Summary - Transport in Plants

Summary

There are two types of transports namely short and long distance in plants to translocate sap and solutes. Based on energy requirement, the transport may either be passive or active. The process of diffusion, facilitated diffusion, imbibition and osmosis are driven by concentration gradient like a ball rolling down to a slope and hence, no energy is needed. The water absorbed (either active or passive) from the soil by root hairs must reach the xylem for further transportation. There are three possible routes to reach the xylem from root hairs. They are i) apoplast ii) symplast and/ or iii) transmembrane. Various theories explain the path of sap in the xylem and Dixon’s Cohesion- tension theory is the most accepted one. Transpiration is mostly carried out by stomata, which has guard cells. The general mechanism of stomatal movement is based on entry and exit of water molecules in guard cells. Many theories are there to explain how water enters and exits from guard cells. The theory of potassium transport enumerates two different reactions separately run for opening and closing of stomata. Contrary to ascent of sap by xylem in an upward direction, the path of solute which consists of the photosynthetic products is always in phloem and translocate multidirectional. The point of origin of translocation is photosynthetic leaves which are the source. On the other hand, point of utilization is called sink. According to Munch mass flow hypothesis, the solutes move along the concentration gradient in a bulk flow.

Although minerals are dissolved in soil water, they do not tend together with water to enter the root hairs during absorption of water. Mineral absorption is independent of water absorption. Minerals are absorbed either actively or passively.