Photosynthesis:
Life from Light and Air
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Trapping Energy from Sunlight
• The process that uses the sun’s energy to make
simple sugars is called photosynthesis.
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AN OVERVIEW OF PHOTOSYNTHESIS
• Photosynthesis is the process by which
autotrophic organisms use sunlight energy to
make sugar and oxygen gas from carbon dioxide
and water.
PHOTOSYNTHESIS
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Plants are energy producers
• Like animals, plants need energy to live
– unlike animals, plants don’t need to eat food to make
that energy
• Plants make both FOOD & ENERGY
– animals are heterotrophs(consumers)
– plants are autotrophs (producers)
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How do plants make energy & food?
• Plants use the energy from the sun
– to make ATP energy
– to make sugars ( -ose)
• glucose, sucrose, cellulose, starch, & more
sun
ATP
sugars
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Building plants from sunlight & air
• Photosynthesis
– 2 separate processes
1. ENERGY building reactions
• collect Sun energy
• use it to make ATP
sun
ATP
H2O
+
CO2
carbon dioxide
CO2
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water
+ HO
2
sugars
C6H12O6
sugars
Building plants from sunlight & air
• Photosynthesis
2. SUGAR building reactions
• take the ATP energy
• collect CO2 from air &
H2O from ground
• use all to build sugars
sun
ATP
H2O
+
CO2
carbon dioxide
CO2
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water
+ HO
2
sugars
C6H12O6
sugars
What do plants need to grow?
• The “factory” for making
energy & sugars
sun
– chloroplast
• Fuels
– sunlight
– carbon dioxide
– water
• The Helpers
– enzymes
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Make ATP!
Make sugar!
I can do it all…
And no one
even notices!
CO2
ATP
enzymes
H2O
sugars
So what does a plant need?
• Take in
– light
– CO2 (leaves)
– H2O (roots)
leaves
• Remove out
– O2 & H20 (leaves)
shoot
• Move around
– sugars
roots
C6H12O6 + 6O2
6CO2 + 6H2O + light

energy
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Photosynthesis
sun
ENERGY
building
reactions
ATP
ADP
SUGAR
building
reactions
H 2O
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used immediately
to synthesize sugars
sugar
CO2
Chloroplasts
Leaf
Leaves
absorb
sunlight & CO2
sun
CO2
Chloroplasts
in cell
Chloroplast
Chloroplasts
contain
Chlorophyll
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Chloroplast
make
ENERGY & SUGAR
Stomata in the leaf
• Function of stomata
– CO2 in
• gets into leaves for photosynthesis
– O2 out
– H2O out
stomate
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Guard Cells in the leaf
• Function of guard cells
– open & close stomata
– protect & secure plant
– controls material flux
guard cell
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An overview of photosynthesis
Chloroplast
Light
CO2
H2O
NADP+
ADP
+P CALVIN
CYCLE
(in stroma)
LIGHT
REACTIONS
(in grana)
ATP
NADPH
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O2
O2
Sugar
Trapping Energy from Sunlight
Photosynthesis happens in two phases:
1. The light-dependent reactions convert light
energy into chemical energy. (ATP)
2. The molecules of ATP produced in the lightdependent reactions are then used to power the
Calvin Cycle or light-independent reactions that
produce simple sugars.
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Trapping Energy from Sunlight
The general equation for photosynthesis is
written as:
• 6CO2 + 6H2O→C6H12O6 + 6O2
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THE LIGHT REACTIONS:
CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY
Visible radiation drives the light reactions
• Certain wavelengths of visible light drive the light
reactions of photosynthesis
Gamma
rays
X-rays
UV
Infrared
Microwaves
Visible light spectrum
Wavelength (nm)
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Radio
waves
Pigments in the chloroplast
To trap the energy in sunlight, the thylakoid membranes
contain pigments, molecules that absorb specific
wavelengths of sunlight.
Wavelengths that are NOT absorbed are reflected
(bounced off) or transmitted (pass through).
