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Surface Area to Volume Ratio investigation.

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Surface Area to Volume Ratio investigation Introduction: * All organisms carry out exchange between themselves and their environment. * There is also exchange by diffusion. The rate of diffusion is affected by: - Temperature - Surface Area - Steepness of the concentration gradient - Distance over which diffusion takes part The larger the surface area or the steeper the concentration gradient or the thinner the membrane of diffusion barrier, the faster is the diffusion. - Diffusion becomes less efficient as the surface area does not increase to an extent that would satisfy the increased demands of a larger volume cell. - As the length of the sides of a cubodial cell increase then: Surface area and volume increase exponentially, volume increases at a greater pace, surface area/volume ratio decreases exponentially. As cell sizes increase, the surface area does not increase in proportion to the volume. Example by means of a cube. * All organisms need to exchange substances such as food, waste, gases and heat with their surroundings. These substances must diffuse between the organism and the surroundings. The rate at which a substance can diffuse is given by Fick's law: Rate of Diffusion a surface area x concentration difference distance The rate of exchange of substances therefore depends on the organism's surface area that is in contact with the surroundings. The requirements for materials depends on the volume of the organism, so the ability to meet the requirements depends on the surface area : volume ratio. As organisms get bigger their volume and surface area both get bigger, but volume increases much more than surface area. A bacterium is all surface with not much inside, while a whale is all insides with not much surface. This means that as organisms become bigger it becomes more difficult for them to exchange materials with their surroundings. In fact this problem sets a limit on the maximum size for a single cell of about 100 mm. ...read more.


The lungs have a very large surface area because of the thousands of tiny air sacs they contain. These air sacs are called alveoli. The alveoli are small with very thin walls. They have a radius of 0.1mm and wall thickness of about 0.2�m. This makes the diffusion pathway between the air in the lungs and the blood very short. The lungs contain about 300 million alveoli, each wrapped in a fine mesh of capillaries. - Gill lamellae comprise the respiratory surface in fish. The total surface area of the lamellae is enormous. The length of the diffusion path for oxygen is very short, because the layer of cells that separate the blood from the surrounding in water is very thin. Water contains only on thirtieth as much oxygen per unit volume as air and water is more difficult to push over the respiratory surface. Fish use also a countercurrent system to maximise the rate of gaseous exchange across the respirartory surface. The blood flows in the opposite direction to water, this helps to maintain a diffusion gradient right along the gill. A result of this more 02 can diffuse from the water to the blood. - The leaves of plants have a large ratio meaning again exchange is carried out more effectively. Very small organisms such as those consisting of a single cell have no special tissues, organs or systems for gaseous exchange. Mammals are large, multi cellular organisms and they have a complex system for gaseous exchange. Mammals needs such a system single celled organism does not. Single celled organisms Mammals * Large surface area to volume (ratio) for diffusion; * short diffusion pathway ( to all parts of organism) * oxygen/ carbon dioxide diffuse in and out. * Small surface area to volume * long diffusion pathway * waterproof/ gastight skinned internal gas exchange surface which is moist with a large surface area The Practical Materials and apparatus * Agar with cresol red, (an indicator: red in alkali, yellow in acid) ...read more.


rate of diffusion of molecules out of any region in a substance is proportional to the concentration of molecules in that region, and the rate of diffusion into the region is proportional to the concentration of molecules in the surrounding regions. Thus, while molecules continuously flow both into and out of all regions, the net flow is from regions of higher concentration to regions of lower concentration. Generally, the greater the difference in concentration, the faster the diffusion. 4. If the gelatine block represents a single cell organism what substances need to diffuse in and which need to diffuse out? Oxygen (&nutrients) needs to diffuse in and carbon dioxide needs to diffuse out. 5. Single cell organisms are less than 1 mm across, the largest may be 2 mm across. Why are they no larger than this? Unicellular organisms like amoeba have a very high surface area to volume ratio. All chemicals needed can pass into the cells directly and all waste can pass out efficiently. Organisms which have a high surface area to volume ratio have no need for special structures like lungs or gills. Nutrients and oxygen passing into an organism are rapidly used up. This gives a limit on the ultimate size to which a microorganism can grow. If vital chemicals did not reach all parts of a cell then death would be a consequence. A unicellular organism may satisfy all its needs by direct diffusion. 6. How can the rate of diffusion be increased in a 10 mm block without changing its volume? The rate of diffusion can be increased by enlarging the surface of the block. In this way, there would not be a straight line, but wavy borders. The larger the surface area, the larger the rate of diffusion. The rate of diffusion can be increased by increasing the concentration gradient, increasing the surface area across which diffusion occurs, increasing the temperature or by decreasing the distance across which molecules need to travel. 1 Anne Kolouschek 12 MA ...read more.

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