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
- 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
- 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.
- 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
- 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 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.
- 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.
- 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.
- 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.