In many organs the anatomical arrangement is such that a steep concentration gradient can be maintained thus maximizing diffusion rates. An example of counter current exchange may be found in the gills of fish. This counter current arrangement maintains a constant concentration gradient between the bloodstream and the water. Water leaving the gills with low oxygen content will contact blood with an even lower oxygen concentration. The net movement of water from a hypotonic solution into a hypertonic plant cell is an example of osmosis.
Osmosis is a special case of diffusion.
Cells in a marine environment are generally isotonic with its salty water. Plants because of their cell walls and large central vacuoles can take advantage of steep water gradients to maintain the shape and rigidity of leaves and stems. The central vacuole is hypertonic to ground water. This effectively removes some water reducing the water gradient allowing more water to take its place higher up the stem. Facilitated Diffusion Like normal diffusion, facilitated diffusion is driven by potential energy of a concentration gradient.