Download Answers to EPQ questions 10 Gas exchange and transport

10 Gas exchange and transport
Answers to Exam practice questions
1 C
[1]
2 C
[1]
3 An explanation that includes points 1, 3 and 5 and any two others:
• Mouth opens and floor of buccal cavity is pulled downwards;
• This increases the volume (lowers pressure) in the buccal cavity and water flows in;
• Mouth and operculum close and floor of buccal cavity is raised;
• This decreases volume (raises pressure) in the buccal cavity, forcing water across gills;
• Muscles in operculum contract increasing volume (causing it to bulge) (decreasing pressure),
drawing more water across the gills;
• Operculum opens to allow water to flow out;
[5]
4 a) An explanation that includes any three of the following points:
• Haemoglobin has four haem groups, which each carry an oxygen molecule;
• The binding of one oxygen molecule is difficult so takes a high ppO2 change/so the initial
gradient is shallow;
• Binding of one oxygen molecule (changes Hb shape) makes it easier to bind with other
oxygen atoms;
• Hence less increase in ppO2 is required/curve becomes steeper;
• To become fully saturated haemoglobin needs a large change in ppO2/change from 8 to 12
KPa
Penalise reference to oxygen atoms once only
[3]
b) Pressure of a mixture of gases is made up of the sum of the pressures exerted by each of the
individual gases;
The contribution of each gas is called its partial pressure;
c) i)
[2]
Fetal = 75%, adult = 65%, difference = 10%
Units needed for mark
ii)
[1]
Fetal haemoglobin has a higher affinity for oxygen (at any given ppO2);
The fetus is supplied with oxygen (across the placenta) from the maternal/adult blood;
This exchange would not take place unless the fetal haemoglobin had a higher affinity;[3]
5 a) An explanation that includes any two the following points:
• Biconcave shape increases surface area;
• Very thin membrane gives short diffusion path;
• No nucleus allows more space for cytoplasm;
• Cytoplasm rich in haemoglobin;
b) i)
[2]
A substance that maintains a stable pH;
© Hodder & Stoughton Limited 2015
10 Gas exchange and transport
Answers to Exam practice questions
Having the ability to accept or donate hydrogen ions;
ii)
[2]
An explanation that includes any three of the following points:
• Hydrogen ions are released/cell contents become more acid when carbon dioxide
dissolves;
• Oxyhaemoglobin takes up hydrogen ions as it releases oxygen;
• Therefore raises/restores pH level/reduces acidity;
• Oxygen – haemoglobin reaction is reversible so can also donate hydrogen ions when
oxyhaemoglobin is formed;
[3]
+
c) More carbon dioxide means more H ions are released;
+
More H ions causes more changes in binding sites and means therefore oxygen is held less
strongly;
Weaker affinity for oxygen means less is carried as carbon dioxide concentration increases; [3]
Stretch and challenge questions
8 It is useful to research all of these pigments. In doing so you will find lots of interesting
information linked both to the niche of each of the animal groups and to the origins of the
pigments. There are several other pigments not illustrated here.
a) Useful points here would be:
• All contain metal ions within a protein structure.
2+
2+
• Haemocyanin has a Cu ion; the others have a Fe ion.
• All have metal ions surrounded by polypeptides.
• Haemoglobin and chlorocruorin have similar (but not the same) polypeptides around the
metal ion (porphyrin rings).
• Haemocyanin has a copper ion surrounded by polypeptide residues made up mainly of
the amino acid histidine.
• Chlorocruorin sub-units like this often join together to form very large structures.
b) i)
The simplest answer here is to suggest that, because they have similar structures, they
have common ancestry. It is possible to suggest that because haemocyanin is less similar
than the other two this may indicate that animals with this pigment are more distantly
related. However, as usual, things are not this simple. Later in the course you will meet
the idea of convergent evolution. This means that animals not very closely related may
evolve similar solutions to common problems. If you think this through even more
carefully it is obvious that compounds already available through the many biochemical
pathways found in cells will be favoured in the evolution of such mechanisms.
ii)
This is another remarkable example of similar compounds with very different uses.
© Hodder & Stoughton Limited 2015
10 Gas exchange and transport
Answers to Exam practice questions
Similarities:
• Both have a typical ring structure (porphyrin) to their polypeptide chains.
• Both have a metal ion contained within this ring.
Differences:
• Chlorophyll has a magnesium ion whereas haemoglobin has an iron(ii) ion.
• Chlorophyll has a long hydrocarbon chain attached.
Given that haemoglobin is an oxygen carrier and chlorophyll a captures photons of light
energy, their structures have evolved accordingly and it does not follow that one
evolved from the other. However, other evidence, such as the universal use of ATP as an
energy transfer system, does seem to indicate that at some very early stage in
evolutionary history cells had some form of common ancestry.
c) In humans red cells are vital. If the concentration of the blood were to suddenly decrease
then water would enter the red cells and they would swell and burst. Even though the same
quantity of haemoglobin was still present, this would be rapidly fatal because of a lack of
oxygen to cells.
• In simple terms the concentration of haemoglobin would be too dilute in the whole
plasma.
• The enzyme carbonic anhydrase is contained within red cells. This too would be diluted
and would also be free to affect all the cells of the body in undesirable ways.
You could research other important balances that are controlled by the transport across the
endothelium of the red cell, e.g. 'chloride shift'. In simple terms there are many interactions that
depend upon this separation of plasma and red cells. This is not to say that pigments in plasma
don't work, as most animals use them in this way, just that in advanced mammals further
refinements are needed to produce the efficiency required for rapid transport of gases.
© Hodder & Stoughton Limited 2015