$\displaystyle \small \bullet$ Transport of substances in plants over longer distances through the vascular tissues (Xylem and Phloem) is called translocation.
$\displaystyle \small \bullet$ The transport of water and minerals occurs through xylem and it is unidirectional, that is from root to the stem and leaves.
$\displaystyle \small \bullet$ The transport of food prepared by leaves occurs through phloem and it is bidirectional.
MEANS OF TRANSPORT
$\displaystyle \small \bullet$ There are mainly three important means of transport of materials into and out of cells:
Diffusion
$\displaystyle \small \circ$ “Movement of molecules from their higher concentration to lower concentration is called Diffusion”.
$\displaystyle \small \circ$ It is the slow movement of gases, liquids and solutes without the energy expenditure.
$\displaystyle \small \circ$ Rate of diffusion is mainly affected by concentration gradient of diffusing substance, permeability of the membrane, temperature, pressure, size (density) of the substances.
Facilitated Diffusion
$\displaystyle \small \circ$ Facilitated diffusion is the passive transport of molecules along their concentration gradient across a biological membrane with the help of special proteins.
$\displaystyle \small \circ$ The porins are proteins that form huge pores in the membrane allowing molecules up to the size of small proteins to pass through.
$\displaystyle \small \circ$ Aquaporins are integral membrane proteins that form water channels, which rotate and release the extracellular molecule bound to the transport protein inside the cell.
$\displaystyle \small \circ$ Some carrier or transport proteins allow diffusion only if two types of molecules move together.
Uniport: A molecule alone moves across a membrane through transport or carrier protein.
Symport: Two molecules together cross the membrane in same direction.
Antiport: Two molecules move in opposite directions.
$\displaystyle \small \circ$ It is the transport of molecules against a concentration gradient (from lower concentrated region to higher concentrated region) with the expenditure energy.
$\displaystyle \small \circ$ It is carried out by membrane-proteins.
$\displaystyle \small \circ$ Active transport is an up-hill transport.
Comparison of Different Transport Mechanism
PLANT- WATER RELATIONS
$\displaystyle \small \bullet$ Water is an universal solvent.
$\displaystyle \small \bullet$ Protoplasm is mainly water in which different molecules are dissolved and suspended.
$\displaystyle \small \bullet$ Soft plant parts mostly contain water. Eg: watermelon has 92% water.
$\displaystyle \small \bullet$ Water is the limiting factor for plant growth and productivity.
Water Potential ($\displaystyle \Psi _{w}$)
$\displaystyle \small \bullet$ The chemical potential of water is called water potential. It is denoted by $\displaystyle \Psi$(Psi) and measured in pascals (Pa).
$\displaystyle \small \bullet$ As the concentration of water in a system increases, its kinetic energy (‘water potential’) also increases. Hence, pure water will have the greatest water potential.
$\displaystyle \small \bullet$ Water potential of pure water at standard temperatures, which is not under any pressure is zero.
Solute Potential ($\displaystyle \Psi _{s}$)
$\displaystyle \small \bullet$ The magnitude of lowering of water potential due to dissolution of a solute is called solute potential.
$\displaystyle \small \bullet$ $\displaystyle \Psi _{s}$ is always negative. The more the solute molecules, the lower (more negative) is the $\displaystyle \Psi _{s}$.
$\displaystyle \small \bullet$ For a solution at atmospheric pressure, water potential ($\displaystyle \Psi _{w}$) = solute potential ($\displaystyle \Psi _{s}$).
Pressure Potential ($\displaystyle \Psi _{p}$)
$\displaystyle \small \bullet$ Magnitude of increase of water potential, when pressure is greater than atmospheric pressure is applied to pure water or a solution.
$\displaystyle \small \bullet$ Pressure potential is usually positive.
$\displaystyle \small \bullet$ Water potential of a cell is affected by solute potential & pressure potential.
$\displaystyle \small \bullet$ The relationship between them is:
$\displaystyle \Psi _{w}=\Psi _{s}+\Psi _{p}$
Osmosis
$\displaystyle \small \bullet$ “The movement of molecules from their higher concentration to lower concentration through semipermeable membrane is called osmosis”.
$\displaystyle \small \bullet$ The net direction and rate of osmosis depend on both the pressure gradient and concentration gradient.
Osmotic Pressure
$\displaystyle \small \bullet$ External pressure applied to prevent the diffusion of water is called osmotic pressure.
$\displaystyle \small \bullet$ Numerically osmotic pressure is equal to osmotic potential.
$\displaystyle \small \bullet$ Osmotic pressure has positive sign and osmotic potential has negative sign.
$\displaystyle \small \bullet$ Osmosis is of two types:
Endosmosis: the inflow of solvent (water) into a cell from outside when placed in hypotonic solution is called endosmosis.
Exosmosis: the outward flow of water from cell when placed in hypertonic solution like sugar solution, the cell tends to shrink and becomes flaccid is called exosmosis.
Plasmolysis
$\displaystyle \small \bullet$ Shrinkage of protoplasm in a cell due to exosmosis when kept in a hypertonic solution is called plasmolysis.
Hypotonic solution: the external solution which is more dilute than the cytoplasm.
Hypertonic solution: the external solution which is more concentrated than the cytoplasm.
Isotonic solution: when the external solution balances the osmotic pressure of the cytoplasm.
Turgor pressure: the pressure built up against the wall due to movement of water inside the cell.
Imbibition
$\displaystyle \small \bullet$ It is a type of diffusion in which water is absorbed by solids (colloids) causing them to increase in volume. Eg: Absorption of water by seeds and dry wood.
$\displaystyle \small \bullet$ Water potential gradient between the absorbent and the liquid imbibed, affinity between the adsorbent and the liquid are necessary for imbibition.
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