Photosynthesis
Light-Dependent Reactions
Calvin Cycle
Observe the animations
highlighted above
Fig. 10-2
• Photosynthesis occurs
in plants, algae,
certain other protists,
and some prokaryotes
BioFlix: Photosynthesis
– These organisms feed
not only themselves but
also most of the living
world
(a) Plants
10 µm
(c) Unicellular protist
(e) Purple sulfur
bacteria
(b) Multicellular alga
(d) Cyanobacteria
40 µm
1.5 µm
Structures of Photosynthesis
• Chloroplasts are structurally similar to and
likely evolved from photosynthetic bacteria
• Leaves are the major
locations of
photosynthesis
• Their green color is from
chlorophyll, the green
pigment within
chloroplasts
• CO2 enters and O2 exits
the leaf through
microscopic pores called
stomata
Fig. 10-3a
Leaf cross section
• Chloroplasts
are found
mainly in cells
of the
mesophyll, the
interior tissue
of the leaf
– A typical
mesophyll
cell has 30–
40
chloroplasts
• Thylakoid
• Grana
• Stroma
Vein
Mesophyll
Stomata
Chloroplast
CO2
O2
Mesophyll cell
5 µm
The Photosynthesis Equation
6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2O
Fig. 10-4
Reactants:
Products:
6 CO2
C6H12O6
12 H2O
6 H2 O
6 O2
• Lightdependent
reactions
– Occurs in
Thylakoid
– Used H2O
and light to
produce ATP,
NADPH, and
O2
– NADPH is an
electron
carrier
• Calvin cycle
– Occurs in stroma
– uses carbon dioxide, ATP, and NADPH to
produce sugars
The Goal of Photosynthesis is to form high energy
sugars.
This requires transforming light energy into usable
chemical energy (ATP)
• ATP is form by the process of:
• Photophosphorylation –
– The production of ATP using energy derived
from the redox reactions of an electron
transport chain.
Redox Reactions
• A chemical reaction involving the transfer of
one or more elections from one reactant to
another; also called oxidation/reduction
reactions
• In oxidation, a
substance loses
electrons, or is
oxidized
• In reduction, a
substance gains
electrons, or is
reduced (the
amount of
positive charge
is reduced)
Fig. 9-UN1
becomes oxidized
(loses electron)
becomes reduced
(gains electron)
Fig. 9-UN2
becomes oxidized
becomes reduced
The Two Stages of Photosynthesis: A Preview
• Photosynthesis consists of the light reactions (the
photo part) and Calvin cycle (the synthesis part)
• The light reactions (in the
thylakoids):
– Split H2O
– Release O2
– Reduce NADP+ to
NADPH
– Generate ATP from ADP
by
photophosphorylation
• The Calvin cycle (in the
stroma) forms sugar from
CO2, using ATP and
NADPH
• The Calvin cycle begins
with carbon fixation,
incorporating CO2 into
organic molecules
Fig. 10-5-1
H2O
Light
NADP+
ADP
+ P
Light
Reactions
Chloroplast
i
Fig. 10-5-2
H2O
Light
NADP+
ADP
+ P
i
Light
Reactions
ATP
NADPH
Chloroplast
O2
Fig. 10-5-3
CO2
H2O
Light
NADP+
ADP
+ P
i
Light
Reactions
ATP
NADPH
Chloroplast
O2
Calvin
Cycle
Fig. 10-5-4
CO2
H2O
Light
NADP+
ADP
+ P
i
Light
Reactions
Calvin
Cycle
ATP
NADPH
Chloroplast
O2
[CH2O]
(sugar)
Fig. 10-7
The light reactions convert solar energy to the
chemical energy of ATP and NADPH
Light
• Chloroplasts
are solarpowered
chemical
factories
– Their
thylakoids
transform
light energy
into the
chemical
energy of
ATP and
NADPH
Reflected
light
Chloroplast
Absorbed
light
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Granum
Transmitted
light
The Nature of Sunlight
• Light is a form of electromagnetic energy
• The electromagnetic spectrum is the entire range of
electromagnetic energy, or radiation
• Visible light consists of wavelengths (including those
that drive photosynthesis) that produce colors we can
see
Wavelength is the
distance between
crests of waves
Wavelength
determines the type
of electromagnetic
energy
Fig. 10-6
10–5 nm 10–3 nm
103 nm
1 nm
Gamma
X-rays
rays
UV
106 nm
Infrared
1m
(109 nm)
Microwaves
103 m
Radio
waves
Visible light
380
450
500
Shorter wavelength
Higher energy
550
600
650
700
750 nm
Longer wavelength
Lower energy
Light and Pigments
• Pigments –
light
absorbing
chemicals
• Chlorophyll –
principle
pigment in
plants
–
–
–
–
Chlorophyll a
Chlorophyll b
Carotenoids
Xanthophyll
Why do leaves change colors?
• Chlorophyll
a
• Chlorophyll
b
Component of a Chloroplast
• Thylakoid – Saclike
photosynthetic
membranes
– Light-dependent
reactions occur here
• Granum – Stack of
thylakoids
• Stroma – Region
outside the thylakoid
membrane
– Reactions of the Calvin
Cycle occur here
The Light-Dependent Reactions
• Photophosphorylation is the process of
creating ATP using a Proton gradient
created by the Energy gathered from
sunlight.
• Chemiosmosis is the process of using
Proton movement to join ADP and Pi. This
is accomplished by enzymes called ATP
synthases or ATPases.
