Transportation in living organisms

Every living organism, whether made of a single cell or many, has to constantly exchange material with its surroundings and move material between different parts of its own body — taking in what it needs and pushing out the wastes it generates. How this transport happens depends heavily on how large and complex the organism is. Where the body's surface area is large relative to its volume and the distance between different parts is very small, simple diffusion and active transport between cells are enough, without any dedicated transport system; this covers transport between the cells of many small or simply built organisms. Within a single cell, transport is often helped along by cyclosis, the streaming movement of cytoplasm seen in organisms like Paramoecium and Amoeba, in root-hair cells of many plants, and in the white blood cells of animals.

Once a body plan gets more complex, simple diffusion between cells is no longer enough, and organisms develop intermediate solutions before a full circulatory system appears. In sponges and coelenterates, the surrounding water itself is drawn through channels and body cavities to reach the cells directly. In flatworms, there is parenchymal circulation — fluid moving through the loosely packed tissue (parenchyma) that fills the body, rather than through dedicated vessels. In roundworms, there are no blood vessels at all; instead the body fluid filling the body cavity is kept moving around the internal organs (viscera) by the contraction of the body wall muscles. Because none of the sponge, flatworm or roundworm solutions send fluid through internal vessels, and material is exchanged directly with cells or tissue, this whole category is described as extracellular transport, distinct from the intracellular transport (cyclosis) that moves material within a single cell.

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