Where It Starts – Photosynthesis
Chapter 7 Part 1
Impacts, Issues:
Biofuels
 Coal, petroleum, and natural gas were once
ancient forests, a limited resource; biofuels from
wastes are a renewable resource
7.1 Sunlight as an Energy Source
 Photosynthetic organisms use pigments to
capture the energy of sunlight
 Photosynthesis
• The synthesis of organic molecules from
inorganic molecules using the energy of light
Properties of Light
 Visible light is part of an electromagnetic
spectrum of energy radiating from the sun
• Travels in waves
• Organized into photons
 Wavelength
• The distance between the crests of two
successive waves of light (nm)
Electromagnetic Spectrum
of Radiant Energy
Fig. 7-2 (a-b), p. 108
shortest wavelengths
(highest energy)
range of most
radiation reaching
Earth’s surface
range of heat
escaping from
Earth’s surface
longest wavelengths
(lowest energy)
visible light
gamma
rays
xrays
ultraviolet
radiation
near-infrared
radiation
400
b
infrared
radiation
500
microwaves
radio
waves
600
700
Wavelengths of visible light (in nanometers)
Fig. 7-2 (a-b), p. 108
Fig. 7-2c, p. 108
The Rainbow Catchers
 Different wavelengths form colors of the rainbow
• Photosynthesis uses wavelengths of 380-750 nm
 Pigment
• An organic molecule that selectively absorbs light
of specific wavelengths
 Chlorophyll a
• The most common photosynthetic pigment
• Absorbs violet and red light (appears green)
Photosynthetic Pigments
 Collectively, chlorophyll and accessory pigments
absorb most wavelengths of visible light
 Certain electrons in pigment molecules absorb
photons of light energy, boosting electrons to a
higher energy level
 Energy is captured and used for photosynthesis
Some Pigments in Photosynthesizers
Two Photosynthetic Pigments
Fig. 7-3a, p. 109
Fig. 7-3b, p. 109
chlorophyll a
β-carotene
Fig. 7-3a, p. 109
7.2 Exploring the Rainbow
 Engelmann identified colors of light that drive
photosynthesis (violet and red) by using a prism
to divide light into colors
• Algae using these wavelengths gave off the most
oxygen
 An absorption spectrum shows which
wavelengths a pigment absorbs best
• Organisms in different environments use different
pigments
Photosynthesis and
Wavelengths of Light
Fig. 7-4a, p. 110
A Light micrograph of photosynthetic cells in a strand of
Chladophora. Engelmann used this green alga to demonstrate that
certain colors of light are best for photosynthesis.
Fig. 7-4a, p. 110
Fig. 7-4b, p. 110
bacteria
alga
Wavelength (nanometers)
B Engelmann directed light through a prism so that bands of colors
crossed a water droplet on a microscope slide. The water held a strand of
Chladophora and oxygen-requiring bacteria. The bacteria clustered around
the algal cells that were releasing the most oxygen—the ones that were
most actively engaged in photosynthesis. Those cells were under red and
violet light.
Fig. 7-4b, p. 110
Fig. 7-4c, p. 110
100
phycoerythrobilin
chlorophyll b
phycocyanobilin
β-carotene
Light absorption (%)
80
chlorophyll a
60
40
20
0
400
500
600
Wavelength (nanometers)
700
C Absorption spectra of a few photosynthetic pigments. Line
color indicates the characteristic color of each pigment.
Fig. 7-4c, p. 110
Animation: T. Englemann’s experiment
7.1-7.2 Key Concepts:
The Rainbow Catchers
 The flow of energy through the biosphere starts
when chlorophylls and other photosynthetic
pigments absorb the energy of visible light
7.3 Overview of Photosynthesis
 Chloroplast
• An organelle that specializes in photosynthesis in
plants and many protists
 Stroma
• A semifluid matrix surrounded by the two outer
membranes of the chloroplast
• Sugars are built in the stroma
Overview of Photosynthesis
 Thylakoid membrane
• Folded membrane that make up thylakoids
• Contains clusters of light-harvesting pigments
that absorb photons of different energies
 Photosystems (type I and type II)
• Groups of molecules that work as a unit to begin
the reactions of photosynthesis
• Convert light energy into chemical energy
Overview of Photosynthesis
 Light-dependent reactions
• Light energy is transferred to ATP and NADPH
• Water molecules are split, releasing O2
 Light-independent reactions
• Energy in ATP and NADPH drives synthesis of
glucose and other carbohydrates from CO2 and
water
Summary: Photosynthesis
Sites of Photosynthesis
in Plants
Fig. 7-5a, p. 111
upper epidermis photosynthetic cells
A Zooming in on a
photosynthetic cell.
leaf vein
lower epidermis
Fig. 7-5a, p. 111
Fig. 7-5b, p. 111
two outer membranes
of chloroplast
stroma
part of thylakoid
membrane system:
thylakoid
compartment,
cutaway view
B Chloroplast structure. No matter how highly folded, its thylakoid
membrane system forms a single, continuous compartment in the stroma.
Fig. 7-5b, p. 111
Fig. 7-5c, p. 111
sunlight
O2
CO2
H2O
CHLOROPLAST
lightdependent
reactions
NADPH, ATP
NADP+, ADP
lightindependent
reactions
sugars
CYTOPLASM
C In chloroplasts, ATP and NADPH form in the light-dependent stage of
photosynthesis, which occurs at the thylakoid membrane. The second
stage, which produces sugars and other carbohydrates, proceeds in the
stroma.
