6. Photosynthesis
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Transcript of 6. Photosynthesis
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Lesson Objective: Give overall outline of photosynthetic
process that leads to the production of
glucose.
TOPIC 6: PHOTOSYNTHESIS
Green plants use sunlight as an energy source,
CO2 & water as raw materials for photosynthesis.
The light energy trapped by greens plant is
converted to chemical energy & stored in thebonds of organic molecules such as
carbohydrates.
O2
is released as a by product.
6CO2 + 12H20 C6H1206 + 6O2 + 6H2O
PHOTOSYNTHESIS
Light energy
Absorb by
chlorophyll
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TOPIC 6: PHOTOSYNTHESIS
1. Chlorophyll pigments inchloroplasts.
2. CO2
3. Water 4. Optimum temperature
(photosynthesis is abiochemical reaction
that involves manyenzyme).
5. Light
6.1: Photosynthesis requires:
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Lesson Objective: List and explain the photosynthetic
pigments involved in photosynthesis
TOPIC 6: PHOTOSYNTHESIS
Thylakoid membranes contain several kinds ofpigments.
Different pigments absorb light of differentwavelength.
Chlorophyll, the mainpigment of photo-
synthesis, absorb
light primarily in blue
& red regions of
visible spectrum.
Chlorophyll
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TOPIC 6: PHOTOSYNTHESIS
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TOPIC 6: PHOTOSYNTHESIS
Chlorophyll a is the most abundant pigment in
plants. It absorbs light mainly in the blue-violet(430nm) & red (662nm) region.
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Chlorophyll b is absorbs light of about 453nm &
642nm. Chlorophyll b helps to increase therange of light a plant can utilize forphotosynthesis.
Carotenoid absorb light maximally in the blue-violet region (460-550nm).
The most common carotene is carotene, anorange pigment found for example in carrots.
Carotenoids act as accessory or antenna
pigments. These pigments pass the light energyto chlorophyll a in the reaction centre. Thecarotenoid also protect the chlorophyll fromoxidation.
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The photosynthetic processinvolves 2 stages;
1. The light dependent reactionwhich requires light energy
& is biochemical in nature.
2. Light independent reaction
which consist of a seriesenzymatic biochemical
reactions that involve thebonding of CO2 tocomplex organiccompounds.
The mechanism of photosynthesis
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Lesson Objective: Explain the photoactivation of chlorophyll
resulting in the conversion of light energy
into ATP and reduced NADPH+.
TOPIC 6: PHOTOSYNTHESIS
These stages occur in the chloroplastthylakoids which contains complexphotosynthetic pigments.
The photosynthetic process involves thefollowing process;
i. Photoactivation
ii. Photolysis of wateriii. Photophosphorylation (produced ATP &
NADPH).
6.3: Light Dependent Reaction
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= when a molecule ofchlorophyll
absorbs a photon, one of the
molecules electrons is elevated toan orbital where it has more
potential energy, the pigment
molecule is said to be in an excitedstate photo-activation process.
i. Photoactivation
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The energy of an absorbed photon isconverted to the potential energy of
an electron raised from the ground
state to an exited state. The chlorophyll & accessory pigments
are group into 2 photosystem;
a) Photosystem I (PSI)
b) Photosystem II (PSII
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Photosystem I has a reaction center
chlorophyll, the P700 center, that has an
absorption peak at 700nm.
Photosystem II has a reaction center with
a peak at 680nm.
These two photosystems work together to
use light energy to generate ATP and
NADPH.
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The phosystems are embedded in thethylakoid membranes of the chloroplast.
A photosystem contains a complex of
about 200-300 pigment molecules(consisting of a cluster of a few
hundred chlorophyll a, chlorophyll b &
carotenoid molecules), primary electron
acceptor, electron transfer system &
enzymes.
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Each photosystem has an
antenna of a few hundredpigment molecules.
When a photon strikes apigment molecule, theenergy is passed from
molecule to molecule until itreached the reactioncenter.
At the reaction center, theenergy drives an oxidation-
reduction reaction. Anexcited electron from thereaction-center chlorophyll iscaptured by a specialisedmolecule called the primaryelectron acceptor.
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Photolysis of water is a process in which water is
split during the light reaction of photosynthesis.
This process occurs in the space of the thylakoid& catalysed by a water splitting enzyme.
The enzyme passes the electron from water to the
reaction center of PS II & forming O2, which is
liberated.
