Table Of ContentAQA – Biology Unit 5
The Essay:
1
Marking the essay
Scientific content
Insert Insert
Is the essay mark Do you want to go to new
Good (12 marks) the intermediate mark
Medium (8 marks) mark?
Poor (4 marks)?
Put a tick in the box for each significant Final mark
reference to material that is beyond that for
required at A-level scientific
content
If the essay has been classed as good, add 2 marks if there is one
tick in the box, or 4 marks if there are two or more ticks.
Breadth
Topic Tick box if Topic Tick box if
at least one at least one Give one
“paragraph” “paragraph” mark for
on topic on topic each box
Physiology Pathogens ticked
Direct Life style
effects
Indirect Genetics
effects
Relevance
Number of Number of
Put a cross in the box crosses marks
for each significantly None 3
irrelevant comment 2 2
More than 2 1
Quality of written communication
Award Mark for quality
3 of written
marks Poor use of either scientific terms or communication
Award
lacking coherence
2
marks Poor use of scientific terminology
Award 2
and lacking coherence
1
mark
What are the examiners looking for:
What you should consider:
Look at this title: ‘Carbon dioxide may affect organisms directly and
indirectly Describe and explain these effects. ‘
Enzymes Bohr
pH Control of heart
effect
rate
Light
independent Cells Physiology
reaction
Carbon
Limiti ng dioxide
factors
Genes
Ecology
Crop production
Selection
Pest numbers
Global
Change in range
warming
3
Try marking this essay....................
Carbon dioxide may affect organisms directly or indirectly. Describe
and explain these effects.
Carbon dioxide is both a vital requirement for producers – the first organisms in any
food chain, and at times a dangerous waste product.
Where plants use carbon dioxide during the Calvin cycle of photosynthesis, at night
they also produce small quantities due to respiration. Fundamentally, carbon dioxide
reacts with ribulose bisphosphate to produce glycerate 3-phosphate in the light
independent reaction. In turn glycerate3-phosphate is reduced to triose phosphate
using reduced NADP and ATP from the light dependent reaction. This follows through
to produce products such as starch, and regenerate RuBP. Without this process,
plants cannot produce the organic molecules which are a critical starting point for all
food chains and webs.
One way that carbon dioxide is released is through respiration. During the Krebs
cycle carbon dioxide is removed when turning citrate into oxaloacetate, removing 2
CO in each cycle. Directly, organisms release 6 CO molecules for each molecule of
2 2
glucose used in the respiratory pathway.
It is important that when breathing, excess carbon dioxide is removed from the
body. In solution, carbon dioxide forms a weak acid called carbonic acid. This lowers
the pH of the blood (in humans) and in return, this change in pH can affect many
enzymes as well as the haemoglobin molecule. The lowered pH causes the shape of the
haemoglobin molecule to change so it can offload more oxygen. This change in
properties of the haemoglobin molecule displaces the oxygen dissociation curve to the
right (Bohr shift) and lowers its affinity for oxygen.
To remove excess carbon dioxide, the sympathetic nerve from the medulla oblongata
will send a series of impulses to the SAN in the heart as a result of chemoreceptors
in the carotid artery detecting an increase in CO concentration. The sympathetic
2
nerve causes the SAN to increase the amount of heart beats per minute, so removing
the carbon dioxide faster by transporting more blood to the lungs.
Another way of carbon dioxide accumulating in the atmosphere is via the carbon cycle
when dead or decaying organisms are broken down by saprobionts and carbon dioxide
is produced by microbial respiration.
Unfortunately, carbon dioxide may indirectly cause adverse effects to the climate in
which organisms live. When radiation from the sun reaches the earth’s crust, it is
4
reflected back. Carbon dioxide along with other greenhouse gases reflects this
radiation and prevents it escaping. As more carbon dioxide accumulates, more heat is
retained leading to rising temperatures and a warmer climate. An increasing
temperature can have a number of effects on living organisms. At first thought, it
might be considered that a combination of a greater concentration of carbon dioxide
and a higher temperature will increase the rate of photosynthesis but this may not be
the case. The enzyme Rubisco is involved in catalysing the reaction in which ribulose
bisphosphate combines with carbon dioxide. Rubisco also combines with oxygen
however in a process called photorespiration. Photorespiration increases rapidly at
higher temperatures so a rise in temperature could result in photosynthesis become
less efficient, not more.
Crop yield is also related to pest damage. A higher temperature is likely to affect
pest numbers. It might do this in a number of ways. The European corn borer is a
pest of maize. Scientists have found that the number of generations of this pest
decrease with distance north. A higher mean temperature will therefore lead to more
generations and greater crop loss. The UK is at the northern limit of distribution for
some pest insects. One of these is the southern green shield bug. This is a serious
pest of some crops in continental Europe. Over the past few years, biologists have
found more of these plant-sucking pests in southern England.
