FSC BIO 124
TOPIC 7
PHOTOSYNTHESIS
NORLAILA NAJWA BT RAHMAN
Content Outline
7.1 Autotroph
7.2 Photosynthetic structure
7.3 Photosynthetic pigment
7.4 Structure of leaf
7.5 Photosynthesis
7.6 Light dependent reactions
7.7 Light independent reactions (Calvin cycle)
NORLAILA NAJWA BT RAHMAN
At the end of the lecture, you will able to
Understand autotrophic nutrition
Describe the structure of photosynthetic leaf
Describe the structure of chloroplast and chlorophyll
Outline the light dependent reaction
Outline the light independent reaction
NORLAILA NAJWA BT RAHMAN
Definition

Photosynthesis is the process by which
autotrophic organisms use light energy to
make sugar and oxygen gas from carbon
dioxide and water
NORLAILA NAJWA BT RAHMAN
Photosynthesis
NORLAILA NAJWA BT RAHMAN
Introduction
Green plants use sunlight as an energy source, carbon
dioxide and water as raw material for photosynthesis
 The light energy trapped by green plant is converted to
chemical energy and stored in the bonds of organic
molecules such as carbohydrates
 Oxygen is released as a by product
 There are 2 types of autotroph : Chemosynthesis and
Photosynthesis

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Autotrophs
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NORLAILA NAJWA BT RAHMAN

Chemosynthesis is the process in which
carbohydrates are manufactured from carbon dioxide
and water using chemical nutrients as the energy
source. Occur only in certain groups of bacteria

Photosynthesis is the process by which autotrophic
organisms use light energy to make sugar and oxygen
gas from carbon dioxide and water. Occur in plants,
algae, some other protists.
NORLAILA NAJWA BT RAHMAN
Chemosynthesis
NORLAILA NAJWA BT RAHMAN
Chemosynthesis
NORLAILA NAJWA BT RAHMAN
Structure of a photosynthetic leaf
Leaf
•Receive light energy from the Sun
•Permit diffusion of gases and water vapour
during photosynthesis
•Contain chlorophyll and other
photosynthetic pigments
•Absorb CO2 and dispose of the waste
product
•Have a water supply and be able to export
manufactured carbohydrate to the rest of
the plant
NORLAILA NAJWA BT RAHMAN
Structure of a photosynthetic leaf
a) Epidermis
•Thin, flat and transparent cell
which allow light to pass through
to the mesophyll cells
•Has stomata
•Secrete waxy transparent cuticle
which forms a watertight layer
NORLAILA NAJWA BT RAHMAN
Structure of a photosynthetic leaf
b) Palisade Mesophyll
• Large number of chloroplasts
• More cell exposure,
maximum light absorption
• Thin cellulose cell walls maximum light penetration
for rapid diffusion of gases
NORLAILA NAJWA BT RAHMAN
Structure of a photosynthetic leaf
c) Spongy Mesophyll
•Smaller number of chloroplasts
•Large airspaces for rapid diffusion of
gases
•Thin cellulose cell walls for maximum
light penetration & rapid diffusion of
gases
NORLAILA NAJWA BT RAHMAN
Structure of a photosynthetic leaf
d) Stomata
•Pores in the epidermis
•Surrounded by guard cell
e) Guard cell
•Alters in shape
•When turgid, thin outer wall bends
more readily than thick inner wall
so the cells becomes curved (pores
between cells open)
NORLAILA NAJWA BT RAHMAN
Structure of a photosynthetic leaf
f) Vascular bundle
•Xylem (brings water for
photosynthesis and cell turgor)
•Phloem (takes products of
photosynthesis to other parts of
the plant)
NORLAILA NAJWA BT RAHMAN
Structure of Chloroplast
f) Chloroplast
 Chloroplast are the sites of photosynthesis in
plant
 Any green part of a plant has chloroplast
 However, the leaves are the major site of
photosynthesis for most plants
 In plants, chloroplast are normally found in the
mesophyll cell of leaves
 A typical palisade cell has about 30 to 40
chloroplasts
 Many chloroplasts are biconvex disc, 3-10μm
long and 2-4μm wide
NORLAILA NAJWA BT RAHMAN
Structure of Chloroplast



