PHOTOSYNTHESIS
What is Photosynthesis?
Photosynthesis is the process that converts
solar energy into chemical energy
Directly or indirectly, photosynthesis
nourishes almost the entire living world
OBTAINING ENERGY
 Organisms can be classified according to how
they get energy
 Organisms that use energy from sunlight are
called autotrophs
 Most autotrophs use the process of
photosynthesis to convert light energy from the
sun into chemical energy in the form of organic
compounds, mostly carbohydrates.
Autotrophs are the producers of the
biosphere, producing organic molecules
from CO2 and other inorganic molecules
Almost all plants are photoautotrophs,
using the energy of sunlight to make
organic molecules
What is photosynthesis?
Photosynthesis involves a series of
chemical reactions (biochemical pathway)
where the product of one reaction is
consumed in the next reaction
Photosynthesis produces chemical energy
in the form of glucose
The ultimate source of energy for all life is
the sun
Plants
Algae
Unicellular
eukaryotes
cyanobacteria
Purple sulfur bacteria
Photosynthesis occurs in plants, algae,
certain other protists, and some prokaryotes
These organisms feed not only themselves
but also most of the living world
The next picture…
 Shows how autotrophs use photosynthesis to
produce organic compounds from carbon
dioxide and water
1. O2 & organic compounds produced are used to
create cellular respiration
2. Cellular Respiration, the CO2 & H2O are
produced
3. The products of photosynthesis are reactants in
cellular respiration, & vice versa
Light
Energy
Photosynthesis
by autotrophs
Carbon dioxide
& water
Organic compounds
& oxygen
Cellular Respiration
by autotrophs
& heterotrophs
Capturing Light Energy
The 1st stage of photosynthesis includes the
light dependent reaction b/c they require
light to happen
The light reactions begin with the
absorption of light in chloroplasts {found
in cells of plants, bacteria & algae}
Internal Membranes of Chloroplasts
Chloroplast have a double membrane:
inner & outer
Stroma is the solution surrounding the
grana
thylakoid are arranged as flatten sacs
Grana are stacks of thylakoid;
– this is where the dark reaction (light
independent reaction) will occur
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chloroplast
Inner membrane
Outer membrane
Granum
Stroma
Thylakoid
Photosynthesis converts light energy to the
chemical energy of food
The structural organization of photosynthetic
cells includes enzymes and other molecules
grouped together in a membrane
This organization allows for the chemical
reactions of photosynthesis to proceed efficiently
Chloroplasts are structurally similar to and likely
evolved from photosynthetic bacteria
Chloroplasts: The Sites of Photosynthesis in
Plants
Leaves are the major locations of
photosynthesis
Their green color is from chlorophyll, the
green pigment within chloroplasts
Chloroplasts are found mainly in cells of
the mesophyll, the interior tissue of the leaf
Each mesophyll cell contains 30–40
chloroplasts
CO2 enters and O2 exits the leaf through
microscopic pores called stomata
The chlorophyll is in the membranes of
thylakoids (connected sacs in the
chloroplast); thylakoids may be stacked in
columns called grana
Chloroplasts also contain stroma, a dense
interior fluid
Leaf cross section
Chloroplasts
Vein
Mesophyll
