Chapter 9 Photosynthesis
The Process that Feeds the Biosphere
 photosynthesis- conversion of light energy into chemical energy that’s stored in sugar & other
organic molecules; consists of the light reactions in which solar energy is captured & transformed
into chemical E, and the Calvin Cycle, in which the chemical E is used to make organic molecules
of food
 an organism acquires the organic compounds it uses for energy & C-skeletons by:
o self feeding. Autotrophs produce their organic molecules from CO2 and other inorganic
raw materials from the environment. They’re the source of organic compounds for all
nonautotrophic organisms. Plants are photoautotrophs, which use light to synthesize
organic substances
o consuming other organisms. Heterotrophs are dependent on autotrophs for food and O2.
10.1 Photosynthesis Converts Light Energy to the Chemical Energy of Food
 The ability of plants to harness light energy and use it to drive the synthesis of organic compounds
emerges from structural organization in the cell: photosynthetic enzymes are grouped together in a
biological membrane, enabling necessary series of chemical reactions to be carried out efficiently
 photosynthesis originated in a group of bacteria that had infolded regions of the plasma membrane
that function similarly to internal membranes of the chloroplast
Chloroplasts: the Sites of Photosynthesis in Plants
 the leaves are the major sites of photosynthesis.
 Chloroplasts are found in the cells of the mesophyll, the tissue in the interior of the leaf
 CO2 enters, O2 exits, by way of microscopic pores called stomata
 Water absorbed by the roots is delivered to the leaves in veins.
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A chloroplast has an envelope of 2 membranes surrounding the stroma, a dense fluid.
Thylakoids are sacs that are suspended within the stroma. They segregate the stroma from the
lumen or thylakoid space inside the thylakoids.
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Chlorophyll is in the thylakoid membrane. Light E absorbed by chlorophyll drives the
synthesis of organic molecules
Tracking Atoms Through Photosynthesis: Scientific Inquiry
 In the presence of light, the green parts of plants produce organic compounds & O2 from CO2 &
H2O in a series of complex chemical reactions.
 Even though the products of cell respiration are the reactants of photosynthesis, and vice versa,
chloroplasts do not synthesize sugars by simply reversing the steps of respiration.
The Splitting of Water
 O2 given off by plants is derived from H2O, not from CO2. The chloroplast splits H2O into H &
O2.
 Van Niel challenged the previously held belief that the O2 came from CO2. He experimented with
bacteria that don’t release O2 but CO2 was still a reactant.
 A significant result of the shuffling of atoms during photosynthesis is the extraction of H from
H2O & its incorporation into sugar. The waste product, CO2 is released to the atmosphere
Photosynthesis as a Redox Process
 Water is split, and e- are transferred along with H+ ions from the H2O & CO2, reducing it to sugar
 Photosynthesis is endergonic, using E from light, because the e- increase in PE as they move from
H2O to sugar.
The two Stages of Photosynthesis: A Preview
1. Light Reactions (photo–) convert solar E into chemical E. H2O is split, providing a source of e- &
p+ (H+) & gives off O2 as a by-product. Light absorbed by chlorophyll drives a transfer of the e- &
H+ from H2) into an e- acceptor, NADP+. ATP is generated using chemiosmosis to power the
addition of a phosphate group to ADP, a process called photophosphorylation. Light E is
converted into chemical E in the form of NADPH, a source of e- as “reducing power” that can be
passed along to an e- acceptor, reducing it, & ATP, energy. Occurs in the chloroplast.
On the outside of the tylakoids, NADP+ & ADP pick up e- & phosphate
2. Calvin cycle (–synthesis) begins with carbon fixation, which incorporates CO2 from the air into
organic compounds. The fixed C is then reduced to a carb by NADPH reducing it and ATP
energizeing it. Occurs in the stroma.