H2O O2 + 2H+ + 2e
H+ ions combine with NADP+ NADPH + H+
ii. Photolysis of Water The Hill
Reaction
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=The process of generating ATP from
ADP + Piduring the light reactions ofphotosynthesis.
Light reaction consist of;
a. non-cyclic photophosphorylationb. cyclic photophosphorylation
iii. Photophosphorylation
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Non-cyclic photophosphorylation involves 2 PS;
- PS I
- PS II
Light energy is absorbed by antenna pigments
of PS II.
The energy is transferred from one antenna
molecule to another molecules then transferredto the reaction center in PS680 in the PS II.
a. Non-cyclic Photophosphorylation
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When an e-reached the bottom of the e-
transport chain, it fills an e-hole in P700
(the chlorophyll a molecule in the reaction
center of PS I).
This replace the e- that light energy drives
from the chlorophyll to the primary e-
acceptor of PS I.
The primary e- acceptor of PS I passesthe photoexited e- to ferredoxin (Fd).
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TOPIC 6: PHOTOSYNTHESIS
An enzyme called NADP+ reductase then
transfers the e- from Fd to NADP+. This is theredox reaction that stores the high-energy e- in
NADPH, the molecules that will provide reducing
power for the synthesis of sugar in the Calvin
cycle.
2H+ + 2e- + NADP+ NADPH + H+
The H+ ions formed in water splitting is used in
formation of NADPH.
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TOPIC 6: PHOTOSYNTHESIS
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TOPIC 6: PHOTOSYNTHESIS
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TOPIC 6: PHOTOSYNTHESIS
Under certain condition, photoexcited e-take an alternative path called cyclic e-flow, which uses PS I but not PS II.
Light energy absorbed by antennamolecules is transferred to P700 in PS I.
Electron from P700 is photoexcited &
passed on to a primary e-
acceptor. Primary e- acceptor passes the e- to
ferredoxin (Fd).
b. Cyclic Photophosphorylation
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TOPIC 6: PHOTOSYNTHESIS
In cyclicphotophosphorylation, analternative route is used.The e- is passed from Fdto cytochrome complex.
Energy is released duringthe e- flow & this is used inchemiosmosis to generatemore ATP.
The e- then transferred toplastocyanin (Pc) & ispassed back to PS I.
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Lesson Objective: Describe the outline of Calvin cycle
involving the light-independent
fixation of CO2.
TOPIC 6: PHOTOSYNTHESIS
Consist of 4 main stages:
a. Carbon dioxide fixationb. Reduction phase
c. Regeneration of CO2 acceptor
d. Product synthesis phase
6.4. Light Independent Reaction/
Calvin Cycle
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TOPIC 6: PHOTOSYNTHESIS
CO2 from the atmosphere diffuse through thestomata into the intercellular spaces of the leaf.It then diffuse into the stroma in the chloroplastof the palisade & spongy mesophyll cells.
A five-carbon acceptor, ribulose biphosphate(RuBP) combines with molecule of CO2 to forman unstable six-carbon sugar.\
The process is catalysed by the enzyme RuBP
carboxylase. The six-carbon compound immediately splitsinto 2 molecules of glycerate-3-phosphate (PGA)(3-carbon compound).
a. Carbon dioxide Fixation
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TOPIC 6: PHOTOSYNTHESIS
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TOPIC 6: PHOTOSYNTHESIS
Glycerate 3-phosphate receives an
additional phosphate group from ATP to
become glycerate 1,3-diphosphate.
Glycerate 1,3-diphosphate combines with
hydrogen atoms from reduces dinucleotide
phosphate (NADPH + H+) & is converted
into glyceraldehyde-3-phosphate(PGAL, triose phosphate).
b.Reduction Phase
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TOPIC 6: PHOTOSYNTHESIS
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TOPIC 6: PHOTOSYNTHESIS
Some of the PGAL molecules are
rearranged in a series of complex
reactions to regenerate ribulosebiphosphate, this process
requires ATP.
c. Regeneration of CO2 acceptor
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TOPIC 6: PHOTOSYNTHESIS
The rest of the glyceraldehyde-3-
phosphate is used to assimilate
organic molecules such asglucose, amino acids, proteins &
lipids.
d. Product synthesis phase
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TOPIC 6: PHOTOSYNTHESIS
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TOPIC 6: PHOTOSYNTHESIS
RuBP carboxylase (Rubisco), theenzyme that catalysed thecarboxylation of ribulose biphosphate,
can also catalyze the oxidation ofRuBP by molecular O2.
