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Photosynthesis
Chapter 10
Overview: The Process
That Feeds the Biosphere
 Photosynthesis is the process that
converts light energy into chemical
energy
 Directly or indirectly, photosynthesis
nourishes almost the entire living world
Photosynthesis
 Autotrophs: Organisms that produce
their own organic substances from
inorganic substances using energy from
the atmosphere
 Plants, algae, and some bacteria are
photoautotrophs
Photoautotrophs
 The first photoautotrophs were the
cyanobacteria
 These organisms were responsible for
oxygenating the Earth’s atmosphere
 Utilize sunlight for energy (sunlight is
converted to ATP to fuel the process of
making sugars)
Photoautotrophs
 Pigments, such as chlorophyll, absorb light
energy
 Photon = A measurable unit of light
 Photosynthetic pigments include chlorophylls
(a, b, c), carotenoids (orange and red) and
xanthophylls (gold and yellow)
Chloroplasts
 Chloroplasts are the
sites of photosynthesis
in plants and most algae
 Chloroplasts are found in
the mesophyll (ground)
tissue of leaves
 A typical mesophyll cell
has 30-40 chloroplasts
Chloroplast Structure
 Chloroplasts have a double membrane (inner
and outer)
 A space filled with fluid lies between the two
membranes
 Inside of the chloroplasts are thylakoids,
arranged in stacks called grana, which are
surrounded by a clear, thick fluid called stroma
Leaf Structure
 The petiole is the attachment point of the leaf
to the plant
 The blade is the “leafy” portion
 Leaves, like all parts of the plant body, consist
of three major tissue types: epidermis, ground
(middle) tissue and vascular (transport) tissue
Leaf Structure
 Openings on the underside of the leaf,
called stomata, allow for the passage of
gasses into and out of the leaf
 Carbon dioxide enters the leaf
 Oxygen, produced by cells in the leaf,
exits
Where does the oxygen
come from?
 Carbon dioxide is absorbed by plants, but
the oxygen that is released comes from
the splitting of water
 6 CO2 + 12 H2O + Light Energy 
C6H12O6 + 6 H2O + 6 O2
A few other things to
know...
 Photosynthesis has two major phases:
the Light Reactions (the photo part) and
the Calvin Cycle (the synthesis part)
 The light reactions (in the thylakoids) split
water, release O2, produce ATP, and form
NADPH
The Two Stages of Photosynthesis:
A Preview
 The Calvin cycle (in the stroma) forms sugar
from CO2, using ATP and NADPH
 Instead of NAD+, a substance called
NADP+ accepts electrons in photosynthesis
(hey, think of it as P for Photosynthesis!)
A few other things to
know...
 Photophosphorylation: Adding a
phosphate group to a molecule using
energy from sunlight.
 Carbon fixation: Incorporation of carbon
from CO2 in the air into organic
molecules such as sugar.
Sunlight
 Light can be measured in packets called
photons
 Photosynthetic pigments, such as
chlorophyll, absorb parts of the visible
light spectrum more readily than others.
