(ii) Gametogenesis occurs at puberty in the human male.
In the female, it begins before birth but arrests at the
primary follicle stage. further development of gametes
resumes at puberty.(iii) In human male, gametogenesis is completed in human
testes, while in a human female, it is completed in the
fallopian tube.
- biofertilizers are microorganisms which bring about
nutrient enrichment of soil by enhancing the availability
of nutrients like nitrogen (N) and phosphorus (P) to
crops. The microorganisms which act as biofertilizers
are bacteria, cyanobacteria (blue green algae) and
mycorrhizal fungi.
(i) Azospirillum bacteria live in close contact with the
roots of cereals like maize and brazilian grasses and
fix nitrogen. Some of the fixed nitrogen is absorbed
by the roots.
(ii) endomycorrhizae stimulate the absorption of
phosphorus, and various other elements by roots.
They enhance nodulation in legumes and secrete
antimicrobial substances to protect the plants from
pathogen attack.
(iii) Rhizobium is one of the most important symbiotic
nitrogen fixing bacteria. It forms nodules on the
roots of legume plants and provides plant with
usable nitrogen.
(iv) Azolla pinnata a small free floating fresh water fern,
harbours a symbiotic blue green algae, Anabaena
azollae in the leaf cavities which fixes nitrogen. a
part of the fixed nitrogen is excreted in the cavities
and becomes available to the fern. The decaying
fern plants release the same for utilization of
the rice plants. When field is dried at the time of
harvesting, the fern functions as the green manure,
being decomposed and thus enriching the field for
the next crop.
OR
(i) Lactobacillus converts lactose sugar of milk into lactic
acid. Lactic acid causes coagulation of milk protein
casein. This property of Lactobacillus has following
useful applications:
(a) Indian curd is prepared by inoculating cream and
skimmed milk with Lactobacillus acidophilus at a
temperature of about 40ºC or less.
(b) yoghurt is produced by curdling milk with the help
of Streptococcus thermophilus and Lactobacillus
bulgaricus. It has a flavour of lactic acid and
acetaldehyde. It is often sweetened and flavoured
with fruit.
(c) Cheese consists of milk curd separated from liquid
part. In preparation of raw cheese, milk is curdled
with the help of lactic acid bacteria.
(ii) Cyclosporin a (an immunosuppressive drug) is obtained
from fungus Trichoderma polysporum while statins
(blood cholesterol lowering agents) are obtained from
yeast Monascus purpureus.- (i) bOD stands for biochemical Oxygen Demand. bOD
is a measure of oxygen required by aerobic decomposers
for the biochemical degradation of organic remains of a
unit mass. Higher the bOD, lower would be the dissolved
oxygen.
(ii) a higher bOD indicates that the particular segment of
the river is highly polluted. as a result, there will be
sharp decline in the dissolved oxygen content in this
part. It will result in mortality of fishes and other aquatic
organisms.
(iii) When the amount of organic matter in the river will
decrease, bOD will decrease too. as a result, aquatic
life will start flourishing.
OR
(i) Ozone shield is the region of stratosphere, where ozone
is present in high concentration. It is called ozone shield
because it protects the living beings from harmful effects
of ultraviolet radiations. The two gases that can cause
damage to ozone shield are chlorofluorocarbons (CfCs)
and methane (CH 4 ). Damage to ozone shield results in
impairment of photosynthetic machinery in plants and
skin cancer in humans.
(ii) Global warming is a warning to mankind because:
(a) rise in temperature is leading to increased melting
of polar ice-caps as well as of other places like the
Himalayan snow caps. This will result in a rise in sea
level that can submerge many coastal areas.
(b) Changes in the environment result in odd weather
and climate changes, e.g., el Nino effect.
nn