NADP+ + e- + Energy  NADPH
• NADP+ (Nicotinamide
adenine dinucleotide
phosphate)
– Electron, hydrogen, and energy
carrier
Light-Dependant
Reactions
1. Photosystem II
• Chlorophyll absorbs light
• Electrons on a chlorophyll molecule (p680)
absorb energy from light and become
“energized”
• High-energy (“energized”) electrons are passed
on to the electron transport chain
– Electrons are passed to pheophytin molecule then to
plastoquinone Qa then to plastoquinone Qb then to ETC.
• Chlorophyll’s electrons are replenished by the
breakdown of H2O
• H2O is broken down into 2H+ ions, O2, and 2 e-.
Electrons are used to replenish chlorophyll’s lost
electrons.
2. Electron Transport Chain
• The molecules of the electron transports
chain use high-energy electrons to push
H+ ions from the stroma into the inner
thylakoid space.
3. Photosystem I
• Chlorophyll absorbs light-energy and reenergized the electrons from photosystem II.
• NADP+ picks up these high-energy
electrons and H+ to become NADPH.
4. Hydrogen Ions
• Chemiosmosis
• Electrochemical Gradient
• Hydrogen ions build up inside the thylakiod
membrane.
– High concentration of H+ inside the membrane (Strong
Positive Charge)
– Low concentration of H+ outside the membrane
(Negative Charge)
– Provides the energy to form ATP
5. ATP formation
• H+ work to reach equilibrium.
• Pass through the ATPsynthase
• Movement of H+ ions through the
ATPsynthase powers ATP production
Do Now…
• What is the function of NADPH?
• How is light energy converted into chemical energy
during photosynthesis?
• Can the complete process of photosynthesis take place
in the dark? Explain your answer.
• Explain what happens to a molecule of water in the light
dependant phase of photosynthesis.
• If O2 is a waste/byproduct of photosynthesis, track where
it came from to where it exits the plant.
Calvin Cycle
The Calvin Cycle
1.
2.
3.
4.
5.
6 CO2 molecules enter the cycle.
Enzyme “rubisco” combines 6 5-carbon (RuBp)
molecules with the carbon from CO2 and forms them
into 12 3-carbon molecules
12 ATP and 12 NADPH form the
12 3-carbon molecules into
12 High-energy 3-carbon molecules (G3P)
2 (G3P)of the 12 3-carbon molecules are combined to
form a 6-carbon sugar
6 ATP molecules are used to convert the 10 remaining
3-carbon molecules back into the 6 5-carbon
molecules the cycle began with (RuBp)
Calvin Cycle
Factors Affecting Photosynthesis
• Water supply
• Amount of sunlight
• Temperature
Types of Photosynthesis
• C3 Photosynthesis
• C4 Photosynthesis
• CAM Photosynthesis
C3 Photosynthesis : C3 plants.
• Called C3 because the CO2 is first incorporated into a 3carbon compound.
• Stomata are open during the day.
• RUBISCO, the enzyme involved in photosynthesis, is
also the enzyme involved in the uptake of CO2.
• Photosynthesis takes place throughout the leaf.
• Adaptive Value: more efficient than C4 and CAM plants
under cool and moist conditions and under normal light
because requires less machinery (fewer enzymes and
no specialized anatomy)..
• Most plants are C3.
C4 Photosynthesis : C4 plants.
• Called C4 because the CO2 is first incorporated into a 4-carbon
compound.
• Stomata are open during the day.
• Uses PEP Carboxylase for the enzyme involved in the uptake of CO2.
This enzyme allows CO2 to be taken into the plant very quickly, and
then it "delivers" the CO2 directly to RUBISCO for photsynthesis.
• Photosynthesis takes place in inner cells (requires special anatomy
called Kranz Anatomy)
• Adaptive Value:
• Photosynthesizes faster than C3 plants under high light intensity and
high temperatures because the CO2 is delivered directly to RUBISCO,
not allowing it to grab oxygen and undergo photorespiration.
• Has better Water Use Efficiency because PEP Carboxylase brings in
CO2 faster and so does not need to keep stomata open as much (less
water lost by transpiration) for the same amount of CO2 gain for
photosynthesis.
• C4 plants include several thousand species in at least 19 plant families.
Example: fourwing saltbush pictured here, corn, and many of our
summer annual plants.
CAM Photosynthesis : CAM plants. CAM stands for
Crassulacean Acid Metabolism
• Called CAM after the plant family in which it was first found
(Crassulaceae) and because the CO2 is stored in the form of an acid
before use in photosynthesis.
• Stomata open at night (when evaporation rates are usually lower) and are
usually closed during the day. The CO2 is converted to an acid and stored
during the night. During the day, the acid is broken down and the CO2 is
released to RUBISCO for photosynthesis
• Adaptive Value:
– Better Water Use Efficiency than C3 plants under arid conditions due to
opening stomata at night when transpiration rates are lower (no sunlight,
lower temperatures, lower wind speeds, etc.).
– May CAM-idle. When conditions are extremely arid, CAM plants can just leave
their stomata closed night and day. Oxygen given off in photosynthesis is
used for respiration and CO2 given off in respiration is used for
photosynthesis. This is a little like a perpetual energy machine, but there are
costs associated with running the machinery for respiration and
photosynthesis so the plant cannot CAM-idle forever. But CAM-idling does
allow the plant to survive dry spells, and it allows the plant to recover very
quickly when water is available again (unlike plants that drop their leaves and
twigs and go dormant during dry spells).
• CAM plants include many succulents such as cactuses and agaves and
also some orchids and bromeliads