Stomata
CO2 O2
Chloroplast
Mesophyll cell
Thylakoid
Granum Thylakoid
Stroma
space
Outer
membrane
Intermembrane
space
Inner membrane
1 m
20 m
Equation for Photosynthesis
Photosynthesis is a complex series of reactions
that can be summarized as the following
equation
6CO2 + 6H2O
 Carbon dioxide
water
 C6H12O6 + 6O2
light
glucose
oxygen
The Splitting of Water
Chloroplasts split H2O into hydrogen and
oxygen, incorporating the electrons of
hydrogen into sugar molecules and
releasing oxygen as a by-product
Reactants:
Products:
6 CO2
C6H12O6
12 H2O
6 H2O
6 O2
Photosynthesis as a Redox Process
Photosynthesis reverses the direction of
electron flow compared to respiration
Photosynthesis is a redox process in which
H2O is oxidized and CO2 is reduced
Photosynthesis is an endergonic process;
the energy boost is provided by light
The Two Stages of Photosynthesis
Photosynthesis consists of the
1. Light Dependent Reactions
(the photo part)
2. Light Independent Reaction/Dark
Reaction/Calvin cycle
(the synthesis part)
Light Dependent Reaction
Light Dependent Reactions: light energy
(absorbed from the sun) is converted to
chemical energy, which is temporarily
stored in ATP and the energy carrier
molecule NADPH
Light reaction takes place in the thylakoid
membrane
Light Dependent Reaction
The light reactions
–
–
–
–
Split H2O
Release O2
Reduce the electron acceptor, NADP  NADPH
Generate ATP: ADP + P  ATP
Calvin Cycle/Dark Reaction
Light Independent Reaction
Calvin cycle forms sugar from CO2, using
ATP and NADPH
The Calvin cycle begins with carbon
fixation, incorporating CO2 into organic
molecules
Calvin cycle takes place in the stroma
Thylakoid
Stroma
The Stroma
The stroma houses
the enzymes needed
to assemble organic
molecules from
CO2, using energy
from ATP &
NADPH
Granum
H2O
CO2
Light
NADP
ADP
 Pi
Light
Reactions
Calvin
Cycle
ATP
NADPH
Chloroplast
O2
[CH2O]
(sugar)
Convert solar energy to ATP
and NADPH
Chloroplasts are solar-powered
chemical factories
Their thylakoids transform light energy
into the chemical energy of ATP and
NADPH
The Nature of Sunlight
Light is a form of electromagnetic
energy, also called electromagnetic
radiation
– Like other electromagnetic energy, light
travels in rhythmic waves
Wavelength is the distance between
crests of waves
– Wavelength determines the type of
electromagnetic energy
Electromagnetic spectrum
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
Light also behaves as though it consists of
discrete particles, called photons
Properties of light
 Light from the sun appears white, but is actually made
of a variety of colors
 White light can be separated into its components by
passing it through a prism
 The resulting array of colors, ranging from red to
violet called the visible light spectrum
 ROY G BIV (rainbow) makes up the visible light
spectrum
Light
 When white light strikes an object, its component
colors can be reflected, transmitted or absorbed by the
object.
 Many objects contain pigments, compounds which
absorb light
– Most pigments absorb certain colors more strongly than
others, which subtracts those colors from the visible
spectrum
– Therefore, the light that is reflected or transmitted by the
pigment no longer appears white
White light
White light contains a variety
of colors. Each color has a
different wavelength
measured in nanometers.
