Chapter 8: Photosynthesis: Energy from the Sun
CHAPTER 8
Photosynthesis: Energy
from the Sun
Chapter 8: Photosynthesis: Energy from the Sun
Chapter 8: Photosynthesis:
Energy from the Sun
Photosynthesis
Identifying Photosynthetic Reactants and
Products
The Two Pathways of Photosynthesis: An
Overview
Properties of Light and Pigments
Chapter 8: Photosynthesis: Energy from the Sun
Chapter 8: Photosynthesis:
Energy from the Sun
Light Reactions: Light Absorption
Making Sugar from CO2: The Calvin–Benson
Cycle
Photorespiration and Its Evolutionary
Consequences
Metabolic Pathways in Plants
Chapter 8: Photosynthesis: Energy from the Sun
Photosynthesis
• Life on Earth depends on the absorption of
light energy from the sun.
4
Chapter 8: Photosynthesis: Energy from the Sun
Photosynthesis
• In plants, photosynthesis takes place in
chloroplasts.
5
Chapter 8: Photosynthesis: Energy from the Sun
Identifying Photosynthetic
Reactants and Products
• Photosynthesizing plants take in CO2, water,
and light energy, producing O2 and
carbohydrate. The overall reaction is
6 CO2 + 12 H2O + light  C6H12O6 + 6 O2 + 6
H2O
The oxygen atoms in O2 come from water, not
from CO2. Review Figures 8.1, 8.2
6
Chapter 8: Photosynthesis: Energy from the Sun
Figure 8.1
Figure 8.1
figure 08-01.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Figure 8.2
Figure 8.2
figure 08-02.jpg
Chapter 8: Photosynthesis: Energy from the Sun
The Two Pathways of
Photosynthesis: An Overview
• In the light reactions of photosynthesis,
electron flow and photophosphorylation
produce ATP and reduce NADP+ to NADPH
+ H+.
Review Figure 8.3
9
Chapter 8: Photosynthesis: Energy from the Sun
Figure 8.3
Figure 8.3
figure 08-03.jpg
Chapter 8: Photosynthesis: Energy from the Sun
The Two Pathways of
Photosynthesis: An Overview
• ATP and NADPH + H+ are needed for the
reactions that fix and reduce CO2 in the
Calvin–Benson cycle, forming sugars.
Review Figure 8.3
11
Chapter 8: Photosynthesis: Energy from the Sun
Figure 8.3
Figure 8.3
figure 08-03.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Properties of Light and
Pigments
• Light energy comes in packets called
photons, but it also has wavelike properties.
Review Figure 8.4
12
Chapter 8: Photosynthesis: Energy from the Sun
Figure 8.4
Figure 8.4
figure 08-04.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Properties of Light and
Pigments
• Pigments absorb light in the visible
spectrum.
Review Figure 8.5
14
Chapter 8: Photosynthesis: Energy from the Sun
Figure 8.5
Figure 8.5
figure 08-05.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Properties of Light and
Pigments
• Absorption of a photon puts a pigment
molecule in an excited state with more
energy than its ground state.
Review Figure 8.6
16
Chapter 8: Photosynthesis: Energy from the Sun
Figure 8.6
Figure 8.6
figure 08-06.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Properties of Light and
Pigments
• Each compound has a characteristic
absorption spectrum which reveals the
biological effectiveness of different
wavelengths of light.
Review Figures 8.7, 8.8
18
Chapter 8: Photosynthesis: Energy from the Sun
Figure 8.7
Figure 8.7
figure 08-07.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Figure
8.8
Figure 8.8
figure 08-08.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Properties of Light and
Pigments
• Chlorophylls and accessory pigments form
antenna systems for absorption of light
energy.
Review Figures 8.7, 8.9, 8.11
21
Chapter 8: Photosynthesis: Energy from the Sun
Figure
8.9
Figure 8.9
figure 08-09.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Light Reactions: Light
Absorption
• An excited pigment molecule may lose its
energy by fluorescence, or by transferring it
to another pigment molecule.
Review Figures 8.10, 8.11
24
Chapter 8: Photosynthesis: Energy from the Sun
Figure
8.10
Figure 8.10
figure 08-10.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Figure 8.11
Figure 8.11
figure 08-11.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Electron Flow, Photophosphorylation, and Reductions
• Noncyclic electron flow uses two photosystems:
• Photosystem II uses P680 chlorophyll, from which
light-excited electrons pass to a redox chain that
drives chemiosmotic ATP production. Light-driven
water oxidation releases O2, passing electrons to
P680 chlorophyll.
• Photosystem I passes electrons from P700
chlorophyll to another redox chain and then to
NADP+, forming NADPH + H+. Review Figure 8.12
26
Chapter 8: Photosynthesis: Energy from the Sun
Figure
8.12 – Part
1
Figure 8.12 – Part 1
figure 08-12a.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Figure
8.12 – Part
2
Figure 8.12 – Part 2
figure 08-12b.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Electron Flow, Photophosphorylation, and Reductions
• Cyclic electron flow uses P700 chlorophyll
producing only ATP.
• Its operation maintains the proper balance
of ATP and NADPH + H+ in the chloroplast.
Review Figure 8.13
29
Chapter 8: Photosynthesis: Energy from the Sun
Figure 8.13
Figure 8.13
figure 08-13.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Electron Flow, Photophosphorylation, and Reductions
• Chemiosmosis is the source of ATP in
photophosphorylation.
• Electron transport pumps protons from stroma into
thylakoids, establishing a proton-motive force.
• Proton diffusion to stroma via ATP synthase
channels drives ATP formation from ADP and Pi.
Review Figure 8.14
31
Chapter 8: Photosynthesis: Energy from the Sun
Figure 8.14
Figure 8.14
figure 08-14.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Electron Flow, Photophosphorylation, and Reductions
• Photosynthesis probably originated in
anaerobic bacteria that used H2S as a
source of electrons instead of H2O.
• Oxygen production by bacteria was
important in eukaryote evolution.
33
Chapter 8: Photosynthesis: Energy from the Sun
Light-Dependent Reactions
Chapter 8: Photosynthesis: Energy from the Sun

