Transcript Document

Chapter 08
Photosynthesis
Section 8.1 Learning Objective
• Know/Understand equation for
photosynthesis
– How is it related to cell respiration
• Understand purpose of photosynthesis
• Describe structure of chloroplast
2
8.1 Photosynthesis Overview
• Ultimate source of energy is the Sun and
is captured by plants, algae, and bacteria
through the process of photosynthesis
6CO2 + 12H2O
C6H12O6 + 6H2O + 6O2
• Oxygenic photosynthesis is carried out by
– Cyanobacteria
– 7 groups of algae
– All land plants – photosynthesis takes place in
chloroplasts
Chloroplast
• Thylakoid membrane – internal membrane
– Contains chlorophyll and other photosynthetic
pigments
– Pigments clustered into photosystems
• Grana – stacks of flattened sacs of
thylakoid membrane
• Stroma lamella – connect grana
• Stroma – semiliquid surrounding thylakoid
membranes
Cuticle
Epidermis
Mesophyll
Vascular bundle Stoma
Vacuole
Cell wall
1.58 mm
Chloroplast
Inner membrane
Outer membrane
Courtesy Dr. Kenneth Miller, Brown University
Photosynthetic Processes
• Light-dependent reactions
– Require light
– Capture energy from sunlight
– Make ATP and reduce NADP+ to NADPH
• Carbon fixation reactions or lightindependent reactions
– Does not require light
– Use ATP and NADPH to synthesize organic
molecules from CO2
Photosynthesis Overview
Sunlight
Light-dependent reactions
• Require light
• Capture energy from
sunlight
• Make ATP and reduce
NADP+ to NADPH
Photosystem
H2O
Thylakoid
O2
Light-Dependent
Reactions
ADP + Pi
CO2
Stroma
ATP
NADP+
Calvin
Cycle
NADPH
Organic
molecules
Carbon fixation reactions
• Does not require light
• Use ATP and NADPH
to synthesize organic
molecules from CO2
Question 1
Which structure is responsible for gas exchange ?
a. Matrix
b. Stroma
c. Stoma
d. Thylakoid
e. Trichomes
Question 3
Where is the chlorophyll located in a plant?
a. Cristae
b. Plasma membrane of the cell
c. Outer membrane of the chloroplast
d. Thylakoid membranes
e. Stroma
Section 8.3 Learning Objectives
• Define pigment, and know how they are
important in photosynthesis
• Relate absorption spectrum of a pigment
with its color
• Know examples of pigments used in
photosynthesis
10
8.3 Pigments
• Molecules that absorb light energy in the
visible range
• Light is a form of energy
• Photon – particle of light
– Acts as a discrete bundle of energy
– Energy content of a photon is inversely
proportional to the wavelength of the light
• Photoelectric effect – removal of an
electron from a molecule by light
The Electro Magnetic Spectrum
• Light is a form of electromagnetic energy
• The shorter wavelength of the light, the greater
is energy
• Visible light represents only a small part of
spectrum, 400 – 700 nm
Increasing energy
Increasing wavelength
0.001 nm
1 nm
Gamma rays
10 nm 1000 nm
X-rays
UV
light
0.01 cm
1 cm
Infrared
1m
100 m
Radio waves
Visible light
400 nm
430 nm
500 nm
560 nm
600 nm
650 nm
740 nm
Absorption spectrum
• When a photon strikes a molecule, its
energy is either
– Lost as heat
– Absorbed by the electrons of the molecule
• Boosts electrons into higher energy level
• Absorption spectrum – range and
efficiency of photons a molecule is
capable of absorbing
Absorption Spectra for Chlorophyll and Carotenoids.
