BCH 3000

2013

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FOTOSINTESIS

Pemerangkapan karbon dioksida semasa fotosintesis. Klorofil. Komponen fotosintesis. Fotosistem I dan II. Fotofosforilasi. Kitaran Calvin. Edaran Hatch-Slack.

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Global Importance of Photosynthesis

1. Photosynthesis by green plants and algae provides nearly all of the energy and organic carbon required by living organisms

2. Photosynthesis provides all of the oxygen required by aerobic organisms

3. The fossil fuels needed for human activity were produced by ancient photosynthetic organisms

4. Photosynthesis is the largest scale biosynthetic process on Earth

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Global Importance of Photosynthesis

5. Photosynthetic organisms remove approximately 2 x 1017

grams of Carbon, or 200 billion tons, per year from the atmosphere. This is about 10% of the total CO2 in the atmosphere.

6. In energy terms, this is roughly 0.1% of the incident visible sunlight

7. Virtually all of the CO2 "fixed" by plants is returned to the atmosphere by microbial, plant, and animal metabolism, and by biomass combustion.

8. Currently, there is more carbon dioxide being returned to the atmosphere than is being taken out by photosynthesis. Thus, atmospheric CO2 levels have been steadily rising for the last 100 years …..???.

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What is photosynthesis?

1. photosynthesis is the process of converting solar energy to chemical energy.

2. Photosynthesis is a complex process, by which atoms from water and carbon dioxide are cleaved and re-formed into organic molecules with high-energy bonds that can be used to fuel cellular processes

PHOTOSYNTYHESIS

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6CO2 + 12 H2O + hv C6H12O6 + 6O2 + 6 H2O

here hv stands for energy from the sun.

This is called oxygenic (oxygen evolving) photosynthesis, because it uses water as an electron donor and produces oxygen gas.

There is another type of photosynthesis found in some bacteria called anoxygenic photosynthesis, which uses hydrogen sulfide (H2S) as an electron donor, producing sulfide as a result

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Light Reactions

Dark Reactions

ATP

NADPH

SUGARS

CO2

O2

H2O + CO2

OVERVIEW OF PHOTOSYNTHESIS

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1. All photosynthesis in green plants and algae occurs in chloroplasts.

2. The chloroplast of plants is surrounded by 2 membranes,

the outer membrane is permeable to small molecules

the inner membrane forms the permeability barrier of the organelle. The inner membrane also contains transporters for phosphate and sugar precursors.

3. A series of membrane-bound pockets called thylakoids are found inside the chloroplast

Where does photosynthesis take place?

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The thylakoid membranes are where the light reactions of photosynthesis take place.

The thylakoid membranes are sac-like structures with a lipid bilayer separating the interior compartment from the stroma

Both photosynthetic prokaryotes and eukaryotes have these flattened sacs/vesicles containing photosynthetic chemicals.

In the land plants, the thlyakoids are usually stacked like pancakes into structures called grana (singular granum).

The space formed by the pockets of the thylakoid membranes is called the lumen

the space surrounding the thylakoids (but inside the inner membrane) is called the stroma

THYLAKOIDS

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Cell

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1. Light Dependent Process (Light Reactions)

a. requires the direct energy of light to make energy carrier molecules that are used in the second process.

b. occur in the grana

Stages of Photosynthesis – 2 stages

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1. The Light Independent Process (or Dark Reactions)

a. occurs when the products of the Light Reaction are used to form C-C covalent bonds of carbohydrates.

b. can usually occur in the dark, if the energy carriers from the light process are present.

c. major enzyme of the Dark Reaction is indirectly stimulated by light term Dark Reaction is not strictly

d. Reactions take place in the stroma of the

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BCH 3000

November 2012

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Overview of the two steps in the photosynthesis process 20

1. light strikes chlorophyll a in such a way as to excite electrons to a higher energy state.

