Physical Properties Diffusion Structure Function Epithelia Epithelial

¶ … physical properties diffusion structure function epithelia epithelial cells digestive circulatory systems animals larger end larger. In answer, surface area volume ratios (SA/V) ratios change organisms larger effect (s) change imposes structure function organisms.

Epithelia: Solving the SA/V (surface-to-area volume) problem

"Epithelium is one of the four primary tissues of the body. It consists of cells usually arranged in sheets or tubules that are attached to the underlying basement membrane" (Epithelia, 2012, SUNY). One of the problems for the epithelium is that as a cell grows larger, the cell's volume increases at an exponentially faster rate than the cell's overall surface area. This could place a limit on the cell's size, since it limits the speed substances coming into the cell can diffuse throughout the entity (Moffatt 1998). However, a number of mechanisms have evolved to cope with the SA/V (surface-to-area volume) problem besides simply 'staying small.'

One solution is the cell changing its shape, either elongating or folding, given that an oval has a larger SA/V ratio than a circle and a folded cell membrane has a larger surface area as well (Moffatt 1998). Another solution is decreasing in overall volume since a "cell with a large water-filled vacuole in the center has much less active cytoplasm than its measurements would suggest. Its metabolic demands are therefore not as great" (Moffatt 1998). Other cells have evolved to develop the capacity of mobility, which enables them to be more proactive search of more food. Single-celled organisms like amoebas and paramecium form "vacuoles, [by which] cells increase their total imports and provide themselves with small, extremely rich bubbles of nutrients" (Moffatt 1998). Improving transit within the cells -- as can be seen in leaf cells which engage in cyclosis -- also addresses the SA/V quandary. This involves "moving nutrients rapidly away from the membrane, [so] concentrations gradients across the membrane can be maintained" (Moffatt 1998).