Photosynthesis
AP Biology
Mrs. Stahl
The Process That Feeds the
Biosphere
• Photosynthesis is the process that
converts solar energy into chemical
energy and occurs in the chloroplast.
• Directly or indirectly, photosynthesis
nourishes almost the entire living world
– All organisms use organic compounds for
energy and for carbon skeletons
– Organisms obtain organic compounds either
through autotrophic nutrition or
heterotrophic nutrition
• Autotrophs sustain themselves without
eating anything derived from other
organisms, they are the main primary
producers of the biosphere
• Autotrophs are the producers of the
biosphere, producing organic molecules
from CO2 and other inorganic molecules
• Almost all plants are photoautotrophs,
using the energy of sunlight to make
organic molecules
Figure 10.1
Figure 10.1a
Other organisms also benefit from
photosynthesis.
• Photosynthesis occurs in plants,
algae, certain other unicellular
eukaryotes, and some
prokaryotes
• These organisms feed not only
themselves but also most of the
living world
(d) Cyanobacteria
(b) Multicellular alga
1 μm
(a) Plants
10 μm
(e) Purple sulfur
bacteria
(c) Unicellular eukaryotes
40
μm
• Heterotrophs obtain their organic
material from other organisms
• Heterotrophs are the consumers of
the biosphere
• Almost all heterotrophs, including
humans, depend on
photoautotrophs for food and O2.
• Heterotrophs can also decompose
and feed on dead organisms or on
organic litter, like feces and fallen
leaves
–Fungi and many prokaryotes feed
via decomposition, also known as
detritivores.
• Earth’s supply of fossil fuels was
formed from the remains of
organisms that died hundreds of
millions of years ago
• In a sense, fossil fuels represent
stores of solar energy from the
distant past
Chemoautotrophs
• Their energy comes from chemicals instead of
sunlight
• Ex- Deep Sea Vents and the existing food webs
that thrive without sunlight and
photosynthesis, also known as
chemosythnesis
Definition from:
http://link.springer.com/referenceworkentry/10.1007
%2F978-3-642-11274-4_271
Chemoautotrophs are organisms that obtain their energy from a
chemical reaction (chemotrophs) but their source of carbon is the
most oxidized form of carbon, carbon dioxide (CO2). The best known
chemoautotrophs are the chemolithoautotrophs that use inorganic
energy sources, such as ferrous iron, hydrogen, hydrogen sulfide,
elemental sulfur or ammonia, and CO2 as their carbon source. All
known chemoautotrophs are prokaryotes, belonging to the Archaea
or Bacteria domains. They have been isolated in different extreme
habitats, associated to deep-sea vents, the deep biosphere or acidic
environments. This form of energy conservation is considered one
of the oldest on Earth. These microorganisms are of astrobiological
interest because they could develop in the extreme conditions
existing in different extraterrestrial planetary bodies, like Mars or
Europa.
