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Photosynthesis
Photosynthesis
• Photosynthesis is the process by which
organisms use the energy of the sun to
synthesize organic compounds (sugars) from
inorganic compounds (CO2 and water)
• Photosynthesis transforms the energy of the sun
into chemical energy (glucose)
• Provides the oxygen we breath, removes CO2,
and provides food, energy
Photosynthesis
• Many organisms participate in photosynthesis:
– Nearly all plants
– Some Protists (Euglena, kelp)
– Some bacteria = cyanobacteria
Photosynthesis
6 CO2 + 6
H2O
Carbon dioxide Water
Light
energy
Photosynthesis
C6H12O6
Glucose
+ 6
O2
Oxygen
• Carbon dioxide and water are waste products of
cellular respiration
• Photosynthesis takes these products and
converts them to the glucose and O2
Photosynthesis
• Photosynthesis occurs on a cellular level
• Chloroplasts are organelles which carry out
photosynthesis
• Chloroplasts contain chlorophyll, a lightabsorbing pigment which give autotrophs their
distinctive color
The Chloroplast
• A chloroplast contains two membranes (as do
mitochondria)
• A thick fluid called the stroma fills the inner
compartment of the chloroplast
• Suspended in the stroma are the thylakoids, a
system of interconnected membranous sacs,
which enclose another compartment known as
the thylakoid space
– The thylakoids are stacked onto one another
to form a granum
Chloroplast
Outer and inner
membranes
Thylakoid
Stroma
Granum
Thylakoid
space
Intermembrane
space
Pigments
• Pigments are light-absorbing molecules built into the
thylakoid membranes of the cholorplast
• Pigments absorb some wavelengths of light, but
reflect others
• We do not see the absorbed wavelengths (because
their energy has been absorbed by the pigment
molecules); we see the wavelengths that the pigment
reflects
• The pigment built into the thylakoid membranes that
allow for photosynthesis to occur is called
chlorophyll
– capture light energy
Chlorophyll
• Chlorophyll absorbs
light mainly in the
blue-violet (high
energy) and red (low
energy)
wavelengths
• Chlorophyll reflects
and transmits light
mainly in the green
(medium energy)
wavelengths
Light
Reflected
light
Chloroplast
Thylakoid
Absorbed
light
Transmitted
light
Increasing energy
10–5 nm 10–3 nm
Gamma
rays
X-rays
1 nm
103 nm
UV
1m
106 nm
Infrared
Microwaves
103 m
Radio
waves
Visible light
380 400
600
500
Wavelength (nm)
700
650
nm
750
Photosynthesis
• Photosynthesis occurs in 2 stages
1. Light reactions convert light energy into
chemical energy (ATP), and split molecules of
water to produce H and O2.
2. Light-independent reactions (the Calvin
Cycle) assembles glucose molecules using CO2
(carbon fixation) and the energy-rich products of
the light reactions
CO2
H2O
Chloroplast
Light
NADP+
ADP
 P
LIGHT
REACTIONS
(in thylakoids)
CALVIN
CYCLE
(in stroma)
ATP
NADPH
O2
Sugar
1. Light (dependent) Reactions
• Light reactions occur in the thylakoid
membranes of the chloroplast
• Light energy absorbed by chlorophyll molecules
is used to split a molecule of water
– gives off protons, electrons and O2 as byproducts
H2O Æ 2H + 1/2 O2
(2 H2O Æ 4H + O2)
• The H are transferred to the coenzyme NADP to
create NADPH to be used in the Calvin Cycle
• Light energy absorbed by chlorophyll molecules
is also used to generate ATP to power the
Calvin Cycle
Photosystems
• In the thylakoid membrane, chlorophyll
molecules are organized into clusters (with other
pigments and proteins) called photosystems
• A photosystem consists of a number of
capturing pigments bound to proteins
surrounded by a reaction center complex
Photosystem
Photon of Light
Light-harvesting
complexes
Reaction
center complex
Thylakoid membrane
Primary electron
acceptor
e–
Transfer
of energy
Pair of
Chlorophyll molecules
Pigment
molecules
How do Photosystems capture light energy?
• The pigments absorb light energy and pass the
electrons (energy) from molecule to molecule until it
reaches the reaction center
• The reaction center complex contains a pair of
chlorophyll molecules and an electron acceptor
• When light is absorbed by the pigments, energy
passes from pigment to pigment molecules until it
reaches the reaction center of the photosystem where
it excites an electron of chlorophyll to a higher energy
state that is transferred to the primary electron
acceptor
2 Separate Photosystems are Required
• There are 2 photosystems: Photosystem II and
Photosystem I, each capable of capturing light energy,
but each photosystem uses that energy differently
• Photosystems I and II were named in order of their
discovery, but Photosystem II functions first in the
sequence of steps that make up the light reactions
Photosystem II
• Photosystem II uses light energy to split H2O into H+
and O2.
• The transfer of an electron from chlorophyll to the
electron acceptor causes a molecule of water to be
split. When water is split, it supplies electrons to the
chlorophyll molecule which lost its electrons to the
primary electron acceptor, releasing O2 and H+ as byproducts
Photosystem II
• The generation of H+ creates a gradient within the
chloroplast that is used to synthesize ATP (just like in
cellular respiration in the mitochondria).
• The electron from Photosystem II is transferred to
Photosystem I to supply an electron to its chlorophyll
following the absorption of light by Photosystem I
Photosystem I
• Photosystem I uses light energy to convert the
coenzyme NADP into NADPH.
• Following the absorption of light, an electron is
transferred from chlorophyll to the electron acceptor.
From there the electron is transferred a molecule of
NADP causing it to become negatively charged.
– the electron lost from chlorophyll in Photosystem I is
replaced by the electron created in Photosystem II
A mechanical analogy of the light reactions
e–
ATP
e–
e–
e–
Mill
makes
ATP
NADPH
e–
n
Photo
e–
Photon
e–
Photosystem II
Photosystem I
2. Light-independent Reactions
• Light-independent reactions occur in the stroma
of chloroplasts
• Consist of the Calvin Cycle, which assembles
sugar molecules using CO2 and H from NADPH
generated during the Light Reactions
• The incorporation of Carbon from CO2 into
organic compounds is called carbon fixation
2. Light-independent Reactions
• The Calvin Cycle does not require light, but
occurs during daylight hours when the light
reactions power the cycle by supplying NADPH
and ATP
• Often called “dark reactions”
• NADPH provides the hydrogens to combine with
CO2 to form glucose
– ATP powers the cycle
Photosynthesis: a review
• Light reactions occur in the thylakoid membrane
– 2 photosystems capture solar energy which
energizes electrons
– Photosystems transfer these excited electrons
through electron transport chains which
produces ATP and NADPH
– Water is split and O2 is released
Photosynthesis: a review
• In the stroma of the chloroplast, sugars are
produced via the Calvin Cycle (light-independent
reactions)
• CO2 is combined with H to form glucose
Photosynthesis: a review
• The sugar produced
by plants during
photosynthesis
provides the
starting materials to
make structural
components such
as cellulose
• 50% of this sugar
goes toward cellular
respiration (plants
respire)