Visible Light
 Energy decreases from violet to red,
while wavelength increases along the
same path
 This means that energy is inversely
proportional to wavelength
Reflected Light
 The portion of the visible light spectrum
that is reflected is the color that the
object appears
 Chlorophyll is green since it absorbs
other colors and reflects green
The Light Reactions
 Begin with the absorption of light energy
by thylakoids, where light energy is
transformed into the chemical energy of
ATP and NADPH
 This is called the excitation of chlorophyll
by light energy
CH3
CHO
in chlorophyll a
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
H2O
Light
LIGHT
REACTIONS
Chloroplast
H2O
Light
LIGHT
REACTIONS
ATP
NADPH
Chloroplast
O2
H2O
CO2
Light
NADP+
ADP
+ Pi
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH
Chloroplast
O2
[CH2O]
(sugar)
Photosystems
 A photosystem consists of a reaction
center surrounded by light-harvesting
complexes
 The light-harvesting complexes (pigment
molecules bound to proteins) funnel the
energy of photons to the reaction center
Photosystems
• A primary electron acceptor in the
reaction center accepts an excited
electron from chlorophyll a
 Solar-powered transfer of an electron
from a chlorophyll a molecule to the
primary electron acceptor is the first step
of the light reactions
Thylakoid
Photosystem
Photon
Thylakoid membrane
Light-harvesting
complexes
Reaction
center
STROMA
Primary electron
acceptor
e–
Transfer
of energy
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
Photosystems
• Photosystem II functions first and is best at
absorbing a wavelength of 680 nm
• Photosystem I is best at absorbing a
wavelength of 700 nm
• The two photosystems work together to use
light energy to generate ATP and NADPH
Noncyclic Electron Flow
 During the light reactions, there are two
possible routes for electron flow: cyclic and
noncyclic
 Noncyclic electron flow, the primary pathway,
involves both photosystems and produces ATP
and NADPH
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Energy of electrons
e–
Light
P680
Photosystem II
(PS II)
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Energy of electrons
Primary
acceptor
2
H+
1/ 2
+
O2
Light
H2O
e–
e–
e–
P680
Photosystem II
(PS II)
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Energy of electrons
Pq
2 H+
+
1/ 2 O 2
Light
H2O
e–
Cytochrome
complex
Pc
e–
e–
P680
ATP
Photosystem II
(PS II)
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
e–
Energy of electrons
Pq
2
H+
1/ 2
+
O2
Light
H2O
e–
Cytochrome
complex
Pc
e–
e–
P700
P680
Light
ATP
Photosystem II
(PS II)
Photosystem I
(PS I)
H2 O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
e–
Pq
Energy of electrons
2
H+
e–
H2O
Cytochrome
complex
+
1/2 O2
Light
Fd
e–
e–
NADP+
reductase
Pc
e–
e–
NADPH
+ H+
P700
P680
Light
ATP
Photosystem II
(PS II)
NADP+
+ 2 H+
Photosystem I
(PS I)
e–
ATP
e–
e–
NADPH
e–
e–
e–
Mill
makes
ATP
e–
Photosystem II
Photosystem I
Cyclic Electron Flow
• Cyclic electron flow uses only
photosystem I and produces only ATP
 Cyclic electron flow generates surplus ATP,
satisfying the demand in the Calvin cycle
Primary
acceptor
Primary
acceptor
Fd
Fd
NADP+
Pq
NADP+
reductase
Cytochrome
complex
NADPH
Pc
Photosystem I
Photosystem II
ATP
The Calvin Cycle
 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 NADPH
The Calvin Cycle
 Carbon enters the cycle as CO2 and
leaves as a sugar named
glyceraldehyde-3-phosphate (G3P)
 For net synthesis of one G3P, the cycle
must take place three times, fixing three
molecules of CO2
The Calvin Cycle
The Calvin cycle has three
phases:
 Carbon fixation (catalyzed by
Rubisco)
 Reduction
 Regeneration of the CO2 acceptor
(RuBP)
H2 O
CO2
Input
Light
(Entering one
CO2 at a time)
3
