11. TRANSPORT IN PLANTS

Plants do not have interstitial fluid and circulatory system. But they need to move various substances (water, mineral nutrients, organic nutrients, plant growth regulators etc) over very long distances through Xylem & Phloem (Translocation).

Direction of transport

o Unidirectional transport: E.g. 
Transport of water and minerals in xylem (from roots to the stems, leaves etc).

o Multidirectional transport: E.g.

 

►Transport of photosynthates (organic compounds).

►Transport of mineral nutrients.

o Sometimes, plant hormones and other chemical stimuli are transported in a strictly polarised or unidirectional manner from where they are synthesized to other parts.

MEANS OF TRANSPORT

1.  Diffusion

-    It is the movement of gases, liquids and solutes from higher concentrated region to lower concentrated region without the energy expenditure.

-    It may be from one part of the cell to the other or from cell to cell, or over short distances.

-    It is a slow process.

-    It is not dependent on a ‘living system’.

-    It is the only means for gaseous movement within the plant body.

-    Factors affecting diffusion rates: o Concentration gradient.

o Permeability of the membrane separating them. o Temperature and pressure.

o Size (density) of the substances. Smaller substances diffuse faster.

o Solubility  in  lipids  of  the  membrane.  Substances

soluble in lipids diffuse through the membrane faster.

2.  Facilitated Diffusion

-    It is the diffusion of hydrophilic substances with the help of membrane protein channels and without expenditure of ATP energy.

-    It also needs a concentration gradient.

-    It is very specific. It allows cell to select substances for uptake. It is sensitive to inhibitors that react with protein side chains.

-    Transport rate reaches a maximum when all of the protein transporters are being used (saturation).

-    Some protein channels are always open; others can be controlled. Some are large, allowing a variety of molecules to cross.

-    Porins are proteins that form huge pores in the outer membranes of plastids, mitochondria and some bacteria. They allow passage of molecules up to the size of small proteins.

-    An extracellular molecule binds to the transport protein. Then it rotates and releases the molecule inside the cell. E.g. water channels – made up of 8 types of aquaporins.

Passive uniports, symports and antiports

 

·   Uniport: A molecule alone moves across a membrane (through transport or carrier protein) independent of other molecules.

·   Symport: Two molecules together cross the membrane in the same direction.

·   Antiport: Two molecules move in opposite directions.

3.  Active Transport

-    It is the transport of molecules against a concentration gradient (from lower concentrated region to higher concentrated region) with the expenditure energy.

-    It is carried out by membrane-proteins.

-    Pumps are proteins that use energy to transport substances across cell membrane (‘uphill’ transport).

-    Transport rate reaches a maximum when all the protein transporters are being used or are saturated.

-    The carrier protein is very specific. These are sensitive to inhibitors that react with protein side chains.

Comparison of Different Transport Processes

Property	Simple	Facilitated	Active
	diffusion	transport	transport
Requires special	No	Yes	Yes
membrane proteins
Highly selective	No	Yes	Yes
Transport saturates	No	Yes	Yes
Uphill transport	No	No	Yes
Requires ATP energy	No	No	Yes

PLANT-WATER RELATIONS

-    Water is a universal solvent essential for all physiological activities of living organisms.

-    Protoplasm is mainly water in which different molecules are dissolved and suspended.

-    Soft plant parts mostly contain water. E.g. A watermelon has over 92% water. Most herbaceous plants have only 10 - 15% of dry matter.

-    Dry seeds and woody parts also contain little water.

-    A mature corn plant absorbs 3 litres of water daily.

-    A mustard plant absorbs water equal to its own weight in about 5 hours.

Water Potential (Ψw)

-    It is the potential energy of water.

-    It is expressed in pressure units such as Pascals (Pa).

-    Water molecules have kinetic energy. In liquid & gaseous form they show random, rapid and constant motion.

-    As the concentration of water in a system increases, its kinetic energy (‘water potential’) also increases. Hence, pure water will have the greatest water potential.

-    Water molecules move from higher energy system (higher water potential) to lower energy system (lower water potential). Such movement of substances down a gradient of free energy is called diffusion.

-    Water potential of pure water at standard temperatures, which is not under any pressure, is zero.

-    If a solute is dissolved in pure water, the concentration of water decreases, reducing its water potential. Hence, all solutions have a lower water potential than pure water. Magnitude of this lowering due to dissolution of a solute is called solute potential (Ψs). Ψs is always negative.

-    The more the solute molecules, the lower (more negative) is the Ψs.

-    For a solution at atmospheric pressure, water potential (Ψw) = solute potential (Ψs).

-    If a pressure greater than atmospheric pressure is applied to pure water or a solution, its water potential increases. It is equivalent to pumping water from one place to another.