Fig. 7-5c, p. 111
Animation: Sites of photosynthesis
7.4 Light-Dependent Reactions
 In the first stage of photosynthesis, light energy
drives electrons out of photosystems
 The electrons may be used in a noncyclic or
cyclic pathway of ATP formation
Capturing Energy for Photosynthesis
 Photons boost electrons in pigments to higher
energy levels
 Light-harvesting complexes absorb the energy
 Electrons are released from special pairs of
chlorophyll a molecules in photosystems
The Thylakoid Membrane
light-harvesting complex
photosystem
Fig. 7-7, p. 112
Cyclic and Noncyclic Pathways
 Electrons from photosystems take noncyclic or
cyclic pathways, forming ATP
ADP + Pi
NADP+
ATP
Light-dependent reactions
(noncyclic pathway)
H2O
ADP + Pi
NADPH
O2
Light-dependent reactions
(cyclic pathway)
ATP
Fig. 7-6, p. 112
Replacing Lost Electrons
 Electrons lost from photosystem II are replaced by
photolysis of water molecules, which dissociate
into hydrogen ions and oxygen
 Photolysis
• Process by which light energy breaks down a
molecule such as water
Electron Flow in a Noncyclic Pathway
 Electrons lost from a photosystem enter an
electron transfer chain in the thylakoid
membrane
 Electron transfer chains
• Organized arrays of enzymes, coenzymes, and
other proteins that accept and donate electrons in
a series
Harvesting Electron Energy
 Light energy is converted to chemical energy
• Entry of electrons from a photosystem into the
electron transfer chain is the first step in lightdependent reactions
 ATP forms in the stroma
• Electron energy is used to build up a H+ gradient
across the membrane
• H+ flows through ATP synthase, which attaches a
phosphate group to ADP
Noncyclic Pathway of Photosynthesis
to second stage of
reactions
The Light-Dependent Reactions of Photosynthesis
light energy
photosystem II
electron
transfer chain
light energy
NADPH
ATP
ATP
synthase
ADP + Pi
photosystem I
NADP+
thylakoid
compartment
stroma
A Light energy drives
electrons out of
photosystem II.
C Electrons from
photosystem II enter an
electron transfer chain.
B Photosystem II pulls
replacement electrons
from water molecules,
which dissociate into
oxygen and hydrogen
ions (photolysis). The
oxygen leaves the cell
as O2.
D Energy lost by the
electrons as they
move through the
chain causes H+ to
be pumped from
the stroma into
the thylakoid
compartment. An H+
gradient forms across
the membrane.
E Light energy drives
electrons out of
photosystem I, which
accepts replacement
electrons from electron
transfer chains.
F Electrons from
photosystem I move
through a second
electron transfer chain,
then combine with
NADP+ and H+. NADPH
forms.
G Hydrogen ions in the
thylakoid compartment
are propelled through the
interior of ATP synthases
by their gradient across
the thylakoid membrane.
H H+ flow causes the ATP
synthases to attach
phosphate to ADP, so
ATP forms in the stroma.
Fig. 7-8, p. 113
Animation: Noncyclic pathway of
electron flow
Electron Flow in a Cyclic Pathway
 When NADPH accumulates in the stroma, the
noncyclic pathway stalls
 A cyclic pathway runs in type I photosystems to
make ATP; electrons are cycled back to
photosystem I and NADPH does not form
7.5 Energy Flow in Photosynthesis
 Energy flow in the light-dependent reactions is
an example of how organisms harvest energy
from their environment
Photophosphorylation
 Photophosphorylation
• A light-driven reaction that attaches a phosphate
group to a molecule
 Cyclic photophosphorylation
• Electrons cycle within photosystem I
 Noncyclic photophosphorylation
• Electrons move from water to photosystem II, to
photosystem I, to NADPH
Energy Flow in
Light-Dependent Reactions
Fig. 7-9a, p. 114
CYCLIC PHOTOPHOSPHORYLATION
e−
A As long as electrons
continue to pass through
this electron transfer
chain, H+ continues to be
carried across the
thylakoid membrane, and
ATP continues to form.
Light provides the
energy boost that keeps
the cycle going.
energy
excited
P700
e−
P700
(Photosystem I)
light energy
Fig. 7-9a, p. 114
Fig. 7-9b, p. 114
NONCYCLIC PHOTOPHOSPHORYLATION
energy
excited
P680
excited
P700
P700
(Photosystem I)
P680
(Photosystem II)
light energy
light energy
B The noncyclic pathway is a one-way fl ow of electrons from water, to
photosystem II, to photosystem I, to NADPH. As long as electrons continue
to fl ow through the two electron transfer chains, H+ continues to be carried
across the thylakoid membrane, and ATP and NADPH keep forming. Light
provides the energy boosts that keep the pathway going.
Fig. 7-9b, p. 114
CYCLIC PHOTOPHOSPHORYLATION
energy
excited
P700
P700
(Photosystem I)
NONCYCLIC PHOTOPHOSPHORYLATION
excited
P700
energy
excited
P680
light energy
P700
(Photosystem I)
P680
(Photosystem II)
light energy
light energy
Stepped Art
Fig. 7-9, p. 114
Animation: Energy changes in
photosynthesis
7.3-7.5 Key Concepts:
Making ATP and NADPH
 Photosynthesis proceeds through two stages in
the chloroplasts of plants and many types of
protists
 In the first stage, sunlight energy is converted to
the chemical bond energy of ATP
 The coenzyme NADPH forms in a pathway that
also releases oxygen