Transport in Plants

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

1

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

2

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.

3

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.

4

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.

5

6

                COMPARISON BETWEEN TRANSPORT PROCESSES

7

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.

20

 

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

21

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.

1 2

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.

 

4

 

 

3

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.

11 1

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.

12

1 3

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’

 

14

16

22 2 e1590515094177

 

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.

 

17 e1590515180626

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.

 

Transport in Plants

 

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.

Transport in Plants