Transcript Photosynthesis - The Open University of Sri Lanka

Department of Botany
The Open University of Sri Lanka
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Content
 Learning Outcomes
 Autotrophic organisms
 Heterotrophs
 Photosynthesis
 Light absorption by the chloroplast
 The stages of photosynthesis
 The C4 plants
 Factors affecting photosynthesis
The measurement of photosynthesis
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Learning Outcomes
After studying this session, you should be able to
 explain the mode of obtaining their energy requirement
for maintenance of their biological activities.
 describe the structure of leaf, chloroplast and different
pigments associated with photosynthesis
 describe the portions of photosynthetically active areas
in the electromagnetic spectrum
 explain that light energy is converted to chemical energy,
and carbon is fixed into organic compounds.
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 describe how light is captured by the plants by means of
pigments and the structural adaptation to do so.
 distinguish between the two processes involved in the
photosynthesis i.e., Phase I and Phase II.
 explain photophosphorylation, the process in which
NADPH and ATP are generated.
 briefly describe the Calvin cycle and its important
reactions. Distinguish between the C3 and C4 pathways of
photosynthesis.
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How do we get energy to do our work?
 From food of course.
Can we synthesize our food within
our bodies?
 No we cannot. So we are referred to as
“heterotrophs”. But there are organisms having the
ability to synthesize their own food within their
bodies.They are called “autotrophs”.
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Categorization of organisms depending
on their food habits
Organisms
Autotrophic
Phototropic
Use energy of sun light
for the synthesis of food
Eg. All green plants,
Cyanobacteria
And some bacteria
Chemotropic
Use energy of a chemical
reaction for the synthesis
of food
Eg. Some bacteria like
Ferrous bacteria
Heterotrophic
Parasitic
Saprophytic
Live within another
organism and obtain
nutrition from the host
Obtain food
from decaying
matters
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What is this process of synthesis of
food within the bodies of all plants?
 It is called “photosynthesis”. And it occurs within
cells of plants.
Sun light
6CO2
+
C6H12O6
6H2O
Glucose
Carbon dioxide
Water
Chlorophyll
+
6O2
Oxygen
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Where does photosynthesis occur?
 In higher green plants or eukaryotes this
reaction takes place in the“chloroplast”.
 But in prokaryotic organism photosynthesis
occur in photosynthetic lamella dispersed in the
cytoplasm.
 As you know prokaryotes like bacteria and
cyanobacteria lack membrane bound organelles.
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What is the structural unit of
photosynthesis?
 It is the
 This structure has the form of a flattened sac or
vesicle.
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Do prokaryotes have thylakoids?
 In eukaryotes the thylakoid may form a part of
the internal membrane structure of the chloroplast.
 To give you a clear picture of the thylakoid
let’s look at the detailed structure of the,
chloroplast.
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Do prokaryotes have thylakoids?
 Yes. They do, however their thylakoid membrane
may form a part of the cell membrane or they
may occur in the cytoplasm
 In cyanobacteria the thylakoid may be a part
of the elaborate internal structure.
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Starch grain
External membrane
Stroma
Inter
Membranous
space
Internal membrane
Channel interconnecting
thylakoid
Grana
Thylakoid
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Chloroplast
 Chloroplasts like mitochondria are bound by
double membranes which are separated by intermembrane space.
 Look at the picture once again and trace the
membrane.
 Inner membrane is smooth in chloroplast
unlike that of mitochondria.
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Chloroplast
 Thylakoids are formed by a third membranous
system which is the interior of the chloroplast.
 Surrounding the thylakoid is a dense solution
which fills the interior of the chloroplast.
This is called the stroma and it’s composition
differs from that of the solution surrounding
other organelles in the cytoplasm.
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Chloroplast
 You know now that thylakoids are flattened
sacs. They are enclosed by additional
compartments called thylakoid spaces which
also contain another solution with yet another
composition.
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Chloroplast
 Under the high power of a light microscope
this is how the chloroplasts appear.
Chloroplasts
 But under an electron microscope we can see
that thylakoids are present in stacks.
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Chloroplast
 They are called “grana”
 In some thylakoid membranes there are
extensions that interconnect grana through
stroma which separate them.
 Now look at the diagram of the chloroplast
again and try to distinguish all the parts.
