Transport in Plants

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Transport in Plants

Why is transportation required in plants?

  • Plants need to move molecules over very long distances, much more than animals do
  • Water taken up by the roots has to reach all parts of the plant, up to the very tip of the growing stem.
  • The photosynthates or food synthesised by the leaves have also to be moved to all parts including the root tips embedded deep inside the soil

Means of Transport in Plants

SIMPLE DIFFUSION

  • Movement by diffusion is passive.
  • Takes place over short distances.
  • Usually movement is from one part of the cell to the other, or from cell to cell.
  • No energy expenditure takes place.
  • Molecules move in a random fashion.
  • Substances move from regions of higher concentration to regions of lower concentration.
  • A concentration gradient is a just a region of space over which the concentration of a substance changes

FACILIATED DIFFUSION

  • It is a passive transport.
  • Very specific.
  • Allows the cell to select substances for uptake.
  • Molecules or ions move across a biological membrane via specific cell membrane constitutes.
  • The diffusion of any substance across a membrane depends on its solubility in lipids.
  • Lipid is the major constituent of membrane.
  • Substances soluble in lipids diffuse through the membrane faster.
  • Transportation of hydrophilic(water loving) substances are difficult to pass through the membrane
  • Internal part of membrane is hydrophobic(water hating) in nature.
  • Their movement is facilitated by membrane proteins.
  • They provide sites for transportation across the membrane.
  • The proteins form channels in the membrane for molecules to pass through.
  • Some channels are always open and others can be controlled.
  • Porins are proteins that form huge pores in the outer membranes of the plastids, mitochondria and some bacteria allowing molecules up to the size of small proteins to pass through.
  • The water channels in the membrane are made up of eight different types of aquaporins.
  • This diffusion takes place without expenditure of ATP energy
  • Facilitated diffusion cannot cause net transport of molecules from a low to a high concentration
  • Transport rate reaches a maximum when all of the protein transporters are being used
  • This is the saturated state.
  • This diffusion is sensitive to inhibitors which react with protein side chains.

PASSIVE SYMPORTS AND ANTIPORTS

  • These are different methods of faciliated diffusion
  • Some membrane proteins allow diffusion only if two molecules move together.
  • In a symport, both molecules cross the membrane in the same direction
  • In an antiport, they move in opposite directions
  • In uniport, molecules move across the membrane independent of other molecules.

ActiveTransport in Plants

  • Active transport occurs across the root so that the plant takes in the ions it needs from the soil around it.
  • Ions are moved into root hairs, where they are in a higher concentration than in the dilute solutions in the soil.
  • This uses energy to pump molecules against a concentration gradient.
  • It is carried out by membrane-proteins.
  • Pumps are proteins that use energy to carry substances across the cell membrane.
  • These pumps can transport substances from a low concentration to a high concentration (‘uphill’ transport).
  • Carrier protein is very specific in what it carries across the membrane
  • Active transport always leads to accumulation of molecules are ions towards one side of the membrane.
  • Transport rate reaches a maximum when all the protein transporters are being used or are saturated.
  • This is also sensitive to inhibitors.

                COMPARISON BETWEEN TRANSPORT PROCESSES

PLANT WATER RELATIONSHIP 

  • Physiological activities of the plant requires water.
  • Water is the medium in which most substances are dissolved.
  • Protoplasm of the cells is equal to water in which different molecules are dissolved and (several particles) suspended.
  • Distribution of water within a plant varies.
  • Woody parts have relatively very little water, while soft parts mostly contain water.
  • Terrestrial plants take up huge amount water daily.
  • Most of the water is lost to the air through evaporation from the leaves, i.e.,
  • For instance, a mature corn plant absorbs almost three litres of water in a day, while a mustard plant absorbs water equal to its own weight in about 5 hours.
  • Due to high demand for water, it is often the limiting factor for plant growth and productivity.

 

