Cross‐Membrane Transport

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.

Passive transport

Simple diffusion. 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 O 2) or small and unchaged polar (like H 2O) 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.

Facilitated diffusion. 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 aquaporins.

Active transport

Moving against a concentration gradient or an electrochemical gradient requires the input of additional energy and so is called the active transport of materials. This is the only kind of transport able to move molecules against a concentration gradient. Like facilitated transport, it depends upon protein transporters in the membrane; in this process these proteins are composed of the membrane‐bound enzyme, H +‐ATPase. Using the typical proton pump reaction, the enzyme catalyzes the hydrolysis of ATP. In primary active transport, the energy released by the hydrolysis is used to modify the membrane protein itself, which then transports the molecule through the protein. In secondary active transport, the energy is used to pump a large quantity of H + across the membrane; as the protons flow back they carry with them other substances. Sucrose is moved in plants similarly by a proton‐sucrose cotransport system.

Vesicle transport

Some materials that move among cells can't be transported in any of the previous ways, but must be transported by enclosing and then moving them in vesicles (small balloon‐like structures). Large proteins and polysaccharides are commonly moved this way; the materials for cell walls, for example, are carried to the construction site in vesicles pinched from the endoplasmic reticulum as are the enzymes used to digest insects captured by carnivorous plants. The process in general is called exocytosis because it conducts materials away (exo‐) from the cell (cyto‐).

Some materials enclosed within vesicles move into the cell in a process of endocytosis (endo meaning within). Once thought to occur only in animals or animal‐like protists, it now is understood to be a process of plants as well where one application carries stray pieces of ready‐made plasma membrane back for recycling.