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Photosynthesis

School Assignment
by

Rizalyne Anne

on 3 February 2014

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Transcript of Photosynthesis

Photosynthesis
Non-cyclic Electron Flow
Cyclic Electron Flow
C4 Photosynthesis - Sugar Cane
CAM Photosynthesis - Pineapples
Photosynthesis
Photosynthesis is a two process reaction that occurs in plants and to some types of algae to make their own food.
The reaction takes place in the leaves
It converts light energy into chemical energy.
It uses carbon dioxide and light to make glucose and oxygen.
C4 - Photosynthesis
CAM - Photosynthesis
Non-cyclic Electron Flow vs. Cyclic Electron Flow
The Calvin Cycle
The chloroplasts splits the water to get electrons
From the equation we can see photosynthesis uses water and carbon dioxide to create a product of organic compounds (glucose) and oxygen.
So where exactly is the oxygen derived from?
Van Niel hypothesized that the oxygen waste product comes from water splitting not from carbon dioxide. He performed an experiment using
hydrogen sulfide
in place of water.
Instead of oxygen being released in the atmosphere, sulfur was released.
Other experiments were performed and this idea was confirmed from other scientists
The Reaction Process
Oxidation of P680:
The absorption of light energy by photosystem II results in the formation of an excited-state P680 molecule. This molecule is rapidly oxidized, transferring a high-energy electron to the primary acceptor.
The Alternative Way of the Light Reaction
Oxidation-reduction of plastoquinone:
From the primary acceptor, the electrons transfer to plastoquinone (PQ) which moves through the lipid bilayer and acts as an electron shuttle between photosystem II and the cytochrome complex. As plastoquinon accepts electrons from photosystem II, it also gains protons (H+) from the stroma. When PQ donates electrons to the cytochrome complex, it also releases protons into the lumen, increasing the proton concentration there.

Electron transfer from the cytochrome complex and shuttling by plastocyanin:
From the cytochrome complex, electrons pass to the mobile carrier plastocyanin, which shuttles electrons from the cytochrome complex to photosystem I
Oxidation-reduction of P700:
When a photon of light is absorbed by photosystem I, an electron is excited and P700 forms. The P700 chlorophyll transfers its electron to the primary electron acceptor of photosystem I, forming P700, P700 can now act as an electron acceptor and is reduced back to P700 by the oxidation of plastocyanin
Electron transfer to NADP+ by ferredoxin:
The first electron from P700 is transported by a short sequence of carriers within photosystem I. It is then transferred to ferredoxin (an iron-sulfur protein). The oxidation of ferredoxin results in the transfer of the electron to NADP+, reducing it to NADP.
Formation of NADPH:
A second electron is transferred to NADP by another molecule of ferredoxin. This second electron and a proton (H+) from the stroma are added to NADP by the NADP+ reductase to form NADPH. NADPH is now carrying two high energy electrons. The concentration of protons in the stroma decreases as a result of this NADPH formation. Along with the movement of protons from stroma to lumen by plastoquinone and the splitting of water into protons. It creates a much higher proton concentration inside the lumen than outside the stroma.
ATP is then synthesized as protons move through the ATP synthase complex.
Photosytem I (P700) can function independently from Photosystem II (P680)
The light is absorbed by P700 and the electron transport from photosystem I to ferredoxin (not followed by electron donation to the NADP+ reductase complex)
Reduced ferredoxin donates electron back to plastoquinone. Plastoquinone gets continually reduced and oxidized and keeps moving protons across the thylakoid membrane without P680.
The net result of cyclic electron transport is that the energy absorbed from light is converted into the chemical energy of ATP without the oxidation of water or the reduction of NADP+ to NADPH
The light energy captured in this cycle is ultimately used to drive the phosphorylation of ADP to ATP.
This alternate process of the light reaction is effective as well because the Calvin cycle requires more ATP than NADPH.
The electrons move from the primary acceptor to plastoquinone (PQ)
From cytochrome complex, electrons travel to plastocyanin.
Plastocyanin is another shuttle for electrons
NADPH:
One formation of NADPH requires two electrons and a proton (from the stroma) and it takes place in the NADP+ reductase.
Noncyclic Electron Transport
Light energy is first absorbed by P680, making the molecule gain energy
The molecule is rapidly oxidized, transferring a high-energy electron to the primary acceptor.
PQ acts as an electron shuttle between P680 and cytochrome complex
PQ also recieves protons from the stroma which gets released into the lumen
Also in this step, a protein splits water into oxygen, H+ and e-
Another light energy is absorbed by P700 and it acts as an electron acceptor
The electrons from P700 is then transferred to ferredoxin (Fd)
The oxidation of ferredoxin results in the transfer of the electron to NADP+
ATP:
Inside the lumen, there is a high concentration of protons. These protons creates a gradient and moves across the chain and goes through the ATP synthase. The ATP synthase uses the free energy released by the protons to create ATP from ADP, but the protons are not consumed in the process.
The ATP and NADPH created in the light reaction move on the Calvin cycle!
Photosytstem I (P680) can work independently from Photosystem II P(700)
Plastoquinone continually gets reduced and oxidized and keeps moving protons across the thylakoid membrane without P680
The only final product of this reaction is ATP. Light energy is absorbed and is used solely to convert chemical enery of ATP without the reduction of NADP+ to NADPH or the oxidation of water.
The light gets absorbed by P700 and electrons gain energy and travel to ferredoxin
Instead of ferredoxin transferring its electrons to NADP+ reductase, it donates it back to plastoquinone
The Calvin Cycle is the
second
reaction that occurs in photosynthesis
At the end of the second phase, one molecule of G3P is released from the cycle as the final product after three turns
Phase 1: Carbon Fixation
It occurs in the
stroma
of chloroplasts
In this reaction, it converts carbon dioxide into carbohydrate molecules
The cycle can be divided into three phases: Carbon fixation, Reduction Reactions and ribulose-1,5-phoshate (RuBP) regeneration
Carbon dioxide is added to a 5-carbon molecule,
ribulose-1,5-phosphate
(RuBP) to form an unstable 6-carbon intermediate
The intermediate instantly splits into two 3-carbon molecule of
3-phosphoglycerate
(PGA)
This reaction is catalyzed by the enzyme
ribulose biphosphate carbolyxase
/ oxygenase (rubisco). Rubisco is a very large enzyme that works very slowly.
Phase 2: Reduction Reaction
Each of the six molecules of PGA is
phosphorylated
by an
ATP
to form six molecules of
1,3-biphosphoglycerate
A pair of electrons from each of six
NADPH
molecules reduces six molecules of 1,3-biphosphoglycerate to six molecules of
glyceraldehyde-3-phosphate (G3P)
Phase 3: RuBP Regeneration
G3P may be converted into glucose and polymerized into starch within the stroma or it may be transported into the cytoplasm and used to produce glucose and sucrose
The remaining five molecules of G3P are rearranged to regenerate three molecules of RuBP to complete the cycle.
Three moleccules of ATP are used in the process
With RuBP regenerated, the cycle may fix more carbon dioxide
Leaf Structure
Stomata:
openings on the surface; allows exchange of gases
Guard Cells:
regulates the size opening of the stomata
Xylem: transport water, minerals and carbohydrates
Upper/Lower Epidermis: protects the plant and allows light to pass through
Cuticle
its water resistant; it protects the leaf from excessive absorption of light and evaporation of water

