Hi this video is part 2 of IGCSE Biology May/June
2020 Paper 4.
Question 4.
Johnson grass, Sorghum halepense, is wind-pollinated.
Part (a).
Fig 4.1 shows some Johnson grass flowers.
State the genus of Johnson grass.
The words in italics stand for genus and species
so the answer is Sorghum.
Make sure you write exactly like the one given
here.
Describe two features visible in Fig 4.1 that
show that Johnson grass flowers are adapted
for wind-pollination.
Since it said visible, we need to find from
this photo.
Well, you can see that there are feathery
stigma, having a large surface area for wind-pollination.
Then, we have anthers hanging outside the
flowers so that wind can easily carry away
the pollen.
Part (b).
Fig 4.2 shows a section through a carpel shortly
after pollination.
State the names of the parts of the carpel
labelled C, D and E.
C is the ovary and D is the ovule surrounded
by ovary wall.
E is called the style, the passageway.
Next.
Complete the sentences.
Pollen grains are formed in anthers.
During their formation the number of chromosomes
in the nuclei is havled by the process of
something.
Okay number of chromosomes being halved.
There's only one answer for that.
It is meiosis.
This means the male nucleus A in the pollen
tube is described as a something nuclues.
What is the name of a nucleus that is halved?
It is a haploid.
When nucleus A something with nulceus B, the
chromosome number doubles to form a something
nucleus.
What can two nuclei A and B do?
They can fuse together and this will form
a diploid, with double the chromosome number.
The name of this process is fertilisation.
Then the something divides by the process
of something to form an embryo.
After fertilisation, zygote is produced so
zygote divides and the process that follows
is mitosis, the usual division of cells.
Part (c).
Discuss the advantages of sexual reproduction
to a wild population of flowering plants such
as Johnson grass.
This question is worth big five points but
no worries we got like 10 points to write
for it.
Okay basically what are the advantages of
sexual reproduction compared to asexual reproduction.
To start with, sexual reproduction gives genetic
variation because the genes of the offspring
will be different from parents.
You are making a new combination from two
different genes so they will not have same
sets of DNA.
This will lead to the offspring having the
ability to adapt to changed environment and
they will eventually evolve and allow natural
selection to occur.
In terms of process of sexual reproduction
in plants, the pollen is exchanged between
individuals so seeds can be dispersed.
So new areas can be colonised by the species
and there will be less competition since the
same set of plants will not stay in the same
area all the time.
Overall, it increases the rate of survival
even in harsh conditions.
Part (d).
Sexual reproduction requires energy.
State three uses of energy in organisms other
than in reproduction.
Plenty of answers for this.
Active transport is the popular example.
Then protein synthesis of course.
And cell divison requires energy as well.
Also, you need it for growth, movement, sensitivity
and so on.
Question 5.
Ciliates are classified in the kingdom Protoctist.
Bacteria are classified in the kingdom Prokaryote.
Part (a).
State two structural features that distinguish
the cells of a protoctist from a prokaryote.
Protoctists are like the algae.
They have nucleus unlike bacteria, the prokaryotes.
Also, they have linear chromosomes but for
prokaryotes, their genetic material just float
around the cytoplasm.
Part (b).
Fig 5.1 shows five species of ciliate that
are found in sewage treatment works.
They all look very different.
Fig 5.2 is a dichotomous key to identify the
ciliates shown in Fig 5.1.
Complete the key in Fig 5.2 by writing suitable
statements.
For box 2 to distinguish species B and E.
For box 4 to distinguish species A and C.
Okay so start, 1 has a ring of cilia at one
end of the organism.
This is correct for B and E. Then to divide
them, we should come out with something that
is correct for B only.
Well, it can be anything as long as it makes
sense.
I'll put the organism has two rings of cilia
since E only has one.
3 has star-like structures inside the organism.
Yes, then D is suitable for this.
4 it needs to be something that is only correct
for A so let's compare A and C.
In A, it's covered in cilia however in C,
only some parts have cilia.
So write organism has a covering of cilia
so that only A will be correct.
Part (c).
Didinium is a predatory ciliate.
A video recording was made of one Didinium
feeding on a Paramecium.
Fig 5.3 shows a sequence of still photographs
taken from the video.