10−5 nm 10−3 nm
Gamma
rays
103 nm
1 nm
X-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
Photosynthetic Pigments: The Light
Receptors
Pigments are substances that absorb visible
light
Different pigments absorb different
wavelengths
Leaves appear green because chlorophyll
reflects and transmits green light
Light
Reflected
light
Chloroplast
Absorbed
light
Granum
Transmitted
light
Spectrophotometer
A spectrophotometer measures a pigment’s
ability to absorb various wavelengths
This machine sends light through pigments and
measures the fraction of light transmitted at each
wavelength
An absorption spectrum is a graph plotting a
pigment’s light absorption versus wavelength
Chloroplast Pigments
Located in the membrane of the thylakoids are
several pigments called chlorophylls
There are several different types of
chlorophyll
2 most common are called
1. chlorophyll a
2. chlorophyll b
Chlorophyll
The absorption spectrum of chlorophyll a
suggests that violet-blue and red light work best
for photosynthesis
Accessory pigments include chlorophyll b and a
group of pigments called carotenoids
An action spectrum profiles the relative
effectiveness of different wavelengths of radiation
in driving a process
Absorption of light
by chloroplast
pigments
Results
Chlorophyll a
Chlorophyll b
Carotenoids
500
600
Wavelength of light (nm)
400
700
(a) Absorption spectra
Rate of
photosynthesis
(measured by O2
release)
Figure 8.9
400
(b) Action spectrum
500
600
700
Aerobic bacteria
Filament
of alga
500
400
(c) Engelmann’s experiment
600
700
Pigments
Chlorophyll a is the main photosynthetic pigment
Accessory pigments, such as chlorophyll b, broaden
the spectrum used for photosynthesis
A slight structural difference between chlorophyll a
and chlorophyll b causes them to absorb slightly
different wavelengths
Accessory pigments called carotenoids absorb
excessive light that would damage chlorophyll
Chlorophyll a & Chlorophyll b
Chlorophyll a absorbs less blue light, but
more red light than chlorophyll b
Neither chlorophyll a nor chlorophyll b
absorbs much green light
– Instead, they allow green light to be reflected
or transmitted
– For this reason, leaves & plants with large
amounts of chlorophyll look green.
Other pigment components
Other compounds found in the thylakoid
membrane include the yellow, orange, and
brown carotenoids, which also function as
accessory pigments
By absorbing colors that chlorophyll a
cannot absorb, the accessory pigments
enable plants to capture more of the energy
in light
Excitation of Chlorophyll by Light
When a pigment absorbs light, it goes from
a ground state to an excited state, which is
unstable
When excited electrons fall back to the
ground state, photons are given off, an
afterglow called fluorescence
If illuminated, an isolated solution of
chlorophyll will fluoresce, giving off light and
heat
Absorption of light
by chloroplast
pigments
Figure 8.9a
Chlorophyll a
Chlorophyll b
Carotenoids
500
600
Wavelength of light (nm)
(a) Absorption spectra
400
700
Theodor W. Engelmann
The action spectrum of photosynthesis was
first demonstrated in 1883 by Theodor W.
Engelmann
In his experiment, he exposed different
segments of a filamentous alga to different
wavelengths
Areas receiving wavelengths
favorable to photosynthesis
produced excess O2
He used the growth of
aerobic bacteria clustered
along the alga as a measure
of O2 production
He discovered the colors of
light (red & violet) that drive
photosynthesis in
photosynthetic algae
In leaves of a plant
 In the leaves of a plant,
the chlorophylls are more
abundant & mask the
color of the other
pigments
 But in the
nonphotosynthetic parts
of a plant (fruits &
flowers) the colors of the
other pigments may be
visible
During the Fall
During the fall many plants lose their
chlorophylls & their leaves take on the rich
hues of the carotenoids.
Why?
–
–
–
–
Shorter day length
Less sunlight
Chlorophyll disintegrates
Other pigments can now be seen
Leaves in the Summer vs. Autumn
Oak leaf in
summer
Oak leaf in
autumn
Light dependent reaction
First stage of photosynthesis
Converting light energy to
chemical energy
Once the pigments in the chloroplast have
captured light energy, the light energy
must then be converted to chemical energy
The chemical energy is temporarily stored
