
Photorespiration
Limiting factor of photosynthesis
Location, Products, Types of plant involved
Differences between Calvin cycle and Hatch-Slack
The function
The flow, The products and its significant, Differences
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Thats all forthis topic.