- Types of Chlorophyll
There are two main types of chlorophylls which are involved in the
process of photosynthesis. They are chlorophyll - a and chlorophyll - b.
Chlorophyll - a is an essential pigment having formula C55H72O5N4Mg.
The formula of chlorophyll - b is C55H70O6N4Mg.
- Structure of Chlorophyll: Molecule of chlorophyll
consists of two parts namely:
A ring called 'porphyrin ring' which
shows magnesium in the centre.
A long hydrocarbon tail called 'phytol' .
There are several side groups which are attached to the porphyrin ring. These groups change the properties of the
pigment. Chlorophyll - a has a methyl group (- CH3) and
while chlorophyll - b has an aldehyde group (- CHO).
- Chlorophyll absorbs blue and
red light, and reflect green light. Thus appears green.
- Carotenoids are the accessory pigments.
a. Carotenes - deep orange in colour (C40H56)
b. Xanthophylls - yellow in colour (C40H56O2).
- Chloroplast Structure: The chloroplast appears as
oval, dark green structure, measuring about 4 - 10 microns in diameter and
1 - 33 microns in thickness.
The number of chloroplasts per cell is a variable factor. Each
chloroplast is bound by a double unit membrane. It is called 'peristromium'. There is a space between two
membranes called as 'periplastidial space'.
- Quantasomes: These are the small
structures present on thylakoid membranes.
These are rich in chlorophyll. Each quantasome
contains 230 chlorophyll molecules. These are the photosynthetic units.
- Kranz Anatomy: Anatomically, C3
and C4 plants are different. C4 plants show Kranz anatomy. Here bundle sheath cells surround the
vascular bundles of veins. There are mesophyll
cell layers present on the outer side. The arrangement of chloroplasts
in the bundle sheath is towards the centre. Chloroplast dimorphism is
observed in C4 plants. The chloroplasts of bundle sheath
cells lack grana while those of mesophyll cells show well developed grana.
- Melvin Calvin could trace
the entire pathway of carbon during its fixation in dark reaction. For
this, he used a radioactive isotope 14C. In 1961, he was
awarded Nobel Prize.
- Emerson reported the
existence of two photosystems.
- Hatch and Slack (1965)
formulated the C4 cycle in plants.
- Ruben and Kamen confirmed that water is the source of oxygen
evolved during photosynthesis and not CO2. For this, they
used radioactive isotope 18O.
- Robert Hill postulated
photolysis of water.
- Van Niel
observed that H2S is the raw material in photosynthetic
bacteria instead of water. When he observed that sulphur
was released by breaking up of H2S, he postulated that during
photosynthesis, water gets dissociated.
- Blackmann found that photosynthesis
rate varied with temperature at high intensities. He gave the law of
- The rate of photosynthesis
is more when light energy is received in flashes instead of continuous
light. The flashing light regenerates ADP and NADP required in light
reaction. While in continuous light, the dark reactions are slow and
thus do not regenerate sufficient ADP and NADP. Thus dark reaction is a
rate limiting step because it is slower than the light reaction.
- Photorespiration: It was earlier believed
that the rate of photosynthesis in light and in dark is the same. But it
was later found that rate of respiration is higher in light.