Transport in plants Class 11
Introduction
l Plant physiology (Physis = nature of life, logos = science) deals with the study of plant functioning (i.e., the various bio-physicochemical phenomena) and the various principles governing it. Father of plant physiology is regarded to Stephan Hales.
l Though plants do not have a circulatory system, they need to move molecules not only to short distance but also to long distances. Water taken up by roots has to reach all parts of the plants, up to the very tip of the growing stem. The photosynthates or food synthesized by the leaves have also to be moved to all parts including the root tips embedded deep in the soil. Transport over longer distances proceeds through the vascular system and is called translocation. Movement across short distances occurs within the cell across the membranes, and from cell to within the tissue. Over small distances, substances move by diffusion and by cytoplasmic streaming supplemented by active transport. The important substances transported in plants are water, minerals, organic nutrients and growth regulators.
l In rooted plants, transport of water and minerals is essentially unidirectional, from roots to the stems. However, translocation of organic and inorganic (mineral) nutrients is multidirectional. Organic compounds are synthesized in the leaves and are exported to all other parts of the plant including storage organs. From the storage organs, they are later re-exported when required. The mineral nutrients are picked up by roots and transported upward into the stem, leaves and the growing regions. Nutrients may be withdrawn from the part undergoing senescence and moved to the growing parts. The plant growth regulators and other chemical stimuli are also transported in very small amounts, either in a strictly polarised unidirectional manner (e.g., auxin basipetally, etc) or diffuse to all parts. Hence, in a flowering plant, there is a complex traffic of compounds moving in different directions, each organ receiving some substances and giving out some others.
Means of Transport
l There are three means of transport of materials across the cells – diffusion, facilitated diffusion (both being passive diffusion) and active diffusion.
Diffusion
l Diffusion is the passive movement of materials across the cells to the cell, or from the intercellular spaces of the leaf to the outside. No energy expenditure takes place. In diffusion, molecules move in a random fashion from regions of higher concentration to lower concentration.
l Diffusion is a slow process and is not dependent on a “living system”. It is more common in gases and liquids, but diffusion in solids rather than of solids is more likely. Diffusion is very important gaseous movement within the plant body.
l Diffusion rates are affected by the gradient of concentration, the permeability of the membrane separating them, temperature and pressure.