Hey guys, welcome to today's biology video.
Today we will be doing Cambridge IGCSE Biology
Past Paper, specimen paper for examination
from 2020.
This is Paper 4, the theory paper.
The first question is about a food web.
Part (a) says we need to complete the food
web by writing the names of the correct organisms
in the boxes in Fig 1.1.
Okay then let's read the information given
here.
The dominant grass species in an African grassland
ecosystem are star grass and red oat grass.
First point, star grass is eaten by antelope
species, such as topi and Thomson's gazelle.
Star grass is over here at the bottom of the
food web and it said it's eaten by topi and
Thomson's gazelle and both are already written
here.
Next point, smaller animals such as mice and
grasshoppers feed on red oat grass.
Red oat grass is also at the bottom and there's
a blank and a grasshopper here.
Since it said mice and grasshopper feed on
red oat grass, mice should be in this box.
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Moving on, topi and Thomson's gazelle are
eaten by predators such as cheetahs, lions
and serval cats.
Lions and cheetahs are already here so this
should be cheetahs.
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Next, grasshoppers and mice are eaten by serval
cats and tawny eagles.
Mice, grasshoppers, serval cats, tawny eagles
are all here.
Lastly, vultures feed on dead mammals.
The mammals in this food web are cheetahs,
lions, thomson's gazelle, serval cats and
tawny eagles and you can see that all the
arrows are pointing to one box.
And that will be vultures.
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Alright part (b).
Name the trophic level of the following species.
star grass.
Star grass was at the bottom of the food web
so it is the producer.
Topi.
Topi was at the second line, right after star
grass, so it is the primary consumer.
Part(c).
State the principal source of energy for the
food web shown in Fig 1.1.
Whether it is this food web or it is some
other food web, the primary source of energy
is always the Sun.
Next.
State what happens to energy when it leaves
a food web.
When energy leaves the food web, it does not
disappear but it changes in the form of heat.
So you can write they change to heat or are
lost as infra-red radiation or even they are
lost to the atmosphere.
Part (d).
Explain why there are no more than four trophic
levels in the food web shown in Fig 1.1.
This is a 3 mark question so we need to give
3 points.
Going back to our food web, we can see that
there are
one, two, three and four levels here.
Why is that so?
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That is because when a new level is formed,
for example from primary consumer to secondary
consumer, energy is being transferred to the
next level.
But the thing is not all the energy is transferred.
In fact, only 10% will be transferred and
the rest all lost as heat.
So by the time you reach the fourth trophic
level, there is only a small amount of energy
and it is not enough to support another level.
If we write these in words,
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We got three points here.
Energy is lost between and within trophic
levels.
only 10% of energy is transferred to the next
level.
Not enough energy in the fourth trophic level
to support another level.
There can be one more point apart from these.
It's that not all energy from Sun can be absorbed
by producers so no matter how much sunlight
you provide to the producers, they will only
make and pass on a certain percentage of energy
to the next level.
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Most energy from Sun is not available and
the energy from Sun is fixed by photosynthesis.
Part (e).
Fish such as salmon can be reared intensively
in fish farms.
They are fed on high protein food made from
animals.
When eating food, the fish are feeding as
secondary consumers.
Describe the disadvantages of intensive farming,
such as salmon farming, for producing human
food.
So this sounds like there's nothing wrong
with it but if you look closely into it, there
are some disadvantages.
Again you only need three points but I will
tell you all the possible answers.
Firstly, it's high protein food made from
animals.
And these are much more expensive than getting
food from nature which is what fish normally
eat.
So the first point, feed is expensive.
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Next, the question said the fish are feeding
as secondary consumers which means they are
at the third trophic level.
Instead, it will be more efficient for humans
to feed on producers like crops which will
have the most amount of energy or just feed
on animals that are used to make fish food
instead of giving it to salmons and having
the salmons later on.
This theory is closely related to the food
web system.
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Then another point is that when salmon are
given their food, they won't be able to consume
every single bits of food given.
Some will be left as waste on the bed of fish
farms.
Therefore, waste feed will cause eutrophication
of water supplies.
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As you can see ,these kind of questions definitely
need some creativity.
Okay now think about how they are reared.
They are put in captivity, a large number
of them put in a small space.
What would happen if a disease or a parasite
happen to be present in one part of the farm?
Yeah they are gonna spread so easily and furthermore,
these can even spread to other organisms in
the wild like who knows a bird or a dog was
passing by and caught the disease.
So you can have two points from this.
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Diseases or parasite spread easily and diseases
may spread to other organisms in the wild.
It's not the end.
I know we have more than three points but
you never know which one might apply to other
questions as well so better learn all of them.
Well the fish farmers will not let their fish
die so easily so they will use some chemicals
like antibiotics in their farms.
And these chemicals are often pollutants.
They are not that safe for the environment.
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Chemicals such as antibiotics used to control
diseases are also pollutants.
Lastly, you can mention about the animal welfare
like these salmon do not get to enjoy the
ocean and live a normal life and you will
still get a point for that.
Question 2.
Fig 2.1 shows a person sitting in a room.
A thermometer shows the temperature of the
room.
The temperature is 12 degree celsius here,
way below the human body temperature.
Part (a).
Give three uses of energy in the body of the
person in Fig 2.1.
Obviously this person will need some energy
to maintain his body temperature because the
room temperature is 12 degree celsius so that's
our first point.
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Then, for the rest of the points, they are
just the general use of energy in resting
human body.
We need energy for muscular contraction or
to pump blood.
You get only one point from this.
