Page 1 - LN-cH-11

Materials also move over short distances, i.e., inside the cell, across the membranes
or even cell to cell. The substances transported are water, mineral, nutrients, organic
nutrients and plant growth regulators. Over short distances the substances move by
diffusion and cytoplasmic streaming supplemented by active transport while, the
long distance transport takes place through xylem and phloem and is called
translocation.

Topic 1 Transport and Plant Water Relations

In rooted plants, water and mineral transport in xylem is unidirectional (roots to
stems). However, translocation of organic and inorganic nutrients is multi
directional. Organic compounds synthesised in the leaves are transported to all parts
including storage organs.
The storage organs later re-export these organic compounds when required.
Important nutrients are also withdrawn from plant parts undergoing senescence and
are supplied to the growing parts. Hormones and plant growth regulators are present
in very minute quantities and are often transported in polarised (unidirectional)
manner.
Means of Transport
Transport in plant is an important phenomenon. It can either be unidirectional or
bidirectional. There are mainly three important methods of transport of materials
into and out of cells, i.e., diffusion, facilitated diffusion and active transport. These
process are given below in details
1. Diffusion
It is a physical process in which passive transport of solvent molecules or solute
ions occur without the expenditure of energy. It is a slow process and is independent
of living system. During the process, the molecules or ions flow in a random fashion
from the region of higher concentration to region of lower concentration be it a gas,
liquid or solids. Rate of diffusion is mainly affected by
(а) Concentration gradient of diffusing substance.
(b) Permeability of the membrane separating them.
(c) Temperature
(d) Pressure
(e) Density
Note:
* Diffusion rate is inversely related to square root of relative density of the diffusing
substance. This is known as Graham’s law of diffusion.

* Diffusion is a very important phenomenon in plants as it is the only means of
transport of gases in them.
* Tendency of different substances to diffuse according to their own partial
pressures or concentration is called independent diffusion.