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Do you know? |
If a cell is broken up, the chloroplasts also break into pieces, so it becomes a very |
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difficult task to isolate them to study the different steps of photosynthesis.It was not |
until 1954, that Daniel I. Arnon was able to break up plant cells so gently that whole |
chloroplasts could be obtained that could carry through photosynthesis. |
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membrane stroma thylakoid It has been found that the chloroplast is |
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lipid globule |
stroma |
a membranous structure, consisting of 3 |
membranes. The third layer forms stacked |
} granum sack like structures called as grana. It is |
believed to be a site for trapping of solar |
energy. The intermediary fluid filled portion |
grana thylakoid is called as stroma. It is believed to be |
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starch grain responsible for enzymatic reactions leading |
to the synthesis of glucose, which in turn |
fig-8: T-S of chloroplast |
join together to form starch. |
Substances found in chloroplast which capture sunlight are called |
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photosynthetic pigments. There are several types of photosynthetic |
pigments involved in the process to produce organic molecules like |
glucose in plants. |
Chlorophyll is such a pigment which contain one atom of magnesium. |
It is similar in structure to the haem of haemoglobin. (The iron containing |
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red pigment that transports oxygen in blood.) Two major kinds of |
chlorophylls are associated with thylakoid membranes. Chlorophyll ‘a’ is |
blue-green in colour and chlorophyll ‘b’ is yellow-green colour. Around |
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250 to 400 pigment molecules are grouped as light harvesting complex or |
photosynthetic unit in each thylakoid. Such innumerable units function |
together in chloroplasts of green plants in the process of photosynthesis. |
During photosynthesis several events occur in the chloroplast some |
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of them are: |
1. Conversion of light energy to chemical energy |
2. Splitting of water molecule (photolysis of water) |
3. Reduction of carbondioxide to carbohydrates |
Light is required to initiate several events while several may continue |
even in absence of it. That would mean, once light energy has been captured |
it can help reactions to continue even in the dark. Light dependent events |
10 X Class Nutrition - Food supplying system |
or reactions are called light reactions and it has been found to take place |
in grana, while the rest are called light independent or dark reactions and |
they occur in the stroma. |
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Mechanism of Photosynthesis |
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1. Light dependent reaction (Photochemical phase) |
In this reaction light plays a key role. A series of chemical reactions |
occur in a very quick succession initiated by light and therefore the phase |
is technically called the photochemical phase or light dependent reaction. |
The light reaction takes place in chlorophyll containing thylakoids called |
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grana of chloroplasts. Several steps occur in the light dependent reaction. |
Step-I : The chlorophyll on exposure to light energy becomes activated |
by absorbing photons. (Photon is the smallest unit of light energy) |
Step-II: The energy is used in splitting the water molecule into two |
component ions named hydrogen (H+), hydroxyl ion (OH-). |
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