To conclude, carbon dioxide is both a harmful waste product and a necessity for the
vital process of photosynthesis.
5
Carbon dioxide may affect organisms directly or indirectly. Describe and
explain these effects.
Carbon dioxide is both a vital requirement for producers – the first organisms in
any food chain, and at times a dangerous waste product.
Where plants use carbon dioxide during the Calvin cycle of photosynthesis, at
night they also produce small quantities due to resp iration. Fundamentally,
carbon dioxide reacts with ribulose bisphosphate to produce glycerate 3-
phosphate in the light independent reaction. In turn glycerate3-phosphate is
reduced to triose phosphate using reduced NADP and ATP from the light
dependent reaction. This follows through to produce products such as starch,
and regenerate RuBP. Without this process, plants cannot produce the organic
molecules which are a critical starting point for all food chains and webs.
One way that carbon dioxide is released is through respiration. Du ring the
Krebs cycle carbon dioxide is removed when turning citrate into oxaloacetate,
removing 2 CO in each cycle. Directly, organisms release 6 CO molecules for
2 2
each molecule of glucose used in the respiratory pathway.
It is important that when breathing, excess carbon dioxide is removed from the
body. In solution, carbon dioxide forms a weak acid called carbonic acid. This
lowers the pH of the blood (in humans) and in return, this change in pH can
affect many enzymes as well as the haemoglobin m olecule. The lowered pH
causes the shape of the haemoglobin molecule to change so it can offload more
oxygen. This change in properties of the haemoglobin molecule displaces the
oxygen dissociation curve to the right (Bohr shift) and lowers its affinity for
oxygen.
To remove excess carbon dioxide, the sympathetic nerve from the medulla
oblongata will send a series of impulses to the SAN in the heart as a result of
chemoreceptors in the carotid artery detecting an increase in CO
2
concentration. The sympathetic nerve causes the SAN to increase the amount
of heart beats per minute, so removing the carbon dioxide faster by
transporting more blood to the lungs.
Another way of carbon dioxide accumulating in the atmosphere is via the carbon
cycle when dead or decaying organisms are broken down by saprobionts and
carbon dioxide is produced by microbial respiration.
6
Unfortunately, carbon dioxide may indirectly cause adverse effects to the
climate in which organisms live. When radiation from the sun reaches the
earth’s crust, it is reflecte d back. Carbon dioxide along with other greenhouse
gases reflects this short-wave radiation and prevents it escaping. As more
carbon dioxide accumulates, more heat is retained leading to rising
temperatures and a warmer climate. An increasing temperature can have a
number of effects on living organisms. At first thought, it might be considered
that a combination of a greater concentration of carbon dioxide and a higher
temperature will increase the rate of photosynthesis but this may not be the
case. The enzyme Rubisco is involved in catalysing the reaction in which ribulose
bisphosphate combines with carbon dio xide. Rubisco also combines with oxygen
however in a process called photorespiration. Photorespiration increases rapidly
at higher temperatures so a rise in temperature could result in photosynthesis
become less efficient, not more.
Crop yield is also related to pest damage. A higher temperature is likely to
affect pest numbers. It might do this in a number of ways. Th e European corn
borer is a pest of maize. Scientists have found that the number of generations
of this pest decrease with distance north. A higher mean temperature will
therefore lead to more generations and greater crop loss. The UK is at the
northern limit of distribution for some pest insects. One of these is the
southern green shield bug. This is a serious pest of some crops in continental
Europe. Over the past few years, biologists have found more of these plant-
sucking pests in southern England.
To conclude, carbon dioxide is both a harmful waste product and a necessity for
the vital process of photosynthesis.
Marks awarded for this essay:
All of the material presented is of a sound A-
level standard fully in keeping with the
specification requirements. It can therefore be
classified as a good essay. In addition, there
Scientific content 16 marks
are two substantial sections in which the
content is more than that expected in the
specification, reflecting a greater depth of
study. This raises the mark to a maximum.
All three areas required in mark scheme have
Breadth 3 marks
been covered.
There is nothing here that might be considered
Relevance 3 marks
to be irrelevant.
Quality of written The essay is coherent and easy to follow, with
3 marks
communication excellent use of terminology.
7
Now have a go at the one on the next page ......................
The causes of disease
The causes of disease in humans are extremely varied. Some diseases are self-
inflicted by a person’s lifestyle. Others are genetic and others are caused by
pathogenic organisms or viruses.
