dissociation2 OCR Assessmenk Bank - Squarespace

Answer all the questions.

1. Fig. 2.1 shows oxygen dissociation curves for both haemoglobin and myoglobin.

The saturation of haemoglobin with oxygen increases as the partial pressure of oxygen is increased.

(i) Use Fig. 2.1 to calculate the fastest rate of change in haemoglobin saturation as oxygen partial pressureincreases. Determine the units for your answer.

answer _ _ _ _ _ _ _ _ _ _ units _ _ _ _ _ _ _ _ _ _ [3]

(ii) Suggest where in the body a partial pressure of oxygen of 13 kPa would be found.

[1]

(iii) On Fig. 2.1, sketch the oxygen dissociation curve for human fetal haemoglobin.

[1]

(iv) Explain the biological significance of the different oxygen dissociation curves of myoglobin and adulthaemoglobin.

© OCR 2017. You may photocopy this page. 1 of 3 Created in ExamBuilder

[2]

END OF QUESTION PAPER


Mark Scheme

1 i Answer (in a range between) = 20 – 22

Units = % kPa– 1

3 1 mark can be awarded if there isevidence that the candidate has identifiedthe steepest part of the slope as (any valuebetween) 3.6–4.6 kPa

ALLOW % per kPa

ii lungs / alveoli 1

iii curve placed anywhere between themyoglobin and haemoglobin curves

1 The fetal haemoglobin curve can mergewith either of the other two lines after 10kPa

iv myoglobin has, greater affinity / AW, foroxygen (at the same kPa)idea that oxygen will be transferred tomyoglobin / muscle, from (adult)haemoglobin

2

Total 7

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Question Answer/Indicative content Marks Guidance

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of 11 | WJEC/CBAC 2017

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(a) The graph below shows the oxygen dissociation curve for three mammals.

Graph A is the curve for a llama which is a mammal that lives at high altitudes. Graph B is the

curve for a domestic cat and graph C is the curve for a mouse which is a small mammal with a

high metabolic rate.

Key:

A = llama

B = domestic cat

C = mouse

(i) What is the percentage saturation of the cat's haemoglobin with oxygen when the partial

pressure of oxygen in the muscle tissue of a cat is 3.0kPa.?

[1]

1.

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(ii) Explain the circumstances under which the partial pressure of oxygen may fall to 3.0kPa in

muscle tissues.

[2]

(b) State which of the three curves represents the haemoglobin with the lowest affinity for oxygen

at 3.0kPa.

[1]

(c) If the pH of the blood became more acidic, the position of curve C for the mouse would

change.

(i) Draw on the graph opposite the new position of curve C when the blood becomes more acidic.

[1]

(ii) Explain the benefits to the mouse of this change in position of curve C.

[3]

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(d) Explain how curve A for the llama shows that its haemoglobin is well adapted for its

environment.

[3]

(e) State which of the three curves could represent the oxygen dissociation curve of a lugworm.

[1]

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2.

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3.

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Marking Scheme

1.

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3.

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Answer all the questions.

1(a). * Describe how the structure of llama hamoglobin is likely to be different from that of camel haemoglobin withreference to the four levels of protein structure.

[6]

(b). Haemoglobin is a protein that carries oxygen in the blood of all mammals. The structure of haemoglobin can varyslightly between species.

Fig. 4.1 shows a llama, a relative of the camel.

Llamas live at high altitudes and camels live at low altitudes.


At high altitudes the partial pressure of oxygen is low.Llama and camel haemoglobin consists of 2 α subunits and 2 β subunits.Each subunit contains a haem group and is able to bind to one molecule of oxygen.In the β subunits, one amino acid present in camel haemoglobin has been replaced by a different amino acidin llama haemoglobin.

Fig. 4.2 shows dissociation curves for llama haemoglobin and camel haemoglobin.

(i) State the partial pressure of oxygen that results in a saturation of 50% in llama haemoglobin.

Answer_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ [1]

(ii) Explain why it is important for the survival of the llama that the llama haemoglobin dissociation curve is to theleft of the camel haemoglobin dissociation curve.

[2]


2. The figure shows the oxygen dissociation curves at different carbon dioxide concentrations.

(i) What name is given to a change in the oxygen dissociation curve due to increasing carbon dioxideconcentration?

[1]

(ii) Letter T in the figure indicates the partial pressure of oxygen in actively respiring tissues.

Explain why the blood off-loads more oxygen to actively respiring tissues than to resting tissues.

[2]


3. The blood circulatory system of a mammal undergoes changes at, or soon after, birth.

(i) One of these changes is that the foramen ovale, a hole in the septum between the right and left atria, closes.In the fetus, the foramen ovale allows blood to flow directly from the right atrium to the left atrium.

Suggest why the foramen ovale is open in the fetus before birth.

[2]

(ii) Another change occurring after birth is that fetal haemoglobin is replaced with adult haemoglobin.

State one difference between fetal haemoglobin and adult haemoglobin and give one reason why thisdifference is essential to the fetus.

difference

reason

[2]

END OF QUESTION PAPER


Mark Scheme

1 a Level 3 (5–6 marks)Describes differences and similarities ofllama and camel haemoglobin at all fourlevels of protein structure with correctreference to bonding.

There is a well-developed line of reasoningwhich is clear and logically structured. Theinformation presented is relevant andsubstantiated.