in ATP & NADPH
O2 is given off
The chlorophylls and carotenoids are
grouped in clusters of a few hundred
pigment molecules in the thylakoid
Each cluster of pigment & the proteins that
the pigment molecules are embedded in are
referred to as photosystem
A Photosystem: A Reaction-Center Complex
Associated with Light-Harvesting Complexes
A photosystem consists of a
reaction-center complex (a type of
protein complex) surrounded by lightharvesting complexes
The light-harvesting complexes
(pigment molecules bound to proteins)
transfer the energy of photons to the
reaction center
Photosystem
 Photosystem II: P680
stimulated by
wavelengths of light
680 nm
 Photosystem I: P700
Stimulated by
wavelengths of light
700nm
Light & Pigments
Photosystem
STROMA
LightReactionharvesting center
complexes complex
Primary
electron
acceptor
e
Transfer
of energy
Special pair of
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
(a) How a photosystem harvests light
Thylakoid membrane
Thylakoid membrane
Photon
Chlorophyll
Protein
subunits
(b) Structure of a photosystem
STROMA
THYLAKOID
SPACE
Intro to how it works
https://www.youtube.com/watch?v=B
K_cjd6Evcw
The light dependent reaction
begins…
 Takes place within the thylakoid membranes
within chloroplasts in leaf cells
 accessory pigment molecules in both
photosystems absorb light
 They acquire some energy carried by the light
which is passed quickly to the other pigment
molecules until it reaches a specific pair of
chlorophyll a molecules
 chlorophyll a molecules can absorb light
PEA
A primary electron acceptor in the
reaction center accepts excited
electrons and is reduced as a result
Solar-powered transfer of an electron
from a chlorophyll a molecule to the
primary electron acceptor is the first
step of the light reactions
Linear Electron Flow
Linear electron flow involves the flow
of electrons through both photosystems
to produce ATP and NADPH using light
energy
Linear electron flow can be broken down
into a series of steps
1. A photon hits a pigment and its energy is
passed among pigment molecules until it
excites P680
2. An excited electron from P680 is transferred to
the primary electron acceptor
3. H2O is split by enzymes, and the electrons are
transferred from the hydrogen atoms to P680,
thus reducing it to P680; O2 is released as a byproduct
The splitting of water inside the thylakoid releases e- which replace
E- that leave photosystem II when it is illuminated.
Splitting of Water
 2H2O  4H + 4e- + O2
 For every 2 water molecules that are split, 4 ebecome available to replace those lost by
chlorophyll molecules in photosystem II
 The p+ that are produced are left inside the
thylakoid, while oxygen diffuses out of the
chloroplast & can leave the plant
 O2 is not needed for photosynthesis to occur,
but is essential for cellular respiration in most
organisms including plants!
H2 O
CO2
Light
NADP
ADP
Calvin
Cycle
Light
Reactions
ATP
NADPH
O2
[CH2O] (sugar)
4. Each electron “falls” down an electron
transport chain from the primary electron
acceptor of PS II to PS I
5. Energy released by the fall drives the creation
of a proton gradient across the thylakoid
membrane; diffusion of H (protons) across the
membrane drives ATP synthesis
6. In PS I (like PS II), transferred light energy
excites P700, causing it to lose an electron to
an electron acceptor (we now call it P700)
•
P700 accepts an electron passed down from PS
II via the electron transport chain
7. Excited electrons “fall” down an electron
transport chain from the primary electron
acceptor of PS I to the protein ferredoxin (Fd)
© 2014 Pearson Education, Inc.
Primary
acceptor
Primary
acceptor
H2O
O2
Fd
Pq
NADP
reductase
Cytochrome
complex
Pc
ATP
Photosystem II
Photosystem I
NADP
 H
NADPH
7. The electrons are transferred to NADP,
reducing it to NADPH, and become
available for the reactions of the Calvin
cycle
•
This process also removes an H from the
stroma
The energy changes of electrons
during linear flow can be
represented in a mechanical
analogy
Mill
makes
ATP
Photosystem II
NADPH
Photosystem I
Restoring Photosystem I
 electrons from chlorophyll molecules in
photosystem II replace electrons that leave
chlorophyll molecules in photosystem I
• If this did not happen, both ETC’s would stop &
photosynthesis would not occur!
