Transcript Photosynthesis: Where it all begins!

Photosynthesis: Where it all
begins!
AP Biology
Us versus Them
• Autotrophs make their
own food (selfnourishing)
• Photoautotrophs use
sunlight as the energy
source
• Heterotrophs must
feed on autotrophs,
one another, or waste
Photosynthesis
• Is the main pathway by which carbon and
energy enter the web of life.
• Where do we find carbon in living things?
An overview
12 H2O + 6 CO2
6O2 + C6H12O6 + 6H2O
Sunlight
Two divisions: Light dependent reactions,
which yields ATP and H+
Light independent reactions, which uses the
products of the light dependent reactions to
make glucose
An Overview
• Takes place in the chloroplasts
• Two outer membranes surrounding a mostly fluid
interior called the stroma
• Another folded membrane is stacked in the stroma.
• The stacks of thylakoid discs are grana (granum)
Stroma
Double
membrane
Thylakoids
Light Dependent Reactions
light
light reactions
ATP
NADPH
During photosynthesis, CO2 will be reduced (gain electrons)
to form glucose. The electrons needed to reduce CO 2 are
temporarily carried by NADPH.
Light | Pigments | Overview | Chloroplast | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return
• Sunlight splits water
molecules
• Oxygen diffuses away
• Its electrons flow
through electron
transfer chains
• This forms ATP
• Coenzyme NADP
picks up the electrons
and hydrogen
Light Dependent Reactions
H2O
light
O2
light reactions
ATP
NADPH
Recall that hydrogen atoms can be used to carry
electrons. NADPH gets its electrons from water.
The oxygen is not used.
Light | Pigments | Overview | Chloroplast | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return
One Needs the Other
H2O
light
light reactions
ATP
C02
O2
NADPH
light-independent reactions
(Calvin cycle)
C6H12O6
The reduction of CO2 to glucose occurs
in the light-independent reactions.
Light | Pigments | Overview | Chloroplast | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return
Light Independent Reactions
H2O
light
O2
light reactions
ATP
C02
•
•
•
•
NADPH
ADP
NADP+
light-independent reactions
(Calvin cycle)
C6H12O6
Light | Pigments | Overview | Chloroplast | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return
Occur in the stroma
Does not require light
ATP gives up energy
Coenzyme NADPH
gives up electrons and
hydrogen
• CO2 is dismantled for
its C and O atoms
• Glucose is made
Not Really Glucose
• Glucose is quickly
changed to sucrose,
cellulose, or starch
Properties of Light
• Light travels in waves
• The distance between two crests is a
wavelength
• The shorter the wavelength, the higher the
energy
• All wavelengths combined appear as white
Light travels in waves. The color of light is
determined by its wavelength. The red light
shown below has a wavelength of 700 nm.
Wavelength
700 nm
Red
470 nm
Notice that blue light has a shorter wavelength.
Blue
Light | Pigments | Chloroplast | Overview | Photosystems | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review |Return
Electromagnetic Spectrum
Visible light is only a part of the
electromagnetic spectrum.
nanometers
10-5
10-3
Gamma
rays
103
1
X-rays
UV
106
Infrared
1m
Microwaves
103 m
Radio waves
Visible light
7
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Photons
• The energy of light
has a particle-like
quality
• Energy, when
absorbed, can be
measured as packets
called photons
• Each photon has a
fixed amount of
energy
Pigments
•
•
•
•
Absorb wavelengths of light
Most absorb only certain wavelengths
Reflect back or transmit the others
Chlorophyll looks green because it does
NOT absorb green wavelengths
Chlorophyll a
absorption
Chlorophyll b
This graph shows the color of
light absorbed by three different
kinds of photosynthetic
pigments. Notice that they do
not absorb light that is in the
green to yellow range.
Carotenoids
400
500
600
Wavelength
700
Light | Pigments | Chloroplast | Overview | Photosystems | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review |Return
Accessory Pigments
• Carotenoids
• Phycobilin
– Phycoerythrin
– Phycocyanin
• Anthocyanins
Light Dependent Reactions
A Closer Look
Produces ATP and NADPH
Photosystems
• In the thylakoid
membrane, pigments
are organized in
clusters called
photosystems
• Photons of light are
absorbed by pigment,
and the pigment’s
electrons get
“excited.”
Excited Electrons
• When electrons of an
atom absorb energy,
they move to a higher
energy level
• Energy entering a
pigment destabilizes
the arrangement of
electrons. (They jump
around)
Photosynthetic Pigments
Light behaves as if it is composed of units or
packets called photons.
Ph
oto
n
Plants have pigment molecules that contain atoms that become
energized when they are struck by photons of light.
Energized electrons move further from the nucleus.
11
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Excited Electrons
• The excited electrons quickly return to a lower
energy level, stabilizes, and some energy is
released in the form of light or heat.
(fluorescence.)
• In photosynthesis, this releasing energy gets
passed on to another pigment in a random “walk”
Some is lost as heat. The remaining energy
matches to a wavelength that the photosystem’s
reaction center can trap.