Lower/Upper Epidermis:
protects the plant and allows light to pass through

Vascular Bundles
transport water, minerals and carbohydrates

Mesophyll Layer
the photosynthetic cells that form the bulk of a plant leaf

Guard Cells
regulates the size opening of the stomata

Stomata
openings on the surface; allows exchange of gases



Photosynthesis happens in the leaves of plants.
Their structure and arrangement on stems and branches maximizes the surface area exposed to sunlight to carry out photosynthesis
The mesophyll layers contain
chloroplast.
The chloroplasts have green pigments called
chlorophyll
, which helps with the absorption of the light energy.
Lets now take a closer look at the structure of a chloroplast!
Chloroplasts are mainly found in

mesophyll cells

(either the palisade or spongy layer).

Each chloroplasts has two membranes around the

stroma.

In the stroma contains membranous sacs,

the thylakoids
.
The thylakoids may be stacked into coloumns called

grana

and connected by the
lamen.


Photosynthesis really occurs in the
thylakoid membrane
!
The Nature of Light
Light is the most important component of photosynthesis .

Light
Light or
electromagnetic (EM) radiation
is a form of energy that travels 3 x 10^8 m/s in the form of wave packets called
photons
(or quanta)
Photons by wavelength are inversely proportional to their energy
Photons with short wavelengths have high energy; those who have long wavelengths have low energy
Photosystems are photosynthetic pigments embedded in the thylakoid membranes of chloroplasts that absorb light energy
Photosytems absorb photons of particular wavelengths and through light reactions, transfer their energy to form ATP and NADPH
Chlorophyll a and b absorb photons with energies in the blue-violet and red regions.
It reflects those in the green region, which explains why leaves have the colour green
The Calvin cycle are performed by
bundle-sheath cells
(surrounded by leaf veins)
The combined effects of the physical arrangement of cells and the C4 pathway establishes a high concentration of carbon dioxide around the rubisco while reducing its exposure to oxygen
Oxaloacetate is then reduced to
malate
by electrons transferred from NADPH.
Then malate diffuses into the bundle-sheath cells, where it enters chloroplasts and is oxidized to
pyruvate
, releasing
carbon dioxide

Carbon dioxide combines with a 3-carbon molecule,
phosphoenol pyruvate
(PEP) to produce 4-carbon
oxaloacetate
Their stomata open only at night when they release oxygen that accumulates from photosynthesis during the day and allow carbon dioxide to enter.
As the temperature increase during the day, the stomata closes, reducing water loss and cutting off the exchange of gases with the atmosphere
The high concentration of carbon dioxide favours the carboxylase (the enzyme that helps with the reaction) allowing the Calvin cycle to proceed efficiently with little loss of carbon dioxide from photorespiration.
C3 Photosynthesis -Plants
There are alternative mechanisms of carbon fixation for plants in hot, arid climates. These intermediates steps are either C4 or CAM
Therefore, photosynthesis is the most important process to the welfare of life on Earth.
Each year photosynthesis synthesizes 160 billion metric tons of carbohydrates.
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