PHOTOSYNTHESIS

Prepared by:
ARGEL JOSEPH C. MAYO, RN, LPT, MAN, MSc.Bio
SHS Teacher III / Nurse
CHLOROPHYLL IS THE MAIN
PHOTOSYNTHETIC
PIGMENT
 CHLOROPHYLL IS THE MAIN
PHOTOSYNTHETIC PIGMENT
• You can see that the green pigment
(chlorophyll) is not evenly distributed
in the cell but is confined to organelles
called chloroplasts
• In plants, the mesophyll (a layer with
many chloroplasts, air spaces, and very
high concentrations of water vapor),
and chloroplasts lie mainly inside the
leaf.
• There are 20 to 100 chloroplasts in
every mesophyll cell
INTERNAL STRUCTURE OF
THE CHLOROPLAST
INTERNAL STRUCTURE OF THE
CHLOROPLAST
CHLOROPHYLL IS FOUND IN
THE THYLAKOID
MEMBRANE
 CHLOROPHYLL IS FOUND IN THE
THYLAKOID MEMBRANE
• Thylakoid membranes contain several
kinds of pigments, which are
substances that absorb visible light.
• The main pigment for photosynthesis,
chlorophyll, absorbs light primarily in
the visible spectrum's blue and red
regions.
• The chlorophyll does not appreciably
absorb green light. Plants usually
appear green because they are
scattered or reflected by some of the
green light that strikes them
 CHLOROPHYLL IS FOUND IN THE
THYLAKOID MEMBRANE
• Thylakoid membranes contain several
kinds of pigments, which are
substances that absorb visible light.
• The main pigment for photosynthesis,
chlorophyll, absorbs light primarily in
the visible spectrum's blue and red
regions.
• The chlorophyll does not appreciably
absorb green light. Plants usually
appear green because they are
scattered or reflected by some of the
green light that strikes them
 THE STRUCTURE OF
CHLOROPHYLL MOLECULE
 THE STRUCTURE OF CHLOROPHYLL
MOLECULE
• Chlorophyll molecule
has two main parts:
(1) PORPHYRIN RING
(2) LONG-SIDE CHAIN
 THE STRUCTURE OF CHLOROPHYLL
MOLECULE
(1) PORPHYRIN RING
• Absorbs light energy
• It consists of smaller rings of carbon
and nitrogen atoms joined together
• The central atom is a magnesium
ion that, during the process, helps
capture and store electrons.
• The magnesium ion also helps red
and blue light to be absorbed by the
porphyrin
 THE STRUCTURE OF CHLOROPHYLL
MOLECULE
(2) LONG-SIDE CHAIN
• The hydrocarbon side chain
makes the molecule highly non-
polar and the membrane anchors
the chlorophyll
PHOTOSYNTHETIC
PIGMENTS
 PHOTOSYNTHETIC PIGMENTS
• Photosynthesis is the process that is used by plants
to harness energy from sunlight and turn it into
chemical energy.
• The primary pigment, chlorophyll, used in
photosynthesis, reflects green light and absorbs
red and blue light most strongly.
• Photosynthesis in plants occurs in chloroplasts that
contain chlorophyll.
 TYPES OF CHLOROPHYLL
(1) Chlorophyll a
- It is the most important pigment because it
initiates the light-dependent reactions of
photosynthesis.

(2) Chlorophyll b
- It is an accessory pigment and participates in
photosynthesis.
 Difference between chlorophyll a and
chlorophyll b
They differ in their functional group on the porphyrin
ring:
- in chlorophyll a, the methyl group (-CH3) is
replaced by a terminal carbonyl group (-CHO) in
chlorophyll b
 Chloroplasts have other accessory
pigments
• Carotenoids, pigments that are yellow
and orange.
• Chlorophyll can be excited by light by
energy passed on to it directly from
the light source, or indirectly by
energy passed on to it from accessory
pigments that have become excited by
light.