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The chloroplast envelope consists of two
membranes, outer membrane and
inner membrane covered the matrix
called stroma
A
system
of
interconnecting
membranous
flatted
sacs
called
thylakoids
At certain places within the chloroplast
the thylakoid membranes are arranged
in stacks called grana
There are about 50 grana in a
chloroplast and each granum is made up
of between 2 to 100 thylakoids.
NORLAILA NAJWA BT RAHMAN
Structure of Chloroplast



The thylakoid membranes and grana
increase the surface area for attachment
of chlorophyll molecules, accessory
pigments and electron carriers involve in
the light reactions
The thylakoid membranes are
surrounded by a semi-fluid, stroma
Stroma contain circular DNA, ribosomes
and enzymes which catalyse dark
reaction, starch granules and lipid
NORLAILA NAJWA BT RAHMAN
Structure of Chloroplast



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The pigments present in thylakoid
membranes consist largely of two kinds of
green chlorophylls, chlorophylls a and
chlorophylls b
Pigments are molecules that absorb
certain wavelengths of light and transmit
others
Chlorophyll a absorbed the red light and
the blue light and it is the most abundant
pigments in plants
Chlorophyll b absorbed only blue light
and transmitted green light, thus the
leaves look green
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Internal Structure of a leaf
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The location and Structure of Chloroplast
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Photosynthesis
Photosystem
•Located in thylakoid membrane
•Arranged in groups of
hundreds molecules
•Function: Capture light energy
for photosynthesis
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Photosynthesis
Two types of Photosystem
• (a) Photosystem I (PS I)
•Has a chlorophyll molecule that absorbs
light at 700nm
•The reaction center for Photosystem I
is chlorophyll a (P700)
• (b) Photosystem II (PS II)
•Has a chlorophyll molecule that absorbs
light at 680nm
•The reaction center for Photosystem II
is chlorophyll a (P680)
NORLAILA NAJWA BT RAHMAN
Photosystem
Photosystem is composed of a reaction
center surrounded by a number of lightharvesting complexes
How a photosystem harvest light?
• 1)When a photon strikes a pigment
molecule in a light-harvesting complex,the
energy is passed from molecule to
molecule until it reaches the reaction
center.
• 2)At the reaction center an exited electron
from one of the two special chlorophyll a
molecules is captured by the primary
electron acceptor
NORLAILA NAJWA BT RAHMAN
Photosynthesis
Photosynthesis can be divided into two stages:
a) Light dependent reaction / Light reaction
•Occurs in the thylakoid membrane
•Require a continuous supply of light
b) Light independent reaction / Dark reaction
•Occurs in the stroma
•Carry out in darkness after light reaction
NORLAILA NAJWA BT RAHMAN
An overview of photosynthesis: Cooperation
of the light reactions and the calvin cycle
1)In chloroplast,the tylakoid
membranes are the sites of the light
reactions whereas the Calvin cycle
occurs in the stroma
2)Light reaction use solar energy to
make ATP and NADPH,which
function as chemical energy and
reducing power,respectively in the
Calvin cycle
3)Calvin cycle incorporates CO2 into
organic molecules,which are
converted to sugar
NORLAILA NAJWA BT RAHMAN
Light Dependent reaction
•Requires sunlight
•Several intermediate compounds
produced are used up in dark reaction.
•Involves 4 stages
i.Absorption of light energy and
photoactivation of chlorophyll complex
ii.ATP synthesize through
photophosphorylation
iii.Production of NADPH
iv.Production of oxygen from photolysis
of water
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Light Dependent reaction
•Products: ATP, NADPH, O2
• Consists of 2 types
• a) Water photolysis
• b) Photophosphorylation(Cyclic and Uncylic)
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Water photolysis


