BSC 1010C
General Biology I
Photosynthesis
Outline
- Plants and other autotrophs are the producers of the biosphere
- Chloroplasts are the sites of photosynthesis in plants
- Evidence that cloroplasts split water molecules enabled researchers
to track atoms through photosynthesis
- The Splitting of Water
- Photosynthesis as a Redox Process
- The light reactions and the Calvin cycle cooperate in transforming
light to the chemical energy of food
- The light reaction transform solar energy to the chemical energy
of ATP and NADPH: a closer look
- The Nature of Sunlight
- Photosynthetic Pigments: The Light Receptors
- The Photoexcitation of Chlorophyll
- Photosystems: Light-Harvesting Complexes of the Thylakoid Membrane
- Noncyclic Electron Flow
- Cyclic Electron Flow
- A Comparison of Chemiosmosis in Chloroplasts and Mitochondria
- The Calvin cycle uses ATP and NADPH to convert CO2
to sugar: a closer look
- Alternative mechanisms of carbon fixation have evolved in
hot, and climates
- Photorespiration: An Evolutionary Relic?
- C4 Plants
- CAM Plants
- Photosynthesis is the biosphere's metabolic foundation:
a review
Photosynthesis transforms solar light energy trapped by chloroplasts
into chemical bond energy stored in sugar and other organic molecules. This
process:
- Synthesizes energy-rich organic molecules from the energy-poor molecules,
CO2 and H2O.
- Uses CO2 as a carbon source and light energy as the energy source.
- Directly or indirectly supplies energy to most living organisms.
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I. Plants and other autotrophs are the producers of the biosphere
Organisms acquire organic molecules used for energy and carbon skeletons by
one of two nutritional modes:
- Autotrophic Nutrition, or
- Heterotrophic Nutrition.
Autotrophic nutrition = (Auto = self; trophos = feed) Nutritional mode
of synthesizing organic molecules from inorganic raw materials.
- Examples of autotrophic organisms are plants, which require only CO2,
H2O and minerals as nutrients.
- Because autotrophic organisms produce organic molecules that enter an ecosystem's
food store, autotrophs are also known as producers.
- Autotrophic organisms require an energy source to synthesize organic molecules.
That energy source may be from light (photoautotrophic) or from the
oxidation of inorganic substances (chemoautotrophic).
Photoautotrophs = Autotrophic organisms that use light as an energy
source to synthesize organic molecules. Examples are photosynthetic organisms
such as plants, algae and some prokaryotes.
Chemoautotrophs = Autotrophic organisms that use the oxidation of inorganic
substances, such as sulfur or ammonia, as an energy source to synthesize organic
molecules. Unique to some bacteria, this is a rarer form of autotrophic nutrition.
Heterotrophic nutrition = (Heteros = other; trophos = feed) Nutritional
mode of acquiring organic molecules from compounds produced by other organisms;
heterotrophs are unable to synthesize organic molecules from inorganic raw materials.
- Heterotrophs are also known as consumers.
- Examples are animals that eat plants or other animals.
- Examples also include decomposers, heterotrophs that decompose and
feed on organic litter. Most fungi and many bacteria are decomposers.
- Most heterotrophs depend on photoautotrophs for food and oxygen (a by-product
of photosynthesis).
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II. Chloroplasts are the sites of photosynthesis in plants
Although all green plant parts have chloroplasts, leaves are the major organs
of photosynthesis in most plants.
- Chlorophyll is the green pigment in chloroplasts that gives a leaf
its color and that absorbs the, light energy used to drive photosynthesis.
- Chloroplasts are primarily in cells of mesophyll, green tissue in
the leaf s interior.
- CO2 enters and O2 exits the leaf through microscopic
pores called stomata.
- Water absorbed by the roots is transported to leaves through veins or vascular
bundles which also export sugar from leaves to nonphotosynthetic parts
of the plant.
Chloroplasts are lens-shaped organelles measuring about 2 - 4 mm
by 4 - 7 mm. These organelles are divided into three
functional compartments by a system of membranes.
- Intermembrane Space. The chloroplast is bound by a double membrane which
partitions its contents from the cytosol. A narrow intermembrane space
separates the two membranes.
- Thylakoid Space. Thylakoids form another membranous system within
the chloroplast. The thylakoid membrane segregates the interior of the chloroplast
into two compartments: thylakoid space and stroma.
Thylakoids = Flattened membranous sacs inside the chloroplast.