 The replacement electrons for photo II are
provided by water molecules
 An enzyme inside the thylakoid splits water into
p+, e- & Oxygen
CHLOROPLAST
STRUCTURE
MITOCHONDRION
STRUCTURE
Intermembrane
space
Inner
membrane
H
Diffusion
Electron
transport
chain
Thylakoid
space
Thylakoid
membrane
ATP
synthase
Matrix
Key

Higher [H ]
Lower [H]
Stroma
ADP  P i

H
ATP
CHLOROPLAST
STRUCTURE
MITOCHONDRION
STRUCTURE
Intermembrane
space
Inner
membrane
H
Diffusion
Electron
transport
chain
Thylakoid
space
Thylakoid
membrane
ATP
synthase
Matrix
Key

Higher [H ]
Lower [H]
Stroma
ADP  P
i

H
ATP
ATP and NADPH are produced on the
side facing the stroma, where the
Calvin cycle takes place
In summary, light reactions generate
ATP and increase the potential energy
of electrons by moving them from H2O
to NADPH
ATP synthesis
Located in the thylakoid membrane
Energy driving this reaction is made by the
movement of p+ from inside the thylakoid
to the stroma
Some of the protons in the stroma are used to make NADPH
from NADP+.
Together NADPH & ATP provide energy for the second set
of reactions in photosynthesis
Electron Transport System
animation
http://www.science.smith.edu/departments/Biolog
y/Bio231/ltrxn.html
Photosystem II
4 H
Light
Cytochrome
complex
Light
NADP
reductase
Photosystem I
3
Fd
Pq
H2O
e−
1
THYLAKOID SPACE
(high H concentration)
e−
NADPH
Pc
2
12
O2
2 H
NADP  H
4 H
To
Calvin
Cycle
STROMA
(low H concentration)
Thylakoid
membrane
ATP
synthase
ADP

P H
i
ATP
Figure 8.16a
Photosystem II
4 H
Light
Cytochrome
complex
Photosystem I
Light
Fd
Pq
−
H2O
e
1
THYLAKOID SPACE
(high H concentration)
STROMA
(low H concentration)
e−
Pc
2
1
2
O2
2 H
Thylakoid
membrane
4 H
ATP
synthase
ADP

P H
i
ATP
Cytochrome
complex
NADP
reductase
Photosystem I
Light
3
NADP  H
Fd
NADPH
Pc
2
4 H
THYLAKOID SPACE
(high H concentration)
To
Calvin
Cycle
ATP
synthase
ADP

P H
i
STROMA
(low H concentration)

ATP
A Comparison of Chemiosmosis in
Chloroplasts and Mitochondria
Chloroplasts and mitochondria generate
ATP by chemiosmosis but use different
sources of energy
– Mitochondria transfer chemical energy from
food to ATP; chloroplasts transform light
energy into the chemical energy of ATP
Chemiosmosis differs between chloroplasts
and mitochondria but also shows similarities
In mitochondria,
– protons are pumped to the intermembrane
space and drive ATP synthesis as they
diffuse back into the mitochondrial matrix
In chloroplasts,
– protons are pumped into the thylakoid
space and drive ATP synthesis as they
diffuse back into the stroma
The Calvin Cycle
The Second phase of photosynthesis
~Light Independent Reaction
~or Dark Reaction
Named for
 Melvin Calvin (19111997), American scientists
who received the Nobel
Prize for biochemistry for
his discovery of the
chemical pathways of
photosynthesis
H2O
Light
CO2
NADP
ADP
 P
i
Light
Reactions:
Photosystem II
Electron transport chain
Photosystem I
Electron transport chain
RuBP 3-Phosphpglycerate
Calvin
Cycle
ATP
NADPH
G3P
Starch
(storage)
Chloroplast
O2
Sucrose (export)
Carbon Fixation
The Calvin Cycle is a series of enzyme-
assisted chemical reactions that make a 3carbon sugar.
A total of 3 CO2 molecules must enter the
Calvin cycle to produce each 3-carbon
sugar that will be used to make the organic
compound.