Wow, interesting photographs.
Complete the table by putting a tick by each
characteristic of life that can be seen in
the still photographs from the video in Fig
5.3.
Excretion, nope.
I don't see anything being released.
Nutrition, yes.
Didinium is feeding on Paramecium so it's
correct.
Growth, nope. the size remained the same.
Reproduction, nope nothing was produced.
Movement, yes the position of two of them
changed constantly.
Respiration, nope we should see something
being released as a product of respiration
but there's nothing here.
Only nutrition and movement are correct.
Part (d).
Fig 5.4 is a food web for some of the microorganisms
in a sewage treatment works.
Didinium at the top predator and photosynthetic
bacteria and decomposer bacteria at the bottom.
Construct one food chain with three trophic
levels that use energy derived from the breakdown
of sewage.
Do not draw the organisms.
From the food web above, it is clear that
bacteria can be used as nutrition for protoctists.
Well, in part (b), it said Didinium and Paramecium
can be found in sewage treatment.
So if we want to draw a food chain from the
breakdown of sewage, it should include Didinium
and Paramecium and it will be like this.
The water that passed out of the sewage works
was often cloudy with suspended matter.
Scientists discovered that ciliates reduce
the cloudiness of water during sewage treatment.
Suggest how the ciliates reduce the cloudiness
of the water using the information in Fig
5.4.
If the ciliates reduced cloudiness, it means
that it got rid of something.
Well, what do ciliates feed on?
We have two things here, photosynthetic bacteria
and decomposer bacteria.
So write ciliates eat bacteria and ciliates
may eat decomposing material.
Explain how sewage treatment reduces the spread
of disease.
This question is about sewage treatment.
First thing they do is the removal of harmful
bacteria from sewage of course, such as cholera.
This stops the spread of pathogens via water.
They can remove the bacteria by using chlorine
or chemical treatment.
Nitrifying bacteria are found in sewage works.
Explain the importance of nitrifying bacteria
in the nitrogen cycle.
Ammonia can often be found in sewage so when
you go through nitrifying, ammonia will be
converted to nitrate or nitrite ions.
Then these can be made available to plants
and plants will absorb them.
This is helpful as nitrate ions are used to
make amino acids by plants.
Question 6.
Colour blindness is a characteristic that
is inherited.
Colour blindness is more common in males than
in females.
Fig 6.1 pedigree diagram showing the inheritance
of colour blindness in a family.
Male with normal colour vision, male with
colour blindness and female with normal colour
vision.
So females here don't have colour blindness.
Part (a).
Define the term inheritance.
Inheritance is the transmission of genetic
information from generation to generation.
Part (b).
Using the symbols B and b, state the genotypes
of individual 5 and individual 8 in the pedigree
diagram.
The capital letter always stands for the dominant
one and colour blindness is a recessive trait,
so small letter b represents having colour
blindness.
5 has colour blindness so it's small letter
b.
Since it's a male, you have to write it as
X small letter b and Y.
For 8, it's a male with normal vision so it's
a capital letter B. It's X capital letter
B and Y.
Individual 3 is a carrier of colour blindness
because she has one copy of the allele for
colour blindness but has normal colour vision.
Describe the evidence from Fig 6.1 that shows
that individual 3 is a carrier.
First of all, you need to point out that colour
blindness is a sex-linked characteristic.
So it's carried with X chromosome and only
females can pass it on to the next generation.
She has passed on the recessive allele to
her sons and the father passed on his Y chromosome.
But she still has normal vision because she
has two X chromosomes which have the gene
for colour vision.
She is heterozygous for the gene, like Bb
and the normal allele allows her to have normal
colour vision.
There was no history of colour blindness in
the parents and grandparents of individuals
1 and 2.
Suggest how colour blindness first occured
in the family in Fig 6.1.
The answer is that mutation could have occured
in person 3 or in one of her parents.
Even thogh you did not have that gene initially,
mutation could have occured to give recessive
allele to the children.
Alright, that's it for this video.
I will be doing more IGCSE May/June 2020 papers
from now on.
Thank you so much for watching.
Please subscribe, like and comment if you
want to watch more videos like this!
Stay safe and God bless you guys.
Bye!
