Autotrophs Are the Producers of The Biosphere
 Autotrophs make their own food without using
organic molecules derived from any other living
thing
– Photoautotrophs : Autotrophs that use the energy of
light to produce organic molecules
– Most plants, algae and other protists, and some prokaryotes
are photoautotrophs
– The ability to photosynthesize is directly related to
the structure of chloroplasts
Copyright © 2009 Pearson Education, Inc.
Chloroplast
Outer and inner
membranes
Thylakoid
Stroma Granum
Thylakoid
space
Intermembrane
space
 Photosynthesis: a process that converts solar energy
to chemical energy
 Plants use water and atmospheric carbon dioxide to
produce a simple sugar and release oxygen
Light
energy
6 CO2 + 6
H2O
Carbon dioxide Water
C6H12O6
Photosynthesis
+ 6
O2
Glucose Oxygen gas
Photosynthesis Occurs in Chloroplasts in Plant Cells
 Leaves contain:
 Stomata = tiny pores in the
leaf; allow CO2 to enter and
O2 to exit
 Veins deliver water &
nutrients absorbed by roots
 Chlorophyll= a light
absorbing pigment
 responsible for the green
color of plants
 located on thylakoid
membrane
Vein
CO2 O2
Chloroplasts
Stoma
Photosynthesis is a Redox Process, as is Cellular
Respiration
 Photosynthesis is a redox (oxidation-reduction)
process
 A loss of electrons = oxidation
 A gain of electrons = reduction *(OIL RIG)
 In photosynthesis: Water is oxidized producing
oxygen and CO2 is reduced producing sugar
Photosynthesis is a Redox Process, as is Cellular
Respiration
Reduction
6 CO2 + 6 H2O
C6H12O6 + 6 O2
Oxidation
CO2
H2O
Chloroplast
Light
NADP+
ADP
P
LIGHT
REACTIONS
CALVIN
CYCLE
(in stroma)
(in thylakoids)
ATP
NADPH
O2
Sugar
Visible Radiation Drives the Light Reactions
 Sunlight is a type of electromagnetic energy (radiation)
 Visible light is a small part of the electromagnetic spectrum
 Light exhibits the properties of both waves and particles
– One wavelength = distance between the crests of two
adjacent waves (shorter λ’s have greater energy)
– Light behaves as discrete packets of energy called
photons (photons contain a fixed quantity of light
energy)
Increasing energy
10–5 nm 10–3 nm
Gamma
rays
X-rays
103 nm
1 nm
UV
106 nm
Infrared
103 m
1m
Microwaves
Radio
waves
Visible light
380 400
600
500
Wavelength (nm)
700
650
nm
750
Visible Radiation Drives the Light Reactions
 Pigments are proteins that
absorb specific wavelengths
of light and transmit others
 Various pigments are built
into the thylakoid membrane
 We see the color of the
wavelengths that are
transmitted (not absorbed)
– Ex. chlorophyll transmits green
Visible Radiation Drives the Light Reactions
 Chloroplasts contain several different pigments
and all absorb light of different wavelengths
– Chlorophyll a: absorbs blue violet and red light and
reflects green
– Chlorophyll b: absorbs blue and orange and reflects
yellow-green
– The carotenoids: absorb mainly blue-green light and
reflect yellow and orange
Photosystems Capture Solar Power
 Solar energy can be released
as heat or light but instead is
conserved and passed from
one pigment to another
 All of the components to
accomplish this are organized
in thylakoid membranes in
clusters called photosystems
 A Photosystem consists of
a light-harvesting complex
surrounding a reaction center
complex
Photosystem
Light-harvesting
complexes
Reaction
center complex
e–
Pigment
molecules
Pair of
Chlorophyll a molecules
Photosystems Capture Solar Power in the Light
Reactions
 Two types of photosystems exist: photosystem I
and photosystem II
– Each type of photosystem has a characteristic reaction
center
– Photosystem II: functions first; is called P680 (chl a best
absorbs light w/ a wavelength of 680, red)
– Photosystem I: functions next; is called P700 (chl a absorbs
wavelength of 700, also red)
Photosystems Capture Solar Power in the Light
Reactions
 When light strikes the photosystem energy
Electron transport chain
NADPH
NADP + H
Photon
energy from
for
is Provides
passed
pigment to pigment within
synthesis of
6
Photosystem I
theby chemiosmosis
photosystem
+
Photon
Photosystem II
Stroma
+
ATP
1
Primary
acceptor
2 e–
Thylakoid
membrane
– Energy from the pigments
is passed to chl. a in
Primary
acceptor it excites electrons
the reaction center where
e
of chl. a
–
4
P680
Thylakoid
space
3
H2O
1

2
5
– Excited e- from chl. a are transferred to the
P700
primary electron acceptor
(reducing it)
O2 + 2 H+
– This solar-powered transfer of an electron from
the reaction center chl. a to the primary
electron acceptor is the first step of the light
reactions
The Light Reactions
– The primary e- acceptor passes electrons to an
electron transport chain (ETC)
– To fill the electron void that chl. a (P680) now has,
chl. a (P680) oxidizes H2O and takes electrons from
water
– This is the step where water is oxidized and oxygen
released
– The ETC is a bridge between photosystems II and I.
The ETC also generates ATP.
Photon
Photosystem II
Stroma
Electron transport chain
Provides energy for
synthesis of ATP
by chemiosmosis
NADP+ + H+
Photon
Photosystem I
1
Primary
acceptor
Primary
acceptor
2 e–
Thylakoid
membrane
e–
4
P700
P680
Thylakoid
space
3
H2O
1