• The reaction center passes the energy in the form
of excited electrons to an electron transport chain
Light energy
A pigment molecule within the antenna absorbs a photon of
light energy. The energy from that pigment molecule is
passed to neighboring pigment molecules and eventually
makes its way to pigment molecule called the reaction
center. When the reaction center molecule becomes excited
(energized), it loses an electron to an electron acceptor.
Thylakoid
membrane
Electron acceptor
Reaction center
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The antenna and electron acceptor are called a photosystem.
There are two kinds of photosystems in plants called photosystem I and photosystem II.
Photosystem I is sometimes called P700 and photosystem II is sometimes P680. The 680
and 700 designations refer to the wavelength of light that they absorb best.
Photosystem
Antenna
Thylakoid
membrane
Electron acceptor
Reaction center
Light | Pigments | Overview | Chloroplast | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return
The Photosystems
• P700 is photosystem I, and can be cyclic.
P700 can cycle alone, or can receive
electrons from P680 as part of a non-cyclic
pathway.
• When it cycles alone, light energy excites
electrons, boosting them to a higher energy
level, and sending them through an electron
transport chain. The end product of the
electron transport chain is ATP.
Photosystems
• P680 is photosystem II and is non-cyclic. It
receives energy from light, boosting
electrons to a higher energy level and
sending them through an electron transport
chain to P700. The electrons it gives up
from the pigments are replaced by the
splitting of water (constant supply) p114
Photosystems
• Both electron transport chains use the
energy from the “falling” electrons to pump
H ions to the inside of the thylakoid
membrane, resulting in a concentration
gradient
• When H ions move through ATP synthase,
the energy is used to attach ADP to P,
making ATP, needed for the light indep.
How ATP is made
• Hydrogen ions from the splitting of water
accumulate in the thylakoid membrane.
• Electron transport chains build up even more
hydrogen ions in the thylakoid.
• Ions are pumped from the stroma into the
thylakoid.
• Sets up a concentration gradient
• When they flow into stroma, they are used to form
ATP from ADP and P (ATP synthase enzyme
required)
Light
Energy
Photosystem II
Chloroplast
Photosystem I
The three blue circles represent the electron transport system. They are proteins
embedded within the thylakoid membrane.
The first protein receives the electron (and energy) from the electron acceptor.
Stroma
Light | Pigments | Overview | Chloroplast | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return
Light
Energy
Chloroplast
H+
H+
H+
H+
H+
H+
H+
H+
As a result of gaining an electron (reduction), the first carrier of the
electron transport system gains energy. It uses some of the energy to
pump H+ into the thylakoid.
Thylakoids
Stroma
Light | Pigments | Overview | Chloroplast | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return
Light
Energy
Chloroplast
H+
H+
H+
H+
H+
H+
H+
The carrier then passes the electron to the next carrier. Because it
used some energy to pump H+, it has less energy (reducing capability)
to pass to the next H+ pump.
Thylakoids
Stroma
Light | Pigments | Overview | Chloroplast | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return
Light
Energy
Chloroplast
H+
H+
H+
H+
H+
H+
H+
H+
This carrier uses some of the remainder of the energy to pump more
H+ into the thylakoid.
Thylakoids
Stroma
Light | Pigments | Overview | Chloroplast | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return
Light
Energy
Chloroplast
H+
H+
H+
H+
H+
H+
H+
H+
The electron is passed to the next carrier which also pumps H+.
Thylakoids
Stroma
Light | Pigments | Overview | Chloroplast | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return
Light
Energy
Chloroplast
H+
H+
H+
H+ H+
H+
H+
H+
The electron transport system functions to create a concentration
gradient of H+inside the thylakoid.
Thylakoids
Stroma
Light | Pigments | Overview | Chloroplast | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return
Light
Energy
Chloroplast
The concentration gradient of H+ is used to synthesize ATP.
ATP is produced from ADP and Pi when hydrogen ions pass
out of the thylakoid through ATP synthase.
H+
H+
H+ H+
H+
H+
H+
ATP
ADP + Pi
H+
Thylakoids
Stroma
Light | Pigments | Overview | Chloroplast | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return
Light
Energy
Chloroplast
This method of synthesizing ATP by using a H+ gradient
in the thylakoid is called photophosphorylation.
H+
H+
H+
H+
H+
H+
H+
ATP
ADP + Pi
H+
Thylakoids
Stroma
Light | Pigments | Overview | Chloroplast | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return
This method of making ATP
• Is called the chemiosmotic model for ATP
production
• Will also happen in the mitochrondria
• In which place do you think more ATP will
be made?
Light Independent Reactions
A Closer Look: takes the ATP and
NADPH from light dependent and
makes glucose
A Cycle
• Called the CalvinBenson cycle.
• ATP drives the
reactions
• NADPH delivers
hydrogen and
electrons
• CO2 provides the
carbon
Photophosphorylation
• Is a specific type of Chemiosmosis
• Chemiosmotic theory refers to the method
of building up H+ concentration gradient to
make ATP.
CO2 Fixation
6 CO2
6 C-C-C-C-C-C
RuBP
Carboxylase
(rubisco)
The enzyme that catalyzes this reaction is
ribulose biphosphate carboxylase (rubisco).