10.2 The Light Reactions Convert Solar E to the Chemical E of ATP & NADPH
The Nature of Sunlight
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Light can be categorized as wavelengths or photons, particles of light that have a fixed quantity of
E.
The amount of E is inversely related to the wavelength
Photosynthetic Pigments: the Light Receptors
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Pigments- substances that absorb visible lights. Wavelengths that are absorbed aren’t visible to us.
Wavelengths that are reflected are visible to us. Leaves appear green because chlorophyll absorbs
violet-blue & red light while transmitting & reflecting green light.
Spectrophotometers measure the wavelengths of light that are absorbed. It measures the light
transmitted at each wavelength. It plots an absorption spectrum
3 different types of pigments in the chloroplasts are:
o chlorophyll α which participates directly in the light reactions; blue green
o chlorophyll ϐ which is an accessory pigment (carotenoid); olive green
Violet-blue & red light work best for photosynthesis, and green is the least effective. This is
confirmed by an action spectrum of photosynthesis.
Engelmann performed an experiment in which he used bacteria to measure rates of photosynthesis
in algae.
carotenoids- accessory pigments; HC that are various shades of yellow & orange (absorbe violet &
blue-green light); broaden the spectrum that can drive photosynthesis. They photoprotect the plant
by absorbing & dissipating excessive light E that could damage chlorophyll or interact with O2,
forming reactive oxidative molecules
Excitation of Chlorophyll by Light
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When pigments absorb a photon of light, one of the molecule’s e- are elevated to an orbital with a
higher PE. It is excited from ground state.
A particular compound absorbs only photons corresponding to a specific wavelength, which is why
each pigment has a unique absorption spectrum
if the illuminated molecule is in isolation, its excited e- immediately drops back down to the
ground-state orbital, & its excess E is given off as heat & fluorescence (light)
A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes
 photosystem- a complex within the membrane that is composed of a reaction-center complex
surrounded by several light-harvesting complexes.
o reaction-center complex- organized association of proteins holding a special pair of
chlorophyll α molecules. Their molecular environment enables them to use energy from light to
boost one of their e- to a higher e- level and to transfer that e- to a different molecule, the
primary e- acceptor
o each light-harvesting complex consists of various pigment molecules bound to proteins. The
number & variety of pigment molecules enable a photosystem to harvest light over a larger
surface area & a larger portion of the spectrum. They act as an antenna for the reaction-center
complex. When a pigment molecule absorbs a photon, the E is transferred from one another
until it’s passes into the reaction-center complex, which accepts the e- & becomes reduced.
 The solar-powered transfer of an e- from the reaction-center chlorophyll α pair to the primary eacceptor is the 1st step. As soon as the e- is excited to a higher E level, the primary e- acceptor captures
it; this is a redox reaction. Each photosystem functions in the chloroplast as a unit.
 The thylakoid membrane is populated by 2 types of photosystems that cooperate in the light reactions.
They’re called photosystem II & photosystem I. Each has a distinct reaction-center complex, a
particular kind of primary e- acceptor next to a special pair of chlorophyll α molecules associated with
specific proteins.
 the reaction center chlorophyll a of PS II is P680 because it best absorbs light having a wavelength of
680 nm (red)
 the reaction center chlorophyll a of PS I is P700 because it best absorbs light having a wavelength of
700 nm (far-red part)
 PS I & PS II associate with different proteins in the thylakoid membrane. This affects the e- distribution
in the 2 pigments & accounts for the slight differences in their light-absorbing properties.
Linear Electron Flow
 Light drives the synthesis of ATP & NADPH by energizing PS I & PS II.
