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Introduction to
Photosynthesis
Chapter 35&10
Developed by Adam F.
Sprague
Chapter 10
1.ALL LIFE REQUIRES ENERGY
2.Animals, fungi, and most protists obtain their energy by
consuming, directly or indirectly, organic food stuffs from their
environment (heterotrophs)
3.Some organisms (autotrophs) have the ability to capture the
energy of the sun to synthesize their own organic food (green
plants, algae)
4.THE ULTIMATE SOURCE OF ALL ENERGY ON EARTH IS THE SUN
5.PHOTOSYNTHESIS is the link between life on earth and the sun
6.It is a set of reactions which convert light energy from the sun
into chemical bond energy of glucose and ATP
Photosynthesis can be summarized
with this chemical equation:
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6CO2 + 12H2O + LIGHT ENERGY -->
C6H12O6 + 6O2 + 6H2O
6CO2 + 12H2O + LIGHT ENERGY
--> C6H12O6 + 6O2 + 6H2O
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The chemical change is the reverse of
cellular respiration
The low energy inorganic compounds
(CO2 and water) are converted into the
high potential organic molecule (glucose)
The Chloroplasts: Sites of
Photosynthesis
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The primary function of this specialized
organelle is to convert light energy into
ATP and NADPH (nicotinamide adenine
dinucleotide phosphate)
Chloroplasts are found mainly in the
cells of the mesophyll (about 50/cell), the
green tissue on the interior of the leaf
Leaf
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Carbon dioxide enters the leaf, and oxygen
exits, by way of microscopic pores called
stomata
The double membrane of the chloroplast
regulates transport of materials in and out
Chloroplasts are filled with an aqueous solution
called the stoma which contains all the
necessary enzymes for photosynthesis
Chloroplast
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The conversion from light energy to ATP and
NADPH occurs in the thylakoid membranes
within the stroma
The thylakoid membranes contain all of the
pigments involved in the process including
chlorophyll (green pigment) and other
carotenoids
The thylakoids are organized into closely
packed stacks called grana
Choloroplast
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Within these thylakoids and grana, light
energy is converted into ATP and
NADPH – these are said to be LIGHTDEPENDENT REACTIONS
The reactions that actually convert CO2
to carbohydrate are LIGHTINDEPENDENT REACTIONS or DARK
REACTIONS
The Light Reactions
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Must take place in the presence of light
Steps that convert solar energy to
chemical energy
Light absorbed by chlorophyll drives a
transfer of electrons from water to an
acceptor named NADP+ which
temporarily stores the energized
electrons
Light Reactions
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Water is split in the process and thus it is the
light reactions of photosynthesis that give off
O2 as a by-product
The light reactions also generate ATP by
powering the addition of a phosphate group to
ADP, a process called photophosphorylation
THE LIGHT REACTIONS PRODUCE NO
SUGAR
The Dark Reactions
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Light is not required directly for these reactions to occur
These reactions incorporate CO2 from the air into organic
material through a process known as carbon fixation
The fixed carbon is then reduced to carbohydrate by the
addition of electrons
The reducing power is provided by NADPH and ATP
provided by the light reactions
Dark reactions in most plants occur during daylight so
that the light reactions can regenerate NADPH and ATP
These reactions occur in the stroma
Light and Pigments
The Nature of Sunlight
The Nature of Sunlight
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light is a form of energy known as
electromagnetic radiation
light travels in rhythmic waves which are
disturbances of electrical and magnetic
fields
The Nature of Sunlight
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the distance between crests of
electromagnetic waves is called the
wavelength
the entire range of radiation is known as
the electromagnetic spectrum
Light Energy
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the narrow range from about 380 to 750nm in wavelength
is detectable by the human eye and is called visible light
the model of light as waves explains many of its
properties, but in certain respects it behaves as though it
consists of discrete particles
these particles called photons act like objects in that each
of them has a fixed quantity of energy
the amount of energy is inversely related to the
wavelength of light (shorter wavelengths have more
energy)
Photosynthetic Pigments
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as light meets matter, it may be
reflected, transmitted or absorbed
substances that absorb light are called
pigments
if a pigment is illuminated in white light,
the color we see is the color most
reflected or transmitted by the pigment
Light perception
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the major pigment in leaves, chlorophyll,
appears green because it absorbs red
and blue light while transmitted and
reflecting green
chlorophyll is actually a family of
pigments with similar chemical structures
Photoexcitation of Chlorophyll
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when energy is absorbed by a molecule
of pigment, one of the molecules
electrons is elevated to from its ground
