chemical stimuli are also transported, though in very small amounts, sometimes in a strictly polarized or unidirectional
manner, from where they are synthesized, to other parts.
MEANS OF TRANSPORT
Passage of materials into and out of the cells is carried out by the following methods – diffusion, facilitated diffusion,
active transport.
Diffusion
Movement by diffusion is passive, slow, and occurs along the concentration
gradient, i.e., from region of higher concentration to region of lower
concentration, provided the cell membrane is permeable to it. No energy
expenditure takes place.
Diffusion is very important to plants, since it is the only means for gaseous
movement within the plant body. The diffusing molecules or ions exert a
pressure called diffusion pressure (DP). This can be defined as the potential
ability of a molecule or ion (be it a solid, liquid or gas) to diffuse from an area
of its greatest concentration to an area of lesser concentration. The diffusion
pressure is directly proportional to the concentration or the number of diffusing
particles, i.e., greater the concentration of diffusing particles in the system, their
diffusion pressure will also be greater and vice versa.
Pure water has the maximum diffusion pressure. When solute particles are added to it, diffusion pressure gets lowered.
Diffusion pressure of a solution, as compared to the pure state, is known as diffusion pressure deficit or DPD. Due
to the presence of diffusion pressure deficit, a solution will always tend to make up the deficit by absorbing water. Hence,
it is also called suction pressure.
Factors influencing rate of diffusion
all the factors which bring about changes in free energy, will ultimately influence diffusion. rise in temperature increases
the rate of diffusion, due to increase in kinetic energy.
The rate of diffusion is inversely proportional to the square root of the density of the diffusing substance (Graham’s
law of diffusion).
where D = diffusion and d = density.rate of diffusion decreases with density of the medium, and is directly proportional to the diffusion pressure gradient.
Importance of diffusion
(i) exchange of gases (CO 2 and O 2 ), between the plant interior and outside air, occurs through diffusion.
(ii) Diffusion keeps the cell walls of the internal plant tissues moist.
(iii) It is the process involved in transpiration.
(iv) It is a means of spreading of ions and other substances throughout the protoplast.
(v) It is an effective means of transport of substances, over a very short distance.
(vi) aroma of flowers is due to diffusion of volatile aromatic compounds and helps to attract pollinating animals.
Facilitated diffusion
The passive absorption of solute, mediated by a carrier, is called facilitated diffusion. Particles which are lipid soluble,
can easily pass directly through the cell membrane, as it is mainly made of lipids. The hydrophilic solutes, find it difficult