- Chlorophyll is found in the thylakoid membranes.
- Thylakoids function in the steps of photosynthesis that initially convert
light energy to chemical energy.
Thylakoid space = Space inside the thylakoid.
Grana = (Singular, granum) Stacks of thylakoids in a chloroplast.
- Stroma. Reactions that use chemical energy to convert carbon dioxide to
sugar occur in the stroma, viscous fluid outside the thylakoids.
Photosynthetic prokaryotes lack chloroplasts, but have chlorophyll built into
the plasma membrane or into membranes of numerous vesicles within the cell.
- These membranes function in a manner similar to the thylakoid membranes
of chloroplasts.
- Photosynthetic membranes of cyanobacteria are usually arranged in parallel
stacks of flattened sacs similar to the thylakoids of chloroplasts.
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III. Evidence that chloroplasts split water molecules enabled
researchers to track atoms through photosynthesis: science as a process
Some steps in photosynthesis are not yet understood, but the following summary
equation has been known since the early 1800's:
6 CO2 + 12 H2O + light energy ®
C6H12O6 + 6 O2 + 6 H2O
|
- Glucose (C6H12O6) is shown in the summary
equation, though the main products of photosynthesis are other carbohydrates.
- Water is on both sides of the equation, because photosynthesis consumes
12 molecules and forms 6.
Indicating the net consumption of water simplifies the equation:
6 CO2 + 6 H2O + light energy ®
C6H12O6 + 6 O2 |
- In this form, the summary equation for photosynthesis is the reverse of
that for cellular respiration.
- Photosynthesis and cellular respiration both occur in plant cells, but plants
do not simply reverse the steps of respiration to make food.
The simplest form of the equation is:
- CH2O symbolizes the general formula for a carbohydrate.
- In this form, the summary equation emphasizes the production of a sugar
molecule, one carbon at a time. Six repetitions produces a glucose molecule.
- The Splitting of Water
The discovery that O2 released by plants is derived from H2O and
not from CO2, was one of the earliest clues to the mechanism of
photosynthesis.
Scientists later confirmed van Niel's hypothesis by using a heavy isotope
of oxygen (18O) as a tracer to follow oxygen's fate during photosynthesis.
- If water was labeled with tracer, released oxygen was 18O:
Experiment 1: CO2 + H2O* ®
CH2O + H2O + O2* |
- If the 18O was introduced to the plant as CO2, the
tracer did not appear in the released oxygen:
Experiment 2: CO2* + H2O ®
CH2O* + H2O* + O2 |
An important result of photosynthesis is the extraction of hydrogen from
water and its incorporation into sugar.
Electrons associated with hydrogen have more potential energy in organic
molecules than they do in water, where the electrons are closer to electronegative
oxygen.
Energy is stored in sugar and other food molecules in the form of these high-energy
electrons.
- Photosynthesis as a Redox Process
Respiration is an exergonic redox process; energy is released from the oxidation
of sugar.
- Electrons associated with sugar's hydrogens lose potential energy as carriers
transport them to oxygen, forming water.
- Electronegative oxygen pulls electrons down the electron transport chain,
and the potential energy released is used by the mitochondrion to produce
ATP.
Photosynthesis is an endergonic redox process; energy is required
to reduce carbon dioxide.
- Light is the energy source that boosts potential energy of electrons as
they are moved from water to sugar.
- When water is split, electrons are transferred from the water to carbon
dioxide, reducing it to sugar.
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IV. The light reactions and the Calvin cycle cooperate in
transforming light to the chemical energy of food: an overview
Photosynthesis occurs in two stages: the light reactions and the
Calvin cycle.
Light reactions = In photosynthesis, the reactions that convert light
energy to chemical bond energy in ATP and NADPH. These reactions:
- Occur in the thylakoid membranes of chloroplasts.
- Reduce NADP+ to NADPH.
ÞLight absorbed by chlorophyll provides the
energy to reduce NADP+ to NADPH, which temporarily stores the
energized electrons transferred from water.
Þ NADP+ (nicotinamide adenine dinucleotide
phosphate), a coenzyme similar to NAD+ in respiration, is reduced
by adding a pair of electrons along with a hydrogen nucleus, or H+.
- Give off O2 as a by-product from the splitting of water.
- Generate ATP. The light reactions power the addition of a phosphate group
to ADP in a process called photophosphorylation.
