Transcript Ch06_lecturestudents

Chapter 6
Capturing Solar
Energy:
Photosynthesis
Lectures by
Gregory Ahearn
University of North Florida
Copyright © 2009 Pearson Education, Inc.
6.1 What Is Photosynthesis?
 Life on earth depends on photosynthesis.
• Photosynthesis is the capturing of sunlight energy and the
conversion of it into chemical energy.
• All present-day organisms that use oxygen as their
respiratory gas depend upon photosynthesis.
• Equally important is the stored sugars from
photosynthesis, since virtually all life depends on this
energy, either directly or indirectly.
Copyright © 2009 Pearson Education Inc.
6.1 What Is Photosynthesis?
 Photosynthesis removes ________ ________
from the atmosphere and adds ________ to it.
• The chemical reaction for photosynthesis:
• 6 CO2 + 6 H20 + light energy  C6H12O6 + 6 O2
• Plants, seaweeds, and single-celled organisms all
show the basic aspects of photosynthesis.
• Energy captured is stored in the bonds of
glucose.
Copyright © 2009 Pearson Education Inc.
6.1 What Is Photosynthesis?
• Leaves are the main location of photosynthesis.
• Plants have _____ leaves so sunlight can penetrate.
• Plant leaves have a large _________ _______ to
expose them to the sun.
• Plant leaves have pores to admit CO2, called
_________ (singular, stoma) that can open and close.
Copyright © 2009 Pearson Education Inc.
6.1 What Is Photosynthesis?
 Inside the leaf are a few layers of cells called the
_____________.
 A system of veins supplies water and minerals to the mesophyll
and brings the sugars they make to other parts of the plant.
 Photosynthesis occurs in ____________ inside the mesophyll.
 Chloroplasts contain a semifluid medium called stroma, which
contains sacs called _____________ within which
photosynthesis occurs.
Copyright © 2009 Pearson Education Inc.
6.1 What Is Photosynthesis?
 An overview of photosynthetic structures
mesophyll
cells
(a) Leaves
outer membrane
inner
membrane
thylakoid
stroma
vein
chloroplasts
stoma
(b) Internal leaf structure
(c) Chloroplast in mesophyll cell
Fig. 6-1
Copyright © 2009 Pearson Education Inc.
6.1 What Is Photosynthesis?
 Photosynthesis consists of light-dependent
and light-independent reactions.
• These reactions occur at different locations
in the chloroplast.
• The two types of reactions are linked by the
energy-carrier molecules adenosine
triphosphatase (ATP) and nicotinamide
adenine dinucleotide phosphate (NADPH).
Copyright © 2009 Pearson Education Inc.
6.1 What Is Photosynthesis?
 An overview of photosynthesis: lightdependent and light-independent reactions
H2O
LIGHT-DEPENDENT
REACTIONS
(thylakoids)
depleted
carriers
(ADP, NADP+)
CO2
Copyright © 2009 Pearson Education Inc.
O2
energized
carriers
(ATP, NADPH)
LIGHT-INDEPENDENT
REACTIONS
(stroma)
glucose
Fig. 6-2
6.1 What Is Photosynthesis?
 Light-dependent reactions
• Occur in the membranes of the ___________.
• Light is captured here and stored in ATP and
NADPH.
• _________is consumed and _________ is
given off.
Copyright © 2009 Pearson Education Inc.
6.1 What Is Photosynthesis?
 Light-independent reactions
• Occurs in the __________.
• ATP and NADPH produced by light-dependent reactions are
used to make __________ and other molecules.
• ____________ ___________ is consumed in the process.
• ATP and NADPH are converted to low-energy ADP and
NADP+.
• They return to the light-dependent reactions to be
recharged into ATP and NADPH
Copyright © 2009 Pearson Education Inc.
LIGHT- DEPENDENT REACTIONS
Copyright © 2009 Pearson Education Inc.
