Gas Diffusion Diffusion of a gas across a membrane occurs according to the laws of physics. The driving force for gas diffusion is the difference in the partial pressure of the gas across the membrane. A high external partial pressure and low internal partial pressure will provide a large difference and will enhance diffusion. Oxygen makes up 20.9 percent of the air, so that 20.9 percent of the atmospheric pressure at sea level (14.72 pounds per square inch) is attributable to oxygen (3.08 pounds per square inch). At higher altitudes, atmospheric pressure and partial pressure of oxygen are reduced. The other factors that determine the diffusion of a gas are the thickness of the membrane across which it diffuses, the membrane's total surface area, and the nature or composition of the membrane. Obviously, a thick membrane will retard diffusion of gas because the gas must move across a greater distance. The distance the gas must diffuse is known as the diffusion distance. Additionally, the total surface area of the membrane available for diffusion has a direct effect on the rate of diffusion from one place to another. The greater the surface area, the greater the quantity of gas that can diffuse in a given time. Finally, the composition of the membrane is of critical importance in determining the diffusion of a gas. The nature of the membrane is referred to as the permeability of the membrane to the gas in question. The greater the permeability, the more easily gas diffuses. A membrane with a layer of minerals (calcium, for example) on the cells will not be as permeable as one without such a layer. Avery important point to note is that gases diffuse according to the difference in the partial pressure of the gas and not according to the concentration of the gas in the liquid. Several scientists have proved this by constructing artificial systems with two dissimilar fluids separated by a membrane. The movement of oxygen is always fromhigh partial pressure to low and not froma high concentration to low. The reason is that pressure is a measure of molecular energy, but concentration of a gas in a liquid depends on the amount of that gas that can dissolve in the liquid-its solubility.
Howler Monkeys: Voices to Be Heard Howler monkeys (primates of the family Cebidae) are among the most fascinating animals of Central America. These heavy-bodied monkeys are the largest primates of the NewWorld tropical rain forests, with a combined head and body length of over 550 millimeters and a prehensile tail that may be even longer. They spend virtually their entire lives in the treetops, where they find food and water and may come down to drink only during the dry season. The fur is long, soft, and luxurious, and the animal almost looks as though it is wearing a cape. Both sexes have a beard, but it is much more developed in males. Color of the furmayrange froma dark, glossy black to a dark reddish brown to a light ochre. As impressive as howler monkeys look, it is not their appearance that makes them so distinctive. Males produce vocalizations which are so strong and loud that the reverberations can be heard up to several miles. If the source of the sound were not known, one would think that a much larger animal, such as a jaguar, was producing it.Whenmaking this extraordinary sound, the howler monkey's tongue is dilated and forms a gobletlike chamber. The chorus may begin with an old male, perhaps the leader of the band. The sound builds up, and there does not appear to be any pause. Other males may join in to create a loud cacophony. Unfortunately, as is true of many animals in the tropical rain forests of Central America, loss of habitat is threatening the species.