high
Light
Absorbtion
carotenoids
chlorophyll a
chlorophyll b
low
400
450
500
550
600
Wavelength (nm)
650
700
Pigments in Photosynthesis
• Organisms have evolved a variety of
different pigments
• Only two general types are used in green
plant photosynthesis
– Chlorophylls
– Carotenoids
• In some organisms, other molecules also
absorb light energy
Chlorophylls
• Chlorophyll a
– Main pigment in plants and cyanobacteria
– Only pigment that can act directly to convert
light energy to chemical energy
– Absorbs violet-blue and red light
• Chlorophyll b
– Accessory pigment or secondary pigment
absorbing light wavelengths that chlorophyll a
does not absorb
Absorption Spectra for Chlorophyll and Carotenoids.
high
Light
Absorbtion
carotenoids
chlorophyll a
chlorophyll b
low
400
450
500
550
600
Wavelength (nm)
650
700
• Structure of
chlorophyll
• porphyrin ring
– Complex ring structure
with alternating double
and single bonds
– Magnesium ion at the
center of the ring
• Photons excite
electrons in the ring
• Electrons are shuttled
away from the ring
H2C
R
H
CH
=
CH3
Chlorophyll b: R
=
CHO
CH2CH3
H3C
Porphyrin
head
Chlorophyll a: R
N
N
Mg
H
N
H
N
H3C
CH3
H
H
H
O
Hydrocarbon
tail
CH2
CO2CH3
CH2
C
O
CH2
CH
CCH3
CH2
CH2
CH2
CHCH3
CH2
CH2
CH2
CHCH3
CH2
CH2
CH2
CHCH3
CH3
O
Light
Absorbtion
high
Oxygen-seeking bacteria
Filament of green algae
low
• Action spectrum
– Relative effectiveness of different
wavelengths of light in promoting
photosynthesis
– Corresponds to the absorption spectrum for
chlorophylls
• Carotenoids
– Carbon rings linked to
chains with alternating
single and double
bonds
– Can absorb photons
with a wide range of
energies
– Also scavenge free
radicals – antioxidant
Oak leaf
in summer
• Protective role
• Phycobiloproteins
– Important in low-light
ocean areas
Oak leaf
in autumn
© Eric Soder/pixsource.com
Question 6
Electromagnetic radiation with a wavelength between
400-700 nm is _____.
a. Radio waves
b. Microwaves
c. X-rays
d. Visible light
e. Ultraviolet
Question 14
What would happen if a leaf lacked carotenoids?
a. The leaf would absorb all energy levels
b. The leaf would turn yellow/orange during the
fall
c. The leaf would not absorb carbon dioxide
d. Photosynthesis would not be as efficient
e. The leaf would require oxygen
Section 8.4-8.5 Learning
Objectives
• Understand structure/function of a photosystem (PS)
• Understand the purpose of light-reactions in
photosynthesis
– Where does it take place
– What goes in, what goes out
– What is the job of the PSs
– How is ATP made
23
8.4 Photosystem Organization
• Antenna complex
– Hundreds of accessory pigment molecules
– Gather photons and feed the captured light
energy to the reaction center
• Reaction center
– 1 or more chlorophyll a molecules
– Passes excited electrons out of the
photosystem
Antenna complex
• Also called light-harvesting complex
• Captures photons from sunlight and
channels them to the reaction center
chlorophylls
• In chloroplasts, light-harvesting complexes
consist of a web of chlorophyll molecules
linked together and held tightly in the
thylakoid membrane by a matrix of
proteins
How the Antenna Complex Works
• When light of proper wavelength strikes any
pigment molecule within a photosystem, the light
is absorbed by that pigment molecule.
• The excitation energy is then transferred from
one molecule to another within the cluster of
pigment molecules until it encounters the
reaction center chlorophyll a.
• When excitation energy reaches the reaction
center chlorophyll, electron transfer is initiated.
