chapter 7 capturing solar energy: photosynthesis 7.1 what is photosynthesis? 7.2 light-dependent...
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Chapter 7Capturing Solar Energy:
Photosynthesis• 7.1 What is photosynthesis?• 7.2 Light-dependent reactions: How is light
energy converted to chemical energy?• 7.3 Light-independent reactions: How is
chemical energy stored in glucose molecules?
• 7.4 What is the relationship between light-dependent and light-independent reactions?
• 7.5 Water, CO2, and the C4 pathway
7.1 What Is Photosynthesis?• ~ 2 billion years ago, some cells acquired the
ability to do photosynthesis (through chance genetic mutation)
• Oxygen levels in the atmosphere increased greatly
• Free oxygen (O2) is corrosive
• But as time went on (more mutation), some organisms used oxygen to break down glucose in cellular respiration
Relationships
• Photosynthesis:6 CO2 + 6 H20 + light energy -> C6H12O6 + 6 O2
• Cellular Respiration:C6H12O6 + 6 O2 -> 6 CO2 + 6 H20 + chemical
and heat energy
7.1 What Is Photosynthesis?
• Leaves and chloroplasts are adaptations for photosynthesis
• A leaf is just a few cells thick so the sun can penetrate
• Stomata (stoma is singular) adjust to let more or less CO2 in
cuticle
Internal leaf structure
upperepidermis
mesophyllcells
lower epidermis
chloroplastsbundle sheath
vascular bundle(vein)
stoma
outer membraneinner membrane
thylakoidstroma
granum(stack ofthylakoids)
channel interconnecting thylakoids
Chloroplast in mesophyll cell
Leaves
cuticle
Internal leaf structure
upperepidermis
mesophyllcells
lowerepidermis
chloroplasts
bundle sheathvascular bundle(vein)
stoma
Figure 7-2c Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.
Mesophyll cell containing chloroplasts
outer membrane
inner membrane
thylakoid
stroma
granum(stack ofthylakoids
channelinterconnectingthylakoids
Chloroplast in mesophyll cell
7.1 What Is Photosynthesis?
• Light-dependent reactions– Chlorophyll captures sunlight energy and
transfer to energy carrier molecules (ATP & NADPH)
– Uses H20 and releases O2
• Light-independent reactions– Enzymes use ATP & NADPH to drive
synthesis of glucose
– Uses CO2 and H20 and releases glucose
LIGHT-DEPENDENTREACTIONS(thylakoids)
LIGHT-INDEPENDENTREACTIONS
(stroma)
depletedcarriers
(ADP, NADP+)
H2O O2
CO2 + H2O glucose
energizedcarriers
(ATP, NADPH)
How Is Light Energy Converted to Chemical Energy?
• Sun emits energy in a broad spectrum of electromagnetic radiation
• A packet of energy is called a photon, and the level of energy corresponds to a wavelength
• Light can be absorbed, reflected or transmitted
Visible light ("rainbow colors")
Wavelength (nanometers)
Visible light
400 450 500 550 600 650 700 750
Gamma rays X-rays UV InfraredMicro-waves
Radiowaves
Sun emits energy in a broad spectrum of electromagnetic radiation
How Is Light Energy Converted to Chemical Energy?
• During photosynthesis, light is first captured by pigments in chloroplasts
• Chloroplasts contain different pigments that can absorb certain wavelengths of photons
chlorophyll b
carotenoids
chlorophyll a
Absorbance of photosynthetic pigments
20
40
60
80
100
400 500 600 700
wavelength (nanometers)
0
light
abs
orpt
ion
(per
cent
)
Chloroplast Pigments
• Chlorophyll a and b strongly absorb violet, blue and red light
• Carotenoids absorb blue and red light• So what colors do you see?
• Carotenoids are also vitamin A and forms the visual pigment in our eyes (captures light energy so we can see!)
The Light-Dependent Reactions Occur Within the Thylakoid Membranes
• Thylakoid membranes contain photosystems I and II
• These are highly organized assemblies of proteins, chlorophyll and accessory pigment molecules
• The pigment molecules absorb light energy (photon)
Copyright © 2005 Pearson Prentice Hall, Inc.