• If a carotenoid molecule is excited, it
can transfer its energy to chlorophyll a
 LIGHT AND
PHOTOSYNTHESIS
 LIGHT AND PHOTOSYNTHESIS
• Gamma rays with very short
wavelengths measured in
fractions of nanometers or
nm are at the end of the
electromagnetic spectrum.
• Radio waves, with
wavelengths so long that
they can be measured in
kilometers, are at the other
end of the spectrum.
 LIGHT AND PHOTOSYNTHESIS
• Light consists of tiny
particles, packets, or
energy called photons.
• A photon's energy is
inversely proportional to its
wavelength: light at a
shorter wavelength has
more energy per photon
than light at a longer
wavelength.
 LIGHT AND PHOTOSYNTHESIS
• One of its electrons
becomes energized when a
molecule absorbs a photon
of light energy, which
means that the electron
shifts from a low-energy
atomic orbital to a high-
energy orbital that is more
distant from the atomic
nucleus
 Why does photosynthesis depend not on some other
wavelength of radiation, but on light detectable by the
human eye (visible light)?
• To excite these biological
molecules, radiation with
wavelengths longer than
those of visible light does not
have enough energy.
• Radiation is so energetic that
it disrupts the bonds of many
biological molecules, with
wavelengths shorter than
those of visible light.
• Visible light causes the kinds
of reversible changes in
molecules that are useful in
photosynthesis
 OVERVIEW OF
PHOTOSYNTHESIS
 OVERVIEW OF PHOTOSYNTHESIS
• During photosynthesis, to power the synthesis of carbohydrates, a
cell uses light energy captured by chlorophyll.
• The overall photosynthesis reaction may be summarized as follows:
 THE REACTIONS OF
PHOTOSYNTHESIS ARE
DIVIDED INTO TWO PHASES
THE REACTIONS OF PHOTOSYNTHESIS
ARE DIVIDED INTO TWO PHASES

Photosynthesis reaction
THE LIGHT – DEPENDENT
REACTIONS
(Phase 1 of Photosynthesis)
 THE LIGHT – DEPENDENT REACTIONS
(Phase 1 of Photosynthesis)
• The light-dependent reactions
start as light is absorbed by
chlorophyll a and /or accessory
pigments, causing one of its
electrons to move to a higher
energy state.
• H2O will replace the electron
when the energized electron is
transferred to the acceptor
molecule.
• Molecular oxygen is released
when H2O splits.
 THE LIGHT – DEPENDENT REACTIONS
(Phase 1 of Photosynthesis)
• For the phosphorylation of
adenosine diphosphate (ADP),
forming adenosine triphosphate
(ATP), some energy from the
energized electrons is used.
• Furthermore, the coenzyme
Nicotinamide Adenine
Dinucleotide Phosphate
(NADP+) is reduced and NADPH
is formed.
• NADPH is a carrier of hydrogen
that can supply high-energy
electrons to power certain
reactions, such as the Calvin
cycle
 OXIDATION-REDUCTION (REDOX) REACTION
• In a redox reaction, one or
more electrons are
transferred from an
electron donor (reducing
agent) to an electron
acceptor (oxidizing agent)
 TWO TYPES OF
PHOTOSYNTHETIC UNITS:
PHOTOSYSTEM I AND II
 TWO TYPES OF PHOTOSYNTHETIC UNITS:
PHOTOSYSTEM I AND II
• Photosystems I and II each
contain a multi-antenna
complex and a reaction
center
• Antenna complexes are units
wherein chlorophylls a and
b, and accessory pigments
are organized with pigment-
binding proteins within the
thylakoid membrane.