 The key to this E transformation is a flow of e- through the PS’s & other molecular components built
into the thylakoid membrane. This is called linear e- flow, which occurs in the light reactions
1. A photon of light strikes a pigment molecule in a light-harvesting complex of PS II, boosting one of its
e- to a higher E level. As this e- falls back to its ground state, an e- in a nearby pigment molecule is
raised to an excited state. The process continues, with the E being relayed to other pigments until it
reaches the P680 pair of chlorophyll a molecules in the reaction-center complex. It excites an e- in this
pair of chlorophylls into a higher E state
2. This e- is transferred from the excited P680 to the primary e- acceptor
3. An enzyme catalyzes the splitting of H2O into 2 e-, 2 H+, & O2. This is called photolysis. The e- are
supplied to the reaction center to regenerate the e- it just lost. The H+ are released into the lumen. The
O2 immediately combines with an O2 atom generated by the splitting of another water molecule,
forming O2.
4. Each photoexcited e- passes from the primary e- acceptor of PS II to PS I via an e- transport chain. The
e- transport chain between PS II & PS I is made up of the e- carrier plastoquinone (Pq), a cytochrome
complex, & a protein called plastocyanin (Pc)
5. the exergonic “fall” of e- to a lower E level provides E for the synthesis of ATP. As e- pass through the
cytochrome complex, H+ are pumped into the lumen, contributing to the proton gradient that is
subsequently used in chemiosmosis
6. Light E has been transferred via light-harvesting complex pigments to the PS I reaction center complex,
exciting an e- of the P700 pair of chlorophyll a molecules located there.
7. The photoexicted e- was then transferred to PS I’s primary e- acceptor, creating an e- hole in the P700
(now P700+, which is now an e- acceptor, accepting e- that reaches the bottom of the e- transport chain
from PSI I.)
8. photoexcited e- are passed in a series of redox reactions from the primary e- acceptor of PS I down
a 2nd e- transport chain through the protein ferredoxin (Fd)
9. The enzyme NADP+ reductase catalyzes the transfer of e- from Fd to NADP+. 2 e- are required
for its reduction to NADPH. Its e- are more readily available for the reactions of the Calvin cycle
than the e- from H2). H+ is also removed from the stroma.
Cyclic Electron Flow
 citric e- flow- alternative pathway for photoexcited e- in which only PS I is used. It’s a short
circuit: the e- cycle back from Fd to the cytochrome compelx & from there continue on to a P700
chlorophyll in the PS I reaction-center complex. Doesn’t produce NADPH, & no release of O2, but
generates ATP
 Evolutionary theory: some photosynthetic bacteria only have PS I, so they generate ATP through
this.
 It is an adaptation for plants that photoprotect it, and it is an adaptation of photosynthesis in C4
plants
A Comparison of Chemiosmosis in Chloroplasts & Mitochondria
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Chloroplast
chloroplasts transform light
energy into chemical
energy in ATP
the source of e- is H2O
don’t need molecules from
food to make ATP; their
PS’s capture light E & use
it to drive the e- from H2O
to the top of the chain
thylakoid membrane of the
chloroplast pumps p+ from
the stroma into the lumen,
which functions as the H+
reservoir
p+ diffuse down their
concentration gradient from
the lumen through ATP
synthase to the stroma,
driving ATP synthesis
ATp forms in the stroma,
where it’s used to help
drive sugar synthesis in the
Calvin cycle
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Both
generate ATP by
chemiosmosis
an e- transport chain
assembled in a membrane
pumps p+ across the
membrane as e- are passed
through a series of carries
that are progressively more
electronegative. In this
way, e- transport chains
transform redox E to a
proton-motive force, PE
stored in H+ gradient
across a membrane
ATP synthase is built into
the membrane. It couples
the diffusion of H+ down
their gradient to the
phosphorylation of ADP
the cytochromes (ironcontaining protein ecarriers) are very similar
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Mitochondria
chemiosmosis is used to
transfer chemical E from
food to ATP
the high E e- dropped down
the transport chain are
extracted from organic
molecules
inner membrane pumps p+
from the matrix out to the
intermembrane space,
which then serves as a
reservoir of H+
p+ diffuse down their
concentration gradient from
the intermembrane space
through ATP synthase to
the matrix, driving ATP
synthesis
The H+ (p+) gradient, or pH gradient, across the thylakoid membrane is important. The lumen has
a higher concentration of H+ and a lower pH (more acidic), while the stroma has a lower
concentration of H+ and a higher pH (less acidic)
10.3 The Calvin Cycle uses the Chemical E of ATP & NADPH to reduce CO2 to Sugar
 anabolic: builds carbs from smaller molecules & consuming E
 C enters the Calvin cycle in the form of CO2 & leaves in the form of sugar
 It spends ATP as an E source & consumes NADPH as reducing power for adding high-E e- to make the sugar
 glyceraldehydes 3-phosphate (G3P) is the sugar directly produced from the Calvin cycle.
