Photosynthesis and Chemosynthesis

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Chapter 23
Photosynthesis and Chemosynthesis
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Photosynthetic pigments
Chlorophyll Chlorophyll a
Chlorophyll Chlorophyll b
Carotenoids Carotene
Carotenoids Xanthophyll
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Chlorophyll a is the most abundant and important
photosynthetic pigment. It is the primary
pigment for photoexcitation.

• The other photosynthetic pigments are called
  accessory pigments.
• They enables the plant to absorb sunlight of
  greater range of wavelength.
• They transfer the energy of the light they have
  absorbed to chlorophyll a for photoexcitation.
• They protect chlorophyll a from destruction by
  absorbing excessive sunlight.

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Absorption Spectrum
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Action Spectrum
The action spectrum is highly correlated to the
absorption spectrum.
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Paper chromatography for separation of
photosynthetic pigments
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Different photosynthetic pigment has different Rf
value.
Rf value of a pigment Distance travelled
by the pigment Distance
travelled by the solvent front
The list below indicates the Rf value of the most
common photosynthetic pigment using Petroleum
ether acetone(91) as a developing
solvent Carotene 0.95 Xanthophyll 0.71 Chloroph
yll a 0.65 Chlorophyll b 0.45
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Chloroplast
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Electromicrograph of Chloroplast
Light dependent stage of photosynthesis takes
place in grana and intergrana. Light independent
stage of photosynthesis takes place in stroma.
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Mechanism of Photosynthesis Light dependent stage
1 Chlorophyll molecule is excitated from its
ground state by a process called
photoexcitation. Ch ? Ch e-
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Mechanism of Photosynthesis Light dependent stage
2 The high energy electron is transferred via a
series of electron transport chain.
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Mechanism of Photosynthesis Light dependent stage
3 The energy is handed over to form ATP, a
process called photophosphorylation.
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Mechanism of Photosynthesis Light dependent stage
4 Water molecule is splitted up to give oxygen,
hydrogen ion and electron, a process called
photolysis. The H together with the e- via the
electron transport chain reduce NADP to NADPH2.
The electron returns the chlorophyll ion to its
ground state.
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Mechanism of Photosynthesis Light dependent stage
Alternative explanation for step 4 Water is
ionized to form hydrogen ion and hydroxide ion
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Mechanism of Photosynthesis Light dependent stage
4 OH- ? 2H2O O2 4e- The electron returns the
chlorophyll ion to the ground state. The H
together with the e- via the electron transport
chain reduce NADP to NADPH2.
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The main products formed by the light dependent
stage are ATP and NADPH2 which will be used in
the light independent stage of photosynthesis. The
light dependent stage is a series of
photochemical reactions which are NOT controlled
by enzymes. The rate is therefore NOT temperature
dependent.
Light dependent stage takes place in grana and
intergrana. Light independent stage takes place
in stroma.
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Light independent stage Carbon Fixation/ Calvin
Cycle
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Products of Photosynthesis
Some triose phosphate is to synthesize glucose
and then sucrose and starch.
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Products of Photosynthesis
Fatty acids and glycerol are produced from the
3-C compound and triose phosphate respectively.
Lipids are formed by combining glycerol and fatty
acids,
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Products of Photosynthesis
Amino acids are formed from the 3-C compounds.
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• Factors affecting the rate of photosynthesis
• Light intensity
• Carbon dioxide concentration
• Temperature

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Concepts of Limiting factor
The rate of photosynthesis is controlled by light
intensity, CO2 concentration and temperature.
The most unfavourable factor affects the rate of
photosynthesis most. This factor is called
Limiting factor. The change in other factors have
no effect on the rate of photosynthesis. When the
limiting factor becomes favourable, it will no
longer be limiting and the rate of photosynthesis
increases accordingly until the other factors
become limiting.
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Chemosynthesis
Autotroph/ autotrophic organisms are able to
synthesize their own food using the raw materials
such as carbon dioxide and water.
Inorganic raw materials ? Organic food
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Examples of Chemo-autotrophs/ Chemosynthetic
organisms
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Chemosynthetic bacteria in Nitrogen Cycle
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Nitrite bacteria
Nitrite bacteria oxidize the ammonium ion in the
soil into nitrite ion with release of energy 2
NH4 3 O2 ? 2 NO2- 2 H2O 4 H Energy The
energy is used to synthesize organic food for
this bacteria.
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Nitrate Bacteria
Nitrate bacteria oxidizes nitrite ion into
nitrate ion with release of energy. 2 NO2- O2 ?
2 NO3- Energy The energy is used to synthesize
organic food from inorganic raw materials.