Download Light Dependant Reactions

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Photosynthesis converts energy of the sun
into chemical energy of glucose
Small portion of Sun’s energy reaches earth
and even smaller portion gets converted into
energy
Still, photosynthesizing organisms synthesize
about 1.4 X 1015 kg of energy-storing
glucose and other sugars in one year
1,400,000,000,000,000
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Most of the glucose is converted to cellulose
and other structural tissues.
Glucose also converted to other sugars as
well as storage forms of carbohydrates
(starch)
Also, sugars produced by photosynthesis are
involved in the synthesis of other cellular
substances
Eg. Amino acids – proteins
Products of photosynthesis account for 95%
of dry weight of plants
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6CO2 (g) + 6H2O(l) + energy → C6H12O6 (s) +
O2(g)
Arrow is misleading, there are over 100
reactions that occur that lead to the end
products
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Photo means capturing light energy
Synthesis means to produce (or in this case,
production of carbohydrates)
Photosynthesis is made up of two types of
reactions
Light-dependant reactions – solar energy is
trapped to generate two high energy
compounds: ATP and NADPH (reduce
nicotinamide adenine dinucleotide
phosphate), has large amounts of reducing
power
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Light independent reactions – the energy of
ATP and the reducing power of NADPH are
used to reduce carbon dioxide to make
glucose which can than be converted into
starch for storage
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Pigments in thylakoid membranes absorb
light energy
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Chlorophyll solution reflects yellow and
green light
Previous figure shows chlorophyll has two
chlorophylls a and b
Also contains another pigment called betacarotene
Beta-carotene is part of large class of
pigments called carotenoids
They absorb blue and green light so they are
red, yellow and orange in colour.
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Chlorophyll is bound to the membranes of
the thylakoids inside the chloroplast
Chlorophyll and other pigments are arrange
in the thylakoid membranes in clusters
called photosystems
Two photosystems
◦ Photosystem I (PSI) (700 nm)
◦ Photosystem II (PSII) (680 nm)
These were named in the order that they
were discovered, NOT in the sequence
they occur
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Made up of pigment molecules that include
a dozen or more chlorophyll molecules as
well as a few carotenoid molecules
Also present is a molecule that accepts
electrons
All pigments molecules can absorb energy
of various wavelengths
However, they always pass the energy to
one specialized, electron accepting
chlorophyll a molecule called the reaction
centre. (antenna analogy)
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Each pigment absorbs light of different
colours
Having a variety of pigments enables a plant
to use a greater percentage of the Sun’s light
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Chlorophyll is bound to membranes of
thylakoids in the chloroplasts
Chlorophyll is arranged in clusters called
photosystems
Plant and algae chloroplasts have two
photosystems
Photosystem I (PSI)
Photosystem II (PSII)
Named after the order scientists discovered
them
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When reaction centre receives the energy, the
electron in the reaction centre becomes
“excited”
This means the electron is raised to a higher
energy level
The electron is then passed on to an
electron-accepting molecule
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A series of progressively stronger electron
acceptors; each time an electron is
transferred, energy is released.
ETS important in energy production in cells.
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The e- leave the reaction center from PS II
and goes to the electron-acceptor
This leaves the reaction center short an eThis electron must be replaced before PSII
can accept light energy to excite ANOTHER
eThe new e- comes from the splitting of a
H2O molecule. O2 that is released by plants
comes from this type of reaction
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Next from the electron-acceptor the
energized e- is transferred along a series of
electron carrying molecules.
Together these are called the Electron
transport system (ETS)
With each transfer, the e- releases a small
amount of energy
This energy released helps to push H+ ions
from the stroma across the thylakoid
membrane and into the thylakoid space
(area inside the thylakoid)
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When hydrogen ions are forced from the
stroma to the thylakoid, they can’t diffuse
back across the membrane
Membrane is impermeable to these ions
Embedded in membrane is a special structure
called ATP synthase
This is the only pathway for the hydrogen
ions to move down their concentration
gradient
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This pathway is linked to a mechanism that
bonds a free phosphate group to ADP to form
ATP.