Calvin cycle = In photosynthesis, the carbon-fixation reactions that
assimilate atmospheric CO2 and then reduce it to a carbohydrate;
named for Melvin Calvin. These reactions:
- Occur in the stroma of the chloroplast.
- First incorporate atmospheric CO2 into existing organic molecules
by a process called carbon fixation, and then reduce fixed carbon to
carbohydrate.
Carbon fixation = The process of incorporating CO2 into
organic molecules.
The Calvin cycle reactions do not require light directly, but reduction Of
CO2 to sugar requires the products of the light reactions:
- NADPH provides the reducing power.
- ATP provides the chemical energy.
Chloroplasts thus use light energy to make sugar by coordinating the two stages
of photosynthesis.
- Light reactions occur in the thylakoids of chloroplasts.
- Calvin cycle reactions occur in the stroma.
- As NADP+ and ADP contact thylakoid membranes, they pick up electrons
and phosphate respectively, and then transfer their high-energy cargo to the
Calvin cycle.
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V. The light reactions transform solar energy to the chemical
energy of ATP and NADPH: a closer look
To understand how the thylakoids of chloroplasts transform light energy into
the chemical energy of ATP and NADPH, it is necessary to know some important
properties of light.
- The Nature of Sunlight
Sunlight is electromagnetic energy. The quantum mechanical model
of electromagnetic radiation describes light as having a behavior that is
both wavelike and particle-like.
- Wavelike properties of light.
- Electromagnetic energy is a form of energy that travels in
rhythmic waves which are disturbances of electric and magnetic fields.
- A wavelength is the distance between the crests of electromagnetic
waves.
- The electromagnetic spectrum ranges from wavelengths that are less
than a nanometer (ganuna rays) to those that are more than a kilometer
(radio waves).
- Visible light, which is detectable by the human eye, is only
a small portion of the electromagnetic spectrum and ranges from about
380 to 750 nm. The wavelengths most important for photosynthesis are
within this range of visible light.
- Particle-like properties of light.
- Light also behaves as if it consists of discrete particles or quanta
called photons.
- Each photon has a fixed quantity of energy which is inversely
proportional to the wavelength of light. For example, a photon of violet
light has nearly twice as much energy as a photon of red light.
The sun radiates the full spectrum of electromagnetic energy.
- The atmosphere acts as a selective window that allows visible light to
pass through while screening out a substantial fraction of other radiation.
- The visible range of light is the radiation that drives photosynthesis.
- Blue and red, the two wavelengths most effectively absorbed by chlorophyll,
are the colors most useful as energy for the light reactions.
- Photosynthetic Pigments: The Light Receptors
Light may be reflected, transmitted or absorbed when it contacts matter.
Pigments = Substances that absorb visible light.
- Different pigments absorb different wavelengths of light.
- Wavelengths that are absorbed disappear, so a pigment that absorbs all
wavelengths appears black.
- When white light, which contains all the wavelengths of visible light,
illuminates a pigment, the color you see is the color most reflected or
transmitted by the pigment. For example, a leaf appears green because chlorophyll
absorbs red and blue light but transmits and reflects green light.
Each pigment has a characteristic absorption spectrum or pattern
of wavelengths that it absorbs. It is expressed as a graph of absorption versus
wavelength.
- The absorption spectrum for a pigment in solution can be determined by
using a spectrophotometer, an instrument used to measure what proportion
of a specific wavelength of light is absorbed or transmitted by the pigment.
- Since chlorophyll a is the light-absorbing pigment that participates
directly in the light reactions, the absorption spectrum of chlorophyll
a provides clues as to which wavelengths of visible light are most effective
for photosynthesi s.
A graph of wavelength versus rate of photosynthesis is called an action
spectrum and profiles the relative effectiveness of different wavelengths
of visible light for driving photosynthesis.
- The action spectrum of photosynthesis can be determined by illuminating
chloroplasts with different wavelengths of light and measuring some indicator
of photosynthetic rate, such as oxygen release or carbon dioxide consumption.
- It is apparent from the action spectrum of photosynthesis that blue and
red light are the most effective wavelengths for photosynthesis and green
light is the least effective.
The action spectrum for photosynthesis does not exactly match the
absorption spectrum for chlorophyll a.
- Since chlorophyll a is not the only pigment in chloroplasts that
absorb light, the absorption spectrum for chlorophyll a underestimates
the effectiveness of some wavelengths.