• This is often consistent with
the presently accepted
model
 TWO TYPES OF PHOTOSYNTHETIC UNITS:
PHOTOSYSTEM I AND II
• Each antenna complex
absorbs light energy and
transfers it to the reaction
center, which contains
chlorophyll molecules and
proteins, including electron
transfer components, that
participate directly in
photosynthesis
• Through the series of
electron transfer reactions,
light energy is converted to
chemical energy within the
reaction centers
TWO TYPES OF PHOTOSYNTHETIC UNITS:
PHOTOSYSTEM I AND II
 TWO TYPES OF PHOTOSYNTHETIC UNITS:
PHOTOSYSTEM I AND II
• When light energy is absorbed
by the pigment molecule, that
energy is passed directly from
one pigment to another within
the antenna complex, through a
process known as resonance,
until it reaches the reaction
center
• When the energy reaches a
molecule of P700 (in the PSI
reaction center) or P680 (in the
PSII reaction center), an
electron is then raised to a
higher energy level
 TWO TYPES OF PHOTOSYNTHETIC UNITS:
PHOTOSYSTEM I AND II
• PSII and PSI are two major
components of the electron
transport chain, which also
includes the cytochrome
complex
• The Cytochrome (Cyt) complex,
an enzyme composed of two
protein complexes that transfer
the electrons from the carrier
molecule Plastoquinone (Pq) to
the protein Plastocyanin (Pc),
thus enabling both the transfer
of protons across the thylakoid
membrane and transfer of
electrons from PSII to PSI.
 HOW DO LIGHT-
DEPENDENT REACTIONS
WORK?
 HOW DO LIGHT-DEPENDENT REACTIONS
WORK?
• The PSII reaction center
(P680) delivers its high-
energy electrons to the
primary electron acceptor,
one at a time, and to PSI
(P700) through the
electron transport chain
(Pq to Cyt complex to Pc).
 HOW DO LIGHT-DEPENDENT REACTIONS
WORK?
• By extracting a low-energy
electron from water, the
missing electron of the
P680 is replaced.
• By splitting 1 molecule of
H2O , the following are being
released:
2 electrons
2 hydrogen atoms
1 oxygen atom
 HOW DO LIGHT-DEPENDENT REACTIONS
WORK?
• To form one molecule of
diatomic O2 gas, the splitting
of two molecules of H2O is
required.
• In order to support oxidative
phosphorylation, about 10%
of the oxygen is used by
mitochondria in the leaf.
• The rest escapes into the
atmosphere where aerobic
organisms use it to support
respiration.
 HOW DO LIGHT-DEPENDENT REACTIONS
WORK?
• Electrons lose energy as they
move through the proteins that
reside between PSII and PSI.
• That energy is used to transfer
hydrogen atoms to the thylakoid
lumen from the stromal side of
the membrane.
• Those hydrogen atoms
accumulate in the thylakoid
lumen, plus those generated by
splitting water, creating an
electrochemical gradient that
will be used in a later stage to
synthesize ATP.
 HOW DO LIGHT-DEPENDENT REACTIONS
WORK?
• Electrons must be re-
energized by PSI because
they have lost energy
upon their arrival at PSI,
so another photon is
absorbed by the PSI
antenna.
• That energy is transmitted
to the reaction center of
the PSI (P700).
 HOW DO LIGHT-DEPENDENT REACTIONS
WORK?
• P700 is oxidized and sends
a high-energy electron
NADP+ to form NADPH.
• Thus, to generate proton
gradients to make ATP, PSII
captures the energy; and
PSI captures the energy to
reduce NADP+ into NADPH
GENERATING AN ENERGY
CARRIER: ATP
 GENERATING AN ENERGY CARRIER: ATP
• An electrochemical gradient is
created by the build-up of
hydrogen ions inside the
thylakoid lumen.
• • To produce ATP, the passive
diffusion of hydrogen ions from a
high concentration area (in the
thylakoid lumen) to a low
concentration area (in the
stroma) is used.
• Because of diffusion and because
they all have the same electrical
charge, the ions build up energy,
repelling each other
 GENERATING AN ENERGY CARRIER: ATP
• The hydrogen ions rush
through any opening in order
to release this energy.
• ATP synthase is a specialized
protein channel that serves as
a passage/opening in the
thylakoid.