6.2 How Is Light Energy Converted To
Chemical Energy?
 Light is first captured by pigments in chloroplasts.
• Membranes of thylakoids contain several types of
pigments (light-absorbing molecules).
• _______________ is one light-absorbing molecule that
absorbs violet, blue, and red light, but reflects green.
• Other accessory pigments include ____________,
which absorb blue and green light, but reflect yellow and
orange.
Copyright © 2009 Pearson Education Inc.
6.2 How Is Light Energy Converted To
Chemical Energy?
 Light, chloroplast
pigments, and
photosynthesis
Absorbance of photosynthetic pigments
light absorption (percent)
100
80
60
carotenoids
40
chlorophyll
20
0
Wavelength (nanometers)
400
450
500
550
600
650
700
750
Visible light
Gamma rays X-rays UV
Infrared Micro- Radio
waves waves
Fig. 6-3
Copyright © 2009 Pearson Education Inc.
6.2 How Is Light Energy Converted To
Chemical Energy?
 Light-dependent reactions take place in
photosystems found in the thylakoid membranes.
• In thylakoids, there are thousands of photosystems of
two types.
• Photosystem I
• Photosystem II
 Each photosystem consists of an assemblage of
proteins, chlorophyll, accessory pigment
molecules, and electron-carrier molecules.
 The light-dependent reactions generate energycarrier molecules.
Copyright © 2009 Pearson Education Inc.
6.2 How Is Light Energy Converted To
Chemical Energy?
 Each photosystem consists of two major parts:
• A light-harvesting complex collects light
energy and passes it on to a specific chlorophyll
molecule called the reaction center.
• An electron transport system (ETS)
transports energized electrons from one
molecule to another.
Copyright © 2009 Pearson Education Inc.
6.2 How Is Light Energy Converted To
Chemical Energy?
 Structures associated with the lightdependent reactions
thylakoids
chloroplast
within thylakoid membrane
PS II
Copyright © 2009 Pearson Education Inc.
ETC
reaction centers
PS I
ETC
Fig. 6-4
6.2 How Is Light Energy Converted To
Chemical Energy?
 Photosystem II generates ________.
• Step 1: The light-harvesting complex passes light to
the reaction center.
• Step 2: Electrons of the reaction center become
energized.
• Step 3: The energized electrons jump to the ETS and
jump from molecule to molecule, releasing energy at
each step.
• Step 4: The released energy powers reactions that
synthesize ATP.
Copyright © 2009 Pearson Education Inc.
6.2 How Is Light Energy Converted To
Chemical Energy?
 Photosystem I generates _________.
• Step 5: The light-harvesting complex passes light to
the reaction center.
• Step 6: Activated electrons from the reaction center
are passed to the ETS and are replaced by electrons
coming from the ETS of photosystem II.
• Step 7: Electrons jump from one molecule of the ETS
to another, until they reach NADP+.
• Step 8: Each molecule of NADP+ picks up two
electrons, forming NADPH.
Copyright © 2009 Pearson Education Inc.
sunlight
7
e–
NADPH
8
NADP+ +
e–
within thylakoid membrane
6
3
energy level of electrons
H+
5
4
2 e–
photosystem I
energy to drive
reaction
center
ATP synthesis
1
e–
photosystem
II
9
2 H+
H2O
1/2 O2
Fig. 6-5
Copyright © 2009 Pearson Education Inc.
6.2 How Is Light Energy Converted To
Chemical Energy?
 Splitting water maintains the flow of electrons
through the photosystems.
• Electrons from the reaction center of photosystem II
flow through the ETS of photosystem II to the
reaction center of photosystem I, forming NADPH.
• Photosystem II’s reaction center must be supplied with
new electrons to keep the process continuing.
• Where do the new electrons come from?
Copyright © 2009 Pearson Education Inc.
6.2 How Is Light Energy Converted To
Chemical Energy?