-    When water enters a plant cell due to diffusion, it causes a pressure against the cell wall. It makes the cell turgid. This increases the pressure potential (Ψp).

-    Pressure potential is usually positive, though 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 Solute potential

&  pressure potential. The relationship between them is:

Ψw = Ψs + Ψp

Osmosis

-    It is the spontaneous diffusion of water across a differentially- or semi-permeable membrane.

-    In plant cells, the cell membrane and the tonoplast (membrane of the vacuole) are important determinants of movement of molecules in or out of the cell. But the cell

wall is not a barrier to movement as it is freely permeable to water and substances in solution.

-    Vacuolar sap in large central vacuole contributes to the solute potential of the cell.

-    The net direction and rate of osmosis depends on the pressure gradient and concentration gradient.

-    Water moves from its region of higher chemical potential (concentration) to its region of lower chemical potential until equilibrium is reached. At equilibrium the two chambers should have the same water potential.

Potato osmometer:

-   Take a potato tuber and make a cavity in it. Pour concentrated sugar solution in the cavity. This setup is called potato osmometer.

-   If it is placed in water, the cavity containing concentrated sugar solution collects water due to osmosis.

A demonstration of osmosis:

-   A thistle funnel is filled with sucrose solution and kept inverted in a beaker containing water.

-   The sucrose solution is separated from pure water in the beaker through a semi-permeable membrane (e.g. egg shell membrane). 

-   Water moves into the funnel, resulting in rise in the level of the solution in the funnel. It continues till the equilibrium is reached (figure a).

-    If an external pressure is applied from the upper part of the funnel, no water diffuses into the funnel through the membrane (figure b).

-    This pressure required to prevent water from diffusing is the osmotic pressure. This is the function of the solute concentration. More the solute concentration, greater will be the pressure required to prevent water diffusion.

-    Numerically osmotic pressure is equivalent to the osmotic potential, but the sign is opposite. Osmotic pressure is the positive pressure applied, while osmotic potential is negative.

Plasmolysis

-    If an external solution balances the osmotic pressure of the cytoplasm, it is called isotonic. When a cell (or tissue) is placed in isotonic solution, there is no net flow of water towards inside or outside (water flow is in equilibrium). Such cells are said to be flaccid.

-    If the external solution is more dilute (higher water potential) than the cytoplasm, it is hypotonic. Cells swell (turgid) in hypotonic solution.

-    If the external solution is more concentrated (more solutes) than the cytoplasm, it is hypertonic.

-    When a cell is placed in a hypertonic solution, water moves from the cell (area of high water potential) across the membrane to outside (area of lower water potential) and the cell shrinks. It is called Plasmolysis. Water is first lost from the cytoplasm and then from the vacuole.

-    During plasmolysis, the cell membrane and protoplast of a plant cell shrinks away from its cell wall. Such cells are said to be plasmolysed.

-    Plasmolysis is usually reversible. When the cells are placed in a hypotonic solution, water diffuses into the

cell. As a result the cytoplasm builds up a pressure against the wall. It is called turgor pressure. The pressure exerted by the protoplasts due to entry of water against the rigid walls is called pressure potential (Ψp). The cell does not rupture due to the rigidity of cell wall. Turgor pressure causes enlargement and extension growth of cells.

Imbibition

-    It is a type of diffusion in which water is absorbed by solids (colloids) causing them to increase in volume. E.g. absorption of water by seeds and dry wood.

-    The pressure due to the swelling of wood can split rocks.

-    Seedlings are emerged out of the soil due to the imbibition pressure.

-    Imbibition requires

o Difference in concentration gradient.

o Water potential gradient between the absorbent and the liquid imbibed.

o Affinity between the adsorbent and the liquid.

LONG DISTANCE TRANSPORT OF WATER

-    Diffusion is a slow process. It accounts for only short distance movement. E.g. movement of molecules across a typical plant cell (about 50 μm) takes about 2.5 s.

-    Long distance transport systems are necessary to move substances faster across long distances.

-    Movement of substances in bulk (en masse) from one point to another due to pressure differences between two points is called Mass (bulk) flow. Water, minerals and food are generally moved by a mass flow system.

-    In mass flow, substances (in solution or in suspension) are swept along at the same pace as in a flowing river. But in diffusion different substances move independently depending on their concentration gradients.

-    Bulk flow is achieved either through a +ve hydrostatic pressure gradient (e.g. a garden hose) or a -ve hydrostatic pressure gradient (e.g. suction through a straw).

-    Bulk movement of substances in long distance through the conducting tissues (xylem & phloem) is called translocation.

Absorption of Water by Plants

-    Absorption of water and minerals occurs by diffusion through millions of root hairs present at the root tips.

-    Root hairs increase the surface area for absorption.

-    The absorbed water is moved deeper into root layers by 2 pathways: Apoplast pathway and Symplast pathway.

