After reading this chapter and attending lecture, the student should be able to:
|Distinguish between autotrophic and heterotrophic nutrition.
|Describe the location and structure of the chloroplast.
|Explain how chloroplast structure relates to its function.
|Write a summary equation for photosynthesis.
|Explain the role of REDOX reactions in photosynthesis.
|Describe the wavelike and particlelike behaviors of light.
|Describe the absorption spectrum of the primary
|List the wavelengths of light that are most effective for
|Explain what happens when chlorophyll absorbs photons of
|List the components of a photosystem and explain their
|Trace electron flow through photosystems II and I.
|Summarize the light dependent reactions with an equation and
describe where they occur.
|Describe important differences between phosphorylation in
mitochondria and in chloroplasts.
|Summarize the light independent reactions (carbon-fixing) of
the Calvin cycle and describe changes that occur in the carbon skeleton of the
|Describe the role of ATP and NADPH in the Calvin cycle.
|Describe what happens to the light independent reactions when
the O2 concentration is much higher than CO2.
|Describe the major consequences of photorespiration.
|Describe two important photosynthetic adaptations that
|Describe the fate of photosynthetic products.
primary electron acceptor
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
Uses CO2 as a carbon source and light energy as the energy source.
Directly or indirectly supplies energy to most living organisms.
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: 1) Autotrophic Nutrition or 2) 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 ecosystems
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
- 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
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.
LEAF CROSS SECTION
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 µm by 4 7 µm. These organelles are divided into three functional
compartments by a system of membranes:
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.
- Intermembrane Space . The chloroplast is bound by a double membrane which partitions its
contents from the cytosol. A narrow intermembrane space separates the two
- Thylakoids. 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.
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:
Indicating the net consumption of water simplifies the equation:
- 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
The simplest form of the equation is: CO2 + H2O ® CH2O + O2
- 6 CO2 + 6 H2O + light energy ® C6H12O6
+ 6 O2
- In this form, the summary equation for photosynthesis is the reverse of that for
- Photosynthesis and cellular respiration both occur in plant cells, but plants do not
simply reverse the steps of respiration to make food.
- 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.
A. 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.
- In the 1930's, C.B. van Niel from Stanford University challenged an early model, which
- O2 released during photosynthesis comes from CO2.
- CO2 is split and water is added to the carbon.
- Van Niel studied bacteria that use hydrogen sulfide (H2S) rather than H2O
for photosynthesis and that produce yellow sulfur globules as a by-product.
- Van Niel deduced that these bacteria split H2S and use H to make sugar. He
generalized that all photosynthetic organisms require hydrogen, but that the source
- Van Niel thus hypothesized that plants split water as a source of hydrogen and release
oxygen as a by-product.
- 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.
- If the 18O was introduced to the plant as CO2, the tracer did not
appear in the released oxygen.
- 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
B. Photosynthesis as a Redox Process
- Respiration is an exergonic redox process; energy is released from the oxidation
- 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
- 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.
IV. The light dependent reactions and the Calvin cycle cooperate in transforming light
to the chemical energy of food: an overview
- Photosynthesis occurs in two stages: the light dependent
reactions and the light independent reactions (known as the Calvin cycle).
- Light dependent 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.
- Light absorbed by chlorophyll provides the energy to reduce NADP+ to NADPH,
which temporarily stores the energized electrons transferred from water.
- 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.
- 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
- 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.
V. The light reactions transform solar energy to the chemical energy of ATP and NADPH: a
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
A. 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
- 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 (gamma rays) to those that are more than a kilometer (radio waves).
(See Campbell, Figure 10.5)
- 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.
- Particlelike 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.
B. Photosynthetic Pigments: The Light Receptors
- Light may be reflected, transmitted or absorbed when it contacts matter. (See Campbell,
- 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
- 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. (See Campbell Methods Box)
- 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 photosynthesis.
- Chlorophyll a has absorption peaks at approximately 440 nm
and 680 nm.
- 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
- 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
- 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
- Carotenoids, yellow and orange hydrocarbons that are built into the thylakoid
membrane with the two types of chlorophyll.
C. 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
- 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
- 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 light of a 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.
D. 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
- 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 one of the many chlorophyll a molecules in each complex can actually transfer
an excited electron to initiate the light reactions. This specialized chlorophyll a
is 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
- 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.
E. 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.
- Occurs in the thylakoid membrane.
- Passes electrons continuously from water to NADP+.
- Produces ATP by noncyclic photophosphorylation.
- Produces NADPH.
- Produces O2.
Electron Flow in Photosystem I
- Light excites electrons from P700, the reaction center chlorophyll in photosystem I.