- Even though only special chlorophyll a molecules can participate directly
in the light reactions, other pigments, called accessory pigments,
can absorb light and transfer the energy to chlorophyll a
The accessory pigments expand the range of wavelengths available
for photosynthesis. These pigments include:
- Chlorophyll b, a yellow-green pigment with a structure similar
to chlorophyll a. This minor structural difference gives the pigments
slightly different absorption spectra.
- Carotenoids, yellow and orange hydrocarbons that are built into
the thylakoid membrane with the two types of chlorophyll.
- The Photoexcitation of Chlorophyll
What happens when chlorophyll or accessory pigments absorb photons?
- Colors of absorbed wavelengths disappear from the spectrum of transmitted
and reflected light.
- The absorbed photon boosts one of the pigment molecule's electrons in
its lowest energy state (ground state) to an orbital of higher potential
energy (excited state).
The only photons absorbed by a molecule are those with an energy state equal
to the difference in energy between the ground state and excited state.
- This energy difference varies from one molecule to another. Pigments have
unique absorption spectra because pigments only absorb photons corresponding
to specific wavelengths.
- The photon energy absorbed is converted to potential energy of an electron
elevated to the excited state.
The excited state is unstable, so excited electrons quickly fall back to
the ground state orbital, releasing excess energy in the process. This released
energy may be:
- Dissipated as heat.
- Reradiated as a photon of lower energy and longer wavelength than the
original light that excited the pigment. This afterglow is called fluorescence.
Pigment molecules do not fluoresce when in the thylakoid membranes, because
nearby primary electron acceptor molecules trap excited state electrons
that have absorbed photons.
- In this redox reaction, chlorophyll is photooxidized by the absorption
of light energy and the electron acceptor is reduced.
- Because no primary electron acceptor is present, isolated chlorophyll
fluoresces in the red part of the spectrum and dissipates heat.
- Photosystems: Light-Harvesting Complexes of the Thylakoid Membrane
Chlorophyll a, chlorophyll b and the carotenoids are assembled
into photosystems located within the thylakoid membrane. Each photosystem
is composed of:
- antenna complex.
- Several hundred chlorophyll a, chlorophyll b and carotenoid
molecules are light-gathering antennae that absorb photons and pass
the energy from molecule to molecule. This process of resonance energy
transfer is called induc tive resonance.
- Different pigments within the antennal complex have slightly different
absorption spectra, so collectively they can absorb photons from a wider
range of the light spectrum than would be possible with only one type
of pigment molecule.
- reaction-center chlorophyll.
- Only two of the many chlorophyll a molecules in each complex can actually
transfer an excited electron to initiate the light reactions.
These specialized chlorophyll as are located in the reaction
center.
- primary electron acceptor.
- Located near the reaction center, a primary electron acceptor molecule
traps excited state electrons released from the reaction center chlorophyll.
- The transfer of excited state electrons from chlorophyll to primary
electron acceptor molecules is the first step of the light reactions.
The energy stored in the trapped electrons powers the synthesis of ATP
and NADPH in subsequent steps.
Two types of photosystems are located in the thylakoid membranes, photosystem
I and photosystem II.
- The reaction center of photosystem I has a specialized chlorophyll a molecule
known as P700, which absorbs best at 700 nm (the far red portion
of the spectrum).
- The reaction center of photosystem II has a specialized chlorophyll a
molecule known as P680, which absorbs best at a wavelength of 680
nm.
- P700 and P680 are identical chlorophyll a molecules, but each is
associated with a different protein. This affects their electron distribution
and results in slightly different absorption spectra.
- Noncyclic Electron Flow
There are two possible routes for electron flow during the light reactions:
noncyclic flow and cyclic flow.
Both photosystem I and photosystem II function and cooperate in noncyclic
electron flow which transforms light energy to chemical energy stored in the
bonds of NADPH and ATP. This process:
- Occurs in the thylakoid membrane.
- Passes electrons continuously from water to NADP+.
- Produces ATP by noncyclic photophosphorylation.
- Produces NADPH.
- Produces O2.
Light excites electrons from P700, the reaction center chlorophylls in photosystem
I. These excited state electrons do not return to the reaction center chlorophylls,
but are ultimately stored in NADPH, which will later be the electron donor
in the Calvin Cycle.
- Initially, the excited state electrons are transferred from P700 to the
primary electron acceptor for photosystem I.
- The primary electron acceptor passes these excited state electrons to
ferredoxin (Fd), an iron-containing protein.
- NADP+ reductase catalyzes the redox reaction that transfers
these electrons from ferredoxin to NADP+, producing reduced coenzyme
- NADPH.