 Step 9: The breakdown of H2O provides the replacement
electrons to keep the process continuing, through the
reaction:
H2O  ½ O2 + 2H+ + 2e–
• The two electrons are donated to photosystem II.
• The hydrogen ions are used to convert NADP+ to NADPH.
• Oxygen atoms combine to form a molecule of oxygen gas
(O2), which is given off to the atmosphere.
Copyright © 2009 Pearson Education Inc.
LIGHT-INDEPENDENT
REACTIONS
Copyright © 2009 Pearson Education Inc.
6.3 How Is Chemical Energy Stored in
Glucose Molecules?
 The ATP and NADPH generated in light-dependent
reactions are used in light-independent reactions to make
molecules for long-term storage.
• These reactions occur in the fluid stroma that surrounds
the thylakoids, and do not require light.
• In the stroma, ATP and NADPH are used with CO2 and H2O
to synthesize the storage form of energy—glucose.
Copyright © 2009 Pearson Education Inc.
6.3 How Is Chemical Energy Stored in Glucose
Molecules?
 The C3 cycle captures carbon dioxide.
• Step 1: CO2 from air combines with a five-carbon sugar,
ribulose biphosphate (RuBP), and H2O to form
phosphoglyceric acid (PGA).
• Step 2: PGA receives energy input from ATP and NADPH
to form glyceraldehyde-3-phosphate (G3P).
• Step 3: Two G3P molecules (three carbons each)
combine to form one molecule of glucose (six carbons).
• Step 4: 10 G3P molecules powered by ATP are used to
regenerate six molecules of RuBP to restart the cycle.
Copyright © 2009 Pearson Education Inc.
6.3 How Is Chemical Energy Stored in
Glucose Molecules?
 The C3 cycle of carbon fixation
1 Carbon
fixation
combines
CO2 with
RuBP
6 CO2 C
6 C C C C C
RuBP
4 RuBP
synthesis uses
energy and
10 G3Ps
6 ADP
6 ATP
3 2 G3Ps available
for synthesis of
glucose
12 C C C
PGA
C3
cycle
2 G3P
synthesis
uses energy
12 ATP
12 ADP
12 C C C
G3P
12 NADPH
12 NADP+
C C C C C C
glucose
Fig. 6-6
Copyright © 2009 Pearson Education Inc.
6.4 What Is The Relationship Between LightDependent And Light-Independent
Reactions?
 Light-dependent reactions capture solar energy;
light-independent reactions use captured energy
to make glucose.
• Energy-carrier molecules provide the link between
these two sets of reactions.
• Light-dependent reactions of thylakoids use light to
charge ADP and NADP+ to make ATP and NADPH.
• ATP and NADPH move to the stroma where they
provide energy to synthesize glucose.
Copyright © 2009 Pearson Education Inc.
6.4 What Is The Relationship Between LightDependent And Light-Independent
Reactions?
Photosynthesis
includes two
separate sets of
reactions (lightdependent and
lightindependent)
that are closely
linked.
energy from
sunlight
O2
CO2
ATP
NADPH
Light-dependent
reactions occur
in thylakoids
ADP
Lightindependent
reactions
(C3 cycle) occur
in stroma
NADP+
H2O
chloroplast
Copyright © 2009 Pearson Education Inc.
glucose
Fig. 6-7
6.5 How Does the Need To Conserve Water
Affect Photosynthesis?
 Photosynthesis requires carbon dioxide; porous leaves
would allow the entry of CO2, but would also result in the
loss of H2O.
 Evolution of the stomata resulted in pores that could open,
letting in _____, but also to close, to restrict _____ losses.
 Closing stomata to prevent H2O loss also restricts the
release of _____, produced by photosynthesis, to the
atmosphere.
Copyright © 2009 Pearson Education Inc.
6.5 How Does the Need To Conserve Water
Affect Photosynthesis?
