A-LevelBiologyGas exchange9700/23 · May/June 2018 · Q414 Marks

Oxygen enters the blood stream from the alveoli in the lungs and carbon dioxide leaves the bloodstream to enter the alveoli. Most of the oxygen is carried by haemoglobin in red blood cells to the body tissues. [Figure 4.1]

(a)Outline how oxygen enters the blood stream from an alveolus.[3]
(b)(i)Use Fig. 4.1 to calculate the difference in percentage saturation of haemoglobin at the lower partial pressure of oxygen of 2.7kPa compared to the higher partial pressure of 13.0kPa. Show your working. difference =[1]
(b)(ii)Explain the advantage of having a difference in percentage saturation of haemoglobin at lower and higher partial pressures of oxygen.[3]
(c)Define the term disease.[2]
(d)Outline the differences between the HbA (normal) and HbS (sickle cell) alleles of the gene coding for the β-globin polypeptide and explain how these differences lead to a change in the haemoglobin molecule formed.[5]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

[3] answers must be in context of oxygen
three from:
1 diffusion
2 movement, down a, concentration / diffusion, gradient / from high(er) to low(er) concentration ; A in terms of partial pressure
3 passive (process) ;
4 through alveolar wall / across alveolar cells / across squamous epithelial cells (of alveolus) / across pavement cells (of alveolus); A squamous cells
5 across, endothelium / endothelial cells / capillary wall ; A squamous, epithelium / cells (in context of capillary wall)
6 ref. to diffusion / AW, through the phospholipid bilayer of cell surface membranes / between the phospholipids of the bilayer (of squamous or endothelial cells);
[1] 65/66/67 (%)
[3] 1 higher partial pressures in lungs / lower partial pressures in (respiring) tissues ; A correct values / range of values, of kPa A in alveoli
advantages of differences (higher v lower partial pressure) only higher partial pressure or lower partial pressure explanation required to gain mark
2 oxygen, binds to v released from, haemoglobin or oxyhaemoglobin, is formed v dissociates / AW ;
3 (so) percentage saturation of haemoglobin (with oxygen) high v low ;
4 affinity of haemoglobin for oxygen high v low ;
5 data from Fig. 4.1 to support ;
6 (body), cells / tissues, need oxygen for aerobic respiration ;
[2] idea of abnormal, change in the body / condition, which produces, poor / ill-, health
or
absence of, good health / well-being;
A disorder / illness
further detail; e.g. having an adverse effect reduces the effectiveness of functions produces, (specific) signs / symptoms lack of social, physical and mental, well-being can be infectious or non-infectious AW
[5] five from:
1 base substitution (in gene coding for, ẞ-globin / polypeptide);
2 different/ altered, mRNA codon ;
3 different tRNA brings a different amino acid (to ribosome) / leads to a change in one amino acid (in the polypeptide chain);
4 altered, primary structure / sequence of amino acids (in ẞ-globin);
5 changed, tertiary / quaternary, structure ;
6 haemoglobin / molecule, less soluble ;
7 (haemoglobin) molecules, stick together / form fibres ;
8 (haemoglobin) less able to bind oxygen / AW ;
points above may be qualified
9 details;; + e.g. thymine / T, replaces, adenine / A
10 in sixth, codon / triplet or sixth amino acid in sequence is changed (non-template strand) GTG instead of GAG / GTA instead of GAA or (template strand) CAC instead of CTC / CAT instead of CTT mRNA codon is, GUG instead of GAG / GUA instead of GAA
11 (amino acid change is) valine instead of glutamic acid ;
12 amino acid with non-polar side chain instead of polar side chain;
13 position of amino acid, is to the exterior / faces cytosol ;