- The oxidized P700 chlorophyll becomes an oxidizing agent as its electron
"holes" must be filled; photosystem II supplies the electrons
to fill these holes.
When the antenna assembly of photosystem II absorbs light, the energy is
transferred to the P680 reaction center .
- Electrons ejected from P680 are trapped by the photosystem II primary
electron acceptor.
- The electrons are then transferred from this primary electron acceptor
to an electron transport chain embedded in the thylakoid membrane. The first
carrier in the chain, plastoquinone (Pq) receives the electrons from
the primary electron acc eptor. In a cascade of redox reactions, the electrons
travel from Pq to a complex of two cytochromes to plastocyanin (Pc) to P700
of photosystem I.
- As these electrons pass down the electron transport chain, they lose potential
energy until they reach the ground state of P700.
- These electrons then fill the electron vacancies left in photosystem I
when NADP+ was reduced.
Electrons from P680 flow to P700 during noncyclic electron flow, restoring
the missing electrons in P700. This, however, leaves the P680 reaction center
of photosystem II with missing electrons; the oxidized P680 chlorophyll thus
becomes a strong oxidiz ing agent.
- A water-splitting enzyme extracts electrons from water and passes them
to oxidized P680, which has a high affinity for electrons.
- As water is oxidized, the removal of electrons splits water into two hydrogen
ions and an oxygen atom.
- The oxygen atom immediately combines with a second oxygen atom to form
O2. It is this water-splitting step of photosynthesis that releases
O2.
As excited electrons give up energy along the transport chain to P700, the
thylakoid membrane couples the exergonic flow of electrons to the endergonic
reactions that phosphorylate ADP to ATP.
- This coupling mechanism is chemiosmosis.
- Some electron carriers can only transport electrons in the company of
protons.
- The protons are picked up on one side of the thylakoid membrane and deposited
on the opposite side as the electrons move to the next member of the transport
chain.
- The electron flow thus stores energy in the form of a proton gradient
across the thylakoid membrane - a proton-motive force.
- An ATP synthase enzyme in the thylakoid membrane uses the proton-motive
force to make ATP. This process is called photophosphorylation because
the energy required is light.
- This form of ATP production is called noncyclic photophosphorylation.
- Cyclic Electron Flow
Cyclic electron flow is the simplest pathway, but involves only photosystem
I and generates ATP without producing NADPH or evolving oxygen.
- It is cyclic because excited electrons that leave from chlorophyll a
at the reaction center return to the reaction center.
- As photons are absorbed by Photosystem I, the P700 reaction center chlorophyll
releases excited-state electrons to the primary electron acceptor; which,
in turn, passes them to ferredoxin. From there the electrons take an alternate
path that sen ds them tumbling down the electron transport chain to P700.
This is the same electron transport chain used in noncyclic electron flow.
- With each redox reaction alone, the electron transport chain, electrons
lose potential energy until they return to their ground-state orbital in
the P700 reaction center.
- The exergonic flow of electrons is coupled to ATP production by the process
of chemiosmosis. This process of ATP production is called cyclic photophosphorylation.
- Absorption of another two photons of light by the pigments send a second
pair of electrons through the cyclic pathway.
The function of the cyclic pathway is to produce additional ATP.
- It does so without the production of NADPH or O2.
- Cyclic photophosphorylation supplements the ATP supply required for the
Calvin cycle and other metabolic pathways. The noncyclic pathway produces
approximately equal amounts of ATP and NADPH, which is not enought ATP to
meet demand.
- NADPH concentration might influence whether electrons flow through cyclic
or noncyclic pathways.
- A Comparison of Chemiosmosis in Chloroplasts and Mitochondria
Chemiosmosis = The coupling of exergonic electron flow down an electron
transport chain to endergonic ATP production by the creation of an electrochemical
proton gradient across a membrane. The proton gradient drives ATP synthesis
as protons diff use back across the membrane.
Chemiosmosis in chloroplasts and chemlosmosis in mitochondria are similar
in several ways:
- An electron transport chain assembled in a membrane translocates protons
across the membrane as electrons pass through a series of carriers that
are progressively more electronegative.
- An ATP synthase complex built into the same membrane, couples the diffusion
of hydrogen ions down their gradient to the phosphorylation of ADP.
- The ATP synthase complexes and some electron carriers (including quinones
and cytochromes) are very similar in both chloroplasts and mitochondria.
Oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts
differ in the following ways:
- Electron Transport Chain
- Mitochondria transfer chemical energy from food molecules to ATP.
The high-energy electrons that pass down the transport chain are extracted
by the oxidation of food molecules.
- Chloroplasts transform light energy into chemical energy. Photosystems
capture light energy and use it to drive electrons to the top of the
transport chain.
- Spatial Organization
- The Inner mitochondrial membrane pumps protons from the matrix out
to the intermembrane space, which is a reservoir of protons that power
ATP synthase.
- The chloroplast's thylakoid membrane pumps protons from the stroma
into the thylakoid compartment, which functions as a proton reservoir.
ATP is produced as protons diffuse from the thylakoid compartment back
to the stroma through ATP synthase com plexes that have catalytic heads
on the membrane's stroma side. Thus, ATP forms in the stroma where it
drives sugar synthesis during the Calvin cycle.
There is a large proton or pH gradient across the thylakoid membrane.
- When chloroplasts are illuminated, there is a thousand-fold difference
in H+ concentration. The pH in the thylakoid compartment is reduced
to about 5 while the pH in the stroma increases to about 8.
- When chloroplasts are in the dark, the pH gradient disappears, but can
be reestablished if chloroplasts are illuminated.
- Andre Jagendorf (1960's) produced compelling evidence for chemiosmosis
when he induced chloroplasts to produce ATP in the dark by using artificial
means to create a pH gradient. His experiments demonstrated that during
photophosphorylation, the function of the photosystems and the electron
transport chain is to create a protonmotive force that drives ATP synthesis.
A tentative model for the organization of the thylakoid membrane includes
the following:
- Proton pumping by the thylakoid membrane depends on an asymmetric placement
of electron carriers that accept and release protons (H+).
- There are three steps in the light reactions that contribute to the proton
gradient across the thylakoid membrane:
- Water is split by Photosystem II on the thylakold side, releasing
protons in the process.
- As plastoquinone (Pq), a mobile carrier, transfers electrons to the
cytochrome complex, it translocates protons from the stroma to the thylakoid
space.
- Protons in the stroma are removed from solution as NADP+
is reduced to NADPH.
- NADPH and ATP are produced on the side of the membrane facing the stroma
where sugar is synthesized by the Calvin cycle.
Summary of the Light Reactions:
During noncyclic electron flow, the photosystems of the thylakoid
membrane transform light energy to the chemical energy stored in NADPH and
ATP. This process:
- Pushes low energy-state electrons from water to NADPH, where they are
stored at a higher state of potential energy. NADPH, in turn, is the electron
donor used to reduce carbon dioxide to sugar (Calvin Cycle).
- Produces ATP from this light driven electron current.
- Produces oxygen as a by-product.
During cyclic electron flow, electrons ejected from P700 reach ferredoxin
and flow back to P700. This process:
- Produces ATP.
- Unlike noncyclic electron flow, does not produce NADPH or O2.
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VI. The Calvin cycle uses ATP and NADPH to convert CO2
to sugar: a closer look
ATP and NADPH produced by the light reactions are used in the Calvin cycle
to reduce carbon dioxide to sugar.
- The Calvin cycle is similar to the Krebs cycle in that the starting material
is regenerated by the end of the cycle.
- Carbon enters the Calvin cycle as CO2 and leaves as sugar.
- ATP is the energy source, while NADPH is the reducing acent that adds high-energy
electrons to form sugar.
- The Calvin cycle actually produces a three-carbon sugar glyceraldehyde
3-phosphate (G3P).
For the Calvin cycle to synthesize one molecule of sugar (G3P), three molecules
of CO2 must enter the cycle. The cycle may be divided into three
phases:
Phase 1: Carbon Fixation. The Calvin cycle begins when each molecule of CO2
is attached to five-carbon sugar, ribulose biphosphate (RuBP).
- This reaction is catalyzed by the enzyme RuBP carboxylase (rubisco)
- one of the most abundant proteins on Earth..
- The product of this reaction is an unstable six-carbon intermediate that
immediately splits into two molecules of 3-phosphoglycerate.
- For every three CO2 molecules that enter the Calvin cycle via
rubisco, three RuBP molecules are carboxylated forming six molecules of
3-phosphoglycerate.
Phase 2: Reduction. This endergonic reduction phase is a two-step process
that couples ATP hydrolysis with the reduction of 3-phosphoglycerate to glyceraldehyde
phosphate.