• ATP synthase allows the energy
released by the hydrogen ion
stream to attach a third
phosphate group to ADP,
which forms ATP
 GENERATING AN ENERGY CARRIER: ATP
• CHEMIOSMOSIS is called
the flow of hydrogen ions
through ATP synthase,
because the ions move
through a semi-permeable
structure from an area of
high concentration to an
area of low concentration
THE REACTIONS OF PHOTOSYNTHESIS
ARE DIVIDED INTO TWO PHASES

Photosynthesis reaction
REVIEW OF
PREVIOUS
LESSON
TRUE OR FALSE?
 •1. A very small portion of
TRUE OR a vast, continuous range
of radiation called the
FALSE? electromagnetic
spectrum represents
visible light.
 •1. A very small portion of
a vast, continuous range
TRUE of radiation called the
electromagnetic
spectrum represents
visible light.
 2. Oxidation is the gain of
TRUE OR electrons.
FALSE? Reduction is the loss of
electrons.
 2. Oxidation is the gain of
FALSE electrons.
Reduction is the loss of
electrons.
 3. The PSI reaction center
(P700) delivers its high-
TRUE OR energy electrons to the
primary electron acceptor,
FALSE? one at a time, and to PSII
(P680) through the electron
transport chain (Pq to Cyt
complex to Pc).
 3. The PSI reaction center
(P700) delivers its high-
energy electrons to the
FALSE primary electron acceptor,
one at a time, and to PSII
(P680) through the electron
transport chain (Pq to Cyt
complex to Pc).
TRUE OR 4. Electrons lose energy
as they move through the
FALSE? proteins that reside
between PSII and PSI.
 4. Electrons lose energy
TRUE as they move through the
proteins that reside
between PSII and PSI.
 5. For the production of
ATP, the passive diffusion
TRUE OR of hydrogen ions from a
high concentration area
FALSE? (in the thylakoid lumen)
to a low concentration
area (in the stroma) is
used.
 5. For the production of
ATP, the passive diffusion
of hydrogen ions from a
TRUE high concentration area
(in the thylakoid lumen)
to a low concentration
area (in the stroma) is
used.
 LIGHT-INDEPENDENT
REACTIONS
(Phase 2 of Photosynthesis)
 LIGHT-INDEPENDENT REACTIONS
(Phase 2 of Photosynthesis)
Other names of this phase:
• Carbon Fixation Reactions;
• Calvin Cycle;
• Calvin-Benson Cycle;
• C3 Pathway
 THREE PHASES OF THE CALVIN CYCLE

• There are three phases in

the Calvin cycle:
(1) CO2 Uptake / Fixation
(2) Carbon Reduction
(3) RuBP Regeneration
 REPRESENTATION OF MOLECULES INVOLVED
IN THE CALVIN CYCLE
• CARBON DIOXIDE
CO2 reacts with RuBP which
is catalyzed by the enzyme
RUBISCO in the initial stage
of the Calvin cycle.
 REPRESENTATION OF MOLECULES INVOLVED
IN THE CALVIN CYCLE
• RIBULOSE BISPHOSPHATE
(RuBP)
five-carbon phosphorylated
compound that reacts with
carbon dioxide in the initial
step of the Calvin cycle.
It consists of two phosphate
groups with five carbon Note: Though the correct formula of
atoms. phosphate is PO4 , here we used P only
for its representation.
 REPRESENTATION OF MOLECULES INVOLVED
IN THE CALVIN CYCLE
• RIBULOSE PHOSPHATE
(RP)
to be phosphorylated by
ATP to produce RuBP.
 REPRESENTATION OF MOLECULES INVOLVED
IN THE CALVIN CYCLE
• PHOSPHOGLYCERATE
(PGA)
Phosphorylated three-
carbon compound.
Consists of one phosphate
with three carbon atoms.
It is involved in the first
phase of the Calvin cycle.
 REPRESENTATION OF MOLECULES INVOLVED
IN THE CALVIN CYCLE
• GLYCERALDEHYDE-3-
PHOSPHATE (G3P)
Phosphorylated three-
carbon compound.