Photosynthesis converts light energy
to the chemical energy of food
• Chloroplasts are structurally similar to and
likely evolved from photosynthetic bacteria
• The structural organization of these
organelles allows for the chemical reactions
of photosynthesis
Tracking Atoms Through
Photosynthesis: Scientific Inquiry
• Photosynthesis is a complex series of
reactions that can be summarized as the
following equation:
6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2O
• The overall chemical change during
photosynthesis is the reverse of the one
that occurs during cellular respiration
Equation for Photosynthesis
6CO2 + 6H2O ------> C6H12O6 + 6O2
Sunlight energy
Where: CO2 = carbon dioxide
H2O = water
Light energy is required
C6H12O6 = glucose
O2 = oxygen
https://www.msu.edu/user/morleyti/sun/Biol
ogy/photochem.html
The Splitting of Water
• Chloroplasts split H2O into hydrogen and oxygen,
incorporating the electrons of hydrogen into sugar
molecules and releasing oxygen as a by-product
Figure 10.5
Products:
12 H2O
6 CO2
Reactants:
C6H12O6
6 H2O
6 O2
Chloroplasts: The Sites of
Photosynthesis
in Plants
• Leaves are the major locations of photosynthesis
• Chloroplasts are found mainly in cells of the
mesophyll, the interior tissue of the leaf
• Each mesophyll cell contains 30–40 chloroplasts
• CO2 enters and O2 exits the leaf through
microscopic pores called stomata
• Veins deliver water from the roots and carry off
sugar from mesophyll cells to nonphotosynthetic
areas of the plant
• A chloroplast has an envelope of two
membranes surrounding a dense fluid called
the stroma, surrounds the thylakoid
• Thylakoids are connected sacs in the
chloroplast which compose a third
membrane system
• Thylakoids may be stacked in columns called
grana
• Chlorophyll, the pigment which gives leaves
their green color, resides in the thylakoid
membranes, plays an important role in the
absorption of light
Leaf cross section
Chloroplasts Vein
Mesophyll
Stomata
CO2
Chloroplast
O2
Mesophyll
cell
20 μm
Figure 10.4b
Chloroplast
Stroma
Thylakoid
Thylakoid
Granum space
1 μm
Outer
membrane
Intermembrane
space
Inner
membrane
Three Stages
• 1. Capture energy from sunlight
• 2. Using the energy to make ATP and to
reduce the compound NADP+, an electron
carrier, to NADPH
• 3. Using the ATP and NADPH to power the
synthesis of organic molecules from CO2 in
the air
F.F. Blackman’s Contribution
• Found out that different light
intensities, CO2 concentrations, and
temperatures alter photosynthetic rates
• Concluded that photosynthesis is a
multi-step process with one portion
requiring light, while the other occurs
with and without light
C.B. van Niels, 1930’s
• Studied purple sulfur bacteria
• They do not release oxygen, instead of using
water they converted hydrogen sulfide into
pure sulfur globules (released as waste)
• He hypothesized that plants split water as a
source of electrons from hydrogen atoms,
releasing oxygen as a byproduct
• He proposed the equation for photosynthesis
Photosynthesis as a Redox Process
• Photosynthesis reverses the direction of
electron flow compared to respiration
• Photosynthesis is a redox process in which
H2O is oxidized and CO2 is reduced
• Photosynthesis is an endergonic process; the
energy boost is provided by light
Figure
10.UN01
becomes reduced
becomes oxidized
The Nature of Sunlight
• Light is a form of electromagnetic energy,
also called electromagnetic radiation
• Like other electromagnetic energy, light
travels in rhythmic waves
• Wavelength is the distance between crests
of waves
• Wavelength determines the type of
electromagnetic energy
Pigments- molecules that absorb light
energy in the visible range
• The color we see is the color that is
not absorbed, but reflected
• The electromagnetic spectrum is the
entire range of electromagnetic energy,
or radiation. Wavelengths of 400-740
nm
• Visible light consists of wavelengths
(including those that drive
photosynthesis) that produce colors we
can see
• Light also behaves as though it consists
of discrete particles, called photons
Figure 10.7
10
−5
nm 10
−3
Gamma
rays
nm
3
1 nm
X-rays
10 nm
UV
1m
(10 9 nm)
6
10 nm
Infrared
Microwaves
10 3 m
Radio
waves
Visible light
380
450
500
Shorter wavelength
Higher energy
550
600
650
700
750 nm
Longer wavelength
Lower energy
Photosynthetic Pigments: The Light
Receptors
• Pigments are substances that absorb visible light
• Different pigments absorb different wavelengths
• Wavelengths that are not absorbed are reflected
or transmitted
• Leaves appear green because chlorophyll reflects
and transmits green light
Figure 10.8
Light
Reflected
light
Chloroplast
Absorbed
light
Granum
Transmitted
light
Animation: Light and Pigments
• A spectrophotometer measures a
pigment’s ability to absorb
various wavelengths
• This machine sends light through
pigments and measures the
fraction of light transmitted at
each wavelength
Refracting
prism
White
light
Chlorophyll
solution
2
1
Slit moves to pass light
of selected wavelength.
