Concentration gradients by which diffusion and osmosis operate are only partially effective when all needs of the plant are considered. Some essential materials, for example, are present in small amounts in the soil but used in greater concentrations by the plant.
How do the plants retain such materials as they accumulate in cells against the concentration gradient? Or,
How is it possible for plants to get rid of unneeded or toxic substances that diffuse inward?
Three means of transport take place: passive, active, and vesicle.
Some materials, like water, simply diffuse across the cell’s membrane as a function of the concentration gradient across the membrane, i.e. they move from a region of their higher concentration to a region of their lower concentration through the phospholipid layer. Materials such as dissolved gases—oxygen and carbon dioxide, for example—also diffuse across, as do lipid-soluble substances. Sufficiently small and nonpolar molecules (like O2) or small and unchaged polar (like H2O) move readily if a concentration gradient exists. The rate of movement depends upon the steepness of the concentration gradient. The energy for simple diffusion is the kinetic energy inherent in all substances.
Charged molecules and polar molecules pass through membranes using protein channels. Like simple diffusion, these channels depend upon concentration gradients between the two sides of the membrane, but unlike simple diffusion, use carrier proteins and channel proteins to assist in the movement of materials. Carrier proteins chemically bind with the moving material, passing it from one binding site to another, literally carrying it through the lipids of the membrane. The channel proteins, on the other hand, open a water-filled passage/channel through which ions move, bouncing from one temporary binding site to another down the channel. The channels are gated—they open and close. Some of the channels appear to facilitate the movement of water exclusively and use channel proteins called aqua porins.