6 C-C-C-C-C
CO2 fixation refers to bonding CO2 to an organic molecule to make a larger
molecule.
Each CO2 is bonded to ribulose biphosphate (RuBP).
C5 + CO2  C6
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C3 Photosynthesis
6 CO2
6 C-C-C-C-C-C
RuBP
Carboxylase
(rubisco)
RuBP 6 C-C-C-C-C
PGA 12 C-C-C
Each of these 6-carbon compounds splits to form
two 3-carbon compounds called phosphoglycerate.
Light | Pigments | Chloroplast | Overview | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return
6 CO2
6 C-C-C-C-C-C
RuBP
Carboxylase
(rubisco)
RuBP 6 C-C-C-C-C
PGA 12 C-C-C
PGAL
12 C-C-C
The two molecules of PGA are reduced to
form PGAL (phosphoglyceraldehyde).
Light | Pigments | Chloroplast | Overview | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return
6 CO2
6 C-C-C-C-C-C
RuBP
Carboxylase
(rubisco)
RuBP 6 C-C-C-C-C
PGA 12 C-C-C
PGAL
12 C-C-C
12 ATP
12 ADP + P
12 NADPH
12 NADP+
Light | Pigments | Chloroplast | Overview | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return
6 CO2
6 C-C-C-C-C-C
RuBP
Carboxylase
(rubisco)
RuBP 6 C-C-C-C-C
PGA 12 C-C-C
6 ADP + P
Two of the PGAL are used6toATP
form glucose phosphate, then glucose.
10 C-C-C
PGAL
12 C-C-C
12 ATP
12 ADP + P
12 NADPH
C-C-C-C-C-C Glucose
12 NADP+
Light | Pigments | Chloroplast | Overview | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return
6 CO2
6 C-C-C-C-C-C
RuBP
Carboxylase
(rubisco)
RuBP 6 C-C-C-C-C
PGA 12 C-C-C
6 ADP + P
The remaining 10 PGAL are
rearranged to form 6 RuBP.
6 ATP
10 C-C-C
PGAL
12 C-C-C
12 ATP
12 ADP + P
12 NADPH
C-C-C-C-C-C Glucose
12 NADP+
Light | Pigments | Chloroplast | Overview | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return
6 CO2
6 C-C-C-C-C-C
RuBP
Carboxylase
(rubisco)
RuBP 6 C-C-C-C-C
PGA 12 C-C-C
6 ADP + P
6 ATP
This process requires
energy in the form of ATP.
10 C-C-C
PGAL
12 C-C-C
12 ATP
12 ADP + P
12 NADPH
C-C-C-C-C-C Glucose
12 NADP+
Light | Pigments | Chloroplast | Overview | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return
H2O
Light
Energy
Chloroplast
NADP+
light
O2
+
H+
NADPH
+
H+ + H H+ H+
H H+
ATP
H+
H2O2e- + 2H+ + ½ O2
Light reactions
ADP + Pi
H+
Thylakoids
Stroma
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ATP
NADPH
ADP
NADP+
6 CO2
C02
RuBP
Carboxylase
(rubisco)
6 C-C-C-C-C-C
6 C-C-C-C-C RuBP
6 ADP + P
6 ATP
10 C-C-C
PGAL
12 C-C-C
C-C-C-C-C-C
Glucose
PGA 12 C-C-C
12 ATP
12 ADP + P
12 NADPH
12 NADP +
Light-independent
reactions
C6H12O6
Light | Pigments | Chloroplast | Overview | Photosystem II | Electron Transport System | Photosystem I | Calvin Cycle | Photorespiration | C4 plants | Review | Return
Cyclic vs Non-cyclic
• Cyclic electron flow
involves the P700
photosystem only
• Electrons are boosted
in energy, passed thru
an electron transport
chain, producing ATP,
and returned to the
P700 photosystem
• Noncyclic: electrons in
P680 are boosted in
energy, go thru the electron
transport chain, producing
ATP. The electrons still
carry some energy, they
move on to P700 and go
through a second ETC,
producing ATP AND
NADPH
Electrons do not return to
the P680. NADP is the
final electron acceptor.
Cyclic vs Non-cyclic
Non-cyclic
C3, CF4, and CAM Plants
• Stomata (openings for
gas and water
exchange) close in dry
weather to conserve
water
• But this means that
CO2 in and O2 out is
also stopped
C3, CF4, and CAM Plants
Halts the light
independent reactions,
but the light dependent
reactions continue
O2 builds up and triggers
an alternate pathway
called photorespiration
Inefficient backdoor way
to make small amounts
of glucose
• This is what happens
to most plants (C3.)
• Diagram on page 117
C4 plants have a better answer
• O2 also builds up here
when stomata close,
but an additional step
keeps the CO2
concentration higher
than the O2
concentration, so the
inefficient
photorespiration is not
triggered.
• Involves two different
cells
• Diagram page 117
CAM plants
• Like C4 plants, CAM
plants use a C4 cycle
and the Calvin-Benson
cycle.
• But the cycles occur in
the same cell, but one
during day and the
other at night.