2. In a series of reactions the energy is converted (along an electron transport process) into ATP and NADPH.

3. Water is split in the process, releasing oxygen as a by-product of the reaction.

4. The ATP and NADPH are used to make C-C bonds in the Light Independent Process (Dark Reactions).

In the Light Reactions

Summary of Photosynthesis

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1. carbon dioxide from the atmosphere (or water for aquatic/marine organisms) is captured and modified by the addition of Hydrogen to form carbohydrates

2. The incorporation of carbon dioxide into organic compounds is known as carbon fixation.

3. The energy for this comes from the first phase of the photosynthetic process. Living systems cannot directly utilize light energy, but can, through a complicated series of reactions, convert it into C-C bond energy that can be released by glycolysis and other metabolic processes.

In the Dark Reactions

Summary of Photosynthesis

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1. Light is composed of particles called photons that act like waves.

2. White light is separated into the different colors (=wavelengths)

3. Wavelength is defined as the distance from peak to peak (or trough to trough)

4. The order of colors is determined by the wavelength of light.

5. Visible light is one small part of the electromagnetic spectrum.

6. The longer the wavelength of visible light, the more red the color.

How can light provide energy for plants?

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1. Plants get the energy for photosynthesis from visible light

2. but how does electromagnetic energy become chemical energy to fuel this process?

3. Photosynthetic organisms produce specialized molecules called pigments that capture the energy from light and use it to fuel a series of redox reactions that store the captured energy in chemical bonds.

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1. A pigment is any substance that absorbs light

2. The color of the pigment comes from the wavelengths of light reflected (in other words, those not absorbed).

3. The most important pigment in plants is chlorophyll-two types of chlorophyll in plants

chlorophyll a (chl a)

chlorophyll b (chl b)

* There is also a third, chlorophyll c found in some algae

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What is a pigment?

- composed of two parts

1. a porphyrin ring with magnesium at its center

2. a hyrophobic phytol tail.- 20 carbon chain that is highly hydrophic and stabilizes the molecule in the hydrophobic core of the thylakoid membrane

Difference between chlorophyl a and chlorophyl b?

STRUCTURALLY

Functional group –

chl a = CH3 group

chl b = CHO group.

Chlorophyll

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CHO

Chlorophyll a

Chlorophyll b

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1. chl a absorbs best at 450 and 680 nm ; chl babsorbs best at 500 and 640 nm

2. chlorophyll a is directly involved in the redox reactions of the light reactions; chl b functions as an accessory pigment - not directly involved in the light reactions

Difference between chlorophyl a and chlorophyl b?

FUNCTIONALLY

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absorb light and pass the energy from the light to the chl a in the reaction center where the first stage of the light reactions take place –other e.g. xanthophylls and carotenoids (-carotene which absorbs different wavelengths than the chlorophylls)

Accessory pigments

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Notice that these main pigments do not absorb green light well

Why?

Chlorophyll a and b both absorb blue light and red light best, resulting in an overall green appearance

-carotene absorbs blue and some green light best resulting in an orange color

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● What is the molecular mechanisms by which pigments and other molecules work together to capture energy and convert into a usable form?.

● The core components of the light reactions are found in two spacially and functionally distinct units called photosystems-

1. Photosystem I (PSI) and

2. Photosystem II (PSII), after the order of their discovery.

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are arrangements of chlorophyll and other pigments packed into thylakoids

Many Prokaryotes have only one photosystem, Photosystem II

Eukaryotes have Photosystem II plus Photosystem I.

Photosystems

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uses chlorophyll a, in the form referred to as P700.

1. Photosystem I (PSI) and

2. Photosystem II (PSII), after the order of their discovery.

Photosystem I

uses a form of chlorophyll a known as P680.

Both "active" forms of chlorophyll a function in photosynthesis due to their association with proteins in the thylakoid membrane

Photosystem II

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The light reactions actually start with Photosystem II, and the products of those reactions are used as

substrates in Photosystem I.

