IMPORTANCE OF PHOTOSYNTHESIS
1. It provides food for some living
organisms such as herbivores and
omnivores (carnivores do not eat
plants).
2. Oxygen given off by green plants
during photosynthesis is used by
animals for respiration.
3. Green plants take in carbon dioxide during photosynthesis, thereby reducing the amount of carbon dioxide in the atmosphere.
4. Photosynthesis produces materials for protein and starch formation.
5. It helps to maintain the oxygen, carbon and water cycle in the atmosphere.
EXPERIMENTS IN PHOTOSYNTHESIS
1. Testing for starch in a green leaf
Label two potted plants, A and B and
put them in the dark for a few days.
This will cause them to use all the
starch they have stored in their leaves.
Put plant A in the light and leave plant
B in the dark for a few days.
Take a leaf from each plant and run a
test for starch on them.
Dip each leaf in boiling water for one
minute to kill and soften it.
Dip each leaf in hot ethanol for one
minutes to remove the green pigment
Wash the leaves in hot water
Add dilute iodine solution to each leaf.
Observation
It would be observed that leaf A would
turn dark blue, which shows the presence
of starch, while leaf B would turn brown,
which shows the absence of starch
because it was kept in the dark.
2. An experiment to show the necessity of
light for photosynthesis
Cut a shape out from a piece of
aluminium foil, making a stencil.
Attach it to a de-starched leaf.
After 4 to 6 hours of daylight, detach
the leaf and test for starch.
Observation
Only areas which received sunlight turn
blue with iodine.
Conclusion
Since starch has not accumulated in the areas without light, it is evident that light plays an important role in photosynthesis.
3. Experiment to show that carbon dioxide is essential for photosynthesis
Water two de-starched potted plants.
Enclose the shoots in polythene bags, one of which contains soda-lime to
absorb carbon dioxide from the air and the other sodium hydrogen-carbonate (sodium trioxocarbonate (IV)) solution to produce extra oxygen.
Place both plants in sunlight or
fluorescent light for several hours.
Detach a leaf from each plant and test for starch.
Observation
The leaf deprived of carbon dioxide will not turn blue while that enriched with carbon dioxide will turn blue.
Conclusion
Since no starch is made in the leaf
deprived of carbon dioxide suggests that carbon dioxide is necessary for
photosynthesis.
4. Experiment to show that oxygen is
produced during photosynthesis
Place a short-stemmed funnel over
some plant shoots (e.g. ceratophyllum)
in a beaker of water.
Invert a test tube filled with water over
the funnel stem.
Raise the funnel above the bottom of
the beaker to allow for free circulation
of water.
Place the set-up in sunlight. Bubbles of
gas appear from the cut stems and is
collected in the test-tube.
Remove the test-tube and introduce a glowing splint.
Observation
The glowing splint bursts into flames.
Conclusion
It could be deduced that the plant has given off a gas which is considerably richer in oxygen than in atmospheric air.
5. Experiment to show that chlorophyll is necessary for photosynthesis
Leave a plant with variegated leaves
(leaves with diverse colours) in light for
2-6 hours.
Remove one leaf and make a labelled
diagram to show the green and nongreen parts.
Test the leaf for starch.
Observation
When the variegated leaf is tested for
starch, the green parts (which contain
chlorophyll) turn blue-black while the nongreen parts (which lack chlorophyll) show
no colour change.
Conclusion
Starch is made only in areas of the leaf
with chlorophyll, showing that chlorophyll
is necessary for photosynthesis.
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