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Agriculture LibreTexts

Section C

Ion transport The plant absorb minerals nutrient from the soil mostly by the roots but in some plants by leaves((carnivorous plants, bromeliads, etc. ) in ionized form.Roots absorb some mineral nutrients selectively , including some which may not be essential for them.

Plants absorb minerals from soil through the root, passes the membrane, in two ways: -

  1. Passive transport:-Movement of molecules from high to low concentration. Needs no energy input. Including Passive diffusion and Facilitated diffusion
  2. Active transport:- Movement of molecules from low to high concentration (opposite the flow of diffusion). Needs input of energy (ATP).

Passive transport :- The main theories and concepts of passive transport are :-

  1. Donann equilibrium theory (Donann 1911):- Entry of ions into the cell across the plasma membrane to maintain electrical equilibrium is known as Donann equilibrium
  2. Interception and contact or Ion exchange theory:- Jenny and Overstreet (1935) Exchange of anions and cations absorbed to the root surface with similarly charged ions of soil solution is known as ion exchange.
  3. Mass flow (Kramer 1956) when solute are transported with the conventional flow of water from the soil to the plant root under the influence of transpiration .

Mechanism of passive transport

  1. Diffusion :-All molecules are in constant motion. As molecules collides into each other, directions are changed, causing random dispersal of the molecules. The random movement of molecules results in diffusion.
  2. Diffusion is the movement of molecules from an area of high concentration to one of low concentration.
  3. This difference in the concentration of molecules across a space is called the concentration gradient, is the difference between the concentration of a solute in one place and its concentration in an adjacent area.
  4. Molecules diffuse faster at higher temperatures than at lower temperatures, and smaller molecules diffuse faster than large molecules.
  5. Diffusion occurs due to the kinetic energy (thermal motion or heat) of the molecules.
  6. The rate of diffusion is variable and depends on temperature, molecular weight, distance to travel, solvent density, and size and the type of molecules that are diffusing.
  7. In diffusion energy is not required by the cell. Most transport of materials into and out of cells occurs by diffusion Only small molecules can cross the cell membrane by simple diffusion.
  8. Only after an even distribution of molecules is occured there is no longer any diffusion because there is no longer a concentration gradient.
  9. Diffusion will eventually cause the concentration of molecules to be the same throughout the space the molecules occupy, the system has reached an equilibrium.

Osmosis :-Movement of water molecules across a semipermeable membrane from an area of high water concentration (low solute) to an area of low water concentration (high solute) is known as osmosis.

  1. Water crosses the membrane toward the higher solute concentration until the concentration gradients of both water and solutes even out.
  2. The net direction of osmosis depends on the relative concentration of solutes on the two sides of the cell membrane.
  3. Hypotonic:- concentration of solute molecules outside cell is lower than the concentration of solutes inside the cell cytosol.In a hypotonic condition plant cell only take up water until the inner and the outer water potential are equal. The solution at the inside of the plant cell is subject to an additional hydrostatic pressure. Plant cells store ions, sugars, organic and amino acids and other substances in considerable concentrations in their vacuoles. The solutes cause an influx of water. In this way can plant cells build up a large positive internal pressure, the turgor pressure. It has a decisive influence on the maintenance of the rigidity and stability of plant tissues. Each cell exerts a pressure on its neighbouring cells. The pressures add up to a large tissue tension.
  4. Hypertonic :-concentration of solutes outside cell is higher than concentration of solutes inside cell cytosol. When a plant cell is immersed in a hypertonic solution ,water is extracted from the protoplasm, it shrinks. This phenomenon is called plasmolysis. The process is reversed as soon as the cells are transferred into a hypotonic solution (deplasmolysis).
  5. Isotonic :-. concentrations of solutes outside and inside cell are equal. In isotonic condition, there is no movement of water into the cell of plant. The cell becomes flaccid (limp), and the plant may wilt.

Facilitated diffusion

  • Plasma membranes of plant made up of phospholipid bilayer interspersed with two type of proteins. It is selective-permeable and allows the passage of specific molecules needed by cell.

  • Lipid bilayer is impermeable to some of the vital molecules like glucose, sodium ions, chloride ions etc. Their transport must therefore be "facilitated" by proteins that span the membrane and provide an alternative route or bypass. Nonelectrolytes (uncharged particles) diffuse through membrane at a rate proportional to their solubilities in lipid and inversely proportional to their molecular size.
  • Due to which, many important gases ,such as, CO2, NH3,O2 and N2 .cross lipid bilayers by dissolving in lipid portion of membrane, diffusing to the other lipid- water interface, and then dissolving in the aqueous phase on the other side of membrane.
  • Facilitated Diffusion is the movement of specific molecules down a concentration gradient, passing through the membrane via a specific carrier protein.
  • It is similar to simple diffusion in the sense that it does not require expenditure of metabolic energy and transport is again down an electrochemical gradient.

Two major groups of integral membrane proteins are involved in facilitated diffusion:-

  • Ion Channels do not really bind the solute, but are hydrophilic pores formed by channel proteins, through the membrane that open and allow certain types of solutes to pass through.
  • Therefore, ion channels mediate only passive transport (down hill) to allow diffusion of ions inorganic ions,such as Na+, K+,Ca++ ,CL- down their electrochemical gradients across a plasma membrane.
  • In general, channels are quite specific for the type of solute they will transport and transport through channels is quite a bit faster than by carrier proteins.
  • Three Essential characteristics: 1. Specific (selective for single nutrient molecule) 2. Passive (requires no input of energy) 3. Saturates (non-linear dependence on concentration).
  • Some ion channels are always open, but others have gates that open to allow ions to pass or close to stop their passage and control the channel's permeability. The gate can controlled by voltage or ion concentration, even by light, hormones or other stimuli. Ion channels :-
    1. Voltage-gated channels
    2. Mechanical gated channels
    3. Ligand-gated channels

  • Carrier proteins(also known as permeases or transporters) :- The carrier proteins are found in all natural membrane and have the two features in common (1) they facilitate movement of solutes in thermodynamically favoured direction and (2) they display affinity and specificity for the solute to be transported.
  • Carrier proteins bind a specific type of solute and are thereby induced to undergo a series of conformational changes which has the effect of carrying the solute to the other side of the membrane. The carrier then discharges the solute and, through another conformational change, reorients in the membrane to its original state. Typically, a given carrier will transport only a small group of related molecules. Carrier protein s are classified as follows :- Uniporters ,when a single solute is transported from one side of the membrane to the other.symporters , when transfer of one solute depends on the simultaneous or sequential transfer of a second solute in the same direction, and antiporters , where transfer of one solute depends on the simultaneous or sequential transfer of a second solute, but in the opposite direction.