A-LevelBiologyGas exchange9700/22 · Oct/Nov 2023 · Q412 Marks

The alveoli of the lungs are the main gas exchange surface in humans.

(a)Explain how blood flow through the alveolar capillaries helps to maintain steep diffusion gradients for gas exchange.[2]
(b)Ventilation of the lungs is the process of inhalation and exhalation. Ventilation helps to maintain steep diffusion gradients. Explain the role of elastic fibres in the alveolar wall during ventilation.[2]
(c)Some cells in the alveolar wall are specialised to secrete surfactant to prevent collapse of the alveoli at the end of exhalation. In these cells, surfactant is stored in membrane-bound organelles known as lamellar bodies. Surfactant is a mixture of lipids, mainly phospholipids, and some proteins. A protein known as ATP-binding cassette transporter A3 (ABCA3) is needed to move surfactant phospholipids into lamellar bodies from the surrounding cytosol (fluid part of cytoplasm). Suggest and explain the features of protein ABCA3 that make it suited to its function.[3]
(d)(i)Fig. 4.1 shows the flow of genetic information in the production of ABCA3. Complete Fig. 4.1 to name the processes occurring at X and Y.[2]
(d)(ii)A triplet of bases codes for one amino acid. This fact only partly explains how the activity of gene ABCA3, which is 80kb long, can result in the protein ABCA3, which is only 1704 amino acids long. Suggest other reasons to explain the difference in the number of base pairs in gene ABCA3 compared with the number of amino acids in protein ABCA3.[3]

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 (Oct/Nov 2023). The mark or mark type is shown in brackets.

[2] any two from: 1 blood arriving, is deoxygenated / has low(er) partial pressure of oxygen A low, concentrations / levels, of oxygen or oxygen-poor or blood arriving has, high(er) partial pressure / AW, of carbon dioxide ; A carbon dioxide-rich
[2] 2 idea that newly oxygenated blood is, constantly / continually, removed ; A oxygenated blood passed to pulmonary vein A oxygen that diffuses, from alveoli / into blood, is transported away (by blood flow)
[2] any two from: allow alveoli to, expand / stretch, on, inhalation / inspiration / breathing in / AW;
[2] prevent alveoli from, overstretching / rupturing / bursting (on, inhalation / inspiration / breathing in) ; I collapse
[2] recoil / recoiling, to help, exhalation / expiration / breathing out/force air out ;
[3] if stated as facilitated diffusion allow max 1 for mp1 or mp2 or mp5
[3] any three from: 1 membrane / carrier / transport, protein (move phospholipids across into lamellar body); R channel protein / cotransporter (protein)
[3] 2 has binding site(s) (specific for surfactant phospholipids) ; R ATP binding site for phospholipids / binds ATP R has receptors to bind to phospholipids
[3] 3 ATP, used / hydrolysed / needed, for active transport / to provide energy (for transport); A ATP binding site for active transport A ref. to ATPase and, active transport/ for active release I has / presence of, ATP energy
[3] 4 (surfactant) phospholipids move, against a concentration gradient/from a low(er) to a high(er) concentration ;
[3] 5 able to carry out conformational change / AW;
[2] X: transcription;
[2] Y: translation ;
[3] any three from: allow ecf if introns and exons consistently the wrong way round
[3] 1 introns, are non-coding / do not code for amino acids / are not involved in polypeptide synthesis ; R introns are triplets / codons, that are non-coding I exons are coding
[3] 2 introns are removed, after transcription / from primary transcript / before mRNA is formed ; R introns removed from, mRNA / primary mRNA
[3] 3 (only) exons (join to) form mRNA ; A mRNA, contains / AW, exons A only exons are translated
[3] if mp 2 and 3 not awarded, allow one mark for: gene splicing / RNA splicing / mRNA does not contain introns or splicing if RNA noted within response – this includes the A for mp4 (UAA etc because these are RNA codons) ; R DNA splicing
[3] 4 some (DNA) triplet(s) / (mRNA) codon(s) are STOP codons ; A UAA/UAG / UGA, is a STOP codon or STOP, codons / triplets, do not code for an amino acid / terminate translation A some, triplets / codons, terminate translation so do not code for amino acids I ref. to premature STOP codons
[3] 5 ref. to met/ first amino acid / amino acid coded for by START codon, (may be) removed after translation;
[3] 6 AVP; e.g. ref. to (non-coding) regulatory sequences