The fist component of Photosystem II is composed of

1. the Light Harvesting Complex (LHC) and

2. the reaction center

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Light Harvesting Complex (LHC)

i. composed of hundreds of molecules of chlorophylls and accessory pigments

ii. Most of the Chl a in a cell is actually involved & constantly in light harvesting/ absorbing light - referred to as antenna chlorophylls

iii. when light of a certain wavelength (<680 nm) is absorbed, the absorbed energy is transferred from one molecule to another until it reaches the reaction center

iv. Because it works optimally with light of <680 nm, the core reaction center of photosystem II is called the P680 complex.

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i. contains a special pair of chlorophylls bound to a number of reaction center proteins in the thylakoid membrane that finally absorb the energy from the light.

ii. This absorbed energy moves one of the P680 chlorophylls into the excited state which is highly unstable.

iii. The Chl a molecule gets rid of this energy by ejecting an electronChl a molecule become oxidized = process is known as photo-oxidation (Why??...)

iv. The high-energy electron is immediately taken up by the Primary electron acceptor also known as the quinone Q

Reaction center

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OXIDATION AND REDUCTION

Now we have areduced Q which is a strong reducing agent and an oxidized chlorophyll a which is a strong oxidizing agent

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1. Photo II absorbs light and energy (photons) which causes the P680 molecule to excite its electron i.e. creates an electron deficiency in Photo II

2. Deficiency of electron in Photo II is filled by an electron from a water molecule, breaking the water into H+ ions and O-2 ions = Photolysis of water

3. This step also releases H+ in to the thylakoid space helping to create a proton gradient. O2 is also released in this step

The Light Reactions

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4. The high-energy electron is immediately taken up by the Primary electron acceptor also known as the quinone Q (plastoquinone)

5. Plastoquinone passes the e- to a cytochrome complex called b6-f complex.

6. This complex passes protons from the stroma into the thylakoid space increasing the proton gradient even more

7. Plastoquinone is found within the membrane - it has a long hydrophobic tail.

8. The electron is then passed to plastocyanin which then transfers the electron to an electron deficient Photo I.

The Light Reactions

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1. Photo I accepts energy from light and then an electron from P700 is excited and passed on to an electron acceptor called FeS.

2. FeS then passes its electron to Ferrodoxin. Ferrodoxin donates its e- to NADP+ reductase.

3. NADP+ reductase donates the electron to a molecule of NADP+ and stabilizes it by adding a proton to form NADPH.

4. This NADPH is then released into the stroma where it becomes part of the dark reactions of biosynthesis.

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● Photolysis of water in Photo II creates proton gradient

● The proton gradient created is used to create ATP.

● H+ in the thylakoid space can only diffuse down it gradient through an enzyme called ATPsynthase.

● ATPsynthase consists of two parts.

1. One is a proton channel that allows the H+ to diffuse into the stroma.

2. The other part couples this process to the phosphorylation of ADP to from ATP

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Cyclic photophosphorylation

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Cyclic photophosphorylation takes place inside a chloroplast, on or in a thylakoid membrane. The figure above represents a single thylakoid membrane. Notice that while PS2 is shown it is not involved in cyclic photophosphorylation. A photon of light energy strikes the leaf and hits photosystem 1. The energy will pass from one antenna pigment molecule to another until it reaches a reaction center molecule (p700). The light energy will then energize 2 electrons.

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The energized electrons now pass to an electron acceptor this creates the electron "hole" within PS1.

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From the electron acceptor the electrons pass to ferredoxin (Fd).

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From ferredoxin (Fd) the electrons pass on to a complex of cytochromes.

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As the electrons move from Fd to the cytochrome complex they release enough energy to power the active transport of hydrogen ions from the stroma

into the thylakoid space. This generates a large hydrogen ion gradient.

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From the cytochrome complex the electrons pass on to Photosystem 1 to fill an electron "hole" in PS1

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The hydrogen ions that have been pumped into the thylakoid space pass down a concentration gradient through the ATP synthetase complexes. As the ions

pass through the synthetase complex their chemiosmotic energy is released.

