Why Do We Need A Transport System 1b4654

Why do we need a transport system? Glucose and oxygen needed for respiration, amino acids and other essential food molecules for cell-building and growth, and the removal of poisonous waste products are all the basic things cells need. These must be transported from outside the organism into the cells. For a unicellular organism such as amoeba, nutrients and oxygen can diffuse directly into the cell from its external environment, and waste substances can diffuse directly out. This is because the surface area of the amoeba is very large in relative to the volume of the inside of its cell. So, its surface area to volume ratio is large. Another example of a species with a large surface area to volume ratio is flatworm. Their body is typically flattened, it is much wider than it is tall, thus increases the surface area to volume ratio. In contrast to unicellular organisms such as an amoeba, larger organisms such as humans, are made up of billions of cells, often organise into specialised organs and tissues. Their surface area to volume ratio is small; therefore substances need to travel a long distance from the outside to reach all body cells. Simple diffusion is not efficient enough as it is not fast enough for nutrients and oxygen to reach the inner cells of the body to sustain the processes of life. This is why complex multicellular organisms have evolved specialised systems to get food and oxygen into their bodies and to remove waste. They also have an internal transport system which carries substances to every cell in the body, delivering oxygen and nutrients and taking away waste quickly to prevent them from damaging the cells. In humans, this transport system is the heart and circulatory system and the blood flows through it. Substances are transported in the flow of the fluid with a mechanism for moving it around the body to reach all body cells and are delivered over short distances from the transport system to individual cells by processes such as diffusion, osmosis and active transport. In addition to the circulatory system, the human body has the lymphatic system. To cope with the portion of blood plasma, also called extracellular fluid, leaking out of capillaries and surround tissue cells, fluid is able to drain into the system of tubes that increase with size from the tiny lymph capillaries to lymph vessels to lymph ducts. The extracellular fluid contains oxygen, glucose, amino acids and other nutrients needed by tissue cells. The fluid eventually collects together into two large collecting ducts, the thoracic duct and the right lymphatic duct, and is drained back into the blood at the right and left subclavian veins prior to the blood returning to the heart at the right atrium. The lymphatic vessels are punctuated at intervals by lymph nodes that remove foreign materials such as infectious microorganisms. The movement of the fluid is aided by the force of the muscles attached to the skeleton. There are also semilunar valves that restrict the back flow of fluid.

http://sciencelearn.org.nz/Contexts/You-Me-and-UV/Science-Ideas-andConcepts/Lymphatic-system http://www.lymphnotes.com/article.php/id/151/ http://www.britannica.com/EBchecked/topic/352770/lymphatic-system http://www.abpischools.org.uk/page/modules/heartandcirculation/heart2.cfm? coSiteNavigation_allTopic=1 Edexcel AS Biology textbook by Ann Fullick