Transport in Plants

Companion cell A type of cell found within the phloem of flowering plants. Each companion cell is usually closely associated with a sieve element. Its function is uncertain, though it appears to regulate the activity of the adjacent sieve element and to take part in loading and unloading sugar into the sieve element. In gymnosperms a similar function is attributed to albuminous cells, which are found closely associated with gymnosperm sieve elements

A plant girdled upto bast will show below the girdled part, the non transportation of the food which occurs through phloem (bast). Water transport from roots to other plant parts via xylem will continue till root cells do not die due to starvation.

The term root pressure was coined by Stephen Hales (1727). The evidences that support the existence of this phenomenon are bleeding and guttation. The magnitude of root pressure is 1-2 atm and very rarely 5 atm.

The main function of sieve tube is the transport of food products from green and storage organs of the plant to other organs of the plant. The sieve clements are sieve tubes arranged one above the other and have sieve plates (obligue or transverse perforated septa) on their end walls. The cytoplasm occur in the form of a thin living layer along the inner side of cellulose wall. The nucleus disappears in mature sieve elements.

The net force with which water is drawn into a cell is equal to difference of OP and TP, known as diffusion pressure deficit or suction pressure. DPD of pure water is maximum (= 1236 atm) solvent moves from cell of low DPD to high DPD. DPD, or SP= OP-TP.

Water always moves from the area of high water potential to the area of low water potential i.e., from less negative potential to more negative potential.

Turgor pressure is the pressure that develops in a cell from time to time due to osmotic diffusion of water molecules into a cell resulting in the stretching of cell wall. Due to turgor pressure, rigid cell wall offers resistance. This resistance works in a direction opposite to turgor pressure. But it is equal to turgor pressure and is called wall pressure. In this condition, no exchange of water takes place.

Good soil is a soil which allows percolating the water slowly from it. Minerals, air, water and organic matter are essential for maintaining a good soil structure that enables adequate air exchange and water drainage. Eg, alluvial soil.

Surface tension does not help in molecule transport. The process of diffusion and osmosis transport molecules is without using energy but in case of active transport energy (ATP) is used.

Jamming of wooden frames during rains is caused by swelling of wood due to imbibition. (The absorption of water by the solid particles of a substance without forming a solution is called imbibition).