Explain how continuous blood flow through the alveolar capillaries maintains steep diffusion gradients for gas exchange.
Ventilation of the lungs consists of inhalation and exhalation. It helps keep diffusion gradients steep. Explain the function of elastic fibres in the alveolar wall during ventilation.
Some alveolar-wall cells are specialised for secreting surfactant to stop the alveoli collapsing at the end of exhalation. In these cells, surfactant is kept in membrane-bound organelles called lamellar bodies. Surfactant is a mixture of lipids, mainly phospholipids, together with some proteins. A protein called ATP-binding cassette transporter A3 (ABCA3) is required to transfer 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.
Fig. 4.1 illustrates the flow of genetic information involved in producing ABCA3. Complete Fig. 4.1 by naming the processes at X and Y.
A triplet of bases codes for one amino acid. This explains only part of why gene ABCA3, which is $80\ \text{kb}$ ($80000$ base pairs) long, produces the ABCA3 protein, which has just $1704$ amino acids. Suggest other reasons for the difference between the number of base pairs in gene ABCA3 and the number of amino acids in protein ABCA3.