In the latter case the resistance is caused by dissipation of mechanical energy when the configuration or the direction of flow is sharply changed, by the formation of vortices and secondary flows as a result of the flow breaking away, by the centrifugal forces, etc.
Animals depend on plants for food as plants produce food and energy which animals can't, so, herbivores eat plants to get energy, carnivores eat herbivores and other carnivores for energy and omnivores eat all four, plants, herbivores, carnivores and also other omnivores for energy.
Plants, on the other hand, depend on their cells to be extremely specialized to perform their life functions. For example, since plants do not have bones for structure and support they must have cell walls around all of their cells. This leads to the need for very unique cells in plants.