WATER POTENTIAL

  • Potentialenergy of water in a system compared to pure water, when both temperature and pressure are kept the same.
  • It is the measure of how freelywater molecules can move in a particular environment or system.
  • Water potential can be denoted as psi (Øw).
  • It is expressed in pressure units such as pascals (Pa)
  • Water molecules possess kinetic energy.
  • The greater the concentration of water in a system,the greater is its kinetic energy or ‘water potential’.
  • Hence, the pure water will have the greatest water potential.
  • The water potential of pure water at standard temperatures,without any pressure, is taken to be zero.
  • It has two main components.
  • Solute potential (Øs) and pressure potential (Øp).
  • Net movement of water molecules takes place from the system with higher energy (higher Øw )to the one with lower energy(lower Øw)
  • The magnitude of the lowering of water potential due to dissolution of a solute is called solute potential (Øs).
  • When any solute is dissolved in pure water, the solution then has fewer free water and the concentration of water decreases,
  • This reduces the water potential of the solution.
  • Hence, all solutions have a lower water potential than pure water.Ψs is always negative.
  • The more the solute molecules, the lower (more negative) is the Øs
  • For a solution at atmospheric pressure (water potential) Øw = (solute potential) Øs.
  • If pressure greater than atmospheric pressure is applied to pure water or a solution, its water potential increases.
  • Pressure potential is the component of water potential due to hydrostatic pressure that is exerted on water in the cell.
  • A pressure is built up against the cell wall, it makes the cell
  • Pressure potential is usually positive
  • In plants negative potential or tension in the water column in the xylem plays a major role in water transport up a stem.
  • Water potential of a cell is affected by both solute and pressure potential.
  • Øw = Øs + Øp

OSMOSIS

  • Diffusion of water across a differentially- or semi-permeable membrane.
  • The net direction and rate of osmosis depends on both the pressure gradient and concentration gradient.
  • Water diffuses from the region of its higher concentration to the region of its lower concentration until equilibrium is reached.
  • At equilibrium the two regions should have the same water potential.
  • The solute concentration of a solution provides the osmotic pressure required to prevent water from diffusing .
  • More the solute concentration, greater will be the pressure required to prevent water from diffusing in.
  • Osmotic pressure is equivalent to the osmotic potential.
  • Osmotic pressure is the positive pressure applied, while osmotic potential is negative.

PLASMOLYSIS

  • Plasmolysis occurs when water moves out of the cell and the cell membrane of a plant cell shrinks away from its cell wall.
  • If the external solution balances the osmotic pressure of the cytoplasm, it is said to be isotonic.
  • If the external solution is more dilute than the cytoplasm, it is
  • If the external solution is more concentrated, it is
  • Cells swell in hypotonic solutions and shrink in hypertonic ones.
  • Plasmolysis occurs when the cell (or tissue) is placed in a solution that is hypertonic (has more solutes)to the protoplasm.
  • Process of plamolysis is usually reversible.
  • Water is first lost from the cytoplasm and then from the vacuole.
  • This causes the protoplast to shrink away from the walls.
  • The cell is then said to be plasmolysed.
  • The hypertonic solution (i.e. a solution with high salt concentration)occupies the space between the cell wall and shrunken protoplast in a plasmolysed cell.
  • In an isotonic solution, there is no net flow of water towards the inside or outside.
  • Water flows into the cell and out of the cell and are in equilibrium
  • The cells are then said to be flaccid.
  • Thepressure potential is equal to the water potential and the water potential is zero in a flaccid cell so the pressure potential tends to be zero.
  • When the cells are placed in a hypotonic solution water diffuses into the cell
  • This causes the cytoplasm to build up a pressure against the wall
  • This is called turgor pressure.
  • This turgor pressure is ultimately responsible for enlargement and extension growth of cells.
  • The fungi and bacteria have cell wall too other than plants.

 

 

 

IMBIBITION

  • The uptake of water by a plant or seed.
  • Special type of diffusion when water is absorbed by solids colloids
  • Imbibition is also diffusion since water movement is along a concentration gradient
  • The seeds and other such materials have almost no

water hence they absorb water easily.

  • The process results in swelling of the substance
  • Imbibition is a property of many biological substances
  • Seeds undergo imbibition swelling when exposed to water.
  • encourages seedlings to emerge out of the soil and establish themselves
  • Water potential gradient between the absorbent and the liquid imbibed is essential for imbibition.

Long-distance transport of water in Transport in Plants

  • Long distance transport of substances within a plant cannot be by diffusion alone.
  • Diffusion is a slow process.
  • It can account for only short distance movement of molecules.
  • Special long distance transport systems

become necessary so as to move substances across long distances and at a much faster rate.

  • Water and minerals, and food are generally moved by

a mass or bulk flow system.

  • This results of pressure differences between the two points.
  • This is unlike diffusion where different

substances move independently depending on their concentration gradients.

  • Bulk flow can be achieved either through a positive hydrostatic pressure gradient (e.g., a garden hose) or a negative hydrostatic pressure gradient (e.g., suction through a straw).
  • The bulk movement of substances through the conducting or vascular tissues of plants is called
  • Xylem is associated with translocation of mainly water, mineral salts, some organic nitrogen and hormones, from roots to the aerial parts of the plants.
  • The phloem translocates a variety of organic and inorganic solutes, mainly from the leaves to other parts of the plants.

