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Write a comparative outline of gaseous exchange in a protozoan, an earthworm and a bony fish

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Write a comparative outline of gaseous exchange in a protozoan, an earthworm and a bony fish (20) All organisms need exchange and transport systems. The gaseous exchange requirements increases as the organism increases in size and complexity. Demands for water, oxygen and nutrient molecules increases with the size. Acellular organisms are active in wet conditions only. Their surface is permeable to oxygen, carbon dioxide and water. They have a large surface area to volume ratio, and diffusion distances in their body are small. Annelida, such as the earthworm, have long, thin, segmented bodies, consisting of three different layers of cells. The surface area to volume ratio is quite large, and diffusion distances are long. Chordates, such as fish, are also made up of three layers of cells. Their surface area to volume ratio is small, and the diffusion distances are very large. In both earthworms and fish, an efficient transport system, containing haemoglobin, is available to carry oxygen around the body. The surface area to volume ratio is a way of expressing the relationship between these parameters as an organism's size changes. Exchange of materials often occurs through the process of diffusion; in which dissolved molecules or other particles move from areas of higher concentration to areas of lower concentration. ...read more.


Fish however, do not have an external surface that is permeable to gases. The thick skin and scales of a fish do not allow gas to diffuse through into their body. Gaseous exchange takes place in the cell membrane of the protozoon, through the skin of the earthworm, and through gill plates and gill lamellae in bony fish. The protozoon's surface area cannot be increased for gaseous exchange. Whereas in the earthworms, they are able to increase their size a small bit due to the fact that they have long and very thin bodies. The amount that it increases however is not enough to make a large difference. In fish, the division of the gill surface into lots of gill lamellae, which all carry a number of gill plates, enables the fish to increase the surface area so that more oxygen is allowed to be extracted from the water. In all three organisms being compared, the exchange surface is both thin and moist. In the fish and the earthworm, not only do they have a thin, moist exchange surface, but it also has a good supply of blood. Large organisms need a circulatory system, as the surface area to volume ratio is small so it cannot keep up with the oxygen demand. ...read more.


The fish has a positive ventilation mechanism for taking in water and passing it out over their gills. They ventilate using a unidirectional respiration system. This is due to the density of water being too great for the fish to breathe like humans. The fish expands its buccal cavity, creating a large surface area for the intake of water. Pressure decreases in the cavity, so it is then lower than that of the external atmospheric pressure, which allows water to enter down a pressure gradient. As the fish closes its mouth, it raises the floor of the cavity, decreasing the volume, but at the same time increasing the pressure. Water is then forced in over the gills. When this occurs, the operculum cavity bulges out, decreasing the pressure, which allows water to be drained over the gills. Protozoa are small enough to be able to rely on diffusion across their cell membrane and to all parts of its body to let gaseous exchange to occur. Earthworms use diffusion across the cells of their skin, but do also have a blood transport system, containing haemoglobin, which allows oxygen to be transported to and from its cells. Fish have gills to give themselves an enormous internal surface. They are able to take in as much oxygen as they need, with the help of a counter current mechanism, and are also able to actively ventilate this surface. Siobhan Hunt 12L1 ...read more.

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