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Water photolysis is a process
of splitting of water molecules
Hydrogen ion will be released
into the stroma to combine
with NADP to formed NADPH
The electron was used to
stabilized the reaction center
NORLAILA NAJWA BT RAHMAN
Photosystem
• Reaction-center complex associated
with light-harvesting complexes.
• How a photosystem harvests light?
• When a photon strikes a pigment
molecule in a light-harvesting
complex, the energy is passed from
molecule to molecule until it reaches
the reaction-center complex.
• Here, an excited electron from the
special pair of chlorophyll a molecules
is transferred to the primary electron
acceptor.
CHEW WEIYUN
Photosystem
• The thylakoid membrane is
populated by two types of
photosystems that cooperate in the
light reactions of photosynthesis:
• photosystem II (PS II)
• photosystem I (PS I)
CHEW WEIYUN
Photosystem
• The reaction-center chlorophyll a
of photosystem II is known as P680
• best at absorbing light of wavelength
of 680 nm (in the red part of the
spectrum).
• The chlorophyll a at the reactioncenter complex of photosystem I is
called P700
• best absorbs light of wavelength 700
nm (in the far-red part of the
spectrum).
CHEW WEIYUN
Photophosphorylation

Is the process where the ATP formed when
the excited electron from the chlorophyll
molecules transferred along the electron
transport chain

ATP is the sources of energy and NADPH
is a reduction agent (dark reaction)

The chlorophyll that lose the electron
become positive charges and unstable

During the transfer of electron from carrier to
another from high energy level to low energy
level, there are a lot of energy released
NORLAILA NAJWA BT RAHMAN
Photophosphorylation ....cont..
• This energy was used to combine the phosphate
molecule to the ADP molecules to form
ATP(phosphorylation) and because of this
energy come from light, this process called
photophosphorylation
• If the excited and released electron come back
to the same chlorophyll to stabilized, this is
known as cyclic photophosphorylation
• If the electron not coming back to the
chlorophyll and this chlorophyll was stabilized
by the electron from water photolysis and this is
known as uncyclic photophosporylation
NORLAILA NAJWA BT RAHMAN
Uncyclic photophosphorylation
• Produces NADPH, ATP and oxygen
NORLAILA NAJWA BT RAHMAN
Edited by CHEW WEIYUN
CHEW WEIYUN
Uncyclic photophosphorylation

Most of the ATP was produced
through uncyclic
photophosphorylation

The process starts when the photon
excited an electron in the chlorophyll
molecules associated with PS II

When the energy is transmitted to the
reaction center, chlorophyll P680
passes on excited electron to primary
electron acceptor (pheophytin)
NORLAILA NAJWA BT RAHMAN
Cont…

The electron then gradually
stepped down in potential
energy through redox reaction
among a series of Pq
(plastoquinone) and cytochrome
complex

PS II is connected to the
cytochrome complex by Pq.

Cytochrome complex is
connected to PS I by a small
protein called Pc (plastocyanin)
NORLAILA NAJWA BT RAHMAN
Cont….
• The protein picks up an electron
from the cytochrome complex,
diffuses along the thylakoid
membrane and donates the
electron to PS I. This chlorophyll
is called P700. The electron from
PS I are transferred to protein Fd
(ferredoxin), which triggers the
reduction of NADP to NADPH.
NORLAILA NAJWA BT RAHMAN
Cont…

This process is continuously
where the lost electron
chlorophyll from PS I will be
stabilized by the electron
from PS II, whereas the lost
electron in the PS II will be
stabilized by electron from
water photolysis
NORLAILA NAJWA BT RAHMAN
Cyclic Photophosphorylation

In cyclic photophosphorylation
 Only
photosystem I is used
 Only ATP is produced
Primary
acceptor
Primary
acceptor
Fd
Fd
Pq
NADP+
reductase
Cytochrome
complex
NADPH
Pc
Figure 10.15
Photosystem II
NORLAILA NAJWA BT RAHMAN
ATP
NADP+
Photosystem I
Cyclic Photophosphorylation
NORLAILA NAJWA BT RAHMAN
Cyclic Photophosphorylation

The excited electron from PS I will all
the way back to the chlorophyll in PS I

One photon of light absorbed by the
light-harvesting pigment complex of
PS I (P700) will be accepted by high
energy primary electron acceptor and
will be carried through the series of
electron carrier (Fd, cytochrome
complex, Pc)