Photoelectric
tube
Galvanometer
3
4
Green
light
The high transmittance (low
absorption) reading indicates that
chlorophyll absorbs very
little green light.
Blue
light
The low transmittance (high
absorption) reading indicates
that chlorophyll absorbs
most blue light.
• An absorption spectrum is a graph
plotting a pigment’s light absorption
versus wavelength
• The absorption spectrum of chlorophyll
a suggests that violet-blue and red light
work best for photosynthesis
• An action spectrum profiles the relative
effectiveness of different wavelengths of
radiation in driving a process
Absorption
of light by
chloroplast
pigments
Chlorophyll a
Chlorophyll b
Carotenoids
400
500
600
700
Wavelength of light (nm)
(a) Absorption spectra
Rate of
photosynthesis
(measured by O2
release)
Figure
10.10
400
(b) Action spectrum
500
600
700
Aerobic bacteria
Filament of alga
400
500
(c) Engelmann’s experiment
600
700
• The action spectrum of photosynthesis was
first demonstrated in 1883 by Theodor W.
Engelmann
• In his experiment, he exposed different
segments of a filamentous alga to different
wavelengths
• Areas receiving wavelengths favorable to
photosynthesis produced excess O2
• He used the growth of aerobic bacteria
clustered along the alga as a measure of O2
production
• Chlorophyll a is the main photosynthetic
pigment
• Accessory pigments, such as chlorophyll b,
broaden the spectrum used for
photosynthesis
• The difference in the absorption spectrum
between chlorophyll a and b is due to a
slight structural difference between the
pigment molecules
• Accessory pigments called carotenoids
absorb excessive light that would damage
chlorophyll
Figure 10.11
CH
3
CH 3 in chlorophyll a
CHO in chlorophyll b
Porphyrin ring:
light-absorbing
“head” of molecule;
note magnesium
atom at center
Hydrocarbon tail:
interacts with hydrophobic
regions of proteins inside
thylakoid membranes of
chloroplasts; H atoms not
shown
Video: Space-Filling Model of
Chlorophyll a
• Accessory pigments called carotenoids
function in photoprotection; they absorb
excessive light that would damage chlorophyll
Excitation of Chlorophyll by Light
• When a pigment absorbs light, it goes from a
ground state to an excited state, which is
unstable
• When excited electrons fall back to the
ground state, photons are given off, an
afterglow called fluorescence
• If illuminated, an isolated solution of
chlorophyll will fluoresce, giving off light and
heat
Figure 10.12
e−
Excited
state
Energy of electron
Heat
Photon
Chlorophyll
molecule
Photon
(fluorescence)
Ground
state
(a) Excitation of isolated chlorophyll molecule
(b) Fluorescence
A Photosystem: A Reaction-Center
Complex Associated with LightHarvesting Complexes
• Photosystem- located in the thylakoid membrane
where chlorophyll is organized along with proteins and
smaller organic molecules.
• A photosystem consists of a reaction-center complex
(a type of protein complex) surrounded by lightharvesting complexes
• The light-harvesting complexes / antenna complexes
(pigment molecules bound to proteins) transfer the
energy of photons to the reaction center chlorophylls
Reaction Center in Photosystems
• Transmembrane protein- pigment complex.
• This is where the light energy is converted into
chemical energy.
• Primary electron acceptor
• An excited electron is passed to an acceptor,
transferring energy away from the chlorophylls.
