Photosynthesis

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Photosynthesis
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
Live within another organism and obtain nutrition
from the host
Obtain food from decaying matters
Use energy of a chemical reaction for the
synthesis of food Eg. Some bacteria like Ferrous
bacteria
Use energy of sun light for the synthesis of
food Eg. All green plants, Cyanobacteria And
some bacteria
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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.

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Where does photosynthesis occur?

chloroplast.

• As you know prokaryotes like bacteria and
• cyanobacteria lack membrane bound organelles.

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What is the structural unit of photosynthesis?
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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
• lets 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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Chloroplast
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Chloroplast

• Chloroplasts like mitochondria are bound by
• double membranes which are separated by
  inter-
• membrane 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 its 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
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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
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Absorption Spectrum of Chlorophyll
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Process of photosynthesis inside the
Chloroplast
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Photosynthesis in summarized form
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(No Transcript)
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Electromagnetic spectrum
Visible range
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CO2 concentration and rate of photosynthesis
Photosynthetic rate in relation to light intensity
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The phase of light reaction
Sun light
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Structure of C4 plant leaf
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• Measurement of Photosynthesis
• There are two methods by which the rate of
• photosynthesis can be measured.
• Harvest Method
• 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