Level 2 (3–4 marks)Describes differences and similarities ofllama and camel haemoglobin in somelevels of protein structure with somereference to bonding.

There is a line of reasoning presented withsome structure. The information presentedis in the most-part relevant and supportedby some evidence.

Level 1 (1–2 marks)Describes a difference or similarity of llamaand camel haemoglobin at a level ofprotein structure.

The information is basic andcommunicated in an unstructured way. Theinformation is supported by limitedevidence and the relationship to theevidence may not be clear.

0 marksNo response or no response worthy ofcredit.

6 Indicative scientific points include:

difference in primary structuredifferent amino acid / polypeptidesequenceone amino acid changed.

amino acid change could causechange to secondary structureinitial coiling or folding of polypeptidechainα-helixβ-pleated sheethydrogen bonding.

amino acid change could causechange to tertiary structurefurther coiling of secondary structureionic bondingdisulphide bondshydrophilic / hydrophobic bonds3D shape.

amino acid change has not changedquaternary structurealpha and beta subunits still able toform haemoglobin in both camel andllama.

b i 2.8 (kPa) 1 ALLOW answer in the range of 2.8–3.0kPa

ii (llama) haemoglobin needs higher affinityfor oxygen (1)(so) can pick up oxygen at lower partialpressure (of oxygen) (1)

2

Total 9



Mark Scheme

2 i Bohr (effect / shift) ✔ 1 Correct spelling onlyACCEPT bohr / Bohr's / bohr's

Examiner's Comments

The vast majority of candidates answered(and spelled) Bohr effect/shift correctly.

ii

in actively respiring tissues1 more / high levels of, carbon dioxide(produced)orhigh pCO2 ✔

2 lowered affinity of haemoglobin foroxygen✔

3 (CO2 results in) dissociation of carbonicacid / increase of H+, leading to the releaseof oxygen ✔4 more oxygen released at same pO2 /suitable data quote from graph ✔

max 2 If symbols used must be correct e.g.CO2 not CO2

1 ACCEPT ORA for resting tissue

2 ACCEPT ‘Hb’ for haemoglobinACCEPT weaker affinity

4 (at, T / 3.2 kPa O2) drops from 40% to24% saturation / 16% reduction

Examiner's Comments

Most candidates described the activelyrespiring cells’ ‘need’ for oxygen and that itis released because the tissues require it.They also stated that actively respiringtissues have a low partial pressure ofoxygen (as they use up oxygen), but failedto make the link to more CO2 beingproduced. A worrying number ofcandidates thought that resting tissues didnot respire or need any oxygen at all, andsome thought that respiring tissuesthemselves have a higher affinity foroxygen. The more able candidatesdescribed the effect of increased carbondioxide in terms of H+ from carbonic acidcausing dissociation of oxygen fromhaemoglobin.

Total 3



Mark Scheme

3 i lungs not, functioning / filled with air;

blood / haemoglobin, is, not oxygenated inthe lungs / oxygenated in placenta(therefore) pulmonary circuit / lungs,bypassed;

2 max ACCEPT fetus not breathing

ACCEPT ref to ‘single circulation’ACCEPT little blood goes to, lungs /pulmonary circuit

DO NOT ACCEPT no blood goes to lungs

Examiner's Comments

Examiners were looking for definitestatements about the flow of blood in thefetus such as ‘less blood flows to the lungs’or ‘the pulmonary circuit is by-passed’.Many candidates were rather vaguemaking statements such as ‘no need forblood to flow to the lungs’ or ‘the fetus getsoxygen from its mother’ without mention ofhaemoglobin or the placenta. Othercommon mistakes included references tomother and fetus sharing blood, or that ‘thefetus doesn’t respire so there is nodeoxygenated blood’! Many candidatesanswered this question carelessly becausethey did not recognise that the placenta isthe only source of oxygen for the fetus. Alarge number of candidates did not discussthe lungs at all in their response, notappreciating that the fetus cannot breathein utero. Instead they made statementssuch as ‘the heart is not developed enoughto pump blood’ or more worryingly ‘themother pumps blood around their body sothey don′t have to’.



Mark Scheme

ii EITHERDifference: (fetal haemoglobin) higheraffinity for oxygen /described/

ORA;

Reason: (fetal haemoglobin) must be ableto bind to oxygen,in low(er) partial pressure /in placenta/when adult oxyhaemoglobin dissociates /when adult haemoglobin dissociates fromoxygen;

OR

Difference: (fetal haemoglobin) containsgamma sub-units;

Reason: creates high( er) affinity foroxygen;

2ACCEPT able to become more saturatedthan adult haemoglobin at low pO2IGNORE gets more saturated at low pO2(ie no comparison to adult haemoglobin)IGNORE ref to saturation curve

CREDIT ‘associate with / combine with /loads’ for bind

IGNORE pick up / take up / gains / absorbs/ attracts / attaches / saturatesDO NOT CREDIT oxygen dissociates orhaemoglobin dissociates

Examiner's Comments

The majority of candidates knew that fetalhaemoglobin has a higher affinity foroxygen than adult haemoglobin. Thosewho did not achieve this mark stated thatfetal haemoglobin had ‘a high affinity’ orfailed to make it clear that it was the affinityfor oxygen. Fewer candidates achieved thesecond mark where examiners werelooking for a clear explanation of thereason for this difference using correcttechnical terminology.

Total 4

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