Gases

Introduction

Diffusion is the movement of particles from an area of higher concentration to an area of higher concentration. The uptake and loss of gases in organisms occurs by diffusion. Diffusion relies on kinetic energy and moves down a concentration gradient, therefore, the oxygen rich atmosphere enters the organism and the high concentration of carbon dioxide in the organism is expelledto the atmosphere.





Discussion

Single celled organisms have a large surface area to volume ratio. This feature allows a quick uptake of gases. Certain types of Cyanobacteria, like the Oscillatoria, are cylindrically shaped, if they grow indefinitely lengthwise, the rate of diffusion will not be affected. Because these type of organisms do not have specialised respiratory systems,having a large surface area to volume ratio and a thin semipermeable membrane facilitates an efficient gas exchange. (Niklas, K. 1997).

Insects are very successful living organisms dependent on diffusion, even though they are small in size, they have developed highly efficient anatomical structures that have enabled them to survive.

Insects are surrounded by a waterproof exoskeleton. Thisexoskeleton permitted them to become terrestrial living things, however, this represents another difficulty; insects lose water easily. Contrary to the postulates of diffusion, there is no semi permeable membrane to diffuse gases. So, the biological dilemma lies on how these creatures obtain oxygen and how they compensate water loss.

Evolution and adaptation have found the right balance toenable these organisms survive. Insects have a system on internal invaginations called tracheae -supported by chitin rings that branch into tracheoles. These tracheoles increase the surface area as they penetrate the insect's tissues. Gas enters through pores called spiracles that open and close, however, to avoid the rapid water loss, spiracles remain closed most of the times.

Fig. 1. Trachealsystem in an insect.



As they open, gas exchange occurs as a concentration gradient is produced when the oxygen inside the insect is used up and carbon dioxide is expelled, consequently, oxygen enters again. This process is made more efficient by abdominal contraction, also called ventilation. Figure 1 shows the tracheal system in the insects. (Barratt, J. 1998)

Insects' small size can beexplained on terms of Fick's Law, as they solely rely on diffusion, their small size provides a short diffusion pathway.

Oxygen concentrations vary in our planet. High altitudes have lower oxygen concentrations, while in the seas, oxygen concentration is generally constant. Some living organisms live in the borderline of water and air but can only breathe in either one of them. (Cameron, J.N.,1989)

Just like insects, fish have a waterproof external barrier, in contrast with insects, fish have a small surface area to volume ratio, therefore, they have developed adequate transport systems to suit their needs.

Fish depend on gills to breathe, these are located behind the fish's head, they can be internal -covered by an operculum, or external gills, that have a larger surfacearea. Gills are made up of filaments resembling the pages of a book. Gills are orientated in such a special way that facilitate the water flow.

To allow efficient ventilation, some species require muscular contraction. Swimming, for example, provides adequate oxygenation. In fish with internal gills there is a coordinate process to pump water in. As water enters the fish's buccal cavity it movestowards the pharynx touching the gill plates. The increase in pressure causes the opercular cavity to open so water can be expelled. The image on the left (Fig. 2) shows this process.

Fig. 2. Buccal cavity of a fish and the entrance and exit
of water through the opercular cavity.






Certain fish that are very active swimmers such as salmons and sharks, keep their mouths...