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BIOLOGY F4 PAPER 2 2009 - ANSWER SCHEME
SECTION B
Item
number
7 (a)
Criteria
How the ozone layer becomes thinner :
 Thinning of the ozone layer is due to the widespread use of CFC
 It is used in aerosol, industrial solvents, electronics and Freon in air conditioners
 Ultraviolet radiation strikes a CFC molecule cause the chlorine atom to break
away
 Then the chlorine atom collides with an ozone molecule and combines with an
oxygen atom to form chlorine monoxide and oxygen
 Then the free atom of oxygen collides with the chlorine monoxide, the two oxygen
atoms form a molecule of oxygen
 The chlorine atom is released and free to destroy more ozone molecules
 The chlorine produced re-enters the cycle
 When the ozone layer becomes thinner, more ultraviolet radiation reaches the
Earth
The effect of excessive ultraviolet radiation on human :
 It causes reduction of the body’s immune system
 It can lead to skin cancer and cataract of the eye.
Effect on plants
 It reduces the rate of growth in plants, therefore reducing crop yields
Effect on aquatic organism
 It causes death of plankton in the water, reduce food supply to aquatic organism,
thus reducing the fisherman’s catch.
Steps to overcome this problem
 Reduce or stop using CFC or chlorine-based products
 Replace CFC with HCFC
 Use wrapping papers instead of polystyrene boxes
 Patch up the holes in the ozone layer by firing frozen ozone balls into the
atmosphere
TOTAL = 10 marks
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(b)
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A : Oxygen decrease, BOD increase :
-
sewage is a food for the bacteria / microorganisms /
decomposers / fungi
availability of abundant food increase the population of
bacteria / microorganisms / decomposers / fungi
microorganisms / decomposers / fungi decomposed sewage
by decomposition process
oxygen being used in the process of decomposition
causes level of oxygen become lower, and BOD increases
decomposition produce simple inorganic substance and
carbondioxide which then flow further downstream
B : oxygen increase, BOD decrease :
-
-
further downstream, there is accumulation of inorganic
substances and carbon dioxide produced during
decomposition process.
The present of inorganic substances act as a nutrient / food for
organisms such as phytoplankton / algae
Present of food and carbon dioxide will cause the population
of phytoplankton / algae to increase
They used food and carbondioxide to undergo photosynthesis
And produced oxygen
Thus level of oxygen increases
And BOD decreases
TOTAL = 10 marks
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