Chapter 23
Photosynthesis and Chemosynthesis
Photosynthetic pigments
Chlorophyll a
Chlorophyll
Chlorophyll b
Carotene
Carotenoids
Xanthophyll
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.
1. They enables the plant to absorb sunlight of greater
range of wavelength.
2. They transfer the energy of the light they have
absorbed to chlorophyll a for photoexcitation.
3. They protect chlorophyll a from destruction by
absorbing excessive sunlight.
Absorption Spectrum
Action Spectrum
The action spectrum is highly correlated to the absorption spectrum.
Paper chromatography
for separation of photosynthetic pigments
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(9:1)
as a developing solvent:
Carotene
0.95
Xanthophyll
0.71
Chlorophyll a
0.65
Chlorophyll b
0.45
Chloroplast
Electromicrograph of Chloroplast
Light dependent stage of photosynthesis takes place in grana and
intergrana.
Light independent stage of photosynthesis takes place in stroma.
Mechanism of Photosynthesis
Light dependent stage
1: Chlorophyll molecule is excitated from its ground state by a
process called photoexcitation.
Ch  Ch+ + e-
Mechanism of Photosynthesis
Light dependent stage
2: The high energy electron is transferred via a series of electron
transport chain.
Mechanism of Photosynthesis
Light dependent stage
3: The energy is handed over to form ATP, a process called
photophosphorylation.
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.
Mechanism of Photosynthesis
Light dependent stage
Alternative explanation for step 4: Water is ionized to form
hydrogen ion and hydroxide ion
Mechanism of Photosynthesis
Light dependent stage
4 OH-  2H2O + O2 + 4eThe electron returns the chlorophyll ion to the ground state. The H+
together with the e- via the electron transport chain reduce NADP to
NADPH2.
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.
Light independent stage: Carbon Fixation/ Calvin Cycle
Products of Photosynthesis
Some triose phosphate is to synthesize glucose and then sucrose and
starch.
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,
Products of Photosynthesis
Amino acids are formed from the 3-C compounds.
Factors affecting the rate of photosynthesis
a. Light intensity
b. Carbon dioxide concentration
c. Temperature
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.
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
Photo-autotroph:
Sunlight energy
Inorganic raw materials  Organic food
Chemo-autotroph: Chemical energy
Inorganic raw materials  Organic food
Examples of Chemo-autotrophs/ Chemosynthetic organisms
Iron bacteria:
Fe2+  Fe3+ + Energy
Inorganic raw materials  Organic food
Colourless sulphur bacteria:
S  SO4= + energy
Inorganic raw materials  Organic food
Chemosynthetic bacteria in Nitrogen Cycle
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.
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.