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
Phase 1: Carbon fixation
NADPH
Rubisco
O2
[CH2O] (sugar)
3 P
Short-lived
intermediate
P
P
6
3-Phosphoglycerate
3 P
P
Ribulose bisphosphate
(RuBP)
6
6 ADP
CALVIN
CYCLE
ATP
H2O
CO2
Input
Light
(Entering one
CO2 at a time)
3
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
Phase 1: Carbon fixation
NADPH
Rubisco
O2
[CH2O] (sugar)
3 P
P
Short-lived
intermediate
3 P
P
6
P
3-Phosphoglycerate
Ribulose bisphosphate
(RuBP)
6
ATP
6 ADP
CALVIN
CYCLE
6 P
P
1,3-Bisphosphoglycerate
6 NADPH
6 NADP+
6 Pi
6
P
Glyceraldehyde-3-phosphate
(G3P)
1
P
G3P
(a sugar)
Output
Glucose and
other organic
compounds
Phase 2:
Reduction
H2O
CO2
Input
Light
(Entering one
CO2 at a time)
3
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
Phase 1: Carbon fixation
NADPH
Rubisco
O2
[CH2O] (sugar)
3 P
P
Short-lived
intermediate
3 P
P
6
P
3-Phosphoglycerate
Ribulose bisphosphate
(RuBP)
6
ATP
6 ADP
3 ADP
3
CALVIN
CYCLE
6 P
ATP
P
1,3-Bisphosphoglycerate
6 NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADP+
6 Pi
P
5
G3P
6
P
Glyceraldehyde-3-phosphate
(G3P)
1
P
G3P
(a sugar)
Output
Glucose and
other organic
compounds
Phase 2:
Reduction
 Calvin cycle animation:
http://www.science.smith.edu/departments/
Biology/Bio231/calvin.html
Alternative mechanisms of
carbon fixation
 Dehydration is a problem for plants,
sometimes requiring tradeoffs with other
metabolic processes, especially
photosynthesis
 On hot, dry days, plants close stomata,
which conserves water but also limits
photosynthesis
Alternative mechanisms of
carbon fixation
 The closing of stomata reduces access to
CO2 and causes O2 to build up
 These conditions favor a seemingly
wasteful process called
photorespiration
Photorespiration: An
Evolutionary Relic?
• In most plants (C3 plants), initial fixation
of CO2, via the enzyme rubisco, forms a
three-carbon compound
• In photorespiration, rubisco adds O2 to
the Calvin cycle instead of CO2
Photorespiration: An
Evolutionary Relic?
 Photorespiration consumes O2 and
organic fuel and releases CO2 without
producing ATP or sugar
• Photorespiration may be an evolutionary
relic because rubisco first evolved at a
time when the atmosphere had far less
O2 and more CO2
Photorespiration: An
Evolutionary Relic?
 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
C4 Plants
 C4 plants minimize the cost of
photorespiration by incorporating CO2 into
four-carbon compounds in mesophyll cells
 These four-carbon compounds are exported
to bundle-sheath cells, where they release
CO2 that is then used in the Calvin cycle
LE 10-19
Photosynthetic
cells of C4 plant
leaf
Mesophyll
cell
PEP carboxylase
Mesophyll cell
CO2
Bundlesheath
cell
The C4 pathway
Oxaloacetate (4 C) PEP (3 C)
Vein
(vascular tissue)
ADP
Malate (4 C)
ATP
C4 leaf anatomy
Stoma
Bundlesheath
cell
Pyruvate (3 C)
CO2
CALVIN
CYCLE
Sugar
Vascular
tissue
CAM Plants
 CAM plants open their stomata at night,
incorporating CO2 into organic acids
 Stomata close during the day, and CO2 is
released from organic acids and used in
the Calvin cycle
Sugarcane
Pineapple
CAM
C4
CO2
Mesophyll
cell
Organic acid
Bundlesheath
cell
CO2
CO2 incorporated
into four-carbon Organic acid
organic acids
(carbon fixation)
CO2
CALVIN
CYCLE
Sugar
Spatial separation of steps
CO2
Organic acids
release CO2 to
Calvin cycle
Night
Day
CALVIN
CYCLE
Sugar
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
The Importance of
Photosynthesis: A Review
 In addition to food production,
photosynthesis produces the oxygen in
our atmosphere
Light reactions
Calvin cycle
H2O
CO2
Light
NADP+
ADP
+ Pi
RuBP
Photosystem II
Electron transport
chain
Photosystem I
ATP
NADPH
3-Phosphoglycerate
G3P
Starch
(storage)
Amino acids
Fatty acids
Chloroplast
O2
Sucrose (export)