• Solar-powered transfer of an electron from a
chlorophyll a molecule to the primary electron
acceptor is the first step of the light reactions
• Each photosystem reaction center chlorophyll
and primary electron acceptor surrounded by
an antenna complex- functions in the
chloroplast as a light harvesting unit
Figure 10.13a
Thylakoid membrane
Photon
Photosystem
Lightharvesting
complexes
Reactioncenter
complex
STROMA
Primary
electron
acceptor
e−
Transfer
of energy
Special pair of chlorophyll a
molecules
Pigment
THYLAKOID SPACE
molecules
(INTERIOR OF THYLAKOID)
(a) How a photosystem harvests light
Figure 10.13b
Thylakoid membrane
Chlorophyll
Protein
subunits
(b) Structure of a photosystem
STROMA
THYLAKOID
SPACE
• There are two types of photosystems in
the thylakoid membrane
• Photosystem II (PS II) functions first (the
numbers reflect order of discovery) and
is best at absorbing a wavelength of 680
nm
• The reaction-center chlorophyll a of PS II
is called P680
• Photosystem I (PS I) is best at
absorbing a wavelength of 700 nm
• The reaction-center chlorophyll a
of PS I is called P700
Differences Between PS II and PS I
• Differences are not in the chlorophyll
molecules, but in the proteins associated with
each reaction center
• These two photosystems work together to use
light energy to generate ATP and NADPH
Noncyclic (Linear) Electron Flow
• During the light reactions, there are two
possible routes for electron flow: cyclic and
linear
• Linear (noncyclic) electron flow, the primary
pathway, involves both photosystems and
produces ATP and NADPH using light energy
• There are 8 steps in linear electron
flow:
1. Photosystem II absorbs a photon of light
and that photon hits a pigment and its
energy is passed among pigment
molecules until it excites P680
2. An excited electron from P680 is
transferred to the primary electron
acceptor (we now call it P680+), leaving
the reaction center oxidized
H 2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
Light
Reaction
Schematic
O2
[CH2O] (sugar)
Figure 10.14-1
Primary
acceptor
e−
1
2
Step 1
and 2
P680
Light
Pigment
molecules
Photosystem II
(PS II)
3. An enzyme extracts electrons from H2O and
supplies them to the oxidized reaction center.
This reaction splits water into two hydrogen
ions and an oxygen atom that combines with
another oxygen atom to form O2 . The electrons
are transferred from the hydrogen atoms to
P680+, thus reducing it to P680.
– P680+ is the strongest known biological
oxidizing agent
– O2 is released as a by-product of this
reaction
Figure 10.14-2
Primary
acceptor
2 H+
+
½ O2
1
H2O
e−
2
3
e−
e−
Step 3
P680
Light
Pigment
molecules
Photosystem II
(PS II)
4. Each electron “falls” down an electron
transport chain from the primary electron
acceptor of PS II to PS I
5. Energy released by the “fall” drives the
creation of a proton gradient across the
thylakoid membrane
–Diffusion of H+ (protons) across the
membrane drives ATP synthesis
Cytochrome Complex
• Connects the two photosystems via complex
systems of electron carriers
• Uses the energy from the passage of electrons
to move protons across the thylakoid
membrane to generate the proton gradient
used by an ATP synthase enzyme.
Figure
10.14-3
4
Primary
acceptor
2 H+
+
½
O2
1
H2O
e−
2
3
e−
e−
Electron
transport
chain
Pq
Cytochrome
complex
Pc
P680
5
Light
ATP
Pigment
molecules
Photosystem II
(PS II)
Step 4 & 5
6. In PS I (like PS II), transferred light
energy excites P700 reaction center,
which loses an electron to an
electron acceptor, creating an
electron “hole” in P700. This hole is
filled by an electron that reaches the
bottom of the electron transport
chain from PS II
Figure 10.14-4
4
Primary
acceptor
2 H+
+
½ O2
1
H2O
e−
2
3
e−
e−
Step 6
Electron
transport
chain
Primary
acceptor
e−
Pq
Cytochrome
complex
Pc
P680
P700
5
Light
Light
6
ATP
Pigment
molecules
Photosystem II
(PS II)
Photosystem I
(PS I)
7. Photoexcited electrons are passed from PS I’s
primary electron acceptor down a second
electron transport chain through the protein
ferredoxin (Fd)
8. The enzyme NADP+ reductase transfers
electrons from Fd to NADP+ . Two electrons are
required for NADP+ ‘s reduction to NADPH.
NADPH will carry the reducing power of these
high energy electrons to the Calvin cycle.