These excited state electrons do not return to the reaction center chlorophyll, 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
Electron Flow in Photosystem II
- 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 acceptor. 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+
- 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 oxidizing 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
- This form of ATP production is called noncyclic photophosphorylation.
F. 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 sends 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 along 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
- 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
G. A Comparison of Chemiosmosis in Chloroplasts and
- 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 diffuse back across the
- Chemiosmosis in chloroplasts and chemiosmosis in mitochondria are similar in several
- 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
- 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:
- 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
- Chloroplasts transform light energy into chemical energy. Photosystems capture light
energy and use it to drive electrons to the top of the transport chain.
- 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 complexes 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 proton-motive force that
drives ATP synthesis.
- For a tentative model of the organization of the thylakoid membrane see Campbell, Figure 10.15.
- Proton pumping by the thylakoid membrane depends on an asymmetric placement of electron
carriers that accept and release protons (H+).
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.
VI. The Calvin cycle uses ATP and NADPH to convert CO2 to sugar: a closer look
An enzyme phosphorylates 3-phosphoglycerate by transferring a phosphate group from ATP.
- 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 agent that adds high-energy
electrons to form sugar.
- The Calvin cycle actually produces a three-carbon sugar glyceraldehyde 3-phosphate
- 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 a
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.
Electrons from NADPH reduce the carboxyl group of 1,3-bisphosphoglycerate to the
aldehyde group of glyceraldehyde 3-phosphate (G3P).
The product, G3P, stores more potential energy than the initial reactant,
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.
The cycle begins with three five-carbon RuBP molecules a total of 15 carbons.
The six G3P molecules produced contain 18 carbons, a net gain of three carbons from CO2.
One G3P molecule exits the cycle; the other five are recycled to regenerate three
molecules of RuBP.
- produces 1, 3-bisphosphoglycerate.
- uses six ATP molecules to produce six molecules of 1,3-bisphosphoglycerate.
- primes 1,3-bisphosphoglycerate for the addition of high-energy electrons from NADPH.
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
- 9 ATP molecules
- 6 NADPH molecules
- G3P produced by the Calvin cycle is the raw material used to synthesize glucose and
- The Calvin cycle uses 18 ATP and 12 NADPH molecules to produce one glucose molecule.
VII. Alternative mechanisms of carbon fixation have evolved in hot, arid climates
- When the O2 concentration in the leaf's air spaces is higher than CO2
concentration, rubisco accepts O2 and transfers it to RuBP.
- This pathway produces no ATP molecules and reduces the number of organic molecules that
can be reduced by the Calvin cycle.
- This metabolic pathway is called photorespiration and it reduces photosynthetic
- 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.
- 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
- 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.
B. 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 precede 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, arid environments.
- Leaf anatomy of C4 plants spatially segregates the Calvin cycle
from the initial incorporation of CO2 into organic compounds.
- In C4 plants there are two distinct types of photosynthetic 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.
- 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. (See Campbell, Figure 10.18)
The steps of C4 carbon fixation.
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 ¾ under hot, dry conditions that cause stomata to close, CO2
concentrations to drop and O2 concentrations to rise.
Step 3: Mesophyll cells then export the four-carbon products (i.e. malate) to the
- 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, minimizing
photorespiration and enhancing sugar production by maintaining a CO2
concentration sufficient for rubisco to accept CO2 rather than oxygen.
C. CAM Plants
- A second photosynthetic adaptation exists in succulent plants adapted to very arid
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
- 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
- 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
- Regardless of whether the plant uses a C3, C4 or CAM pathway, all
plants use the Calvin cycle to produce sugar from CO2.
VIII. Photosynthesis is the biospheres 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
- Light reactions capture solar energy and use it to:
- produce ATP
- transfer 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
- The disaccharide sucrose is the transport form of carbohydrate in most plants.
- Sucrose is the raw material for cellular respiration and many anabolic pathways in
- 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 of C3 plants
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.
Atkins, P.W. Atoms, Electrons, and Change. New York, Oxford: W.H. Freeman and
Company, 1991. Chapter 9, "Light and Life" is a witty, imaginative description
of photosynthesis. Though written for a lay audience, it is probably best appreciated by
someone already familiar with photosynthesis.
Campbell, N. Biology. 4th ed. Menlo Park, California: Benjamin/Cummings, 1996.
Lehninger, A.L., D.L. Nelson and M.M. Cox. Principles of Biochemistry. 2nd ed.
New York: Worth, 1993.
Matthews, C.K. and K.E. van Holde. Biochemistry. 2nd ed. Redwood City,
California: Benjamin/Cummings, 1996.