 Phases of the Calvin Cycle:
1. Carbon fixation: CO2 molecule is incorporated one at a time by attaching to a 5-C sugar named
ribulose biphosphate (RuBP). Rubisco (RuBP carboxylase) is the enzyme that catalyzes this first step.
The product of this reaction is a 6-C intermediate so unstable that it immediately splits in half, forming
2 molecules of 3-phosphoglycerate (P-GAL)
2. Reduction: each molecules of PGAL receives an additional phosphate group from ATp, become 1biphosphoglycerate. Then, a pair of e- donated from NADPH reduces 1, 3-phosphoglycerate, which
also loses a phosphate group, becoming G3P. For every 3 molecules of CO2 that enter the cycle, there
are 6 molecules of G3P formed.Only one molecule of G3P can be counted as a net gain of catb. The
cycle bagan with 15 C worth of carbs in the form of 3 molecules of the 5-C RuBP. Now there are 18 C
worth of C in the form of 6 G3P. One molecule exits the cycle to be used by the plant cell. The other 5
must be recycled to regenerate the 3 molecules of RuBP
3. Regeneration of the CO2 acceptor (RuBP)- The C-skeletons of 5 G3P are rearranged by the last
steps of the Calvin Cycle into 3 RuBP. The cycle spends 3 more ATP to accomplish this. The RuBP is
now prepared to receive O2 again, and the cycle continues
 Input: 1 G3P molecule, 9 ATP, 6 NADPH
 Output: G3P (becomes starting material for metabolic pathways that synthesize other organic compounds)
10.4 Alternative Mechanisms of C-fixation have Evolved in Hot, Arid Climates
 dehydration is a major problem. There is a compromise between photosynthesis and the prevention
of excessive water loss from the plant. The CO2 required for photosynthesis enters a leaf via
stomata, the pores on the leaf surface. But stomata are also the main avenues of transpiration, the
evaporative loss of H2O from leaves. On hot, dry days, plants close their stomata to conserve
water, but this also reduces photosynthetic yield by limiting access to CO2. Concentration of O2
released from the light reactions being to increase. This is photorespiration
 C3 plants (majority of plants) initially fix C via rubisco; called C3 because the first organic
product of C fixation is a 3-C molecule, PGAL.
o ex: rice, wheat, soybeans
o on hot, dry days when their stomata close, they produce less sugar because the declining
level of CO2 in the leaf starves the Calvin cycle. Rubisco has an affinity for O2, so it binds
to it in place of CO2 to the Calvin cycle. The product splits, and a 2-C compound leaves
the chloroplast, releasing CO2. This is called photorespiration because it occurs in the
light & consumes O2 while producing CO2. It consumes ATP & produces no sugar. It
decreases photosynthetic output by draining off organic material from the Calvin cycle &
releasing CO2 that would otherwise be fixed
 Photorespiration could’ve came from an ancient atmosphere that had less O2 & more CO2.
 Photorespiration can protect plants. Plants that can’t undergo photorespiration are more susceptible
to damage by excess light. Photorespiration neutralizes the otherwise damaging products of the
light reactions, which build up when a low CO2 concentration limits the progress of the Calvin
cycle
 Rising CO2 levels would benefit C3 plants the most by lowering the amount of photorespiration.