Apoplast pathway:

-    It is the system of adjacent cell walls that is continuous throughout the plant, except at the casparian strips of the endodermis in the roots.

-    It 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.

-     As water evaporates into the intercellular spaces or the atmosphere, tension develops in the continuous stream of water in the apoplast. Hence mass flow of water occurs

due to the adhesive and cohesive properties of water.

Symplastic pathway:

-     It is the system of interconnected protoplasts.

-     Neighbouring cells are connected through cytoplasmic strands that extend through plasmodesmata.

-     During symplastic movement, 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. E.g. cytoplasmic streaming in cells of the Hydrilla leaf. The movement of chloroplast due to streaming is easily visible.

-    Most of the water flow in the roots occurs via the apoplast since the cortical cells are loosely packed, and hence offer no resistance to water movement. However the endodermis is impervious to water because of a band of suberised matrix called the casparian strip.

-    Water molecules are unable to penetrate the 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 water movement through the root layers is ultimately symplastic in the endodermis. This is the only way water and solutes can enter the vascular cylinder.

-    Once inside the xylem, water is again free to move between cells as well as through them. In young roots, water enters directly into the xylem vessels and tracheids. These are non-living conduits and so are parts of the apoplast. 

-    Some plants have additional structures for water and mineral absorption. E.g. 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 absorb mineral ions and water from soil. The roots provide sugars and N-containing compounds to mycorrhizae. Some plants have an obligate association with the mycorrhizae. E.g. Pinus

seeds cannot germinate and establish without the presence of mycorrhizae.

Water Movement up a Plant

Water moves up a stem against gravity. So it needs energy.

Root Pressure

-    As 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.

-    It helps to push up water to small heights in the stem.

Experiment to prove existence of root pressure:

-    Choose a small soft-stemmed plant. On an early morning having plenty of atmospheric moisture, cut the stem horizontally near the base. Drops of solution ooze out of the cut stem. This is due to the positive root pressure.

-    At night and early morning evaporation is low. So 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. Such water loss in liquid phase is called guttation.

-    Root pressure can only provide a modest push in the water transport. They have no a major role in water movement up tall trees. Root pressure re-establishes the continuous chains of water molecules in the xylem which often break under the tensions created by transpiration.

-    In most plants, majority of water transport occurs by transpiration pull.

Transpiration pull

-    In plants, the water flow upward through the xylem achieves fairly high rates (up to 15 m /hr).

-    Water is mainly ‘pulled’ through the plant, and that the driving force for this process is transpiration. This is known as cohesion-tension-transpiration pull model of water transport.

TRANSPIRATION 

-    It is the evaporative loss of water by plants through the stomata in the leaves.

-    Less than 1% of the water reaching the leaves is used in photosynthesis and plant growth. The remaining is lost by transpiration.

-    Transpiration can be studied using cobalt chloride paper, which turns colour on absorbing water.

-    During transpiration, exchange of O₂ and CO₂ in the leaf also occurs.

-    Normally, stomata are open in the day time and close during the night.

-    Opening or closing of the stomata is due to change in the turgidity of the guard cells.

-    The inner wall of guard cell lining stomatal aperture is thick and elastic and the outer wall is thin.

-    When turgidity of guard cells increases, the outer walls bulge out and pull the inner walls into a crescent shape.

-    Cellulose microfibrils in the guard cells are oriented radially rather than longitudinally making it easier for the stoma to open.

-    The guard cells lose turgidity due to water loss (or water stress) and the inner walls regain their original shape. As a result the stoma closes.

-    Usually the lower surface of a dorsiventral (dicotyledonous) leaf has a greater number of stomata while in an isobilateral (monocotyledonous) leaf they are about equal on both surfaces.

Factors affecting transpiration:

-    External factors: Temperature, light, humidity, wind speed etc.

-    Plant factors:

o Number and distribution of stomata. o Number of stomata open.

o Water status of the plant.

o Canopy structure etc.

-    The transpiration driven ascent of xylem sap depends mainly on the following physical properties of water:

• Cohesion: mutual attraction between water molecules.

• Adhesion: attraction of water molecules to polar surfaces (such as the surface of tracheary elements).

• 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 (ability to resist a pulling force) and high capillarity (ability to rise in thin tubes). In plants capillarity is aided by the small diameter of the tracheary elements – the tracheids and vessel elements.

-    The xylem vessels from the root to the leaf vein supply the water for photosynthesis. As water evaporates through the stomata, since the thin film of water over the cells is continuous, water pulls into the leaf from the xylem. The concentration of water vapour in the atmosphere is lower as compared to the substomatal cavity and intercellular spaces. This also helps water to diffuse into the surrounding air. This creates a ‘pull’.