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The energy released by the hydrogen ions is used to help convert ADP and a phosphate group into ATP. Some of this ATP is going to be utlized in the Calvin

Cycle (dark reactions).. 68

Non-cyclic photophosphorylation

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Non-cyclic photophosphorylation takes place inside a chloroplast, on or in a thylakoid membrane. The figure above

represents a single thylakoid membrane.

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A photon of light energy strikes the leaf and hits photosystem 2. The energy will pass from one

antenna pigment molecule to another until it reaches a reaction center molecule (p680). The light energy

will then energize 2 electrons.71

The energized electrons now pass to an electron acceptor this creates an electron "hole" within PS2..

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Water is split inside the thylakoid, providing the electrons to fill the "hole" for photosystem 2. The hydrogen ions, that for the

moment are inside the thylakoid, and oxygen which will diffuse out of the chloroplast and the cell.

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From the electron acceptor the electrons pass to plastoquinone (PQ).

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From plastoquinone (PQ) the electrons pass on to a complex of cytochromes.

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As the electrons move from PQ to the cytochrome complex they release enough energy to power the active transport of hydrogen ions from the stroma

into the thylakoid space. This generates a large hydrogen ion gradient. 76

From the cytochrome complex the electrons pass on to Photosystem 1 to fill an electron "hole" in PS1.

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A photon of light energy strikes the leaf and hits photosystem 1. The energy will pass from one antenna pigment molecule to

another until it reaches a reaction center molecule (p700). The light energy will then energize 2 electrons.

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The energized electrons now pass to an electron acceptor this creates the electron "hole" within PS1.

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From the electron acceptor the electrons pass to ferredoxin (Fd).

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From ferredoxin (Fd) the electrons and 2 hydrogen ions are used to reduce NADP+ to NADPH + H+. The NADPH + H+ is going to be utlized in the Calvin Cycle

(dark reactions).81

The hydrogen ions that have been pumped into the thylakoid space pass down a concentration gradient through the ATP

synthetase complexes. As the ions pass through the synthetase complex their chemiosmotic energy is released.

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The energy released by the hydrogen ions is used to help convert ADP and a phosphate group into ATP. Some of this ATP

is going to be utlized in the Calvin Cycle (dark reactions)..

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Summary of cyclic electron flow -characteristics

1. No NADPH produced

2. No water is oxidized – since there is no reduction of NADP+, no need for splitting of water

3. No evolution of water

4. ADP is phosphorylated to ATP

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Phase 2: Reduction

1. This reaction occurs in two steps:

a. phosphorylation of PGA by ATP to form a "bis-phosphate“= 1,3-bisphosphoglycerate

b. reduction of di-PGA by NADPH to form triose phosphate, a simple 3-carbon carbohydrate = glyceraldehyde-3-phosphate (G3P)

2. This reaction requires BOTH ATP and NADPH, the high energy chemical intermediates formed in the light reactions.

PGA + ATP + NADPH → G3P + ADP + Pi + NADP+

Dark Reactions : 3 phases

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3. The NADP+ and ADP formed in this process return

to the thylakoids to regenerate NADPH and ATP in

the light reactions.

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Phase 3: Regeneration

1. The final stage in the Calvin cycle is the regeneration of RuBP

2. involves a series of reactions that convert triose phosphate

i. first Ru5P (ribulose 5-phosphate),

ii. then phosphorylation of Ru5P to regenerate RuBP (ribulose-bisphosphate).

3. This final step requires ATP formed in the light reactions.

Dark Reactions : 3 phases

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3. The triose phosphate formed in the Calvin cycle can

remain in the chloroplast where it is converted to

starch (reason why chloroplasts form starch grains)

4. Alternatively, triose phosphate can be exported from

the chloroplast where it is converted to sucrose in

the cytosol

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1. Called C3 because the CO2 is first incorporated into a 3-carbon compound.