Self-inflicted diseases are caused by the lifestyle choice of the individuals or
the individuals surrounding them. One such example is lung cancer caused by
smoking. While some people develop cancer without ever smoking, the vast
majority of lung cancer sufferers are smokers. Emphesema is another disease
linked to smoking as chemicals cause your air passages to block up. The cilia also
shrivel up and there is no way to reverse this process as they cannot be
repaired. Many other diseases are caused by a poor diet and lack of exercise.
Heart disease is one example.
Another cause of disease is genetics. Some diseases are inherited directly from
a parent who is a carrier or a sufferer from the disease. Most of the time their
genetic diseases are recessive traits so they require two doses of the recessive
faulty gene for the illness to express itself as carriers have a faulty gene but
no symptoms. Some genetic diseases such as haemophilia are sex-linked and the
disease only expresses itself if a carrier female and a male sufferer produce
offspring.
Other genetic diseases are not inherited but instead are caused by a problem in
the genetic code. Some of these problems involve changing the base sequence
but this may have no effect on the protein produced. This is because the new
sequence of bases, for example AAA, may code for the same amino acid as the
original one which might be AAT. This means that the amino acid is not changed
so the protein is the same and the person does not get a disease.
Diseases can also be caused by pathogenic organisms and viruses. These can
enter the body through various interfaces such as breaks in the skin, the
digestive system, respiratory system and reproductive system. Such organisms
invade the body and infect host cells. Many viruses work in the same way only
they surround themselves with the host cell making it difficult to destroy them
and rendering antibiotics useless against them. Pathogens which cause disease in
humans can find their way into the body in many ways. Airborne pathogens which
are expelled from an infected person can remain in the air in water droplets
expelled by coughing or sneezing. Pathogens can also be picked up on the skil
from unclean surfaces and can make their way into the body through cuts or
damaged skin.
8
The causes of disease
The causes of disease in humans are extremely varied. Some diseases are self-
inflicted by a person’s lifestyle. Others are genetic and others are caused by
pathogenic organisms or viruses.
Self-inflicted diseases are caused by the lifestyle choice of the individuals or
the individuals surrounding them. One such example is lung cancer caused by
smoking. While some people develop cancer without ever smoking, the vast
majority of lung cancer sufferers are smokers. Emphesema is another disease
Q
linked to smoking as chemicals cause your air passages to block up. The cilia also
Q
shrivel up and there is no way to reverse this process as they cannot be
repaired. Many other diseases are caused by a poor diet and lack of exercise.
Heart disease is one example.
Another cause of disease is genetics. Some diseases are inherited directly from
a parent who is a carrier or a sufferer from the disease. QMo st of the time their
genetic diseases are recessive traits so they require two doses of the recessive
faulty gene for the illness to express itself as carriers have a faulty gene but
no symptoms. Some genetic diseases such as haemophilia are sex-linked and the
disease only expresses itself if a carrier female and a male sufferer produce
offspring.
Other genetic diseases are not inherited but instead are caused by a problem in
the genetic code. Some of these problems involve changing the base sequence
but this may have no effect on the protein produced. This is because the new
sequence of bases, for example AAA, may code for the same amino acid as the
original one which might be AAT. This means that the amino acid is not changed
so the protein is the same and the person does not get a disease.
Diseases can also be caused by pathogenic organisms and viruses. These can
enter the body through various interfaces such as breaks in the skin, the
digestive system, respiratory system and reproductive system. Such organisms
invade the body and infect host cells. Many viruses work in the same way only
Q they surround themselves with the host cell making it difficult to destroy them
and rendering antibiotics useless against them. Pathogens which cause disease in
humans can find their way into the body in many ways. Airborne p athogens which
are expelled from an infected person can remain in the air in water droplets
expelled by coughing or sneezing. Pathogens can also be picked up on the skin
from unclean surfaces and can make their way into the body through cuts or
damaged skin.
9
Marks awarded for this essay:
Fits descriptor for four marks. Most of this is
very superficial but the only real errors relate
to the section in which the candidate says that
“Some genetic diseases such as haemophilia
Scientific content 4 marks
are sex-linked and the disease only
expresses itself if a carrier female and a
male sufferer produce offspring.”
All three areas required in mark scheme have
Breadth 3 marks
been covered.
There is a section on mutation that is clearly
Relevance irrelevant, but as it is only one in an essay of 3 marks
normal length, three marks is still awarded.
The essay is coherent and easy to follow but
Quality of written there is some poor use of terminology with
2 marks
communication phrases such as “cause your air passages to
block up” and cilia “shrivelling up”
10
Description:Global warming . 4 Try marking this essay.. Carbon dioxide may affect organisms directly or indirectly. Describe and explain these effects. Carbon dioxide
