Final answer:
Closing the valve would cause a sustained collapse of the lung in the glass bell jar model because air cannot enter to reinflate the lung. The presence of surfactant is vital for lung inflation and preventing collapse, especially in premature infants.
Step-by-step explanation:
When the valve on a glass bell jar representing a lung model is closed, the effect on a collapsed lung would be a sustained collapse. This is because closing the valve prevents air from entering the lung and reinflating it. Normally, pulmonary surfactant plays a crucial role in lung function by reducing the surface tension in the alveoli, making it easier to inflate the lungs and preventing their collapse. Without surfactant — as in the case of premature infants who might not produce enough — one would expect difficulty inflating the lungs. Surfactant is not produced in enough quantities until around 26 weeks of gestational age; therefore, babies born before this period have a high risk of developing respiratory distress syndrome.