Transport in Plants Test - 6

The apoplastic movement of water occursexclusively through the intercellular spaces and the walls of the cells. Movement through the apoplast does not involve crossing the cellmembrane. This movement is dependent on the gradient. The apoplast does not provide any barrier to water movement and water movement is through mass flow.

Long distance transport of substances within a plant cannot be by diffusion alone. Diffusion is a slow process. It can account for only short distance movement of molecules. For example, the movement of a molecule across a typical plant cell (about 50 μm) takes approximately 2.5 s.

The apoplast is the system of adjacent cell walls that is continuous throughout the plant, except at the casparian strips of the endodermis in the roots.

Symplastic movement may be aided by cytoplasmic streaming. We can observed cytoplasmic streaming in cells of the Hydrilla leaf.

Plasmodesmata is a narrow thread of cytoplasm that passes through the cell walls of adjacent plant cells and allows communication between them.

Relative humidity of atmosphere also influence the opening and closing of stomata. If relative humidity of atmosphere comes below 50% stomata close down.

The flow of water upward through the xylem in plants can achieve fairly high rates, up to 15 metres per hour. The water is mainly pulled through the plant and that the driving force for this process is transpiration from the leaves. This is referred to as the cohesion-tension-transpiration pull model.

When two solution are separated by a semipermeable membrane, water enters the sugar solution. The movement of water always occurs from higher concentration to lower concentration. Such solution are called osmotically active.

Terrestrial plants take up huge amount water daily but most of it is lost to the air through evaporation from the leaves, i.e., transpiration.