– This process also removes an H+ from the
stroma
Figure
10.14-5
4
Primary
acceptor
2 H+
+
½
O2
1
H2O
e−
2
3
Electron
transport
chain
e− −
e
8
Fd
e−
Pq
Cytochrome
complex
Pc
P680
Electron
transport
chain
Primary
acceptor
e−
e−
7
NADP+
reductase
NADP+
+ H+
NADPH
P700
5
Light
Light
6
ATP
Pigment
molecules
Photosystem II
(PS II)
Photosystem I
(PS I)
Step 7
&8
• The light reactions use the solar
power of photons absorbed by both
photosystem I and photosystem II to
provide chemical energy in the form
of ATP and reducing power in the
form of the electrons carried by
NADPH
Cyclic Electron Flow
• Under certain conditions, photoexcited electrons from
PS I, but not PS II, can take an alternative pathways =
cyclic electron flow
– Excited electrons cycle from their reaction center to a
primary acceptor, along an ETC, and return to the oxidized
P700 chlorophyll
– As electrons flow along the ETC, they generate ATP by cyclic
photophosphorylation.
– There is no production of NADPH and no release of oxygen
– The purpose of this flow is that it allows the chloroplast to
generate enough surplus ATP to satisfy the higher demand
for ATP in the Calvin cycle.
Figure 10.16
Primary
acceptor
Primary
acceptor
Fd
Fd
Pq
NADP+
reductase
Cytochrome
complex
NADPH
Pc
Photosystem I
Photosystem II
ATP
NADP+
+ H+
Cyclic
Electron
Flow
• Some organisms such as purple sulfur
bacteria have PS I but not PS II
• Cyclic electron flow is thought to have
evolved before linear electron flow
• Cyclic electron flow may protect cells from
light-induced damage
Cyclic versus Noncyclic
Cyclic
• Allows the chloroplast to
generate enough surplus
ATP to satisfy the higher
demand for ATP in the
Calvin cycle.
Noncyclic
• Produces ATP and NADPH in
roughly equal amounts
Chemiosmosis
• The process in which ATP is generated in the
mitochondria and chloroplast
• Defined as: Energetic electrons excited by light
(chloroplasts) or extracted by oxidation in the
Krebs cycle (mitochondria) are used to drive
proton pumps, creating a proton gradient.
When protons subsequently flow back across
the membrane, they pass through channels
that couple their movement to make ATP.
A Comparison of Chemiosmosis in
Chloroplasts and Mitochondria
• Chloroplasts and mitochondria generate ATP by
chemiosmosis, but use different sources of energy
• Mitochondria transfer chemical energy from food to
ATP; chloroplasts transform light energy into the
chemical energy of ATP
• ETC pumps protons across a membrane as electrons
are passed along a series of increasingly
electronegative carriers. This transforms redox energy
to a proton-motive force in the form of H+ gradient
across the membrane. ATP synthase molecules harness
the proton-motive force to generate ATP as H+ diffuses
back across the membrane.