2
5
O2 + 2 H+
6
NADPH
The Light Reactions
 As the first protein of the ETC accepts electrons it
pumps H+ into the thylakoid space. This generates
a proton (H+) gradient.
 H+ are concentrated in the thylakoid space and less
concentrated in the stroma
 Protons flow down their gradient through an
enzyme called ATP synthase.
 As H+ flow through the ATP synthase it
phosphorylates ADP forming ATP
 The phosphorylation of ADP is endergonic; the
flow of protons down their gradient is exergonic.
Chemiosmosis Powers ATP Synthesis in the Light
Reactions
 Chemiosmosis is a mechanism where the cell
couples exergonic and endergonic reactions
 Ex. An ATP synthase couples the flow of H+ down their
gradient (exergonic) to the phosphorylation of ADP
(endergonic)
 The chemiosmotic production of ATP in photosynthesis is
called photophosphorylation
 This step produces ATP in the stroma
Two Photosystems Connected by an Electron
Transport Chain Generate ATP and NADPH
– Electrons moving down the ETC are passed to
P700 of Photosystem I, and ultimately to NADP+
by the enzyme NADP+ reductase
– This step produces NADPH in the stroma
Stroma (low H+
concentration)
Light
H+
Light
H+
ADP + P
NADP+ + H+
H+
ATP
NADPH
H+
H2O
1

2
H+
O2 + 2 H+
Photosystem II
Electron
transport
chain
Thylakoid space
(high H+ concentration)
H+
H+
H+
H+
H+
Photosystem I
H+
H+
H+
H+
ATP synthase
THE CALVIN CYCLE:
CONVERTING CO2 TO SUGARS
ATP and NADPH Power Sugar Synthesis in the
Calvin Cycle
 The Calvin cycle makes sugar
within a chloroplast
 Atmospheric CO2, ATP, and NADPH
are required to produce sugar
 Using these three ingredients, a
three-carbon sugar called
glyceraldehyde-3-phosphate (G3P)
is produced
 A plant cell may then use G3P to
make glucose and other organic
molecules
CO2
Input ATP
NADPH
CALVIN
CYCLE
Output:
G3P
ATP and NADPH Power Sugar Synthesis in the
Calvin Cycle
 The Calvin Cycle Has Three Phases:
 1. Carbon Fixation-Atmospheric carbon in the form of
CO2 is incorporated into a molecule of ribulose
bisphosphate (RuBP) via rubisco
 2. Reduction Phase – NADPH reduces 3PGA to G3P
(requires 3 molec’s. of CO2 for 1 G3P)
 3. Regeneration of Starting Material –RuBP is
regenerated and the cycle starts again
Copyright © 2009 Pearson Education, Inc.
Step 1 Carbon fixation
Input:
3
CO2
Rubisco
1
Step 2 Reduction
P
3 P
P
6
RuBP
3-PGA
6
3 ADP
ATP
6 ADP + P
Step 3 Release of one
molecule of G3P
3
ATP
CALVIN
4
2
CYCLE
6 NADPH
6 NADP+
Step 4 Regeneration of RuBP
P
5
P
6
G3P
G3P
3
Output: 1
P
G3P
Glucose
and other
compounds
PHOTOSYNTHESIS REVIEWED
AND EXTENDED
Copyright © 2009 Pearson Education, Inc.
H2O
CO2
Chloroplast
Light
NADP+
ADP
+ P
Photosystem II
Thylakoid
membranes
RuBP
CALVIN
CYCLE 3-PGA
(in stroma)
Electron
transport
chains
Photosystem I
ATP
NADPH
Stroma
G3P
O2
Sugars
LIGHT REACTIONS
CALVIN CYCLE
Cellular
respiration
Cellulose
Starch
Other organic
compounds
EVOLUTION CONNECTION: Adaptations that
save water in hot, dry climates evolved in C4
and CAM plants
 In hot climates, plant stomata close to reduce
water loss so oxygen builds up
– Rubisco adds oxygen instead of carbon dioxide to RuBP
in a process called photorespiration
– Photorespiration consumes oxygen, produces CO2, and
produces no sugar, or ATP
Copyright © 2009 Pearson Education, Inc.
EVOLUTION CONNECTION: Adaptations that
save water in hot, dry climates evolved in C4
and CAM plants
 Some plants have evolved a means of carbon
fixation that saves water during
photosynthesis
– C4 plants partially shut stomata when hot and dry
to conserve water
– This reduces CO2 levels, so contain PEP
carboxylase to bind CO2 at low levels (carbon
fixation); They’re called C4 plants because they
first fix CO2 into a four-carbon compound.
– This allows plant to still make sugar by
photosynthesis
Copyright © 2009 Pearson Education, Inc.
EVOLUTION CONNECTION: Adaptations that
save water in hot, dry climates evolved in C4
and CAM plants
 Another adaptation to hot and dry
environments has evolved
– CAM open their stomata at night thus
admitting CO2 in w/o loss of H2O
– CO2 enters, and is fixed into a fourcarbon compound, (carbon fixation)
– It is released into the Calvin cycle
during the day
Copyright © 2009 Pearson Education, Inc.
CO2
Mesophyll
cell
CO2
4-C compound
4-C compound
CO2
CO2
CALVIN
CYCLE
CALVIN
CYCLE
Bundlesheath
cell 3-C sugar
3-C sugar
C4 plant
CAM plant
Night
Day
e–
Excited state
Heat
Photon
Photon
(fluorescence)
Ground state
Chlorophyll
molecule