Photosynthesis
Capturing and using solar energy
Quick, think!
• Does photosynthesis:
• create energy?
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No
• use energy?
Yes
• “store” energy?
Yes
• release energy?
No
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True or False?
• Plants do photosynthesis but not
cellular respiration.
• Photosynthesis is a plant’s way of
creating ATP for its cells.
False
False
• Plants make sugar and other carbon
compounds so that animals can eat. False
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Photosynthesis
• What photosynthesis does:
• Converts sunlight into stored chemical
energy.
• Makes carbon compounds that can be
broken down for energy or used to
build tissue.
Photosynthesis is an ___ process.
100%
1. Endergonic
2. Exergonic
0%
1
2
Photosynthesis is endergonic
because:
80%
1. Energy is
consumed by the
process.
2. Energy is given
off by the process.
3. Energy is made
by the process.
20%
0%
1
2
3
Remember this?
energy
input
C6H12O6 + O2
(glucose) (oxygen)
6 CO2 + 6 H2O
(carbon
(water)
dioxide)
Photosynthesis is an endergonic process.
Photosynthesis takes in energy and uses it to build carbon
compounds.
Energy is
captured
from
sunlight.
Carbon dioxide
is absorbed
from the air.
Oxygen is
released.
photosynthesis
Water is absorbed
from soil, used in
photosynthesis, and
stored in cells.
Sugar is
synthesized
and used in
plant tissues.
plant
tissues,
growth
Inorganic mineral nutrients
(nitrate, phosphate) are
absorbed from soil and
used in plant tissues.
This diagram shows that
photosynthesis is an
endergonic reaction
because it takes in
energy.
Carbon for making
carbon compounds
(such as sugar) comes
from the atmosphere.
Oxygen, hydrogen, and
minerals are needed also.
Oxygen and hydrogen come
from water. Minerals comes
from the soil
Plants make carbon-based molecules from raw
inorganic compounds.
(chloroplast)
photosynthesis
H2O CO2
ATP
sugar
O2
cellular
respiration
(mitochondrion)
Plants use the organic carbon compounds as
“food” and to build cell parts.
Organic molecules are:
1. Natural, not manmade.
2. Molecules that
contain carbon.
3. Molecules that
contain carbon
AND hydrogen.
4. Any molecule from
a living organism.
40%
20%
1
20%
2
20%
3
4
Internal leaf structure
cuticle
upper
epidermis
mesophyll
cells
lower
epidermis
stoma
vascular bundle bundle
sheath
(vein)
chloroplasts
Chloroplast
outer membrane
inner membrane
thylakoid
stroma
channel
interconnecting
thylakoids
The function of the chloroplast
is to:
100%
1. Produce energy.
2. Gather light
energy and
convert it to
chemical energy.
3. Break sugars
down for energy.
0%
1
0%
2
3
energy from
sunlight
O2
CO2
ATP
Light energy is “captured”
by chlorophyl, which is
embedded in the
thylakoid membranes.
NADPH
Light-dependent
reactions are
associated with
thylakoids.
ADP
Lightindependent
reactions
(C3 cycle) occur
in stroma.
NADP+
H2O
chloroplast
G3P
Energy from the
light-dependent
reactions drives the
reactions where
carbon compounds
are produced.
H2O
LIGHT-DEPENDENT
REACTIONS
(in thylakoids)
DEPLETED
CARRIERS
(ADP, NADP+)
CO2
O2
ENERGIZED
CARRIERS
(ATP, NADPH)
LIGHT-INDEPENDENT
REACTIONS
(in stroma)
G3P
ATP and NADPH are
used to move energy
from one part of the
chloroplast to
another.
ATP made in the
chloroplast is ONLY
used to power
production of carbon
compounds. It is not
available to the rest of
the cell.
Overall, the light-dependent
reactions do what?
80%
1. Make energy.
2. Capture energy.
3. Make carbon
compounds.
4. Break down carbon
compounds.
20%
0%
1
0%
2
3
4
Overall, the light-independent
reactions do what?
60%
1. Make energy.
2. Capture energy.
3. Make carbon
compounds.
4. Break down carbon
compounds.
20%
20%
0%
1
2
3
4
• Suppose for a moment that the ATP
made in the chloroplast was available to
the cell, and was the ONLY source of
ATP for the cell. Could the plant
survive? (Remember that ATP is an
unstable molecule that cannot be stored
longer than a few minutes.)
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Light-dependent reactions convert light energy into
temporary chemical energy.
energy level of electrons
sunlight
2e–
reaction
center
HO 9
2e– 2
photosystem II
energy to drive
ATP synthesis
NADPH
NADP+
H+
2e–
2e–
2e–
photosystem I
2H+
1/2 O2
The electron transport chain produces NADPH and drives
ATP synthesis.
ATP synthase uses energy from the diffusion of H+ to
drive ATP synthesis.
Energy from energized
electrons powers active
transport of H+ by ETC.
PSII
ETC
PSI
Energy-carrier
molecules power
the C3 cycle.
stroma
ETC
C3
cycle
Energy from
energized
electrons powers
NADPH synthesis.
thylakoid space
High H+ concentration
generated by active
transport.