• Spatial organization of chemiosmosis differs
between chloroplasts and mitochondria but also
shows similarities
• In mitochondria, protons are pumped to the
intermembrane space and drive ATP synthesis as
they diffuse back into the mitochondrial matrix
• In chloroplasts, protons are pumped into the
thylakoid space and drive ATP synthesis as they
diffuse back into the stroma
Figure 10.17
CHLOROPLAST
STRUCTURE
MITOCHONDRION
STRUCTURE
Intermembrane
space
Inner
membrane
H+
Diffusion
Electron
transport
chain
Thylakoid
space
Thylakoid
membrane
ATP
synthase
Stroma
Matrix
ADP + P i
Key
[H+]
Higher
Lower [H+]
H+
ATP
• ATP and NADPH are produced on the side
facing the stroma, where the Calvin cycle takes
place
• In summary, light reactions generate ATP and
increase the potential energy of electrons by
moving them from H2O to NADPH
Figure 10.18
Photosystem II
4 H+
Light
Cytochrome
complex
NADP+
reductase
3
Photosystem I
Light
Fd
Pq
H2O
THYLAKOID SPACE
(high H+ concentration)
e
e
NADPH
Pc
2
1 ½ O2
+2 H+
NADP+ + H+
4 H+
To
Calvin
Cycle
STROMA
(low H+ concentration)
Thylakoid
membrane
ATP
synthase
ADP
+
Pi
H+
ATP
The Two Stages of Photosynthesis: A
Preview
• Photosynthesis consists of the light reactions
(the photo part) and Calvin cycle (the synthesis
part)
• The light reactions (in the thylakoids)
– Split H2O
– Release O2
– Reduce the electron acceptor NADP+ to NADPH
– Generate ATP from ADP by photophosphorylation
• The Calvin cycle (in the stroma) forms sugar from
CO2, using ATP and NADPH
• The Calvin cycle begins with carbon fixation,
incorporating CO2 into organic molecules
Figure Light
10.6-1H O
2
NADP+
LIGHT
REACTIONS
Thylakoid
Chloroplast
ADP
+
Pi
Stroma
Figure Light
10.6-2H O
2
NADP+
LIGHT
REACTIONS
ADP
+
Pi
ATP
Thylakoid
NADPH
Chloroplast
O2
Stroma
Figure Light
10.6-3H O
CO2
2
NADP+
LIGHT
REACTIONS
ADP
+
Pi
ATP
Thylakoid
NADPH
Chloroplast
O2
CALVIN
CYCLE
Stroma
Figure Light
10.6-4H O
CO2
2
NADP+
LIGHT
REACTIONS
ADP
+
Pi
CALVIN
CYCLE
ATP
Thylakoid
Stroma
NADPH
Chloroplast
O2
[CH2O]
(sugar)
BioFlix: The Carbon Cycle
BioFlix: Photosynthesis
The light reactions convert solar
energy to the chemical energy of ATP
and NADPH
• Chloroplasts are solar-powered chemical
factories
• Their thylakoids transform light energy into the
chemical energy of ATP and NADPH
The Calvin cycle uses the chemical
energy of ATP and NADPH to reduce CO2
to
sugar
• The Calvin cycle, like the citric acid cycle, regenerates its
•
•
•
•
•
•
starting material after molecules enter and leave the cycle
The cycle builds sugar from smaller molecules by using ATP
and the reducing power of electrons carried by NADPH
Anabolic-> uses energy to build sugars from smaller
molecules
Carbon enters the cycle as CO2 and leaves as sugar (three
carbon sugar called glyceraldehyde-3-phosphate (G3P)
Each turn in the cycle fixes one carbon
For the net synthesis of one G3P molecule, the cycle must
take place three times, fixing three molecules of CO2
To make one glucose molecule requires six cycles and the
fixation of six CO2 molecules.
• The Calvin cycle has three phases
1. Carbon fixation (catalyzed by
rubisco)
2. Reduction
3. Regeneration of the CO2 acceptor
(RuBP)
Figure
H 2O
10.UN03
CO2
Light
NADP+
ADP
CALVIN
LIGHT
CYCLE
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Figure
10.19-1
Input 3 CO2, entering one per cycle
Phase 1: Carbon fixation
Rubisco
3 P
3 P
P
6
P
P
3-Phosphoglycerate
RuBP
Calvin
Cycle
Each CO2 molecule is attached to a five carbon sugar,
ribulose biphosphate (RuBP), which is catalyzed by
Rubisco (enzyme). Rubisco is the most abundant
protein in chloroplasts and possibly the most
abundant on Earth.
The six carbon intermediate is unstable and splits in
half to form two molecules of 3-phosphoglycerate for
each CO2
Figure 10.19-2
Input 3 CO2, entering one per cycle
Phase 1: Carbon fixation
Rubisco
3 P
3 P
RuBP
Each 3-phosphoglycerate receives
another phosphate group from ATP to
form 1,3 biphosphoglycerate. A pair of
electrons from NADPH reduces each
1,3 biphosphoglycerate to G3P.