As hydrogen moves down the concentration
gradient through the ATP synthase, the
energy of the gradient is used to generate
ATP molecules.
The linking of the movement of hydrogen
ions to the production of ATP is called
chemiosmosis
We will see this again in the mitochondria
during C/R
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Plants use chlorophyll to trap solar energy
and convert it to chemical energy.
We’ve figured out how to do this as well, in
a large scale, our space stations have large
solar panels
On small scale, solar calculators
On Earth though, solar cells cannot provide
enough energy that society needs
We use fossil fuels for energy but produce
to much CO2
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If we could use hydrogen cells it would be
great for a fuel source as its byproduct is
water
Unfortunately there is Not enough H2(g) in
environment
It takes more energy to split water than
hydrogen combustion releases
Scientists are trying to mimic PSII
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When there is enough NADPH and ATP in the
stroma, the energy from these molecules can
be used to synthesize glucose
Series of reactions that synthesize glucose is
called the Calvin-Benson cycle
Named after Melvin Calvin and Andrew
Benson
Used a radioactive carbon tracer to discover
the reaction
3 steps in this reaction
Consumption: Water
Formation: ATP, NADPH+, and oxygen
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Step 1 – Fixing Carbon Dioxide: carbon atom
in carbon dioxide is chemically bonded to a
pre-existing molecule in the stroma
The molecule is a five-carbon compound
called ribulose bisphosphate, aka: RuBP
The resulting six carbon compound is
unstable and immediately breaks down into
two identical three carbon compounds
These three carbon compounds are the first
stable products of the products
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These three carbon compounds are the first
stable products of the process
Plants that demonstrate this process are
called C3 plants
Fixing Carbon Dioxide
CO2 + RuBP → unstable C6 → 2 C3
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Step 2 – Reduction: The newly three carbon
compounds are in a low energy stage
To convert them into a higher energy state they
are first activated by ATP and then reduced by
NADPH
The result of this reaction is two molecules of
PGAL (short for glyceraldehyde -3 –phosphate)
In their reduced state some of the PGAL
molecules leave the cycle and may be used to
make glucose
The remaining PGAL molecules move on to the
third stage
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Step 3 – Replacing RuBP: most of the reduced
PGAL molecules are used to make more RuBP
molecules
Energy is required to break and reform the
chemical bonds to make the five carbon RuBP
from PGAL
This cycle must be completed 6 times to
synthesize one molecule of glucose.
Of the 12 PGAL molecules made, 10 are used
to regenerate RuBP, 2 are used to make
glucose
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The process of
moving from PSII to
PSI, is not considered
to be “cyclic” because
the electrons that are
first excited in PSII are
not recycled through
the system.
Water replenishes the
supply at PSII
NADPH carries efrom PSI to the CalvinBenson cycle.
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In the Calvin cycle there is
a larger demand for ATP
than for NADPH.
Thus, when NADPH
concentrations are
sufficient, the cell will
switch to cyclic
photophosphorylation.
This means the PSII will
not be used in order to
avoiding the splitting of
water and addition NADPH
production.
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Two types of reactions:
◦ Light-dependant and light-independent
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Two photosystems (PSI and PSII)
PSII releases electrons when it is excited by
photons of light – it transfers the e- to and then it
is passed in the ETS
Energy released in the ETS is used to force H ions
across the thylakoid membrane
Energy from this gradient is used to help generate
ATP from ADP and phosphate by means of
chemiosmosis
As H ions move through the gradient, they drive
the reaction that generates ATP to be used in the
‘Calvin-Benson cycle
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An e- from water that is split replaces the e- released
in the PSII the oxygen molecule is converted to O2
and released
When an e- from PSI is excited it is eventually used to
reduce NADP+ to NADPH
Calvin Benson cycle occurs in the stroma and
synthesizes carbohydrates
CO2 combines with RuBP to form a 6 C compound the
splits into 2 Three-carbon compounds
ATP and NADPH from the light dependent reactions
provide energy and reducing power to for PGAL from
the newly formed 3 carbon compounds
6 cycles produces 12 PGAL molecules 10 which
regenerate PGAL and 2 which form glucose