state to a higher orbital around the
nucleus (excited state)
Photoexcitation of Chlorophyll
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the only photons absorbed are those whose
energy is exactly equal to the energy
difference between the ground state and an
excited state
the energy of the photon is converted to the
potential energy of an electron, making the
electron less stable
generally, when pigments absorb light, their
excited electrons drop back down to the
ground state very quickly releasing their
energy as heat and/or light (fluorescence)
Light Dependent Reactions
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Photosynthetic Unit
Photosynthetic Unit
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in its native environment of the thylakoid membrane, chlorophyll
is organized along with proteins, pigments, and other kinds of
smaller organic molecules into photosystems
the proteins of these chloro-protein complexes affect the
absorption properties of the photosystem
a photosystem has a light gathering "antenna complex"
consisting of a few hundred chlorophyll a, chlorophyll b, and
carotenoid molecules
the number and variety of pigment molecules allows for the
absorption of light over a larger surface area and larger portion
of the spectrum
all of the antenna molecules absorb photons of light and the
energy is transmitted from pigment molecule to pigment
molecule until it reaches the reaction center
Photosystems I and II
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Emerson found that when plants were exposed to long (>680 nm) and
short (<680 nm) wavelengths of light, the rate of photosynthesis was
much greater than the sum of the rates of production for each
individual range (to explain this "Emerson Enhancement Effect", it
must be assumed that there are two photosystems in the thylakoid
membranes, photosystem I and photosystem II
the reaction center of photosystem I is known as P700 because its
pigment is best at absorbing light with an average wavelength of 700
nm (far-red)
photosystem II has pigment in it reaction center, P680, which best
absorbs light with an average wavelength of 680 nm (red)
the chlorophyll a in both photosystems is identical, it is their
association with different proteins that affects their light absorbing
properties
ATP Synthesis in Chloroplasts
ATP Synthesis in Chloroplasts
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chloroplasts and mitochondria generate ATP by
the same basic mechanism of chemiosmosis
an electron transport chain embedded in the
thylakoid membrane pumps protons across the
membrane as electrons are passed through a
series of carriers producing a proton-motive
force (potential energy stored in the proton
gradient)
ATP Synthesis in Chloroplasts
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ATP synthase in the membrane couples the diffusion of
hydrogen ions down their gradient to the phosphorylation
of ADP
in contrast to oxidative phosphorylation in mitochondria,
chloroplasts use light energy (not chemical energy in
food) to drive electrons to the top of the transport chain
the proton pump of the thylakoid membrane moves
hydrogen ions from the stroma to the thylakoid space
which functions as the H+ reservoir
the membrane makes ATP in the stroma as hydrogen
ions diffuse back down their gradient through ATP
synthase
Dark Reactions
Dark Reactions
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The "Dark Reactions" include the
biochemical, enzyme-catalyzed
reactions involved in the synthesis of
carbohydrate from carbon dioxide; these
are collectively know as the CalvinBenson cycle
The Reactions
The Reactions
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THE FIRST step (carbon fixation) of the reaction pathway is
when a molecule of CO2 is added to a compound named
ribulose bisphosphate (RuBP), a five-carbon sugar with a
phosphate group at each end
This reaction is catalyzed by the enzyme RuBP carboxylaseoxygenase, ("RUBISCO" for short) the most abundant protein in
chloroplasts (and on earth!)
The product of the reaction is a six-carbon intermediate that is
so unstable that it immediately splits in half to form two
molecules of 3-phosphogrlyceric acid/phosphoglycerate
For every three CO2 that enter the Calvin-Benson cycle via
rubisco, a total of six molecules of 3-phosphoglyerate are made
Dark reactions
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IN THE SECOND step (reduction) of the cycle, each molecule of 3phosophglyceric acid receives and additional phosphate group
An enzyme transfers the phosphate group from ATP forming 1,3diphophoglyceric acid (glycolysis?)
For every three (3) molecules of CO2 incorporated into the cycle, six molecules
of ATP must be used to produce six (6) molecules of 1,3-diphosphoglycerate
IN THE NEXT step, the NADPH (from the light reactions) reduces the
diphosphoglycerate to phosphoglyceraldehyde (PGAL) (6 for every 3 CO2)
Some of these molecules (1 PAL/3 CO2) are converted into glucose but most
are used to regenerate RuBP
The stromal reactions to convert the 3-carbon PGAL to the 5-carbon RuBP are
dependant on the presence of 3 more molecules of ATP/3 CO2 in the cycle
The five (5) remaining PGAL (3-C) are re-arranged into three (3) RuBP (5-C)
molecules
The Calvin-Benson cycle….
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produces three-carbon intermediates
used to synthesize glucose
produces three-carbon intermediates
used to regenerate the initial carbon
dioxide-acceptor molecule
The Calvin-Benson cycle….