 HOW DOES PLANT ABSORB WATER

  • Water is absorbed along with mineral solutes, by the root hairs, purely by diffusion.
  • Root hairs are thin-walled slender extensions of root epidermal cells that greatly increase the surface area for absorption.
  • Watermove deeper into root layers by two distinct pathways:apoplast pathway and symplast pathway
  • Inside a plant, theapoplast is the space outside the plasma membrane within which material can diffuse freely
  • It is interrupted by the Casparian strip in roots
  • The apoplastic movement of water occurs

exclusively through the intercellular spaces and the walls of the cells.

  • Movement through the apoplast does not involve crossing the cell membrane
  • This movement is dependent on the gradient.
  • The apoplast does not provide any barrier to water movement and water movement is through mass flow.
  • Mass flow of water occurs due to the adhesive and cohesive properties of water
  • Water flow in the roots occurs via the apoplast
  • Symplast is a continuous network of interconnected plant cell protoplasts.
  • Neighbouring cells are connected through cytoplasmic strands that extend through
  • The water travels through the cells – their cytoplasm
  • Intercellular movement is through the plasmodesmata.
  • Water has to enter the cells through the cell membrane
  • Hence the movement is relatively slower.
  • Movement is again down a potential gradient
  • Symplastic movement may be aided by cytoplasmic streaming.
  • Water molecules are unable to penetrate the suberised casparian strip layer
  • So they are directed to wall regions that are not suberised, into the cells proper through the membranes.
  • The water then moves through the symplast and again crosses a membrane to reach the cells of the xylem.
  • The movement of water through the root layers is symplastic in the endodermis.
  • This is the only way water and other solutes can enter the vascular cylinder.
  • Some plants have additional structures associated with them that help in water (and mineral) absorption.
  • A mycorrhiza is a symbiotic association of a fungus with a root system.
  • The fungal filaments form a network around the young root or they penetrate the root cells.
  • The hyphae have a very large surface area that absorb mineral ions and water from the soil.
  • The fungus provides minerals and water to the roots, in turn the roots provide sugars and N-containing compounds to the mycorrhizae.
  • Pinus seeds cannot germinate and establish without the presence of mycorrhizae. It has an obligate association with mycorrhizae.

Water movement up a plant during Transport in Plants

  • Various ions from the soil are actively transported into the vascular tissues of the roots
  • Water follows its potential gradient and increases the pressure inside the xylem.
  • This positive pressure is called root pressure
  • Responsible for pushing up water to small heights in the stem.
  • Effects of root pressure is even observable at night and early morning when evaporation is low
  • Excess water collects in the form of droplets around special openings of veins near the tip of grass blades, and leaves of many herbaceous parts.
  • This water loss in its liquid phase is known as guttation.
  • Root pressure do not play a major role in water movement up tall trees.
  • Due to enormous tensions created by transpiration chains of water molecules in the xylem often break
  • Root pressure re establishes this continous chain of water molecules in xylem.
  • Most plants meet their need by transpiratory pull instead of root pressure.

 

Transpiration pull after transport in plants

  • The flow of water upward through the xylem in plants can be achieved upto 15 metres per hour
  • Water is mainly ‘pulled’ through the plant the driving force for this process is transpiration from the leaves.
  • Referred to as the cohesion-tension-transpiration pull model of water transport.
  • Less than 1 per cent of the water reaching the leaves is used in photosynthesis and plant growth.
  • Most of it is lost through the stomata in the leaves.
  • This water loss is known as transpiration.