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We need energy to carry out metabolic reactions
like digestion or excretion.
These processes are happening all the time.
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Our cells are going through mitosis or cell
division for growth or repair of our body
cells.
Two points from this.
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You can also say that active transport is
being carried out for example for nerve impulses
to pass on, and energy is required for it.
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Way more than three points here but it's good
to know all of them when studying for the
exam because you don't know how they are gonna
twist the question.
Part (b).
Name the process carried out by the person
in Fig 2.1 that releases energy.
The one famous process we learn in biology
that releases energy.
It's respiration.
Next.
State the balanced chemical equation that
describes this process.
You are supposed to memorise this equation.
C6H12O6 plus 6O2 equals to 6CO2 plus 6H2O.
For photosynthesis, it's the opposite of this.
The reactants go to the products' side and
the products to the reactants' side.
Part(c).
The person leaves the room and runs very fast
for 200m.
When the person stops running, his breathing
rate and his heart rate remain high for several
minutes.
Explain why the person's breathing rate and
heart rate remain high.
The question is saying that when you start
running, your breathing rate and heart rate
will increase.
But just because you stop running, the rates
do not suddenly decrease but you will still
be panting for few minutes.
Why is that so?
That is because there is this thing called
oxygen debt.
Before when you were running, your body needed
extra energy so anaerobic respiration has
occured.
Anaerobic respiration is the method of producing
energy without using oxygen but you will have
to pay back for it afterwards.
In other words, when the person stops running,
this person takes time for oxygen debt to
be repaid and as a result, more oxygen will
be needed than usual.
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Also, another product of anaerobic respiration
is lactic acid.
This substance builds up in muscles during
anaerobic respiration and it needs to be broken
down in order to prevent it from harming your
body.
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Remember the concept of oxygen debt and building
up of lactic acid due to anaerobic respiration.
Part (d).
The run results in changes in the skin involving
the blood vessels and the sweat glands.
Describe what happens to the blood vessels
and sweat glands during the run.
Explain why these changes happen.
Okay a 5 mark question but it's an easy one
if you know what they are talking about.
This person is running and they never mentioned
this in the question but when you run, your
body temperature increases.
So basically, it's about the effect of homeostasis
on blood vessels and sweat glands when the
body temperature increases.
Let's do it one by one.
Blood vessels first.
When the body temperature increases, vasodilation
of arterioles will occur.
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What's vasodilation?
It's the temporary expanding of blood vessels
to let more blood flow.
So this vasodilation will allow increase in
supply of blood to skin capillaries, resulting
in loss of heat by radiation to the atmosphere
and lowering the body temperature.
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You just got 6 points from this.
But of course you still need to talk about
sweat glands.
Similarly, this increase in body temperature
which caused vasodilation of arterioles will
also cause the increase in blood flow to sweat
glands.
Sweat glands are where your sweat come out
so there will be increased production of sweat.
This will help lower your body temperature
by the evaporation of sweat.
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Question 3.
Part (a).
Define the term sensitivity.
Definition time.
Sensitivity is the ability to detect and respond
to changes in the environment.
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Make sure you know all the definitions of
Mrs Gren at least.
Moving on, Fig 3.1 shows the reflex arx involved
in a simple reflex action.
We have a diagram of spinal cord, sinal nerve
and a finger about to touch the nail.
Part (b).
On Fig 3.1 use label lines to identify and
name the three types of neurone shown.
Keep in mind that we have three types of neurone
in our body, sensory neurone, relay neurone
and motor neurone.
This is the sensory neurone, where it senses
everything.
This is the relay neurone which delivers the
impulse.
And this is the motor neurone which will probably
cause muscle contraction.
Part (c).
A reflex is an involuntary action.
Explain what is meant by the term involuntary
action.
When there is an involuntary action, your
body will act without even thinking about
it and always act in the same way.
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It's automatic action which the stimulus always
leads to the same response.
Another definition but first understand the
concept first and you will be able to memorise
it.
Part (d).
Define the term synapse.
Synapse is a junction between two neurones.
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Junction and neurones are keywords here always
include these two words.
Next.
Describe how impulses are transmitted across
a synapse.
Impulse needs to go past a junction between
two neurones and how does that occur?
An impulse will trigger the release of chemical
called neurotransmitters into the gap.
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Then, this will diffuse across the gap and
bind to the receptor on the opposite side
of the junction, allowing the impulse to travel.
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Part (e).
When a body senses danger, more adrenaline
is secreted from the adrenal glands.
Describe two ways in which the hormone adrenaline
affects the body in preparation for action.
Meaning, what is the effect of adrenaline
on human body?
Firstly, it makes your heart beat faster and
breathe faster.
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It will also try to increase blood glucose
concentration to have extra energy, so it
will stimulate the breakdown of glycogen in
liver.
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Also, the pupils in your eyes will dilate.
Your eyes will allow more light to see better.
And finally, you will have increased sensitivity
to be more alert of the surrounding.
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Part (f).
State one difference between nervous and hormonal
control systems.
Nervous system is something to do with your
nerve system, impulse and all and hormonal
system is something to do with hormones like
adrenaline.
So we actually have two differences.
Nervous control is faster than hormonal control.
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They act within miliseconds but for hormones,
they may take few minutes.
However, hormonal control lasts for a longer
period of time than nervous control.
Their effects will be present longer.
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That's it for this video.
I will cover the next half of the paper in
my next video.
So subscribe and stay tuned!
Thank you for watching.
Stay safe and God bless you guys.
Bye!