2. Stomata are open during the day.

3. RUBISCO, the enzyme involved in photosynthesis, is also the enzyme involved in the uptake of CO2.

4. Photosynthesis takes place throughout the leaf.

5. Adaptive Value: more efficient than C4 plants under cool and moist conditions and under normal light because requires less machinery (fewer enzymes and no specialized anatomy

6. Most plants are C3.

C3 Photosynthesis : C3 plants

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1. Plants must cope with a competing reaction that

inhibits photosynthesis known as photorespiration.

2. Photorespiration can be defined as the light-

dependent uptake of O2 in the chloroplast.

3. Unlike photosynthesis, this process involves the

uptake of oxygen and the release of carbon dioxide

Photorespiration

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4. Rubisco is an unusual enzyme.

a. During photosynthesis it catalzyes the carboxylation of RuBP to give two molecules of PGA

b. However, it can also catalyze the oxygenation of RuBP to give one molecule of PGA and one molecule of a 2 carbon compound called phosphoglycolate

5. This reaction occurs because O2 can compete with CO2 at the active site of Rubisco

Photorespiration

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1. Since oxygenation of RuBP competes with carboxylation, it lowers the efficiency of photosynthesis - 25% of the carbon in phosphoglycolate is lost as CO2.

2. Plants must use energy to "recover" the remaining 75% of this carbon, which further limits the efficiency of photosynthesis.

3. to compensate, plants evolved a pathway, known as the photorespiratory pathway, to recover at least some of the carbon that would otherwise be lost. This pathway involved biochemical reactions in the chloroplast, mitochondria, and peroxisome.

Photorespiration

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3. The importance of photorespiration is easily demonstrated by the fact that nearly all plants grow better under high CO2 versus low CO2.

4. Conditions that favor carboxylation (photosynthesis) over oxygenation (photorespiration) include i. high CO2

ii. moderate light intensities, and iii. moderate temperatures.

5. Conditions that favor oxygenation over carboxylation include i. low CO2 levels, ii. high temperatures, and high light intensities.

Photorespiration

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Certain groups of plants evolved mechanisms to overcome the constraints imposed by photorespiration -known as C4 and CAM photosynthesis.

Both mechanisms act as "CO2 pumps" that increase the concentration of CO2 at the active site of Rubisco.

As a result, photorespiration does not occur in these plants.

Plants with either of these pathways are generally better adapted to hot, dry environments than C3 plants

How plants adapt to this conditions??

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1. Called C3 because the CO2 is first incorporated into a 3-carbon compound.

2. Stomata are open during the day.

3. RUBISCO, the enzyme involved in photosynthesis, is also the enzyme involved in the uptake of CO2.

4. Photosynthesis takes place throughout the leaf.

5. Adaptive Value: more efficient than C4 plants under cool and moist conditions and under normal light because requires less machinery (fewer enzymes and no specialized anatomy

6. Most plants are C3.

C3 Photosynthesis : C3 plants

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1. Plants must cope with a competing reaction that

inhibits photosynthesis known as photorespiration.

2. Photorespiration can be defined as the light-

dependent uptake of O2 in the chloroplast.

3. Unlike photosynthesis, this process involves the

uptake of oxygen and the release of carbon dioxide

Photorespiration

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4. Rubisco is an unusual enzyme.

a. During photosynthesis it catalzyes the carboxylation of RuBP to give two molecules of PGA

b. However, it can also catalyze the oxygenation of RuBP to give one molecule of PGA and one molecule of a 2 carbon compound called phosphoglycolate

5. This reaction occurs because O2 can compete with CO2 at the active site of Rubisco

Photorespiration

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1. Since oxygenation of RuBP competes with carboxylation, it lowers the efficiency of photosynthesis - 25% of the carbon in phosphoglycolate is lost as CO2.

2. Plants must use energy to "recover" the remaining 75% of this carbon, which further limits the efficiency of photosynthesis.

3. to compensate, plants evolved a pathway, known as the photorespiratory pathway, to recover at least some of the carbon that would otherwise be lost. This pathway involved biochemical reactions in the chloroplast, mitochondria, and peroxisome.