Photosynthesis Overview
Sunlight
Light-dependent reactions
• Require light
• Capture energy from
sunlight
• Make ATP and reduce
NADP+ to NADPH
Photosystem
H2O
Thylakoid
O2
Light-Dependent
Reactions
ADP + Pi
CO2
Stroma
ATP
NADP+
Calvin
Cycle
NADPH
Organic
molecules
Carbon fixation reactions
• Does not require light
• Use ATP and NADPH
to synthesize organic
molecules from CO2
How the Antenna Complex Works
Photosystem
Photon
Chlorophyll
molecule
e– Electron
e–
donor
Electron
acceptor
Reaction center
chlorophyll
Thylakoid membrane
Reaction center
• Transmembrane protein–pigment complex
• When a chlorophyll in the reaction center
absorbs a photon of light, an electron is
excited to a higher energy level
• Light-energized electron can be
transferred to the primary electron
acceptor, reducing it
• Oxidized chlorophyll then fills its electron
“hole” by oxidizing a donor molecule
Excited
chlorophyll
molecule
Light
Electron
donor
Electron
acceptor
e–
e–
e–
e–
Chlorophyll
reduced
Donor
oxidized
Chlorophyll
oxidized
Acceptor
reduced
–
+
e–
e–
+
e–
–
e–
Question 7
What happens at the reaction center of a photosystem?
a. Light is absorbed
b. An electron is energized
c. NADP+ is reduced
d. ATP is formed
e. Carbon fixation occurs
Photosynthesis Overview
Sunlight
Light-dependent reactions
• Require light
• Capture energy from
sunlight
• Make ATP and reduce
NADP+ to NADPH
Photosystem
H2O
Thylakoid
O2
Light-Dependent
Reactions
ADP + Pi
CO2
Stroma
ATP
NADP+
Calvin
Cycle
NADPH
Organic
molecules
Carbon fixation reactions
• Does not require light
• Use ATP and NADPH
to synthesize organic
molecules from CO2
8.5 Light-Dependent Reactions
– Photon of light is captured by a pigment molecule
2. Charge separation
– Energy is transferred to the reaction center; an
excited electron is transferred to an acceptor
molecule
3. Electron transport
– Electrons move through carriers to reduce NADP+
4. Chemiosmosis
– Produces ATP
Capture of light energy
1. Primary photoevent
Cyclic Photophosphorylation
• In sulfur bacteria, only one photosystem is
used
• Generates ATP via electron transport
• Anoxygenic photosynthesis
• Excited electron passed to electron
transport chain
• Generates a proton gradient for ATP
synthesis
Energy of electrons
High
Low
Photon
Excited reaction center
e–
Electron
acceptor
b-c1
complex
e–
Reaction
center (P870)
e–
Electron
acceptor
Photosystem
ATP
Chloroplasts Have Two
Connected Photosystems
• Oxygenic photosynthesis
• Photosystem I (P700)
– Functions like sulfur bacteria
• Photosystem II (P680)
– Can generate an oxidation potential high enough to
oxidize water
• Working together, the two photosystems carry
out a noncyclic transfer of electrons that is
used to generate both ATP and NADPH
Absorption Spectra for Chlorophyll and Carotenoids.
high
Light
Absorbtion
carotenoids
chlorophyll a
chlorophyll b
low
400
450
500
550
600
Wavelength (nm)
650
700
The Two Photosystems Work Together
• Photosystem I transfers electrons
ultimately to NADP+, producing NADPH
• Electrons lost from photosystem I are
replaced by electrons from photosystem II
• Photosystem II oxidizes water to replace
the electrons transferred to photosystem I
• 2 photosystems connected by cytochrome/
b6-f complex
Noncyclic Photophosphorylation
• Plants use photosystems II and I in series
to produce both ATP and NADPH
• Path of electrons not a circle
• Photosystems replenished with electrons
obtained by splitting water
• Z diagram
Noncyclic Photophosphorylation
Z Diagram
Excited reaction center
2. The electrons pass through the b6-f
complex, which uses the energy
released to pump protons across
the thylakoid membrane. The proton
gradient is used to produce ATP by
Excited reaction center
chemiosmosis.
