Some Points
Introduction
In a flowering plant the substances that would need to be transported are water, mineral nutrients, organic
nutrients and plant growth regulators.
On the basis of distance travelled by substances, transport is of two types :
(i) Short distance transport (ii) Long distance transport
(i) Short distance transport :
Over small distances, substances move by diffusion and by cytoplasmic streaming. Cytoplasmic streaming
is supported by active transport.For short distance transport vascular tissues (xylem and phloem) are not required.·(ii) Long distance transport :
Transport over long distances proceeds through the vascular system (xylem and phloem) and is called translocation.Over long distances substances move by bulk flow or mass flow.
Direction of transport :
In a flowering plant there is a complex traffic of compounds (but probably very orderly) moving in different
directions, each organ receiving some substances and giving out some others.
In rooted plants, transport of water and minerals in xylem is essentially unidirectional, from roots to the stems.
Organic and mineral nutrients however, undergo multidirectional transport
Means of transport
(1) Simple diffusion :
Movement of ions (particles) and molecules of solids, liquids and gases from region of higher concentration to
regions of lower concentration till equilibrium established is called diffusion.Features of diffusion :(i) It is a downhill process (passive process) because no energy expenditure takes place.
(ii) It is a random kinetic motion.
(iii) It is a slow process.
(iv) Driving force is concentration gradient.
(v) It is not dependent on a living system.
(vi) It is non selective process.
(vii) It is not sensitive to inhibitors.
Factors affecting rate of diffusion :
(i) Gradient of concentration : In diffusion substances move from regions of high concentration to regions
of lower concentration.
(ii) Permeability of the membrane : The rate of diffusion is directly proportional to the permeability of the
membrane.
(iii) Temperature : Increase in temperature, increase the kinetic energy. Thus, causes increase in rate of diffusion.
(iv) Pressure : The rate of diffusion is directly proportional to the gradient of diffusion pressure.
(v) Density : The rate of diffusion is inversely proportional to the square root of density of particles.
(Graham's diffusion law)
Facilitated diffusion :
The diffusion of any substance across a membrane also depends on its solubility in lipids. (Lipid is the major
structural constituent of the membrane). Substance soluble in lipids diffuse through the membrane faster (simple
diffusion), while substances that have a hydrophillic moiety find it difficult to pass through the membrane;
their movement has to be facilitated.
In other words "this is a movement of substances with hydrophillic moiety from regions of high concentration
to regions of low concentration through transmembrane proteins."
Site for facilitated diffusion :
Transmembrane proteins or tunnel proteins.
Features of facilitated diffusion :
(i) It is a downhill process (passive process) because no energy expenditure takes place.
(ii) Driving force is concentration gradient.
(iii) It is dependent on the living system (for transmembrane protein).
(iv) Facilitated diffusion can not cause net transport of molecules from a low to a high concentration.
(v) Transport rate reaches a maximum when all of the protein transporters are being used. This is called saturation.
(vi) It is sensitive to inhibitors which react with protein side chains.
(vii) It is very specific because it allows cell to select substances for uptake.
Some facts about facilitated diffusion :
· Transmembrane proteins (channel proteins) do not set up a concentration gradient: a concentration gradient
must already be present for molecules to diffuse even if facilitated by the proteins.
· Some protein channels are always open while others can be controlled.
· Porins : They are protein channels that form huge pores in the outer membranes of the plastids,
mitochondria and some bacteria. They allow molecules upto the size of small proteins to pass through.
· In plants for the bulk movement of water, water channels are present. They made up of eight different
types of aquaporins.
Passive symport, antiport and uniport :
Some transport proteins allow facilitated diffusion only if two types of molecules move together.
(i) Symport : Two molecules cross the membrane in the same direction.
(ii) Antiport : Two molecules cross the membrane in opposite directions.
(iii) Uniport : When a molecule moves across a membrane independent of other molecules, the process is
called uniport.
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