A mechanical analogy for the light reactions
MillmakesATP
ATP
e–
e–e–
e–
e–
Pho
ton
Photosystem II Photosystem I
e–
e–
NADPH
Pho
ton
Figure 10.14
sunlight
ener
gy le
vel o
f el
ectr
ons
+ H+NADP+
1/2 O2 + 2 H+ H2O
2e–
NADPH2e–
photosystemII
reactioncenter
within thylakoid m
embrane
2e–
synthesisenergy to driveATP
photosystem I
electron transport chain
2e–
NADPH
1/2 O2 + 2 H+
H2O
2e–
photosystemII
photosystem I
within thylakoid m
embrane
synthesis
energy to driveATP
+ H+
2e–
2e–
2e–
NADP+
sunlight
ener
gy le
vel o
f el
ectr
ons
reactioncenter
electron transport chain
How Is Light Energy Converted to Chemical Energy?
• Photosystem II generates ATP
• Photosystem I generates NADPH
• Splitting water maintains the flow of electrons through the photosystems
• Chemiosmosis: creating the hydrogen ion gradient
• Chemiosmosis: ATP synthesis
• Oxygen is a by-product of photosynthesis
Figure 7-8 (part 1) Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.
chloroplast
thylakoid
Figure 7-8 (part 2) Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.
C3
cycle
PSII PSIETCstroma
ETC
thylakoid space
Energy fromenergizedelectrons powersNADPH synthesis.
Flow of H+ downconcentration gradientpowers ATP synthesis.
Energy from energizedelectrons powers activetransport of H+ by ETC.
High H+ concentrationgenerated by activetransport.
H+ channel coupledto ATP-synthesizingenzyme.
Energy-carriermolecules powerthe C3 cycle.
Figure E7-1 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.
1 Energy is released aswater flows downhill.
2 Energy isharnessed torotate turbine.
3 Energy of rotatingturbine is used togenerate electricity.
Light-Independent Reactions: How Is Chemical Energy Stored in Glucose Molecules?
• The C3 cycle captures carbon dioxide
• The C3 (3 carbons) cycle of carbon fixation
• It is also called the Calvin-Benson cycle
• Carbon is fixed during the C3 cycle is used to synthesize glucose
• You do not need to know the details of this cycle, nor Figure 7-10 (ed. 7 Figure 7-6)
ADP12
ATP
ATP
12
NADPH12
NADP+12G3P
12 C C C
6 H2O
12
PGAC3
cycle
RuBP
ADP6
6
6 CO2
6
C C C C CC
glucose(or other organic
compounds)
C C C C C
C
C CC
O2H2O
H20
ATP
ADP
NADPH
glucose
NADP+
CO2
Light-dependentreactions occurin thylakoids.
energy fromsunlight
Light-independentreactions(C3 cycle) occurin stroma.
chloroplast
cuticle
upperepidermis
mesophyllcells
lowerepidermis
chloroplasts
bundle sheathvascular bundle(vein)
stoma
Water, CO2, C3 and C4 Pathways
7.5 Water, CO2, and the C4 Pathway
• When stomata are closed to conserve water, CO2 cannot enter, and O2 cannot leave– A wasteful process called photorespiration
occurs when it is too hot for C3 plants
– Glucose is not made, and seedlings may die.
– Some plants have a C4 pathway
– Compare pathways of C3 and C4 plants
CO2
O2
PGA
C3
CYCLE
CO2
G3P
CO2
CO2
O2
PGA
C3
CYCLE RuBP
within chloroplast in mesophyll cell
within chloroplast in mesophyll cell
within chloroplast in bundle-sheath cell
PEP
4-carbonmolecule
AMP
ATP
pyruvate
bundle-sheathcells
bundle-sheath cells
G3P
CO2
C4
Pathway
C3 plants use the C3 pathway
RuBP
glucose
C4 plants use the C4 pathway
glucose