Electrons reduce a carboxyl group to
the aldehyde group of G3P (stores
more potential energy). If our goal was
the net production of one G3P, we
would start with 3CO2 (3C) and 3 RuBP
(15C) . After fixation and reduction we
would have 6 molecules of G3P (18C).
One of these three 6 G3P (3C) is a net
gain of carbohydrate. This molecule
can exit the cycle and be used by the
plant cell.
P
6
P
P
3-Phosphoglycerate
6
ATP
6 ADP
Calvin
Cycle
6 P
P
1,3-Bisphosphoglycerate
6 NADPH
6 NADP+
6 P
6
P
G3P
1
P
G3P
Output
i
Phase 2:
Reduction
Glucose and
other organic
compounds
Regeneration: The other five G3P
(15C) remain in the cycle to
regenerate RuBP. In a complex
series of reactions, the carbon
skeletons of five molecules of
G3P are rearranged by the last
steps of the Calvin cycle to
regenerate 3RuBP
Input 3 CO2, entering one per cycle
Phase 1: Carbon fixation
Rubisco
3 P
3 P
P
6
P
P
3-Phosphoglycerate
RuBP
6
ATP
6 ADP
Calvin
Cycle
3 ADP
3
ATP
6 P
P
1,3-Bisphosphoglycerate
6 NADPH
Phase 3:
Regeneration
of RuBP
6 NADP+
6 P
i
P
5
G3P
For the net synthesis of one G3P
molecule, the Calvin cycle consumes
nine ATP and six NADPH. The light
reactions regenerate ATP and NADPH.
The G3P from the Calvin cycle is the
starting material for metabolic
pathways that synthesize other organic
compounds, including glucose and
other carbs.
6
P
G3P
1
P
G3P
Output
Phase 2:
Reduction
Glucose and
other organic
compounds
Alternative mechanisms of carbon
fixation have evolved in hot, arid
climates
• Dehydration is a problem for plants, sometimes
requiring trade-offs with other metabolic
processes, especially photosynthesis
• On hot, dry days, plants close stomata, which
conserves H2O but also limits photosynthesis
• The closing of stomata reduces access to CO2 and
causes O2 to build up
• These conditions favor an apparently wasteful
process called photorespiration.
Photorespiration: An Evolutionary Relic?
• In most plants (C3 plants), initial fixation of CO2, via
rubisco, forms a three-carbon compound
(3-phosphoglycerate)
• In photorespiration, rubisco adds O2 instead of CO2
in the Calvin cycle, producing a two-carbon
compound = photorespiration
– The two carbon fragment is exported from the
chloroplast and degraded to CO2 by the mitochondria
and peroxisomes
• Photorespiration consumes O2 and organic fuel and
releases CO2 without producing ATP or sugar, but it
actually consumes ATP
• Photorespiration may be an evolutionary relic
because rubisco first evolved at a time when the
atmosphere had far less O2 and more CO2
• Photorespiration limits damaging products of light
reactions that build up in the absence of the Calvin
cycle
• In many plants, photorespiration is a problem
because on a hot, dry day it can drain as much as
50% of the carbon fixed by the Calvin cycle. Certain
plant species have evolved alternate modes of
carbon fixation to minimize photorespiration.
C4 Plants
• C4 plants minimize the cost of photorespiration by
incorporating CO2 into four-carbon compounds.
• Ex- sugarcane and corn
• There are two distinct types of cells in the leaves of
C4 plants:
– Bundle-sheath cells are arranged in tightly packed
sheaths around the veins of the leaf
– Mesophyll cells are loosely packed between the bundle
sheath and the leaf surface
Sugar production in C4 plants
occurs in a three-step process:
1. The production of the four carbon precursors is
catalyzed by the key enzyme PEP carboxylase in the
mesophyll cells
1.
PEP carboxylase has a higher affinity for CO2 than rubisco does (i.e., on
hot, dry days when the stomata are closed); it can fix CO2 even when CO2
concentrations are low
2. These four-carbon compounds are exported to bundle-sheath
cells
2.