- An enzyme phosphorylates 3-phosphoglycerate by transferring a phosphate
group from ATP.
- This reaction:
Þ produces 1, 3-bisphosphoglycerate.
Þ uses six ATP molecules to produce six
molecules of 1,3-bisphosphoglycerate.
Þ primes 1,3-bisphosphoglycerate for the
addition of high-energy electrons from NADPH.
- Electrons from NADPH reduce the carboxyl group of 1,3-bisphsphoglycerate
to the aldehyde group of glyceraldehyde 3-phosphate (G3P).
Þ The product, G3P, stores more potential
energy than the initial reactant, 3-phosphoglycerate.
Þ G3P is the same three-carbon sugar produced
when glycolysis splits glucose.
- For every three CO2 molecules that enter the Calvin cycle, six G3P molecules
are produced, only one of which can be counted as net gain.
Þ The cycle begins with three five-carbon
RuBP molecules - a total of 15 carbons.
Þ The six G3P molecules produced contain
18 carbons, a net gain of three carbons from CO2.
ÞOne G3P molecule exits the cycle; the other
five are recycled to regenerate three molecules of RuBP.
Phase 3: Regeneration of Starting Material (RuBP). A complex series of reactions
rearranges the carbon skeletons of five G3P molecules into three RUBP molecules.
- These reactions require three ATP molecules.
- RuBP is thus regenerated to begin the cycle again.
For the net synthesis of one G3P molecule, the Calvin cycle uses the products
of the light reactions:
- 9 ATP molecules
- 6 NADPH molecules
G3P produced by the Calvin cycle is the raw material used to synthesize glucose
and other carbohydrates.
- The Calvin cycle uses 18 ATP and 12 NADPH molecules to produce one glucose
molecule.
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VII. Alternative mechanisms of carbon fixation have evolved
in hot, and climates
- Photorespiration: An Evolutionary Relic?
A metabolic pathway called photorespiration reduces the yield of
photosynthesis.
Photorespiration = In plants, a metabolic pathway that consumes oxygen,
evolves carbon dioxide, produces no ATP and decreases photosynthetic output.
- Occurs because the active site of rubisco can accept O2 as
well as CO2. Produces no ATP molecules.
- Decreases photosynthetic output by reducing organic molecules used in
the Calvin cycle.
When the O2 concentration in the leafs air spaces is higher
than CO2 concentration, rubisco accepts O2 and transfers
it to RUBP. The "photo" in photorespiration refers to the fact that this pathway
usually occurs in light when photosynthesis reduces CO2 and raises
O2 in the leaf spaces. The "respiration" in photorespiration refers
to the fact that this process uses O2 and releases CO2.
Some scientists believe that photorespiration is a metabolic relic from
earlier times when the atmosphere contained less oxygen and more carbon dioxide
than is present today.
- Under these conditions, when rubisco evolved, the inability of the enzyme's
active site to distinguish carbon dioxide from oxygen would have made little
difference.
- This affinity for oxygen has been retained by rubisco and some photorespiration
is bound to occur.
Whether photorespiration is beneficial to plants is not known.
- It is known that some crop plants (e.g. soybeans) lose as much as 50%
of the carbon fixed by the Calvin cycle to photorespiration.
- If photorespiration could be reduced in some agricultural plants, crop
yields and food supplies would increase.
Photorespiration is fostered by hot, dry, bright days.
- Under these conditions, plants close their stomata to prevent dehydration
by reducing water loss from the leaf.
- Photosynthesis then depletes available carbon dioxide and increases oxygen
within the leaf air spaces. This condition favors photorespiration.
Certain species of plants, which live in hot arid climates, have evolved
alternate modes of carbon fixation that minimize photorespiration. C4
and CAM are the two most important of these photosynthetic adaptations.
- C4 Plants
The Calvin cycle occurs in most plants and produces 3-phosphoglycerate,
a three-carbon compound, as the first stable intermediate.
- These plants are called C3 plants, because the first stable
intermediate has three carbons.
- Agriculturally important C3 plants include rice, wheat and
soybeans.
Many plant species preface the Calvin cycle with reactions that incorporate
carbon dioxide into four-carbon compounds.
- These plants are called C4 plants.
- The C4 pathway is used by several thousand species in at least
19 families including corn and sugarcane, important agricultural grasses.
- This pathway is adaptive, because it enhances carbon fixation under conditions
that favor photorespiration, such as hot, and environments.