Photosynthesis begins when pigments in
photosystem II absorb light, increasing
their energy level.
Photosystem II
Chapter 8: Photosynthesis: Energy from the Sun

These high-energy electrons are passed
on to the electron transport chain.
Photosystem II
High-energy
electron
Electron
carriers
Chapter 8: Photosynthesis: Energy from the Sun

Enzymes on the thylakoid membrane
break water molecules into:
Photosystem II
2H2O
High-energy
electron
Electron
carriers
Chapter 8: Photosynthesis: Energy from the Sun
hydrogen ions
oxygen atoms
energized electrons
Photosystem II
+
O2
2H2O
High-energy
electron
Electron
carriers
Chapter 8: Photosynthesis: Energy from the Sun

The energized electrons from water
replace the high-energy electrons that
chlorophyll lost to the electron transport
chain.
Photosystem II
+
2H2O
High-energy
electron
O2
Chapter 8: Photosynthesis: Energy from the Sun

As plants remove electrons from water,
oxygen is left behind and is released
into the air.
Photosystem II
+
2H2O
High-energy
electron
O2
Chapter 8: Photosynthesis: Energy from the Sun

The hydrogen ions left behind when
water is broken apart are released inside
the thylakoid membrane.
Photosystem II
+
2H2O
High-energy
electron
O2
Chapter 8: Photosynthesis: Energy from the Sun

Energy from the electrons is used to
transport H+ ions from the stroma into
the inner thylakoid space.
Photosystem II
+
2H2O
O2
Chapter 8: Photosynthesis: Energy from the Sun

High-energy electrons move through
the electron transport chain from
photosystem II to photosystem I.
Photosystem II
+
O2
2H2O
Photosystem I
Chapter 8: Photosynthesis: Energy from the Sun

Pigments in photosystem I use
energy from light to re-energize the
electrons.
+
O2
2H2O
Photosystem I
Chapter 8: Photosynthesis: Energy from the Sun

NADP+ then picks up these highenergy electrons, along with H+ ions,
and becomes NADPH.
•
+
O2
2H2O
2 NADP+
2
2
NADPH
Chapter 8: Photosynthesis: Energy from the Sun

As electrons are passed from
chlorophyll to NADP+, more H+ ions
are pumped across the membrane.
+
O2
2H2O
2 NADP+
2
2
NADPH
Chapter 8: Photosynthesis: Energy from the Sun

Soon, the inside of the membrane fills
up with positively charged hydrogen
ions, which makes the outside of the
membrane negatively charged.
+
O2
2H2O
2 NADP+
2
2
NADPH
Chapter 8: Photosynthesis: Energy from the Sun

The difference in charges across the
membrane provides the energy to make
ATP
+
O2
2H2O
2 NADP+
2
2
NADPH
Chapter 8: Photosynthesis: Energy from the Sun
H+ ions cannot cross the membrane
directly.