3.
The bundle sheath cells strip a carbon from the four carbon compound
as CO2, and return the three carbon remainder to the mesophyll cells.
The bundle sheath cells then use rubisco to start the Calvin cycle with an
abundant supply of CO2
3. In effect, the mesophyll cells pump CO2 into the bundle sheath
cells, keeping CO2 levels high enough for rubisco to accept CO2
and not O2
C4 Photosynthesis
• Minimizes photorespiration and enhances sugar
production
• Thrive in hot regions with intense sunlight
• Since the Industrial Revolution in the 1800s, CO2
levels have risen greatly
• Increasing levels of CO2 may affect C3 and C4
plants differently, perhaps changing the relative
abundance of these species
• The effects of such changes are unpredictable
and a cause for concern
C4 leaf anatomy
Photosynthetic
cells of
C4 plant
leaf
Mesophyll
cell
The C4 pathway
Mesophyll
cell PEP carboxylase
Bundlesheath
cell
Oxaloacetate (4C)
Vein
(vascular
tissue)
PEP (3C)
ADP
Malate (4C)
Stoma
ATP
Pyruvate
CO2 (3C)
Bundlesheath
Calvin
cell
Cycle
Sugar
Vascular
tissue
Figure 10.20
CO2
CAM Plants
• Some plants, including succulents, use crassulacean acid
metabolism (CAM) to
fix carbon
• Ex- cacti and pineapple
• CAM plants open their stomata at night, incorporating CO2
into organic acids
– Temperatures are typically lower at night and close them during
the day.
– During the night, these plants fix CO2 into a variety of organic
acids in mesophyll cells
• Stomata close during the day, and CO2 is released from
organic acids and used in the Calvin cycle
– Light reactions supply ATP and NADPH to the Calvin cycle, and CO2
is released from the organic acids
Sugarcane
Pineapple
C4
Mesophyll
cell
CO2
Organic
acid
CO2 2
Bundlesheath
cell
1
CO2
Organic
acid
CO2
Calvin
Cycle
Calvin
Cycle
Sugar
Sugar
(a) Spatial separation
of steps
1
CAM
Night
2
Day
(b) Temporal separation
of steps
The Importance of Photosynthesis: A Review
• The energy entering chloroplasts as sunlight
gets stored as chemical energy in organic
compounds
• Sugar made in the chloroplasts supplies
chemical energy and carbon skeletons to
synthesize the organic molecules of cells
• Plants store excess sugar as starch in structures
such as roots, tubers, seeds, and fruits
• In addition to food production, photosynthesis
produces the O2 in our atmosphere
O2
CO2
Figure 10.22a
H2O
Sucrose
(export)
Mesophyll cell
H2O
Chloroplast
CO2
Light
NADP+
LIGHT
REACTIONS:
Photosystem II
Electron transport
chain
Photosystem I
Electron transport
chain
H2O
O2
ADP
+
P i
ATP
NADPH
3-Phosphoglycerate
RuBP CALVIN
CYCLE
G3P
Starch
(storage)
Sucrose (export)
Figure 10.22b
LIGHT REACTIONS
• Are carried out by molecules
in the thylakoid membranes
• Convert light energy to the
chemical energy of ATP
and NADPH
• Split H2O and release O2
to the atmosphere
CALVIN CYCLE REACTIONS
• Take place in the stroma
• Use ATP and NADPH to convert
CO2 to the sugar G3P
• Return ADP, inorganic phosphate,
and NADP+ to the light reactions
Figure
10.UN06
3 CO2
Carbon fixation
3 x 5C
6 x 3C
Calvin
Cycle
Regeneration of
CO2 acceptor
5 x 3C
Reduction
1 G3P (3C)
CAMPBELL
BIOLOGY
TENTH
EDITION
Reece • Urry • Cain • Wasserman • Minorsky • Jackson
10
Photosynthesis
Lecture Presentation by
Nicole Tunbridge and
Kathleen Fitzpatrick
© 2014 Pearson Education, Inc.