2
e–
Energy of electrons
–
2 e
NADP
reductase
NADP+ + H+
PC
Reaction
center
Proton gradient formed
for ATP synthesis
Photosystem II
1. A pair of chlorophylls in the reaction center absorb
two photons of light. This excites two electrons that
are transferred to plastoquinone (PQ). Loss of
electrons from the reaction center produces an
oxidation potential capable of oxidizing water.
Photon
3. A pair of chlorophylls in the reaction
center absorb two photons. This
excites two electrons that are passed to
NADP+, reducing it to NADPH. Electron
transport from photosystem II replaces
these electrons.
H2O
2H+ + 1/2O2
NADPH
Plastocyanin
H+
2
e–
Fd
2 e–
b6-f
complex
Photon
Ferredoxin
Plastoquinone
PQ
Reaction
center
2
e–
Photosystem I
• The enhancement effect: photosynthesis
is carried out by two systems that acts in
series
Rate of Photosynthesis
high
low
Far-red light on
Off Red light on
Time
Off Both lights on Off
Light-Dependent
Reactions
ADP + Pi
ATP
NADP
Photon
Photon
H+
ATP
NADPH
ADP
NADPH
Calvin
Cycle
Antenna
complex
Thylakoid
membrane
H+ + NADP+
Fd
2 e–
PQ
2 e–
Stroma
22 e–
2 e–
PC
H2O
Thylakoid
space
Plastocyanin
Plastoquinone
Water-splitting
enzyme
Ferredoxin
Proton
gradient
H+
H+
H+
H+
1/
2O2
2H+
Photosystem II
1. Photosystem II
absorbs photons,
exciting electrons
that are passed to
plastoquinone (PQ).
Electrons lost from
photosystem II are
replaced by the
oxidation of water,
producing O2
b6-f complex
2. The b6-f complex
receives electrons
from PQ and passes
them to plastocyanin
(PC). This provides
energy for the b6-f
complex to pump
protons into the
thylakoid.
Photosystem I
NADP
reductase
3. Photosystem I absorbs
photons, exciting
electrons that are
passed through a
carrier to reduce
NADP+ to NADPH.
These electrons are
replaced by electron
transport from
photosystem II.
ATP
synthase
4. ATP synthase uses
the proton gradient
to synthesize ATP
from ADP and Pi
enzyme acts as a
channel for protons
to diffuse back into
the stroma using this
energy to drive the
synthesis of ATP.
Chemiosmosis
• Electrochemical gradient can be used to
synthesize ATP
• Chloroplast has ATP synthase enzymes in
the thylakoid membrane
– Allows protons back into stroma
• Stroma also contains enzymes that
catalyze the reactions of carbon fixation –
the Calvin cycle reactions
Production of additional ATP
• Noncyclic photophosphorylation generates
– NADPH
– ATP
• Building organic molecules takes more energy
than that alone
• Cyclic photophosphorylation used to produce
additional ATP
– Short-circuit photosystem I to make a larger proton
gradient to make more ATP
Question 8
Which of the following is made during the light reactions?