CO2 & O2 are alternative substrates
that compete with each other for thesame active sites on the enzyme.
PHOTORESPIRATION
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TOPIC 6: PHOTOSYNTHESIS
When the concentration of CO2 is high & that
of O2 is low, CO2 occupies the active sites,carboxylation is favored & carbohydrate
synthesis by the Calvin cycle proceeds.
But when the concentration of CO2
is low &
that of O2 is high, the site is occupied by O2,
oxidation is favored orphotorespiration occurs.
This occurs on hot dry days, which cause water
stress in plants. As a result of water stress, plants close their
stoma ( help the plant to conserve water)
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TOPIC 6: PHOTOSYNTHESIS
Once the stoma close, O2 produced during
photosynthesis accumulates in the chloroplast. So rubisco binds RuBP to O2 instead of CO2.
O2 + RuBP Phosphoglycolate +glycerate-3-phosphate
Two phosphoglycolate molecules then undergo
a series of reactions requiring O2 & ATP to
produce one molecule of glycerate-3-phosphate
& the release
(2 carbon compound)
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TOPIC 6: PHOTOSYNTHESIS
Photorespiration is wasteful because organiccarbon (phosphoglycolate) is converted into CO
2
with no net production of ATP or other energyrich metabolites.
Photorespiration can reduce the potentialphotosynthetic yield from between 30-40%.
This degradation process is calledphotorespiration because;
i. it occurs during the daylight
ii. it requires O2 (like aerobic respiration)iii. It produces CO2 & H2O (like aerobic
respiration)
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TOPIC 6: PHOTOSYNTHESIS
Eg. of C4 plants:
- maize plant (Zea mays)
- sugar cane (Saccharum officinale)
- sorghum (Sorghum bicolar)
- sunflower (Helianthus spp)
6.5: Hatch-Slack Pathway (C4 plants)
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TOPIC 6: PHOTOSYNTHESIS
In C4 plants, there are 2 distinct types of
photosynthetic cells:a. bundle-sheath cells
b. mesophyll cells.
Bundle sheath cells are arranged intotightly packed sheets around the veins ofthe leaf.
Between the bundle sheets & the leafsurface are the more loosely arrangedmesophyll cells.
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TOPIC 6: PHOTOSYNTHESIS
The
arrangement
pattern of
the cells
around thevascular
bundle is
known asKrantz
anatomy.
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TOPIC 6: PHOTOSYNTHESIS
CO2 in the atmosphere diffuse into the
mesophyll cells of C4 plants where they
combine with phosphoenolpyruvate
(PEP,3C acceptor) to produce
oxaloacetate (4C). This process is catalysed by the enzyme
phosphoenol-pyruvate carboxylase
(Pepco) which has higher affinity for CO2at low CO2 concentration.
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TOPIC 6: PHOTOSYNTHESIS
Oxaloacetate is reduced to malate (4C). Malate
is shunted through plasmodesmata into thebundle sheath cells.
Malate is then oxidised to pyruvate (3C) by theremoval of hydrogen & CO2.
This increases the concentration of CO2 in thebundle cells & the CO2 undergoes fixation as inthe normal C3 pathway.
High CO2
concentration in the bundle sheathcells inhibits photorespiration & as a result,production of carbohydrate (glucose) increases20-25%.
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TOPIC 6: PHOTOSYNTHESIS
Pyruvate diffuses into the mesophyll
cells & phosphorylated to regenerate
phosphoenol-pyruvate.
In general, C4 plants are found in hotclimates (tropical).
Under hot conditions, C4 plants attain
a higher photosynthetic ratecompared to C3 plants.
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TOPIC 6: PHOTOSYNTHESIS
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TOPIC 6: PHOTOSYNTHESIS
Differences between C3 & C4 Plants
Aspect C3 plants C4 plants
Examples
CO2
fixation
Tomato,tobacco,
legume, wheat.
Once, only in
mesophyll cells.
Sugar cane,
Sorghum, maize.
Twice, first in
mesophyll cells &
then bundle sheath
cells.
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TOPIC 6: PHOTOSYNTHESIS
CO2
acceptor
RuBP ( 5C) PEP (3C) in
mesophyll cells. RuBP in bundle
sheath cells.
Enzyme Rubisco whichis inefficient at
low CO2
concentration.
