Chapter 7:
PHOTOSYNTHESIS
1. Overview of Photosynthesis
2. The “Light” Reactions
3. The “Dark” Reactions
1. Overview of Photosynthesis
What is Photosynthesis?
The process of converting light energy (kinetic)
into energy stored in the covalent bonds of
glucose molecules (potential).
Light
energy
6 CO2
+
Carbon dioxide
6
H2O
Water
C6H12O6
PHOTOSYNTHESIS
Glucose
+
6
O2
Oxygen gas
• carried out by photoautotrophs
• plants, phytoplankton, cyanobacteria (any
photosynthetic organism)
• the basis of almost all ecosystems
• all “food energy” ultimately comes from the sun
• source of all atmospheric oxygen (O2)
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Photosynthesis vs Respiration
essentially the
reverse
of each other
Photosynthesis occurs in Chloroplasts
Leaf Cross Section
Mesophyll Cell
Leaf
Chloroplast
CO2
O2
Chloroplast
The Chloroplast
outer membrane
inner membrane
stroma
thylakoid
granum
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Photosynthesis consists of
2 sets of Reactions
The light-dependent or “Light” Reactions:
H2O
CO2
Chloroplast
Light
NADP+
ADP
+ P
CALVIN
CYCLE
(in stroma)
LIGHT
REACTIONS
(in thylakoids)
ATP
Ele
ctr
on
s
O
NADPH
Sugar
• convert sunlight
energy into
chemical energy
(stored in ATP &
NADPH)
“Dark” Reactions
(Calvin cycle):
• use chemical
energy from
light reactions
to make glucose
2. Light-dependent
(“Light”) Reactions
Light Reactions occur in Thylakoids
A variety of
light-absorbing
pigments &
electron transport
proteins are
embedded within
the thylakoid
membrane
3
The Pigments absorb “Visible” Light
Chlorophyll a & b:
• the major pigments (absorb red, blue…, reflect green)
Carotenoids (e.g., β-carotene)
• accessory pigments (absorb green, blue, reflect red, yellow)
Absorption Range for each Pigment
blue
red
Chlorophyll absorbs
“non-green” light energy
Light
Reflected
light
Chloroplast
Absorbed
light
• green light
passes on
through or
is reflected,
causing the
leaves to
appear
green
Transmitted
light
4
H2O
½ O2 + 2 H+ + 2 *ePS I
1
PS II
2
e- transport chain
(ETC) pumps H+
into thylakoid
4
PS II
2 e- to
NADPH
PS I
3
ATP Synthase
uses H+ flow to
make ATP
Light Energy absorbed by Pigments Fuels
4 General Steps of the “Light Reactions”:
1) H2O split to O, 2 H+ & 2 high energy e- (*e-) in PS II
H2O
sunlight
O2 + H+ + *e-
2) Energy released by a series of *e- transfers is
used to generate H+ gradient
• H+ accumulates inside the thylakoid membrane
3) H+ gradient used to make ATP (chemiosmosis)
4) *e- “re-energized” in PS I, passed on to NADP+
• *e- ends up in NADPH (an electron carrier)
5
Analogy of
Light reactions
e–
ATP
e–
e–
NADPH
e–
e–
e–
ton
Pho
Mill
makes
ATP
Photon
e–
Photosystem II
Photosystem I
Summary of the
“Light” Reactions
Chloroplast
Stroma (low H+ concentration)
H+
Light
Light
H+
ADP +
H+
NADP+
+ H+
P
ATP
NADPH
H+
Thylakoid
membrane
H2O
H+
1
O2 + 2 H +
H+
H+
H+
H+
H+
H+
2
Photosystem II
Electron
transport chain
Photosystem I
H+
H+
H+
ATP synthase
Thylakoid space
(high H+ concentration)
3. Light-independent
(“Dark”) Reactions
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The “Dark” Reactions
A series of reactions called the Calvin cycle
that synthesize glucose from CO2 and H2O:
CO2 + H2O
ATP, NADPH
C6H12O6 (glucose)
• uses energy stored in ATP and NADPH
• produced by the light reactions
• can occur in dark (doesn’t require light directly)
• also occurs during daylight!
• takes place in the stroma of chloroplasts
• outside the thylakoids
“Dark” Reactions
Involves an anabolic
pathway known as the
Calvin cycle:
Calvin
cycle
• endergonic reactions of
this pathway are fueled by
ATP & NADPH from the
“light” reactions
Don’t
memorize
this!!
• resulting sugars can be
used as a source of
energy or to build other
organic molecules
Summary of Photosynthesis
stroma
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Key Terms for Chapter 7
• photoautotroph
• chloroplast, thylakoid, stroma
• chlorophyll, carotenoids
• ATP, NADPH
• electron transport chain (ETC)
• ATP synthase
• Light reactions, dark reactions, Calvin cycle
Relevant Review Questions:
1-6, 8-10, 12
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