Leaf anatomy of C4 plants spatially segregates the Calvin cycle
from the initial incorporation of CO2 into organic compounds. There
are two distinct types of photosynthetic cells:
- Bundle-sheath cells.
- Are arranged into tightly packed sheaths around the veins of the leaf.
- Thylakoids in the chloroplasts of bundle-sheath cells are not stacked
into grana.
- The Calvin cycle is confined to the chloroplasts of the bundle sheath.
- Mesophyll cells.
- Are more loosely arranged in the area between the bundle sheath and
the leaf surface.
The Calvin cycle of C4 plants is preceded by incorporation of
CO2 into organic compounds in the mesophyll.
Step 1: CO2 is added to phosphoenolpyruvate (PEP) to form oxaloacetate,
a four-carbon product.
- PEP carboxylase is the enzyme that adds CO2 to PEP.
Compared to rubisco, it has a much greater affinity for CO2
and has no affinity for O2.
- Thus, PEP carboxylase can fix CO2 efficiently when rubisco
cannot - under hot, dry conditions that cause stomata to close, CO2
concentrations to drop and O2 concentrations to rise.
Step 2: After CO2 has been fixed by mesophyll cells, they convert
oxaloacetate to another four-carbon compound (usually malate).
Step 3: Mesophyll cells then export the four-carbon products (i.e. malate)
through plasmodesmata to bundle-sheath cells.
- In the bundle-sheath cells, the four carbon compounds release CO2,
which is then fixed by rubisco in the Calvin cycle.
- Mesophyll cells thus pump CO2 into bundle-sheath cells, nuninuzmg
photorespiration and enhancing sugar production by maintaining a CO2
concentration sufficient for rubisco to accept CO2 rather than
oxygen.
- CAM Plants
A second photosynthetic adaptation exists in succulent plants adapted to
very and conditions. These plants open their stomata primarily at night and
close them during the day (opposite of most plants).
- This conserves water during the day, but prevents CO2 from
entering the leaves.
- When stomata are open at night, CO2 is taken up and incorporated
into a variety of organic acids. This mode of carbon fixation is called
crassulacean acid metabolism (CAM).
- The organic acids made at night are stored in vacuoles of mesophyll cells
until morning, when the stomata close.
- During daytime, light reactions supply ATP and NADPH for the Calvin cycle.
At this time, CO2 is released from the organic acids made the
previous night and is incorporated into sugar in the chloroplasts.
The CAM and C4 pathways:
- are similar in that CO2 is first incorporated into organic
intermediates before it enters the Calvin cycle.
- differ in that the initial steps of carbon fixation in C4 plants
are structurally separate from the Calvin cycle; in CAM plants, the two
steps occur at separate times.
Regardless of whether the plant uses a C3, C4 or CAM
pathway, all plants use the Calvin cycle to produce sugar from CO2.
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VIII. Photosynthesis is the biosphere's metabolic foundation:
a review
On a global scale, photosynthesis makes about 160 billion metric tons of carbohydrate
per year. No other chemical process on Earth is more productive or is as important
to life.
- Light reactions capture solar energy and use it to:
Þ produce ATP
Þtransfer electrons from water to NADP+
to form NADPH
- The Calvin cycle uses ATP and NADPH to fix CO2 and produce sugar.
Photosynthesis transforms light energy to chemical bond energy in sugar molecules.
- Sugars made in chloroplasts supply the entire plant with chemical energy
and carbon skeletons to synthesize organic molecules.
- Nonphotosynthetic parts of a plant depend on organic molecules exported
from leaves in veins.
ÞThe disaccharide sucrose is the transport
form of carbohydrate in most plants.
Þ Sucrose is the raw material for cellular
respiration and many anabolic pathways in nonphotosynthetic cells.
- Much of the sugar is glucose - the monomer linked to form cellulose,
the main constituent of plant cell walls.
Most plants make more organic material than needed for respiratory fuel and
for precursors of biosynthesis.
- Plants consume about 50% of the photosynthate as fuel for cellular respiration.
- Extra sugars are synthesized into starch and stored in storage cells of
roots, tubers, seeds, and fruits.
- Heterotrophs also consume parts of plants as food.
Photorespiration can reduce photosynthetic yield in hot dry climates. Alternate
methods of carbon fixation minimize photorespiration.
- C4 plants spatially separate carbon fixation from the Calvin
cycle.
- CAM plants temporally separate carbon fixation from the Calvin cycle.
Course Pages maintained by
Dr. Graeme Lindbeck
.