TRANSPORT IN PLANTS:
· The primary primordial soup of an ancient ocean from which life originated several billions of years ago on the earth, had water as the solvent in a physiologically active cell, more than 90-95% of the protoplasm is composed of water.
· In many plants that grow on land like Bryophyta and Pteridophyta, water is essential for movement of the motile male gamete.
· Plant absorbs most water and minerals through their roots. The roots of a plant can be very extensive, as much as 50kms of roots can be found in 1 square meter of soil. Once in the plant, the water and minerals within it, is moved from the bottom of the tallest tree to the top by the process of transpiration. It is estimated that approximately 99% of the water absorbed by the roots of plant in a given time period is lost from the leaves by transpiration.
· Plants develop vascular tissue; xylem (transports water and minerals throughout the plant) and phloem (transports sugars made by the plant).
· Plants transports water, minerals, plant growth regulators and more.
Transportation in plants can be occurring in following three types:
b) Facilitated diffusion
c) Active transport
· In diffusion process, water molecules move from higher concentration to lower concentration across permeable membrane.
· It is a slow process.
· There is no expenditure of energy.
· Diffusion process depends upon concentration gradient, permeability of membrane, temperature, pressure and size of substance.
· It is a passive process.
b) Facilitated diffusion:
· Facilitated diffusion is a process of diffusion of hydrophilic molecule along concentration gradient without the expenditure of energy.
· Membrane transport proteins are involved in this diffusion process. These proteins formed channels which may be opened or closed.
· Facilitated diffusion is very specific.
· There are two types of proteins which are involved in diffusion process: Porins, proteins that form huge pores in outer membrane of mitochondria and plastids. Aquaporins, proteins that facilitate diffusion of water molecule.
· Transport can be of three types: Symport, both transport molecule move in same direction. Antiport, here both molecule move in opposite direction at the same time. Uniport, independent movement of molecule and single molecule is moved across the membrane.
c) Active transport:
· In active transport special membrane proteins are required which are very specific in nature.
· This requires energy to pump molecule against concentration gradient.
· Pumps are proteins which uses energy to carry substances across the cell membrane.
· When all molecules require are saturated, it leads to maximum transport.
· The chemical potential of water is known as water potential.
· It is denoted by Ψ and measured in pascals (Pa).
· Greater the concentration of water in a system, greater is its kinetic energy and greater is the water potential.
· If two systems are iin contact, then there is movement of water from the solute of greater water potential to lower water potential.
· Solute potential (Ψs): Magnitude of lowering water potential where solute is added to water.
· Pressure potential (Ψp): Magnitude of increase of water potential where pressure is greater than the atmospheric pressure. It is applied to pure water and solution.
· Water potential of pure water is zero.
· Solute potential is always negative and water potential is always positive.
· Ψw= Ψs+ Ψp
· Water potential of cell is affected by solute potential and pressure potential.
Water is the most abundant constituent of all physiologically active plant cells. Several physiological processes of plants like osmosis, imbibition, plasmolysis are accomplished by water.
Transpiration explains how water moves up the plant against gravity in tubes via a xylem. Plants pump the water up from the soil to deliver nutrients to their leaves. It is influenced by several factors.
Opening and closing of stomata is controlled by concentration of solutes in guard cells. Some factors like light, temperature, CO2, O2,hormones regulate the opening and closing of stomata.
Diffusion is the movement of particles from the region of higher concentration to the region of lower concentration. It is affected by temperature, pressure, density of medium, distance,number of particles per unit volume.
Minerals are move into the roots by using energy in the form of ATP. After the uptake minerals are transported to all parts by transpiration pull. Later they are absorbed by active or passive transport.
Food is synthesized through the process of photosynthesis. It can be transported to different parts of the plant, this function is performed by phloem.
Water transport is important for the uptake of mineral nutrients from the soil. There are three pathways which facilitate the movement of water in plants like apoplast, symplast, trans-membrane pathways.
Mass flow hypothesis states that the mass flow of solute take place from the source(mesophyll cells) to the sink (where photosynthesis needed). This theory was proposed by Ernst Munch.