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Without the presence of ATP and
NADPH from the light-dependent photochemical reactions, the conversion of
carbon dioxide to glucose can not occur
The Metabolic Fates of Glucose:
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About 50% of the glucose formed is used immediately to meet the plants
energy needs
Excess glucose can be converted to starch within the stroma of the chloroplast
or in specialized storage cells of roots, tubers, seeds, and fruits
REMEMBER, plants actively metabolize glucose (cellular respiration) and grow
in the dark and in the light
The glucose may be converted to sucrose (glucose + fructose) for transport
(via the phloem cells) to the non-photosynthetic leaves, roots, and stems
The formation of sucrose takes place in the cytoplasm, NOT in the chloroplast
the sucrose provides raw material for cellular respiration and many other
anabolic pathways that synthesize proteins, lipids, and other products
The glucose may be converted to CELLULOSE, to build cell walls, especially in
plant cells that are still growig and maturing
This conversion also takes place within the cytoplasm
Photosynthetic Induction
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In the dark, carbon fixation will stop in a plant when the
chloroplast has consumed all the ribulose bisphosphate
and PGAL
When the plant is exposed to light, maximum rates of
carbon dioxide fixation can not take place until all the
intermediates of the Calvin cycle have been replenished
to an optimal level
This lag time between exposure to light and maximum
photosynthetic rates is called photosynthetic induction
The enzymes which catalyze the steps of the CalvinBenson cycle also rely on products of the light-dependent
reactions to maintain their "active" form
Photorespiration
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Plants that produce three-carbon phosphoglycerate as
the first product of the light-independent reactions are
referred to as C3 plants
The active site of Rubisco can utilize O2 or CO2 with a
preference for CO2
If the air spaces in a leaf have a much higher
concentration of O2 than CO2, the active site of rubisco
will accept O2
When this occurs, a two-carbon molecule of
phosphoglycerate is produced, leaves the chloroplasts
and is metabolized in the peroxisomes and mitochondria
resulting in the release of carbon dioxide
Photorespiration
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Photorespiration consumes oxygen, released carbon
dioxide and generally occurs only in the light
The environmental conditions that foster photorespiration
in C3 plants are hot, dry, bright days
On such days, plants close their stomata to reduce water
loss and the plant soon depletes its CO2 and increases
O2 within the leaf
Photorespiration generates no ATP, decreases
photosynthetic output by siphoning organic material from
the Calvin cycle, produces no food, and seemingly has no
known benefit to plants
Alternate Photosynthetic
Pathways
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Photorespiration
Photorespiration
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Plants are constantly evolving to ensure they
are optimally adapted to their environments
plants have adapted anatomically and
metabolically to thrive in their terrestrial domain
of major concern to plants is dehydration via
transpiration through the stomata of the leaf
surface
on hot, dry days, plants close their stomata to
reduce water loss but at the same time, their
limiting the intake of carbon dioxide which will
reduce photosynthetic yield
Photorespiration
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with the stomata closed, carbon dioxide concentrations will quickly
decrease and oxygen concentrations will rise
plants that produce three-carbon 3-phosphoglycerate as the first stable
product of the Calvin cycle are called C3 plants (ie. rice, wheat)
these plants produce less food when their stomata close on hot, dry
days
the active site of Rubisco can bind oxygen or carbon dioxide with a
preference for CO2
•
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Ribulose-1,5-bisphosphate carboxylase/oxygenase, most commonly known
by the shorter name RuBisCO, is an enzyme (EC 4.1.1.39) that is used in the
Calvin cycle to catalyze the first major step of carbon fixation, a process by
which the atoms of atmospheric carbon dioxide are made available to
organisms in the form of energy-rich molecules such as sucrose. RuBisCO
catalyzes either the carboxylation or oxygenation of ribulose-1,5-bisphosphate
(also known as RuBP) with carbon dioxide or oxygen.
if the air spaces in a leaf have a much higher concentration of oxygen
than carbon dioxide, the active site of Rubisco will accept oxygen
C4 and CAM Plants
C4 and CAM Plants
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certain plants have evolved alternate
mode of carbon fixation forming a fourcarbon compound as its first product
a unique leaf anatomy is correlated with
the mechanism of C4 photosynthesis
including two distinct types of
photosynthetic cells; bundle sheath cells
and mesophyll cells
C4 and CAM Plants
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bundle sheath cells are arranged into tightly packed sheaths
around the veins of the leaf
between the bundle sheath and the leaf surface are the more
loosely arranged mesophyll cells
the Calvin cycle is confined to the chloroplasts of the bundle
sheath
the cycle is preceded by incorporation of carbon dioxide into
three-carbon phosphoenolpyruvate (PEP) to form four-carbon
oxaloacetate
the enzyme involved, PEP carboxylase has a much higher
affinity for carbon dioxide than does Rubisco
after the CO2 is "fixed", the mesophyll cells export oxaloacetate
to the bundle sheath cells where the CO2 is released and is
introduced into the Calvin cycle
C4 and CAM Plants
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CAM plants have adapted to dry conditions by opening
their stomata during the night and closing them during the
day, opposite to how other plants behave
when the stomata are open CO2 is incorporated into a
variety of organic acids in a method of carbon fixation call
crassulacean acid metabolism (CAM)
the mesophyll cells of CAM plants store the organic acids
they make during the night in their vacuoles until morning
when the stomata close
CO2 is released from the acids during the day for
incorporation into the Calvin cycle