 When stomata are closed to conserve water, wasteful
__________________ occurs.
• In hot, dry conditions, plant stomata are closed much
of the time, reducing internal CO2 concentrations and
increasing O2 concentrations.
• Increased O2 reacts with RuBP (instead of CO2) in a
process called photorespiration.
• Photorespiration does not produce useful cellular energy,
and prevents the C3 synthesis of glucose.
Copyright © 2009 Pearson Education Inc.
6.5 How Does the Need To Conserve Water
Affect Photosynthesis?
 Some plants have evolved metabolic pathways that
reduce photorespiration.
 These plants can produce glucose even under hot
and dry conditions.
 The two most important alternative pathways are:
• The C4 pathway
• Crassulacean acid metabolism (CAM)
Copyright © 2009 Pearson Education Inc.
 Typical plants (C3 plants) fix carbon and synthesize
glucose as a result of the C3 cycle in mesophyll
cells.
 C4 plants are plants that use a supplementary method
of CO2 uptake which forms a 4-carbon molecule
instead of the two 3-carbon molecules of the Calvin
cycle.
 These plants are able to handle higher temperatures
and higher light intensity than C3 plants. C4 plants do
require more ATP than C3 plants but also make more
glucose per given leaf area and grow more quickly.
Copyright © 2009 Pearson Education Inc.
6.5 How Does the Need To Conserve Water
Affect Photosynthesis?
 C3 plant
bundlesheath
cell
mesophyll cell
In a C3 plant, carbon capture and
glucose synthesis are in mesophyll cells
(a) C plant
3
Fig. 6-9a
Copyright © 2009 Pearson Education Inc.
6.5 How Does the Need To Conserve Water
Affect Photosynthesis
 The C4 pathway includes two stages that
take place in different parts of the leaf.
• In the first stage, CO2 is captured in mesophyll
cells in the presence of high O2, producing a fourcarbon molecule.
• The four-carbon molecule is transferred from
mesophyll cells to the bundle-sheath cells where
the four-carbon molecule is broken down to CO2.
Copyright © 2009 Pearson Education Inc.
6.5 How Does the Need to Conserve Water
Affect Photosynthesis
 C4 plant
bundlesheath
cell
mesophyll cell
In a C4 plant, carbon capture is in
mesophyll cells, but glucose is
synthesized in bundle-sheath cells
(b) C4 plant
Fig. 6-9b
Copyright © 2009 Pearson Education Inc.
6.5 How Does the Need to Conserve Water
Affect Photosynthesis
 C4 plants capture carbon and synthesize
glucose in different places.
• In the sheath-bundle cells, the released CO2
proceeds to the second stage of the pathway—
the regular C3 cycle—without excess O2
interfering with the process.
• Many C4 plant species are grasses, and are
agriculturally important species such as sugar
cane, corn, and sorghum.
Copyright © 2009 Pearson Education Inc.
6.5 How Does the Need to Conserve Water
Affect Photosynthesis
 CAM plants capture carbon and synthesize
glucose at different times.
• In CAM plants, photorespiration is reduced by fixing carbon
in two stages that take place in the same cells but at
different times of the day.
• At night, with open stoma, reactions in mesophyll cells
incorporate CO2 into the organic acid molecules that are
stored in vacuoles.
• During the day, with stoma closed, the organic acids release
their CO2 and the regular C3 cycle proceeds.
• Many desert species are CAM plants
Copyright © 2009 Pearson Education Inc.
6.5 How Does the Need to Conserve Water
Affect Photosynthesis
C4
CAM
CO2
CO2
night
mesophyll
cell
C C C C
CO2
bundle-sheath
cell
C3
cycle
C4
(a) Steps in separate places
1 CO2 is
incorporated
into four-carbon
molecules
2 Four-carbon
molecules
release CO2 to
the C3 cycle
C C C C
CO2
day
mesophyll
cell
C3
cycle
CAM
(b) Steps at separate times
Fig. 6-10
Copyright © 2009 Pearson Education Inc.