-    The forces generated by transpiration can create pressures sufficient to lift a xylem sized column of water over 130 metres high.

Transpiration & Photosynthesis – a Compromise

-   Photosynthesis is limited by available water which can be swiftly depleted by transpiration. The humidity of rainforests is mainly due to this cycling of water from root to leaf to atmosphere and back to the soil.

-   The evolution of the C4 photosynthetic system can be considered as a strategy for maximising the availability of CO2 and minimising water loss. C4 plants are twice as efficient as C3 plants in terms of fixing carbon (making sugar). However, a C4 plant loses only half as much water

as a C3 plant for the same amount of CO2 fixed.

Uses of Transpiration:

• Creates transpiration pull for absorption and transport.

• Supplies water for photosynthesis.

• Transports minerals from soil to all parts of the plant.

• Cools leaf surfaces, sometimes 10 - 15^o, by evaporation.

• Maintains the shape and structure of the plants by keeping cells turgid.

UPTAKE AND TRANSPORT OF MINERAL NUTRIENTS

Uptake of Mineral Ions

-    Most minerals are actively absorbed by the roots because

(i)     Minerals occur in the soil as charged particles (ions) which cannot move across cell membranes.

(ii)   The concentration of minerals in the soil is usually lower than the concentration of minerals in the root.

-    The active uptake of ions is partly responsible for the water potential gradient in roots, and therefore for the uptake of water by osmosis.

-    Some ions are absorbed passively.

-    The specific membrane proteins of root hair cells actively pump ions from the soil into the epidermal cells.

-    Endodermal cell membrane also has transport proteins. They allow some solutes cross the membrane, but not others. These proteins are control points, where a plant adjusts quantity and types of solutes that reach the xylem.

-    The suberin in the root endodermis allows the active transport of ions in one direction only.

Translocation of Mineral Ions

-    The ions reached in xylem are further transported to all parts of the plant through the transpiration stream.

-    The chief sinks for the mineral elements are

o Growing regions such as apical and lateral meristems. o Young leaves.

o  Developing flowers, fruits and seeds. o Storage organs.

-    Unloading of mineral ions occurs at the fine vein endings through diffusion and active uptake by these cells.

-    Mineral ions are also frequently remobilized, particularly from older, senescing parts (e.g. older dying leaves) to younger leaves.

-    Elements most readily mobilized are phosphorus, sulphur, nitrogen and potassium. Some elements that are structural components like calcium are not remobilized.

-    Some of the nitrogen travels as inorganic ions while most of it is carried in the organic form such as amino acids and related compounds.

-    Small amounts of P and S are also carried as organic compounds. There is also exchange of materials between xylem and phloem. Hence, we cannot clearly say that xylem transports only inorganic nutrients while phloem transports only organic materials.

PHLOEM TRANSPORT: FLOW FROM SOURCE TO SINK

-    It is the long distance movement of organic substances (food, primarily sucrose) from a source (region of synthesis the food i.e., the leaf) to a sink (region of storage or utilization of food) through the phloem.

-    The source and sink may be reversed depending on the season, or the plant’s needs. E.g. In early spring, the sugar stored in roots is moved to the tree buds for growth and development of photosynthetic apparatus. Thus root becomes the source and buds the sink.

 

-    The direction of movement in the phloem can be upwards or downwards, (bi-directional). In xylem, the movement is always upwards (unidirectional). Hence, food in phloem sap can be transported in any direction.

-    Phloem sap is mainly water and sucrose, but other sugars, hormones and amino acids are also translocated.

The Pressure Flow (Mass Flow) Hypothesis

-    It is the hypothesis that explains the mechanism of translocation of sugar (phloem transport).

-    The glucose prepared at the source (by photosynthesis) is converted to sucrose (a disaccharide).

-    Sucrose is moved into the companion cells and then into the living phloem sieve tube by active transport (loading). It produces a hypertonic condition in phloem (water potential decreases). Sieve tube cells form long columns with holes in sieve plates. Cytoplasmic strands pass through the holes in the sieve plates, so forming continuous filaments.

-    Water in the adjacent xylem moves into the phloem by osmosis. As osmotic pressure/hydrostatic pressure builds up, the phloem sap moves to areas of lower osmotic pressure (sink).

-    The sucrose from the phloem sap actively moves into the cells. The cells convert the sugar into energy, starch, or cellulose (complex carbohydrates).

-    As sugars are removed, the osmotic pressure decreases (water potential increases) and water moves out of the phloem.

Identification of the tissue that transports food (girdling)

o Carefully remove a ring of bark (including phloem layer)

from a tree trunk.

o After a few weeks, the portion of the bark above the ring on the stem becomes swollen. This is due to the absence of downward movement of food.

o This shows that phloem is the tissue responsible for translocation of food; and that transport takes place in one direction, i.e., towards the roots.

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