H+ channel coupled
to ATP-synthesizing
enzyme.
Flow of H+ down
concentration gradient
powers ATP synthesis.
What’s important in the light-dependent (“photo”) reactions:
• The ETC uses light energy to produce
NADPH.
• Energy from the ETC concentrates H+
ions. The energy released as they
diffuse through ATP synthase makes
ATP.
• ATP and NADPH are used to power the
light-independent reactions.
In photosynthesis, water is split in
order to:
60%
1. Release oxygen.
2. Get protons and
electrons.
3. Make energy.
4. Make chlorophyll.
40%
0%
1
2
3
0%
4
In photosynthesis, light energy is
captured by:
80%
1.
2.
3.
4.
Protons
Oxygen
Chlorophyll
ATP
20%
0%
1
2
0%
3
4
The membrane protein that
makes ATP is:
60%
1.
2.
3.
4.
ATP synthase
Chlorophyll
ADP
Oxygen
20%
20%
0%
1
2
3
4
Light-independent reactions. Notice where ATP
and NADPH are used up.
1 Carbon
fixation
combines
CO2 with
RuBP.
6 CO2
2 G3P
synthesis
uses energy.
6
6 RuBP
3 RuBP
synthesis
uses energy
and 10 G3P.
12
C3 cycle
(CalvinBenson
cycle)
PGA
12 ATP
12 ADP
6
ADP
6
ATP
4 G3P available for
synthesis of carbon
compounds such
as glucose.
12 NADPH
12
G3P
glucose
(or other molecules)
12 NADP+
What’s important in the light-independent (“synthesis”)
reactions:
• Energy carried by ATP and NADPH is
used to power synthesis of G3P.
• G3P can be used to make glucose as
well as other monomers.
• These monomers can be used to build
polymers, or may be broken down to
make ATP for the cell.
The source of carbon to make carbon
compounds in photosynthesis is:
100%
1.
2.
3.
4.
Glucose
G3P
Carbon dioxide
Water
0%
1
0%
2
0%
3
4
Which of these happens in the
C3 cycle?
60%
1. ATP is made from
ADP and P.
2. ATP is broken down
to power molecule
synthesis.
3. ATP becomes the
monomer of other
compounds.
20%
1
20%
2
3
Plant cells need ATP to run cell processes.
Where does that ATP come from?
1. Sunlight.
2. The light-dependent
reactions of
photosynthesis.
3. The lightindependent
reactions of
photosynthesis.
4. Cellular respiration
80%
20%
1
0%
0%
2
3
4
IMPORTANT!!!
• Photosynthesis does NOT supply
energy to the cell. Photosynthesis USES
light energy to make organic
compounds.
• To get energy for the cell, plant cells
must use cellular respiration to break
down glucose and make ATP.
An analogy:
• Photosynthesis is like going to the
grocery store and buying food to store in
your cupboard. The food and the energy
in it is stored.
• Cellular respiration is like eating the food
when you are hungry and need energy.
The energy in food is released only by
eating the food.
While animals can only do cellular respiration, plants
make “food” using photosynthesis and break the “food”
down for energy in cellular respiration.
(chloroplast)
photosynthesis
H2O CO2
ATP
cellular
respiration
(mitochondrion)
sugar
O2
Animals
Plants
Photosynthesis
Cellular Respiration
Cellular Respiration
Day
Day
Night
Night
Cellular Respiration
Cellular Respiration
Photosynthesis supplies the “food” that plants need to
carry out cellular respiration.
C3 plants use the C3 pathway
Much photorespiration
occurs under hot, dry
conditions.
CO2
O2
PGA
C3
Cycle
rubisco
CO2
RuBP
G3P
glucose
stoma
within mesophyll chloropast
bundlesheath
Little glucose
cells
is synthesized.
In a C3 plant, mesophyll cells
contain chloroplasts; bundlesheath cells do not.
C3 plants are at a disadvantage in hot, dry climates.
CO2 is captured with
a highly specific enzyme.
C4 plants use the C4 pathway
CO2
PEP
AMP
C4
Pathway
4-carbon
molecule
ATP
pyruvate
PGA
stoma
bundlesheath
cells
CO2 O2
rubisco
C3
Cycle
G3P
glucose
In a C4 plant, both mesophyll
and bundle-sheath cells contain
chloroplasts.
within mesophyll
chloropast
CO2
RuBP
Almost no
photorespiration
occurs in hot, dry
conditions.
within bundle-sheath
chloropast
Lots of glucose is synthesized.
C4 plants essentially store carbon for hot times of the
day. Guess what pathway many weeds use?
• Fill in the blanks in this generalized
diagram showing what goes into and
what comes out of the chloroplast.
Chloroplast
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Recap
• Think of photosynthesis as an energy
“storing” process, not an energymaking or energy-releasing process.
• The products of photosynthesis can be:
• used to build cell parts.
• broken down to make ATP for the cell.
Photosynthesis animations
• Electron transport chains
• Light-dependent and light-independent
reactions