Page 1 - LN-PHOTOSYNTHESIS IN PLANTS

All animals including human beings depend on plants for their food. Green plant
synthesize the food they need, and for all other organisms depend on them for their
needs. Green plants carry out photosynthesis, a physio-chemical processor which
they use the light energy to derive the synthesis of organic compounds.

Topic 1 Introduction to Photosynthesis

The energy required by all living organisms comes directly or indirectly from the
sunlight.
Thus, sunlight plays an important role in fixation of CO 2 through which, conversion of
solar energy into chemical energy takes place. Water plays a significant role during
this process.
Hence, photosynthesis is the process by which plants, some bacteria and some
protistans uses the energy from sunlight to produce sugar, which through cellular
respiration produce ATP, the fuel used by all living organisms.
Photosynthesis is an important phenomenon due to the following two reasons
(i) It is the primary source of all food on the earth.
(ii) It is also responsible for the release of oxygen into the atmosphere by green
plants.
Photosynthesis is the only phenomenon of biological importance that can harvest
the energy of sunlight.
Requirements of Photosynthesis
On the basis of outline knowledge about the role of light, green plants, CO 2 etc in the
process of photosynthesis, several simple experiments might be performed
indicating that chlorophyll, light and carbon dioxide are essential components for
photosynthesis to take place.
Necessity of Chlorophyll (Green Pigment of Leaf)
To start with this experiment two leaves are taken, one is a variegated leaf or a leaf
that must partially covered with black paper and another leaf that must be exposed
to light. When these leaves are tested for presence of starch, it was observed that
photosynthesis had occurred only in the green parts of the leaves in the presence of
light, which ensures that chlorophyll is essential for photosynthesis.
Necessity of Carbon Dioxide
To initiate the half leaf experiment (given by Moll), a part of leaf is enclosed in a test
tube. The test tube contains some cotton soaked in KOH (KOH is used because it
absorbs carbon dioxide) and the another half of leaf is exposed to light.
The set up is then allowed to stand in light for about few hours, when starch test was
done, it was observed that the exposed part of leaf was tested positive for starch
while, the portion that was enclosed in the tube tested negative. This indicates that,
the CO 2 is also essential for photosynthesis to take place.
The difference between the two parts of the leaves in the Moll’s half leaf experiment
was due to the difference in the availability of CO 2 outside and its absence inside the
bottle.
Necessity of Light
The rate of photosynthesis is directly proportional to the intensity of light. The
necessity of light for photosynthesis can be shown by fixing a dark paper on leaf of
well watered, but destarched plant (plant can be destarched, by placing it in dark for