Photorespiration

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3. The importance of photorespiration is easily demonstrated by the fact that nearly all plants grow better under high CO2 versus low CO2.

4. Conditions that favor carboxylation (photosynthesis) over oxygenation (photorespiration) include i. high CO2

ii. moderate light intensities, and iii. moderate temperatures.

5. Conditions that favor oxygenation over carboxylation include i. low CO2 levels, ii. high temperatures, and high light intensities.

Photorespiration

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How plants adapt to this conditions??

Certain groups of plants evolved mechanisms to overcome the constraints imposed by photorespiration -known as C4 and CAM photosynthesis.

Both mechanisms act as "CO2 pumps" that increase the concentration of CO2 at the active site of Rubisco.

As a result, photorespiration does not occur in these plants.

Plants with either of these pathways are generally better adapted to hot, dry environments than C3 plants

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C4 Pathway

1. Also called the Hatch-Slack pathway after its

discoverers

2. They found that the first labeled product of

photosynthesis was a C4 acid (oxaloacetate) rather

than PGA. This compound is formed by the addition of

one molecule of CO2 to one molecule of

phosphoenolpyruvate (PEP)

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3. C4 plants have a unique leaf anatomy ("Kranz" =

wreath) which includes two types of

photosynthetic cells: bundle-sheath cells and

mesophyll cells. The Calvin cycle occurs only in the

bundle-sheath cells.

4. The initial carboxylation reaction occurs in the

mesophyll cells which contain the unique plant

enzyme PEP carboxylase that fixes carbon dioxide

into oxaloacetate.

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5. Oxaloacetate is then converted into another C4 acid,

malate, which exits the mesophyll cells and enters

the bundle sheath cells

6. ln the bundle sheath cells malate is decarboxylated

releasing CO2.

7. effect = concentrating CO2 in the bundle sheath

cells where it can be fixed into carbohydrate by the

Calvin cycle.

8. Carbohydrate is then moved into the vascular

bundle and translocated to the rest of the plant

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9. 7. In essence, the mesophyll cells act to minimize

photorespiration by supplying a steady flow of

carbon dioxide to the Calvin cycle even when

stomatal conductance is low

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4. Adaptive Value: Photosynthesizes faster than C3 plants under high light intensity and high temperatures because the CO2 is delivered directly to RUBISCO, not allowing it to grab oxygen and undergo photorespiration.

5. Has better Water Use Efficiency because PEP Carboxylase brings in CO2 faster and so does not need to keep stomata open as much (less water lost by transpiration) for the same amount of CO2 gain for photosynthesis.

6. C4 plants include several thousand species in at least 19 plant families. E.g. sugarcanes; corn

C4 Photosynthesis : C4 plants.

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To summarize, the steps in C4 photosynthesis are

1. carbon assimilation (in mesophyll) into a C4 acid

2. transport of C4 acid to bundle sheath cell

3. decarboxylation of C4 acid to produce CO2

4. transport of C3 product back to the mesophyll and regeneration of the CO2 acceptor

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CAM Photosynthesis : CAM plants

i. CAM = Crassulacean Acid Metabolism

ii. after the plant family Crassulaceae and because the CO2

is stored in the form of an acid before use in photosynthesis.

iii. Stomata open at night - evaporation rates are usually lower

iv. closed during the day.

v. The CO2 is converted to an acid and stored during the night.

vi. During the day, the acid is broken down and the CO2 is released to RUBISCO for photosynthesis

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i. Better Water Use Efficiency than C3 plants under arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.)

ii. When conditions very dry, CAM plants can just leave their stomata closed night and day. Oxygen given off in photosynthesis is used for respiration and CO2

given off in respiration is used for photosynthesis-allow the plant to survive dry spells, and it allows the plant to recover very quickly when water is available again

iii. E.g. cactus

Adaptive Value:

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CAM plants-

Cactus