During this process, the energy was
released and used in ATP synthesis
NORLAILA NAJWA BT RAHMAN
Cyclic Photophosphorylation
• The cyclic photophosphorylation is
important due to the production of
additional ATP for Calvin cycle
used
• Light reaction is important due to
the ATP production which is the
source of energy for dark reaction
• The end product of light reaction
are ATP, NADPH and oxygen
NORLAILA NAJWA BT RAHMAN
CHEW WEIYUN
Photosynthetic pigments
• When light meets matter, it may be reflected, transmitted, or
absorbed.
• Substances that absorb visible light are known as pigments.
• Consider this:
• Why does leaves appeared green?
CHEW WEIYUN
Photosynthetic pigments
• Chloroplasts have several pigments.
• Each pigments absorb different wavelengths of light.
• Chlorophylls (primary pigments)
• Carotenoids (accessory pigments)
CHEW WEIYUN
Photosynthetic pigments
• Chlorophyll a and b
CHEW WEIYUN
Photosynthetic pigments
• An absorption
spectrum is a
graph of the
absorbance of
different
wavelengths of
light by a pigment.
CHEW WEIYUN
Photosynthetic pigments
• An action spectrum is a
graph of the rate of
photosynthesis at
different wavelengths of
light.
CHEW WEIYUN
Dark Reaction

Occur in the stroma of chloroplast and involve the
reduction of carbon dioxide to form carbohydrate.

Dark reaction need the energy that was supplied by ATP
(from light reaction)

The reduction agent is the NADPH (product from light
reaction)

Dark reaction involve the series of enzymatic reaction
and known as Calvin Cycle
NORLAILA NAJWA BT RAHMAN
Dark reaction

The actual sugar product of the Calvin cycle is
not glucose (C6H12O6), but a three-carbon
sugar, glyceraldehyde-3-phosphate (G3P)

Each turn of the Calvin cycle fixes one carbon

For the net synthesis of one G3P molecule,
the cycle must take place three time, fixing
three molecule of CO2
To make one glucose molecules would require
six cycle and the fixation of CO2 molecules


Dark reaction consists of 3 main parts:
1.
2.
3.
Carbon fixation
Reduction
Regeneration of CO2 acceptor (RuBP)
NORLAILA NAJWA BT RAHMAN
Dark reaction
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1. Carbon fixation

It is the first step in dark
reaction

Each CO2 (1C) molecule is
attached to a RuBP (5C) to
form unstable six-carbon
(6C)

This is catalyzed by Rubisco
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NORLAILA NAJWA BT RAHMAN
 2.

Reduction
During reduction, each 3-PGA receives
another phosphate group from ATP to
form 1,3 bisphosphoglycerate

Electron from NADPH reduce 1,3
bisphophoglycerate to G3P
(glyceraldehyde-3-phosphate)

If our goal was to reproduce one G3P
net, we would start with 3 CO2 (3C) and 3
RuBP (15C)
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• Cont.... (Reduction)
• After fixation and reduction
we would have six molecule
G3P (18C)
• One of these six G3P (3C) is
a net gain of carbohydrate
• The other five G3P (15C)
must remain in the cycle to
regenerate three RuBP
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•3. Regeneration

In the last phase, these five G3P
molecules are rearranged to form 3
RuBP molecule.

To do this, the cycle must spend 3
molecule of ATP to complete the
cycle and prepare for the next.