a. ADP and NADP+
b. Glucose and ATP
c. ATP and NADPH
d. NADPH and Carbon Dioxide
e. Carbon Dioxide and Water
Section 8.6 Learning Objectives
• Understand the purpose of the Calvin
Cycle (Dark Reactions)
– Where does it take place
– What goes in, what comes out
– How is glucose made
46
8.6 Carbon Fixation – Calvin Cycle
• To build carbohydrates cells use
• Energy
– ATP from light-dependent reactions
– Cyclic and noncyclic photophosphorylation
– Drives endergonic reaction
• Reduction potential
– NADPH from photosystem I
– Source of protons and energetic electrons
Calvin Cycle
• Named after Melvin Calvin (1911–1997)
• Also called C3 photosynthesis
• Key step is attachment of CO2 to RuBP
(ribulose 1, 5 bisphosphate) to form PGA
• Uses enzyme ribulose bisphosphate
carboxylase/oxygenase or rubisco
Three Phases of Calvin Cycle
1. Carbon fixation
– RuBP + CO2 → PGA
2. Reduction
– PGA is reduced to G3P
3. Regeneration of RuBP
– PGA is used to regenerate RuBP
•
•
3 turns incorporate enough carbon to produce a
new G3P
6 turns incorporate enough carbon for 1
glucose
Stroma of chloroplast
6 molecules of
Light-Dependent
Reactions
ADP+ Pi
ATP
NADP+
Carbon
dioxide (CO2)
NADPH
Calvin
Cycle
12 molecules of
Rubisco
6 molecules of
3-phosphoglycerate (3C) (PGA)
Ribulose 1,5-bisphosphate (5C) (RuBP)
12 ATP
12 ADP
6 ADP
12 molecules of
Calvin Cycle
1,3-bisphosphoglycerate (3C)
6 ATP
12 NADPH
4
Pi
12 NADP+
10 molecules of
12 Pi
Glyceraldehyde 3-phosphate (3C)
12 molecules of
Glyceraldehyde 3-phosphate (3C) (G3P)
2 molecules of
Glyceraldehyde 3-phosphate (3C) (G3P)
Glucose and
other sugars
Output of Calvin Cycle
• Glucose is not a direct product of the
Calvin cycle
• G3P is a 3 carbon sugar
– Used to form sucrose
• Major transport sugar in plants
• Disaccharide made of fructose and glucose
– Used to make starch
• Insoluble glucose polymer
• Stored for later use
Energy Cycle
• Photosynthesis uses the products of respiration
as starting substrates
• Respiration uses the products of photosynthesis
as starting substrates
• Production of glucose from G3P even uses part
of the ancient glycolytic pathway, run in reverse
Heat
Sunlight
Photosystem
II
ADP + Pi
ATP
O2
Photosystem
I
NADP+
H2O
NADPH
Electron
Transport
System
ATP
ADP + Pi
NAD+
NADH
Calvin
Cycle
CO2
Pyruvate
Glucose
ATP
Krebs
Cycle
ATP
ADP + Pi
Question 9
What is the function of Rubisco?
a. Absorption of photon energy
b. Harvesting electrons from water
c. Reduction of NADP+
d. Chemiosmosis
e. Carbon fixation
Question 13
If a plant had non-functioning mitochondria, it could still
successfully complete photosynthesis.
a. This is true
b. This is false
Section 8.7 Learning Objectives
• Understand photorespiration
– What problem does rubisco have?
• How can some plants deal with
photorespiration
56
Photorespiration
• Rubisco has 2 enzymatic activities
– Carboxylation
• Addition of CO2 to RuBP
• Favored under normal conditions
– Photorespiration
• Oxidation of RuBP by the addition of O2
• Favored when stoma are closed in hot conditions
• Creates low-CO2 and high-O2
• CO2 and O2 compete for the active site on
RuBP
• Photorespiration reduces the carbohydrate
yield of photosynthesis
• Under hot, arid conditions, leaves lose water by
evaporation through openings in the leaves called
stomata.
• The stomata close to conserve water but as a result, O2
builds up inside the leaves, and CO2 cannot enter the
leaves, favoring photorespiration.
Leaf
epidermis
Heat
H2O
H2O
Stomata
O2
CO2
O2
CO2
Question 12
Where could a botanist expect to find C4 plants?
a. Canada
b. Costa Rica
c. Only in trees
d. Tundra
e. Mount Everest
Question 17
What would happen to the rate of photosynthesis if
the light levels remained the same and the carbon
dioxide levels were increased?
a. Rate of photosynthesis would increase
b. Rate of photosynthesis would decrease
c. Rate of photosynthesis would remain the
same
d. There is no relationship between the two