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Chloroplast
The thylakoid of the chloroplast is oriented
parallel to each other. Thus by swinging towards
the light chloroplast simultaneously aims all of
its million of pigment molecules for optimum
reception.
Photosynthetic pigments
 You have observed that most of the leaves are
green in color.
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Why is that ?
 Because leaves have pigments
What are pigments?
They are substances which absorb light
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What are these pigments?
 They are ,
1. Chlorophyll a
2. Chlorophyll b
3. Carotene
4. Xanthophylls
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What are these pigments?
 Among these, chlorophyll is the major and
the most important pigment for
photosynthesis.
 Out of chlorophyll, chlorophyll a is present in
all photosynthesizing organisms and is
essential for the process of photosynthesis.
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Where are these pigments
present?
 They are arranged on the lamella of chloroplast in
a very thin layer to facilitate maximum
absorption of light.
 They are more concentrated in the thylakoid
disc of the grana.
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How can you relate the presence of
pigments and photosynthesis?
 Presence of pigment is essential for the
photosynthesis to take place.
Because these pigments must absorb light.
Mainly sunlight to provide energy for the reaction
to proceed.
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Transverse section of a plant leaf
Cuticle
Upper epidermis
Palisade sheath cell
Spongy mesophyll cell
Lower epidermis
Stoma
Sub-stomatal air space
Guard cell
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Absorption Spectrum of Chlorophyll
Chlorophyll a
400
Chlorophyll b
500
600
700
800
Wavelength (nm)
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Process of photosynthesis inside the
Chloroplast
NADP+
ADP
RUBP
3-Phosphoglycerate
ATP
NADPH
G3P
O2
Starch
Amino acid
Fatty acid
Sucrose
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Photosynthesis in summarized form
CO2
Light
Reaction
ATP
NADPH
Dark
Reaction
Glucose
O2
H22O
O
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CO2 1C
Ribulose 1,55C
biphosphate
33C
phosphoglycerate
3ATP
3ADP
1,33C
diphosphoglycerate
Ribulose 55C
phosphate
2Pi
6AT
P
6ADP
C3 Cycle
Glyceraldehyde 3C
3-phosphate
6NADP
H6NADP
Glyceraldehyde
3C
3-phosphate
Glyceraldehyde
3-phosphate3C
6Pi
Sugar, Fatty acid,
Amino acids
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Electromagnetic spectrum
Visible range
Gamma
rays
10-14
X-rays
10-12
Ultraviolet
rays
10-10
10-8
Infrared
rays
10-6
Radar
10-4
10-2
FM
TV
Short
wave
AM
102
1
Wavelength (m)
400
500
600
700
Wavelength (nm)
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Rate of photosynthesis
CO2 concentration and rate of photosynthesis
Carbon dioxide concentration
Rate of photosynthesis
Photosynthetic rate in relation to light intensity
Light intensity
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Cyclic electron flow
The phase of light reaction
ATP
- 0.4
ADP + P
X
FRS
Ferredoxin
Ferredoxin
reductase
Cytochrome
Cytochrome
0
b
b
PS
II
Q
NADP-
PS I
+0.4
2H2O
O2 + 4H+
Plasto
-Quinone
CytoChrome
f
ADP + P
Non cyclic electron flow
+0.8
Plastocyanin
P 700
Z
P 680
B
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Structure of C4 plant leaf
Vascular membrane
(Vein)
Bundle sheath cell
Air space
beneath stoma
Mesophyll cells
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Measurement of Photosynthesis
There are two methods by which the rate of
photosynthesis can be measured.
1. Harvest Method
2. Gas Exchange method
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Harvest Method
 In this method the rate is determined by the increase
of dry weight of the plant, assuming that when CO2
is fixed to form sugars the dry weight is increased.
 By measuring the dry weight of the plant at the
beginning and at the end of a given period the rate
can be determined.
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Harvest Method
 To get accurate results weights have to be obtained
over a long period of time.
 However this method has its draw backs because
respiration, the oxidation of the produced sugars can
take place simultaneously, leading to decrease in dry
weight.
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Gas Exchange method
Air bubbles
Hydrilla plant
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Author – Indrani Amarasinghe
Web Content Developer – Chameera Kendaragama
Produced by The Open University of Sri Lanka
2014
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