 Rising temperatures increase photorespiration.
C4 Plants
 C4 plants preface the Calvin cycle with an alternate mode of C-fixation that forms a 4-C
compound as its 1st product.
 ex: sugar cane, corn
 There are 2 distinct types of photosynthetic cells:
1) bundle sheath cells, which are arranged into tightly packed which around the veins of the
leaf. The Calvin cycle is confined to their chloroplasts.
2) mesophyll cells, which are between the bundle sheath and the leaf surface. CO2 is
incorporated into organic compounds, which are then sent to the Calvin cycle
1. PEP carboxylase, an enzyme only present in mesophyll cells, adds CO2 to PEP, forming the 4-C
product oxaloacetate (OAA). PEP carboxylase has a very high affinity for CO2 & none for O2. So,
it can fix C efficiently when it’s hot & dry & stomata are partially closed, causing CO2
concentration in the leaf to fall & O2 to rise.
2. the mesophyll cells export their 4-C products to bundle-sheath cells through plasmodesmata
3. Within the bundle-sheath cells, the 4-C compounds release CO2, which is reassimilated into
organic material by rubisco & the Calvin cycle.
a. The same reaction regenerates pyruvate, which is transported to mesophyll cells. There,
ATP is used to convert pyruvate to PEP, allowing the reaction to continue. This ATP is the
“price” of concentration CO2 in the bundle-sheath cells. To regenerate this extra ATP,
bundle-sheath cells carry out cyclic e- flow, only carrying out PSI, which is their only
photosynthetic mode of generating ATP
 mesophyll cells of C4 plants pump CO2 into the bundle sheath, keeping the CO2 concentration in
the bundle-sheath cells high enough for rubisco to bind to CO2 rather than to O2.
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The cyclic series of reactions involving PEP carboxylase& the regeneration of ATP is a CO2concentrating pump that’s powered by ATP. So, C4 minimizes photorespiration & enhances sugar
production.
C4 plants are unaffected by increasing CO2 levels or temperature
CAM plants
 open their stomata during the night & close it during the day. Closing it at day helps conserve
water, but it also prevents Co2 from entering the leaves. During the night, when their stomata are
open, they take up CO2 & incorporate it into a variety of organic acids. This mode of C fixation is
called CAM.
 The mesophyll of CAM plants store the organic acids they make during the night in their vacuoles
until morning, when the stomata close.
 During the day, when the light reactions can supply ATP & NADPH for the Calvin cycle, CO2 is
released from the organic acids made the night before to come incorporated into sugar in the
chloroplasts.
 ex: succulent (water-storing) plants, cacti, pineapples
C4
 the initial steps of C
fixation are separated
structurally from the Calvin
cycle
Both
 CO2 is 1st incorporated into
organic intermediates
before it enters the Calvin
cycle
 use the Calvin cycle to
make sugar from CO2
CAM
 the 2 steps occur at separate
times, but within the same
cells
The Importance of Photosynthesis: A Review
 The light reactions capture solar energy & use it to make ATP & transfer e- from H2O to NADP+,
forming NADPH.
 The Calvin cycle uses the ATP & NADPH to produce sugar from CO2.
 The energy that enters the chloroplasts as sunlight becomes stored as chemical E in organic
compounds.
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The sugar made in the chloroplasts supplies the entire plant with chemical E & C-skeletons for the
synthesis of all major organic molecules of cell plants.
Organic molecules exported from leaves via veins are transported out of the leaves in the form of
sucrose, which provides raw material for cell respiration & anabolic pathways for
nonphotosynthetic cells.
Several glucoses are linked to make cellulose, which is the main ingredient of cell walls
Photosynthesis is responsible for the presence of O2 in the atmosphere.
Produces billions of tons of carbs per year
Researchers are seeking ways to capitalize on photosynthetic production to produce alternative
fuels