It consists of one phosphate
with three carbon atoms;
involved in the second
phase of the Calvin cycle.
THREE PHASES OF THE CALVIN CYCLE
PHASE 1: CO2 UPTAKE / FIXATION
 THREE PHASES OF THE CALVIN CYCLE
PHASE 1: CO2 UPTAKE / FIXATION
• In the first stage of the
Calvin cycle, CO2 molecule
reacts with ribulose
bisphosphate (RuBP)
which is a phosphorylated
five-carbon compound.
• The ribulose bisphosphate
carboxylase/oxygenase
(also known as rubisco) is
an enzyme that catalyzed
this reaction.
 THREE PHASES OF THE CALVIN CYCLE
PHASE 1: CO2 UPTAKE / FIXATION
• An unstable six-carbon
intermediate is the
product of this reaction,
which immediately breaks
down into
phosphoglycerate (PGA)
molecules with three
carbons each.
 THREE PHASES OF THE CALVIN CYCLE
PHASE 1: CO2 UPTAKE / FIXATION
• This time, the carbon has
been fixed, because the
carbon which was
originally part of the
molecule of CO2 is now
part of the carbon
skeleton.
• In this phase, we have 12
molecules of PGA.
 THREE PHASES OF THE CALVIN CYCLE
PHASE 1: CO2 UPTAKE / FIXATION
• C3 Pathway is the other
term for the Calvin cycle,
because a three-carbon
compound is the product
of the initial carbon
fixation reaction.
• Plants initially fix carbon in
this way
THREE PHASES OF THE CALVIN CYCLE
PHASE 2: CARBON REDUCTION
 THREE PHASES OF THE CALVIN CYCLE
PHASE 2: CARBON REDUCTION
• In the second stage of the
Calvin cycle, PGA
molecules are converted
to glyceraldehyde-3-
phosphate (G3P) with the
help of energy and
reducing power from ATP
and NADPH which are
both products of light-
dependent reactions.
 THREE PHASES OF THE CALVIN CYCLE
PHASE 2: CARBON REDUCTION
• For carbohydrate
synthesis, six carbons can
leave the system as two
molecules of G3P.
• There should be six
carbons that will enter the
cycle as CO2
• Each of these G3P (three-
carbon molecules) is
essential half a hexose.
 THREE PHASES OF THE CALVIN CYCLE
PHASE 2: CARBON REDUCTION
• Recall that hexose is a six-
carbon sugar with a
chemical formula of
C6H12O6 (examples are
glucose and fructose which
are both
monosaccharides).
 THREE PHASES OF THE CALVIN CYCLE
PHASE 2: CARBON REDUCTION
• Recall that hexose is a six-
carbon sugar with a chemical
formula of C6H12O6 (examples
are glucose and fructose which
are both monosaccharides).
• In this phase, we have 10
molecules of G3P.
• This time, there is a reduction
in the number of carbons.
THREE PHASES OF THE CALVIN CYCLE
PHASE 3: RuBP REGENERATION
 THREE PHASES OF THE CALVIN CYCLE
PHASE 3: RuBP REGENERATION
• Although two (2) G3P
molecules are removed from
the cycle,
ten (10) G3P molecules remain,
representing a total of thirty
(30) atoms of carbon.
 THREE PHASES OF THE CALVIN CYCLE
PHASE 3: RuBP REGENERATION
• These thirty (30) carbons and
their associated atoms are
rearranged into six (6) molecules
of ribulose phosphate through a
series of 10 reactions that make
up the third phase of the Calvin
cycle, each of which is
phosphorylated by ATP to
produce ribulose bisphosphate
(RuBP), the five-carbon compound
that the cycle began with.
 THREE PHASES OF THE CALVIN CYCLE
PHASE 3: RuBP REGENERATION
• Once again, these RuBP
molecules begin the CO2
fixation process and
eventual production of G3P