Pepco which hashigh affinity for
CO2 at low
concentration.RuBP which is
efficient at high
CO2
concentration.
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TOPIC 6: PHOTOSYNTHESIS
First product
formed.
Glycerate 3-
phosphate
(a C3acid)
Oxaloacetate,
(a C4 acid).
Photo-
respiration
O2 acts as
competitive
inhibitor.
Photorespiration
occurs.
Photorespiration
is inhibited by
high
concentration of
CO2.
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TOPIC 6: PHOTOSYNTHESIS
Leaf
anatomy
Krantz
anatomy
absent.
Only
one type of
chloroplast
in mesophyll
cells.
Krantz anatomy
present with the
columnar palisade
cells arranged in a ring
around the bundle
sheath cells & the
vascular tissue.
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TOPIC 6: PHOTOSYNTHESIS
Leaf
anatomy
There are 2 different
forms of chloroplasts.
Mesophyll palisade cells
contain few small
chloroplasts, have many
large well-developed
grana but not starch
grains.
Bundle sheath cells have
many large chloroplasts with
fewer poorly developed grana& contain starch grains.
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TOPIC 6: PHOTOSYNTHESIS
E.g. of CAM plants are succulent plantssuch as the cacti & pineapples.
This succulent (Crassulaceae), avoidwater loss in their hot environment byclosing their stomata during the day &opening them at night.
CAM plants only contain mesophyll cells &no Krantz anatomy.
CAM Plants = Crassulacean Acid
Metabolism
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TOPIC 6: PHOTOSYNTHESIS
Photosynthetic pathway is the same as C4 plants butfixation of CO2 takes place at night when stomata are
open. Phosphoenolpyruvate (PEP) 3C
combines with CO2 to produce oxaloacetate
(OAA) 4C.
Malate is stored in the cell vacuole at nightto prevent pH changes in the cytoplasm.
During daytime, malate is oxidised producing pyruvate& CO2.
Concentration of CO2 increases in the mesophyll cell &
photorespiration is prevented. CO2 is used in Calvin Cycle producing organic
molecules.
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TOPIC 6: PHOTOSYNTHESIS
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TOPIC 6: PHOTOSYNTHESIS
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TOPIC 6: PHOTOSYNTHESIS
a) Light Intensity
At low light intensity there is no photosynthesis
6.6: FACTORS LIMITING THE RATE OF
PHOTOSYNTHESIS
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TOPIC 6: PHOTOSYNTHESIS
As in figure, a higher intensity of light increases
the rate of photosynthesis, unless influenced by
other limiting factors, such as very low CO2
concentration.
Under ideal conditions with no other limitingfactors, the rate of photosynthesis is directly
proportional to the intensity of light.
Increase in CO2 concentration increases the rate
of photosynthesis until saturation point is again
reached.
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TOPIC 6: PHOTOSYNTHESIS
At this point, CO2 concentration oranother new factor may limit the
photosynthetic process.
Light intensity only effects the light-dependent reactions & not the light-
independent reaction, which is
catalysed by specific enzymes.
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TOPIC 6: PHOTOSYNTHESIS
Visible light of differentwavelengths or differentcolour affect the rate ofphotosynthesis
differently. Red light of around 650-750nm & blue light of430-500nm are mosteffective for
photosynthesis whereasgreen light of 540nm isleast effective.
b) Waveleng th of Ligh t
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TOPIC 6: PHOTOSYNTHESIS
This is because red & blue lights areabsorbed by the chlorophyll whereas
green light is not absorbed at all.
The effects of different wavelengthsof light on photosynthesis can be
seen from the action spectrum below.
The action spectrum is correspondingto the absorption spectrum.
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TOPIC 6: PHOTOSYNTHESIS
c) Temperature
The rate of photosynthesis is temperature-
dependent because it is a series of reaction that
depend on enzymes.
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TOPIC 6: PHOTOSYNTHESIS
d) CO2Concentration
The rate of photos. is directly proportionalto the CO2 concentration, in conditionwhere the light intensity & temp. are not
limiting factors. The concentration of CO2 in the
atmosphere is only 0.035%. When thisvalue increases, the rate of photos. wouldincrease until a maximum is reached atapproximately 1.0%.
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A concentration of CO2 that exceeds 1.0%will stimulate the closing of the stoma &
will reduce the rate of photosynthesis.
Rateo
fphotosynthes
is