For the net synthesis of one G3P
molecule, the Calvin cycle consumes
9 ATP and 6 NADPH
NORLAILA NAJWA BT RAHMAN
Factors affecting rate of photosynthesis
Light intensity
 Green plants require sunlight to
carry out photosynthesis
 The most effective light for
photosynthesis is red and blue
light
 The light intensity influenced the
light reaction in photosynthesis
 If there is not limited factors, the
rate of photosynthesis is increase
followed the light intensity
NORLAILA NAJWA BT RAHMAN
Edited by CHEW WEIYUN
Factors affecting rate of photosynthesis
Temperature
• The rate of dark reaction was influenced by temperature
because dark reaction was catalyzed by the enzyme
• The rate of photosynthesis is maximum at the room
temperature or medium temperature of 20o – 35oC
• If the temperature above of 40oC, it will denaturalize the
enzyme and the photosynthesis slowly stopped
NORLAILA NAJWA BT RAHMAN
Factors affecting rate of photosynthesis
• At high light intensity the rate
of photosynthesis increases as
the temperature is increased
over a limited range.
• At low light intensity,
increasing the temperature
has little effect on the rate of
photosynthesis.
CHEW WEIYUN
Factors affecting rate of photosynthesis
CO2 concentration
• The rate of photosynthesis is directly proportional to the
carbon dioxide concentration
• The concentration of CO2 in the atmosphere is only 0.035%.
• When value increase, the rate of photosynthesis also
increase until a maximum level 1.0%.
• A concentration of CO2 that exceeds 1.0% will stimulate the
closing of the stomata and will reduce the rate of
photosynthesis.
NORLAILA NAJWA BT RAHMAN
Factors affecting rate of photosynthesis
• The rate of photosynthesis
at different temperatures
and different carbon
dioxide concentrations.
(0.04% CO2 is about
atmospheric
concentration.)
CHEW WEIYUN
Photorespiration
• Occurs on hot, dry, bright days
• -> Stomata closes
• Produce less sugar because of declining level of CO2
• O2 build up
• Fixation of O2 instead of CO2 on enzyme Ribulose bisphosphate carboxylase
• Produces 2-C molecules (Glycolate) instead of 3-C sugar molecules
(Glycerate)
• Produces Oxygen but no sugar molecules or no ATP
• RuBP is being ‘wasted’.
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C-C-C=C-C
O-C=O
Enzyme RuBP
carboxylase
In the presence of CO2, 2
molecules of G3P is formed
(a) CO2 fixation in C3 plants
In the presence of O2;
o 1 molecules of G3P is formed
o 1 molecule of phosphoglycolate
C-C-C=C-C
(2C) is formed
O-O
o O2 acts as a competitive inhibitor
to CO2
CHEW WEIYUN (b) photorespiration
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CHEW WEIYUN
Photorespiration
Because of photorespiration, plants have special adaptations
to limit the effect of photorespiration:
1. C4 plants
2. CAM plants
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C4 Plants
• Hot, moist environments
• In tropics high light intensity, low [CO2]
• Low [CO2] activates photorespiration
• 15% of plants (grasses, corn, sugarcane)
• Photosynthesis occurs in 2 places
• Light reaction - mesophyll cells
• Calvin cycle - bundle sheath cells
• The bundle sheath cells have no direct contact
with the air inside the leaf.
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C4 Pathway
• Low [CO2] limits
photosynthesis
• Mesophyll cells contain
PEP carboxylase
Glucose, ect
PEP carboxylase
Phloem
Tissue
CHEW WEIYUN
• Phophoenolpyruvate
(PEP) has higher affinity
towards low [CO2]
• [CO2] is increase in
bundle sheath cell for
Calvin cycle
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C4 Pathway
• CO2 in atmosphere diffuses into the mesophyll cells of C4 plant where
they combine with phosphoenolpyruvate (PEP) producing oxaloacetate
(4C)
• Oxaloacetate is reduced to malate (4C)
• Malate is shunted through plasmodesmata into bundle sheath cells
• Malate is oxidize into pyruvate (3C) acid – by removal of H and CO2
• CO2 concentration in stroma (chloroplast) increases and enters Calvin
cycle
• High CO2 concentration in the bundle sheath cells inhibits
photorespiration, and increases the rate of photosynthesis
• Pyruvate diffuses into the mesophyll cells and is phosphorylated to
regenerate phosphoenolpyruvate
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Carbon dioxide fixation in C3 and
C4 plants
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CAM Plants
• Crassulacean Acid Metabolism
• Hot, dry environments
• 5% of plants (cactus and ice plants)
• Stomata closed during day
• Stomata open during the night
• Light reaction - occurs during the day
• Calvin Cycle - occurs when CO2 is present
• Low [CO2] promotes photorespiration
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CAM Plants
• During night, malate is
produced and stored in
vacuole
• During day, malate is
utilised to provide CO2 for
Calvin cycle
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CHEW WEIYUN
CAM Plants
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Why do CAM plants close their
stomata during the day?
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