• The Calvin cycle occurs in the stroma of the
chloroplast
Carbon fixation by the Calvin
cycle
In the Calvin cycle, carbon atoms from
CO2 in the atmosphere are bonded or
“fixed” into organic compounds
This incorporation of CO2 into organic
compounds is known as carbon fixation
The Calvin cycle uses the chemical energy of ATP and
NADPH to reduce CO2 to sugar
The Calvin cycle, like the citric acid cycle,
regenerates its starting material after
molecules enter and leave the cycle
– Calvin cycle builds sugar from smaller
molecules by using ATP and the reducing
power of electrons carried by NADPH
Carbon enters the cycle as CO2 and
leaves as a sugar named (G3P)
For net synthesis of one G3P, the cycle
must take place three times, fixing three
molecules of CO2
Phases of the Calvin Cycle
The Calvin cycle has three phases
– Carbon fixation
– Reduction
– Regeneration of the CO2 acceptor
Calvin cycle: step 1
CO2 diffuses into the stroma from the
surrounding cytosol
An enzyme combines a CO2 molecule with
a 5-carbon carbohydrate called RuBP
The 6-carbon molecule that results are very
unstable & they each immediately split into
2 3-carbon molecules called 3phosphoglycerate (3-PGA)
Input 3
as 3 CO2
Phase 1: Carbon fixation
Rubisco
3 P
3 P
P
RuBP
P
6
P
3-Phosphoglycerate
Calvin
Cycle
Calvin cycle: step 2
 Each molecule of 3-PGA is converted into
another 3-carbon molecule, glyceraldehyde 3phosphate (G3P) in a 2 part process:
– 1st each PGA molecule receives a phosphate group
from a molecule of ATP
– The resulting compound then receives a p+ from
NADPH & releases a phosphate group, producing
G3P
– The ADP, NADP+, and PO4 can be used again in
light reaction to make more ATP & NADPH
Input 3
as 3 CO2
Phase 1: Carbon fixation
Rubisco
3 P
3 P
P
P
6
P
3-Phosphoglycerate
RuBP
6
ATP
6 ADP
Calvin
Cycle
6 P
P
1,3-Bisphosphoglycerate
6 NADPH
6 NADP
6 Pi
6
P
G3P
1
P
G3P
Output
Phase 2:
Reduction
Glucose and
other organic
compounds
Calvin cycle
One of the G3P molecules leaves the
Calvin cycle & is used to make organic
compounds (carbohydrates)-stored for later
use glucose
The remaining G3P molecules are
converted back into RuBP through the
addition of P from ATP
Input 3
as 3 CO2
Phase 1: Carbon fixation
Rubisco
3 P
3 P
P
P
6
P
3-Phosphoglycerate
RuBP
6
ATP
6 ADP
Calvin
Cycle
3 ADP
3
ATP
6 P
P
1,3-Bisphosphoglycerate
6 NADPH
Phase 3:
Regeneration
of RuBP
6 NADP
6 Pi
P
5
G3P
6
P
G3P
1
P
G3P
Output
Phase 2:
Reduction
Glucose and
other organic
compounds
In conclusion
By regenerating the RuBP that was
consumed in step 1, the reactions of step 3
allow the Calvin cycle to continue
operating
Some PGAL (G3P) molecules are not
converted into RuBP, but leave the cycle &
can be used by the plant cell to make other
organic compounds (cellular respiration)
Calvin cycle animation
http://www.science.smith.edu/departments/Biolog
y/Bio231/calvin.html
Balance sheet for photosynthesis
How much ATP & NADPH required to
make 1 molecule of PGA from CO2?
