Symbiosis National Aptitude Test SNAP 2012 Paper

DIRECTIONS (Qs.71-75): Read the following passage and answer the questions that follow.
The bird egg is a self-contained life-support system for the developing bird embryo. All the nutrients, minerals, energy sources and water utilized by the embryo during its incubation are already present in the freshly laid egg, so that the egg requires only warming by the parents and periodic turning to prevent the adhesion of the embryo to the shell membranes. Still, the egg lacks one crucial requirement: oxygen, which drives the metabolic machinery of the embryonic cells so that they can execute the complex maneuvers of development. How does to egg breathe, taking up oxygen from the surrounding atmosphere and discharging carbon dioxide, the waste product of respiration?
Gas exchange is usually associated with the periodic inhalation of a fluid medium (air or water), which carries oxygen to the capillaries of the lungs or the gills and removes carbon dioxide from the respiratory organ with each exhalation. The lungs or the gills are driven by muscles whose rate of pumping is determined by metabolic demand and controlled by the nervous system. Yet the eggs of birds and other organisms (such as insects, spiders, amphibians and reptiles) show no respiratory movements, and there are no air currents within the egg that could transport oxygen to the capillaries of the growing embryo. Instead the egg “breathes” by diffusion through thousands of microscopic pores in the shell.
Gas moves through the pores by the passive process of diffusion: the tendency for a high concentration of a molecule to run downhill to an area of lower concentration. Diffusion takes place because of the kinetic energy of gas molecules and does not require the direct expenditure of metabolic energy by the embryo: the lower concentration of oxygen inside the egg brings new oxygen in through the pores from the outside, where the concentration is higher. Conversely, the concentration of carbon dioxide inside the egg causes those molecules to diffuse toward the outside, where there are essentially none. These diffusion processes are governed by the available pore area of the shell, the length of the pores and the concentration differences of the gases diffusing across the shell.
The water content of the air within the egg is greater than that of the air outside it, and so the pores will also allow water molecules (which are smaller than oxygen molecules) to diffuse out. Animals have evolved many specialized adaptations for conserving water, but bird eggs seems designed to lose it at a controlled rate. Most of the energy needed for embryonic development is taken from the fat stores of the yolk, and for every gram of fat burned an almost equal mass of metabolic water is generated. Therefore, the relative water content of the egg will increase during incubation unless water is lost. If the relative water content at hatching is to equal that of the freshly laid egg, about 15 percent of the initial mass of the egg must be lost as water. As breeders of domestic fowl well know, this amount of water loss is essential for successful hatching.