ATP synthase
+
O2
2H2O
2 NADP+
2
2
NADPH
Chapter 8: Photosynthesis: Energy from the Sun
The cell membrane contains a protein
called ATP synthase that allows H+ ions to
pass through it

ATP synthase
+
O2
2H2O
2 NADP+
2
2
NADPH
Chapter 8: Photosynthesis: Energy from the Sun
As H+ ions pass through ATP synthase, the
protein rotates.

ATP synthase
+
O2
2H2O
2 NADP+
2
2
NADPH
Chapter 8: Photosynthesis: Energy from the Sun
As it rotates, ATP synthase binds ADP and a
phosphate group together to produce ATP.

ATP synthase
+
O2
2H2O
ADP
2 NADP+
2
2
NADPH
Chapter 8: Photosynthesis: Energy from the Sun
Because of this system, light-dependent
electron transport produces not only highenergy electrons but ATP as well.

ATP synthase
+
O2
2H2O
ADP
2 NADP+
2
2
NADPH
Chapter 8: Photosynthesis: Energy from the Sun
Making Sugar from CO2: The
Calvin–Benson Cycle
• The Calvin–Benson cycle makes sugar from
CO2.
• This pathway was elucidated through use of
radioactive tracers.
Review Figure 8.15
34
Chapter 8: Photosynthesis: Energy from the Sun
Figure
8.15
Figure 8.15
figure 08-15.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Making Sugar from CO2: The
Calvin–Benson Cycle
• The Calvin–Benson cycle has three phases:
fixation of CO2, reduction and carbohydrate
production, and regeneration of RuBP.
• RuBP is the initial CO2 acceptor, 3PG is the
first stable product of CO2 fixation.
• Rubisco catalyzes the reaction of CO2 and
RuBP to form 3PG.
Review Figures 8.16, 8.17
36
Chapter 8: Photosynthesis: Energy from the Sun
Figure
8.16
Figure 8.16
figure 08-16.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Figure
8.17
Figure 8.17
figure 08-17.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Photorespiration and Its
Evolutionary Consequences
• Rubisco catalyzes a reaction between O2
and RuBP in addition to that of CO2 and
RuBP.
• Photorespiration significantly reduces
photosynthesis efficiency.
• Reactions that constitute photorespiration
are distributed over chloroplast, peroxisome,
and mitochondria organelles.
39
Chapter 8: Photosynthesis: Energy from the Sun
Photorespiration and Its
Evolutionary Consequences
• At high temperatures and low CO2
concentrations, the oxygenase function of
rubisco is favored.
40
Chapter 8: Photosynthesis: Energy from the Sun
Photorespiration and Its
Evolutionary Consequences
• C4 plants bypass photorespiration.
• PEP carboxylase in mesophyll chloroplasts
initially fixes CO2 in four-carbon acids, which
diffuse into bundle sheath cells, where their
decarboxylation produces locally high
concentrations of CO2.
Review Figures 8.19
41
Chapter 8: Photosynthesis: Energy from the Sun
Figure 8.19
Figure 8.19
figure 08-19.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Photorespiration and Its
Evolutionary Consequences
• CAM plants operate much like C4 plants, but
their initial CO2 fixation by PEP carboxylase
is temporally separated from the Calvin–
Benson cycle, rather than spatially
separated.
Review Figure 8.21
43
Chapter 8: Photosynthesis: Energy from the Sun
Figure
8.21
Figure 8.21
figure 08-21.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Metabolic Pathways in Plants
• Plants respire in light and darkness, but
photosynthesize only in light.
• A plant must photosynthesize more than it
respires, giving it a net gain of reduced
energy-rich compounds.
45
Chapter 8: Photosynthesis: Energy from the Sun
Metabolic Pathways in Plants
• Photosynthesis and respiration are linked
through the Calvin–Benson cycle, the citric
acid cycle, and glycolysis.
Review Figure 8.22
46
Chapter 8: Photosynthesis: Energy from the Sun
Figure
8.22 – Part
1
Figure 8.22 – Part 1
figure 08-22a.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Figure
8.22 –
Part 2
Figure 8.22 – Part 2
figure 08-22b.jpg