• Plants lack both interstitial fl uid as well as a regular circulation system. Even then they have to move (transport) various types of substances (gases, minerals, water, hormones, photosynthates and organic solutes) not only to short distance (from one cell to another or from one tissue to another) but also to very long distances such as water from roots to tops of plants or photosynthates from leaves to tips of roots.
• Substances move over short distances through diffusion and active transport supplemented by cytoplasmic streaming. The movement over longer distances occur through the vascular system, the xylem & the phloem. Such type of movement is called translocation.
• One of the important aspect of translocation is its direction. In rooted plants, transport of water & minerals through xylem is unidirectional (which occurs from roots to stems), and transport of organic compounds & mineral nutrients is multidirectional. Organic compounds (synthesized in leaves) are exported to all other parts including every living cell, growing points, fruits and storage organs. From storage organ they are later re-exported.
Mineral nutrients are primarily picked up by roots. They are passed out upwardly to leaves, stem and growing regions through vascular system (xylem and phloem).
• Senescent organs and leaves pass out most of their nutrients, especially the minerals, before falling down from the plant.
• Plant hormones and other chemical stimuli are transported in very small amounts. Therefore, a complex traffi c of materials is going on in fl owering plants, some moving to different directions, some passing out in polarized manner, with most organs, receiving some substances and giving out some others.
MEANS OF TRANSPORT
Diffusion
• Diffusion is passive and may be from one part of the cell to the other, or from cell to cell, or over short distances, like, from the inter-cellular spaces of the leaf to the outside. No energy expenditure takes place.
• Diffusion is a random movement of individual molecules from a region of higher concentration to a region of lower concentration. It is a slow process and independent of living system.
• Larger the difference in concentration, more rapid is the fl ow of molecules.
• Uptake and distribution of water, gases and solutes occur in plants as a result of diffusion.
• Diffusion is common in gases and liquids, but diffusion in solids rather than of solids is more likely. E.g. diffusion of CO2 from atmosphere to leaves.
• Diffusion rates are affected by concentration gradient, membrane permeability, temperature and pressure.
• Diffusion pressure, DP (term coined by Meyer, 1938), is the pressure exerted by the tendency of the particles to diffuse from the area of higher concentration to the area of their lower concentration. Diffusion pressure is directly proportional to the concentration of particles of the diffusing substance and temperature.
• The extent to which a membrane permits or restricts the movement of a substance is called membrane permeability. It depends on the membrane composition and chemical nature of the solute.