TRANSPIRATION

  • Evaporative loss of water by plants. It occurs mainly through the stomata in the leaves
  • Exchange of oxygen and carbon dioxide in the leaf also occurs through pores called stomata
  • Transpiration was first measured byStephen Hales (1677–1761), an English botanist and physiologist.
  • Stomata are open in the day time and close during the night.
  • Stomata consist of two guard cells that form a small pore on the surfaces ofleaves.
  • The immediate cause of the opening or closing of the stomata is a change in the turgidity of the guard cells.
  • The inner wall of each guard cell, towards the pore or stomatal aperture, is thick and elastic.
  • The swelling of guard cells(turgor) due to absorption of water causesopening of stomatal pores while shrinking of guard (flacid)cells closes the pores.
  • When turgidity increases within the two guard cells the thin outer walls bulge out and force the inner walls into a crescent shape.
  • Orientation of the microfibrils in the cell walls of the guard cells is responsible for the opening of the stoma
  • Microfibrils are oriented radially rather than longitudinally making it easier for the stoma to open.
  • In dorsiventral (often dicotyledonous) leaf the lower surface has a greater number of stomata
  • In an isobilateral (often monocotyledonous) leaf they are about equal on both surfaces
  • Transpiration is affected by several external factors: temperature, light, humidity, wind speed.
  • Plant factors that affect transpiration include number and distribution of stomata, number of stomata open, per cent, water status of the plant, canopy structure (organization or spatial arrangement(three-dimensional geometry) of a plantcanopy)
  • A number of otheradaptations  help to reduce water loss from transpiration.
  • Plants that live in areas with lowhumidity commonly have leaves with less surface area so that evaporation is limited
  • Plants in humid areas, especially those in low light conditions may have large leaves because the need for adequate sunlight is heightened and the risk of water loss is low.
  • Some plants have evolvedalternative photosynthetic pathways,  to minimize transpiration losses.
  • These plants open their stomates at night to take in carbon dioxide
  • Close them during the day when conditions are commonly hot and dry.
  • The transpiration driven ascent of xylem sap depends mainly on cohesion, adhesion, surface tension
  • Cohesion – mutual attraction between water molecules.
  • Adhesion – attraction of water molecules to polar surfaces
  • Surface Tension – water molecules are attracted to each other in the liquid phase more than to water in the gas phase.
  • These properties give water high tensile strength and high capillarity.
  • High tensile strength is an ability to resist a pulling force.
  • High capillarity is the ability to rise in thin tubes.the small diameter of the tracheary elements i.e. the tracheids and vessel elements are responsible for the capillarity.
  • Water evaporates through the stomata. it results in pulling of water, molecule by molecule, into the leaf from the xylem.
  • This creates a ‘pull’

 

 

UPTAKE OF MINERAL IONS

  • All minerals cannot be passively absorbed by the roots.
  • Minerals are present in the soil as charged particles (ions)
  • Minerals cannot move across cell membranes
  • The concentration of minerals in the soil is usually lower than the concentration of minerals in the root
  • Most minerals must enter the root by active absorption into the cytoplasm of epidermal cells.
  • This needs energy in the form of ATP.
  • The active uptake of ions is partly responsible for the water potential gradient in roots
  • This results in the uptake of water by osmosis.
  • Specific proteins in the membranes of root hair cells actively pump ions from the soil into the cytoplasms of the epidermal cells.
  • Transport proteins of endodermal cells are control points, where a plant adjusts the quantity and types of solutes that reach the xylem.

 

TRANSLOCATION OF MINERAL IONS

  • The mineral ions after reaching xylem transport up the stem to all parts of the plant is through the transpiration stream.
  • Mineral elements enter the growing regions of the plant, such as the apical and lateral meristems, young leaves, developing flowers, fruits and seeds, and the storage organs.
  • Mineral ions get unloaded at the fine vein endings through diffusion and active uptake by these cells.
  • Mineral ions are frequently remobilised, particularly from older, senescing parts.
  • Older dying leaves export their mineral content to younger leaves.
  • Phosphorus, sulphur, nitrogen and potassium are more mobilised mineral ions.
  • Calcium are not remobilised.
  • Nitrogen is carried in the organic form as amino acids and related compounds.
  • Small amounts of P and S are carried as organic compounds.
  • Small amount of exchange of materials does take place between xylem and phloem.

 


PHLOEM TRANSPORT

  • Food is transported by the vascular tissue phloem

from a source to a sink

  • Source is that part of the plant which synthesises the food
  • Sink is the part that needs or stores the food. the source-sink relationship is variable
  • The direction of movement in the phloem can be upwards or downwards, i.e., bi-directional.
  • Food in phloem sap can be transported in any required direction
  • Sugars other than sucrose, hormones and amino acids are also transported or translocated through phloem.

 

 

THE PRESSURE FLOW OR MASS FLOW HYPOTHESIS

  • The accepted mechanism used for the translocation of sugars from source to sink is called the pressure flow hypothesis.
  • First glucose is prepared at the source by photosynthesis.
  • Then it is converted to sucrose (a dissacharide).

sucrose is moved into the companion cells and then into the living phloem sieve tube cells.

  • This takes place by active transport.
  • This produces a hypertonic condition in the phloem.
  • Due to this water in the adjacent xylem moves into the phloem by osmosis.
  • The phloem sap will move to areas of lower pressure due to osmotic pressure.
  • At the sink osmotic pressure is to be reduced.
  • Active transport is necessary to move the sucrose out of the phloem sap and into the cells which will use the sugar converting it into energy, starch, or cellulose.
  • Then the osmotic pressure decreases and water moves out of the phloem.
  • As hydrostatic pressure in the phloem sieve tube increases, pressure flow begins, and the sap moves through the phloem.
  • At the sink incoming sugars are actively transported out of the phloem and removed as complex carbohydrates.
  • Loss of solute produces a high water potential in the phloem.
  • Then water passes out.