Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
Answers to self-assessment questions
or tube to fertilise. The embryo is kept for a
few days in culture.
Sperm bank – sperm are collected from
males, mixed with an albumen solution and
put into thin tubes known as straws. These
are kept at –196 o C until required.
Embryo transfer – embryos formed during IVF
are inserted into the uterus for implantation
to take place. They could be inserted into
the female that donated the eggs or into
another female which has been treated with
hormones so is prepared for implantation
(embedding in the endometrium).
Surrogacy – one or more embryos are
placed into the uterus of a female who did
not provide the female gamete. Pregnancy
therefore occurs in a female who is
biologically unrelated to the embryo / fetus.
This is to save the female who is the source of
the eggs from the risks of pregnancy.
‘Frozen zoo’ – a store of sperm, eggs and
embryos of animals, many / all of which are
endangered species. Stored material can
be made available to zoos anywhere in the
world. The frozen zoo is a store of genetic
diversity to prevent inbreeding.
c To prevent inbreeding. This reduces the
chances of them becoming increasingly
homozygous over several generations. The
health and fertility of animals is likely to
decrease if they are inbred. For example, the
risk of developing genetic diseases caused by
recessive alleles increases.
17 Some possible answers:
Zoos provide a refuge for endangered
animals, when their habitat no longer exists
or is too fragmented to support populations
in the wild. They can keep animals in
better heath than in the wild. They conduct
research on the best ways to breed them
to increase the populations also ensuring
that health is maintained or improved by
restricting inbreeding. They allow people to
see animals that they would not otherwise
see (except on video or TV). Zoos provide an
important role in conservation education.
18a Many wild plants are threatened with habitat
loss and the effects of climate change. Seeds
can be kept for a long time – possibly until
suitable places and conditions are available
for the plants to be reintroduced. Seed
banks keep the genetic material that would
otherwise be lost.
b Seeds that can be stored in seed banks are
orthodox seeds. Some plants do not produce
this type of seed. Their seeds are known as
recalcitrant. The plants concerned have to
grow as plants in a gene bank, e.g. cocoa and
coconut.
19a Selection pressures in the natural habitat
might include the ability of the adult plants
to survive grazing, wide variations in rainfall
or competition with other species. In the
seed bank, none of these selection pressures
would apply. In the seed bank, the greatest
selection pressure will become the ability
of the seeds to survive the conditions in
which they are stored for a long period of
time. Seeds from store are germinated every
few years to check on their viability. Also
some plants may be grown in protected
environment to set seeds to replenish the
stock in cold store.
b It is possible that the plants that grow from
the seeds that have been saved will not
have characteristics that will allow them
to survive the selection pressures they will
encounter in their natural habitat. This could
reduce the chances of success in returning
them to the wild.
c Providing enough space to grow all the plants
necessary to have a collection of the genetic
diversity in the species. Keeping plants
disease–free. There are problems involved
in maintaining the collection in areas that
might be susceptible to natural catastrophes,
such as hurricanes, severe storms, flooding
and drought.
20a Some possible answers
Alien species may be carnivores so will prey
on many animals. They will compete with
existing predators in the ecosystem. They
may be herbivores in which case they will
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