Light
Reflected
light
Chloroplast
Absorbed
light
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Pigments in the chloroplast
Materials in the pigment have the appearance to be the
color of the wavelengths that are NOT absorbed.
Photosynthetic pigments can absorb light energy &
make it available for conversion to chemical energy.
Light
Reflected
light
Chloroplast
Absorbed
light
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Pigments in the chloroplast
• Chlorophyll A: most common pigment in chloroplast
(absorbs blue & red light wavelengths and reflects
green light wavelengths thus giving the chloroplast a
green color)
• (chloro = green; phylla + leaf)
• Accessory pigments: additional pigments that absorb
different light wavelengths (carotene, chlorophyll B,
& xanthophyll)
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Light-Dependent Reactions
Absorption of light energy by chlorophyll
• As sunlight strikes the chlorophyll molecules in a
photosystem of the thylakoid membrane, the energy
in the light is transferred to electrons.
• These highly energized, or excited, electrons are
passed from chlorophyll to an electron transport
chain, a series of proteins embedded in the thylakoid
membrane.
• At each step along the transport chain, the electrons
lose energy.
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Energy from the energized electrons pump
H+ ions and change NAPD+ to NADPH.
The H+ ions move from high to low and turn
the ATP synthase and change ADP + P to ATP
Energized electrons
Oxygen
by-product
Photolysis
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Light-Dependent Reactions
• Chemiosmosis: This “lost” energy can be used to
make ATP from ADP, or to pump hydrogen ions into
the center of the thylakoid disc.
• The electrons are transferred to the stroma of the
chloroplast. To do this, an electron carrier molecule
called NADP is used.
• NADP can combine with two excited electrons and a
hydrogen ion (H+) to become NADPH.
• NADPH will play an important role in the lightindependent reactions.
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Restoring electrons
To replace the lost electrons, molecules of water
are split in the first photosystem. This reaction is
called photolysis.
The O2 liberated by photosynthesis is made from
the oxygen in water.
Chlorophyll
O2 + 2H+
2
H2O + +
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O2 + 2e-
2e-
H2 O
Restoring electrons
The oxygen produced by photolysis is released into
the air and supplies the oxygen we breathe.
The electrons are returned to chlorophyll.
The hydrogen ions are pumped into the thylakoid,
where they accumulate in high concentration.
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Light Dependent Reactions: Summary
• Absorptions of light energy by chlorophyll takes place in
the thylakoid
1. Split water molecule (PHOTOLYSIS); Oxygen
combines with other oxygen to produce O2 which is
given off as a waste product
2. Hydrogen produced by splitting of water is attached
to hydrogen carrier NADP  NADPH (energy in this
molecule)
3. Then energy from “excited electrons” is used to take
ADP + P ATP
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Light Independent Reactions
(The Calvin Cycle)
• CO2 put into organic molecules by process of Carbon
fixation.
• Does not require light but must have ATP and NADPH
which are produced by light dependent reactions.
• Called Calvin Cycle (or Dark Cycle)
• Takes place in stroma of chloroplast.
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Light Independent Reactions
(The Calvin Cycle)
• The Calvin cycle constructs G3P
(a sugar) using:
– carbon from atmospheric
CO2;
– electrons and H+ from
NADPH;
– energy from ATP
• Energy-rich sugar (G3P) is then
converted into glucose.
INPUT
CALVIN
CYCLE
OUTPUT:
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An overview of photosynthesis
Chloroplast
Light
CO2
H2O
NADP+
ADP
+P
LIGHT
REACTIONS
(in grana
ATP
CALVIN
CYCLE
(in stroma
NADPH
O2
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Sugar
Factors Affecting Photosynthesis
Factors affecting the rate of photosynthesis:
1. Temperature: increases rate up to a certain point
2. Light Intensity: increases rate up to a certain point
3. CO2 level: increases rate up to a certain point
4. Water: decrease water, decrease photosynthesis
5. Minerals: Magnesium, Nitrogen, Phosphorous, …
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