Each turn of the Calvin cycle fixes 1-CO2
Since G3P is a 3-carbon compound-it takes 3
turns of the cycle to produce each molecule of
G3P
Each turn, 2-ATP & 2-NADPH are used, 1 for
each molecule of PGA produced & 1 more
ATP
Total
Three turns of the Calvin cycle use 9
molecules of ATP & 6 molecules of
NADPH
Carbon Fixing Plants
Evolution of Alternative Mechanisms of Carbon
Fixation in Hot, Arid Climates
Adaptation to dehydration is a problem for land
plants, sometimes requiring trade-offs with
other metabolic processes, especially
photosynthesis
– On hot, dry days, plants close stomata, which
conserves H2O but also limits photosynthesis
– The closing of stomata reduces access to CO2 and
causes O2 to build up
– These conditions favor an apparently wasteful
process called photorespiration
Alternative pathways
Calvin cycle is the most common pathway
for carbon fixation
Plant species that fix carbon thought the
Calvin cycle are known as C3 Plants
because of the 3-carbon compound (G3P)
that is initially formed
C3 Plants
C3 Plants are those that only use the
Calvin Cycle to fix carbon.
Most all Plants
- They are called C3 plants, since they fix CO2
into a compound with 3 carbons (G3P).
- initial fixation of CO2, by RuBP, forms a
three-carbon compound (G3P)
Photorespiration
 In the presence of light-plant consumes O2 and
releases CO2 (in stead of fixing carbon dioxide)
during photosynthesis
– results in a decrease in photosynthetic output
since no ATP is produced and carbon is lost
Favored when
stomata’s are closed
in hot conditions
C3 plants suffers
Photorespiration on Hot days
RuBP adds O2 instead of CO2 in the Calvin
cycle, producing a two-carbon compound
 Photorespiration decreases photosynthetic output
by consuming ATP, O2, and organic fuel and
releasing CO2 without producing any ATP or sugar
Photorespiration may be an
evolutionary relic because RuBP first
evolved at a time when the atmosphere
had far less O2 and more CO2
Other carbon fixation plants
Other plant species fix carbon through
alternative pathways & then release it to
enter the Calvin cycle
These are generally found in hot, dry
climates.
Under these conditions, plants rapidly lose
water to air
Stomata
 Most of the water loss from
a plant occurs through
small pores called stomata
which are found on the
underside of leaves
Problems in the Stomata
 Are passageways for CO2 enters and O2 exits
plant leaves.
 When stomata’s are partly closed, the level of
CO2 in the plant falls as CO2 is consumed in the
Calvin cycle.
 At the same time, O2 rises b/c the light reactions
split water and generate O2
 Both conditions inhibit carbon fixation by the
Calvin cycle < low CO2 & high O2
C4 pathways
 C4 pathways use an enzyme which fix CO2 into
4-carbons, and then transported to other cells
where CO2 is available to then use the Calvin
Cycle- known as C4 plants
 C4 plants have their stomata partially closed the
hottest part of the day
C4 Plants
 Ex: corn, sugar cane, & crabgrass.
 They lose about half as much water as C3 plants
C4 Plants
C4 plants minimize the cost of
photorespiration by incorporating CO2 into a
four-carbon compound
– An enzyme in the mesophyll cells has a high
affinity for CO2 and can fix carbon even when
CO2 concentrations are low
– These four-carbon compounds are exported to
bundle-sheath cells, where they release CO2 that
is then used in the Calvin cycle
CAM pathways
 CAM pathways open their stomata only at night
to reduce water loss and close them during the
day
 At night- they take in CO2 and fix it into a variety
of organic compounds
 Day-CO2 is released from these compounds &
enters the Calvin cycle
 In low temperatures, they grow fairly slow, but
they lose less water than C3 or C4 plants
CAM plants
Cactuses, pineapples and certain plants that
have a different adaptation to hot, dry
climate
The Importance of Photosynthesis
The energy entering chloroplasts as sunlight
gets stored as chemical energy in organic
compounds
Sugar made in the chloroplasts supplies
chemical energy and carbon skeletons to
synthesize the organic molecules of cells
– Plants store excess sugar as starch in the
chloroplasts and in structures such as roots, tubers,
seeds, and fruits
– In addition to food production, photosynthesis
produces the O2 in our atmosphere