Transport In Plants

Over small distances, substances move by diffusion and by cytoplasmic streaming supplemented by active transport. Transport over longer distances proceeds through the vascular system and is called translocation. In rooted plants, transport in xylem is essentially unidirectional, from roots to the stems. Organic and mineral nutrients however, undergo multidirectional transport. Hormones for plant growth regulators and other chemical stimuli are also transported, though in very small amounts, sometimes in a strictly polarized or unidirectional manner from where they are synthesized to other parts.

Movement by diffusion is passive, and maybe from one part of the cell to the other or from cell to cell, or over short distances, say, from the intercellular spaces of the leaf to the outside. Molecules move in a random fashion, net result being substances moving from regions of higher concentration to regions of lower concentration. No energy expenditure takes place. It is a slow process and is not dependent on a living system. It is obvious in gases and liquids, diffusion in solids rather than of solids is more likely. The only means for gaseous movement within the plant body is diffusion. Diffusion rates are affected by the gradient of concentration, the permeability of the membrane separating them, temperature and pressure.

It continues till equilibrium is reached. Direction of diffusion of one substance is independent of the movement of another substance.

The diffusing particle create a certain pressure called as diffusion pressure (DP) which is directly proportional to the number or concentration of diffusion particles. The molecules move from higher diffusion pressure to lower diffusion pressure.

Substances soluble in lipids diffuse through the membrane faster. Substances that have a hydrophilic moiety, find it difficult to pass through the membrane; their movement has to be facilitated. Membrane proteins provide sites at which such molecules cross the membrane. They do not set up a concentration gradient; our concentration gradient must already be present for molecules to defuse even if facilitated by the proteins. This process is called facilitated diffusion.

Special proteins help move substances across membranes without expenditure of ATP energy. Facilitated diffusion cannot cause net transport of molecules from a low to high concentration as this would require input of energy. Transport rate reaches a maximum when all of the protein transporters are being used (saturation). Facilitated diffusion is very specific: it allows cell to select substances for uptake. It is sensitive to inhibitors which react with protein side chains. Proteins form channels in the membrane for molecules to pass through it. Some channels are always open whereas others can be controlled. The porins are proteins that forms huge pores in the outer membrane of the plastids, mitochondria and some bacteria allowing molecules up to the size of small proteins to pass through. Extracellular molecule bound to the transport proteins; the transport proteins then rotates and releases the molecule inside the cell, e.g., water molecules- made up of 8 different types of aquaporins.

In the symport, both molecules across the membrane in the same direction; in an antiport, they move in opposite directions. When a molecule moves across the membrane independent of other molecules, the process is called uniport.

Active transport uses energy to pump molecules against the concentration gradient. Active transport 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 high concentration (uphill transport). Transport rate reaches a maximum when all the protein transporters are being used or are saturated. It is very specific and sensitive to inhibitors that react with protein side chains.

Water provides the medium in which more substances are dissolved. The protoplasm of the cells is nothing but water in which different molecules are dissolved and suspended.

Terrestrial plants take up huge amount of water daily but most of it is lost to the air through evaporation from the leaves, i.e., transpiration. a mature corn plant absorbs almost 3 litres of water in a day, while the mustard plant absorbs water equal to its own weight in about 5 hours. Water is often the limiting factor for plant growth and productivity.

Water potential is a concept fundamental to understanding water movement. Free energy of water is referred to as water potential. It is also defined as chemical potential of water. It is directly proportional to concentration of water and atmospheric pressure. Solute potential and pressure potential are the two main components that determine water potential. Water molecules possess kinetic energy. In liquid and gaseous form, they are in random motion that is both rapid and constant. The greater the concentration of water in a system, the greater is its kinetic energy or water potential. Pure water will have the greatest water potential. If two systems containing water are in contact, random movement of water molecules will result in that movement of water molecules from the system with higher energy to the one with lower energy. Thus water will move from the system containing water at higher water potential to the one having lower water potential. This process of movement of substances down a gradient of free energy is called diffusion. Water potential is denoted by the Greek symbol Psi and is expressed by pressure units such as Pascals. By convention, the water potential of pure water at standard temperature, which is not under any pressure, is taken to be zero.

If some solute is dissolved in pure water, the solution has fewer free water and the concentration of water decreases, reducing its water potential. Hence, all solutions have a lower water potential than pure water; the magnitude of the lowering due to dissolution of a solute is called solute potential. It is always negative. The more the solute molecules, the lower (more negative) is the solute potential. For a solution at atmospheric pressure, water potential = solute potential. If a pressure greater than atmospheric pressure is applied to pure water or a solution, its water potential increases.

Pressure can build up in a plant system when water enters a plant cell due to diffusion causing a pressure built up against the cell wall, it makes the cell turgid; this increases the pressure potential. Pressure potential is usually positive, do 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. Relationship between them is:-

Water potential = solute potential + pressure potential.

Cell wall is freely permeable to water and substances in solution, hence is not a barrier to the movement. Cells usually contain a large central vacuole, whose contents, the vacuole sap, contribute to the solute potential of the cell. The cell membrane and the membrane of the vacuole, the tonoplast together are important determinants of movement of molecules in or out of the cell. Osmosis is the term used to refer specifically to the diffusion of water across a differentially semipermeable membrane. It occurs spontaneously in response to a driving force. The net direction and rate of osmosis depends on both the pressure gradient and concentration gradient. Water will move from its region of high chemical potential to its region of lower chemical potential until equilibrium is reached. At equilibrium, the two chambers should have the same water potential.

Pressure required to prevent water from diffusing is in fact, the osmotic pressure and this is the function of the solute concentration; more the solute concentration, greater will be the pressure required to prevent water from diffusing in. 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.

Factors affecting osmotic pressure:-

1) concentration of solute particles

2) ionization of solute molecules

3) temperature

4) hydration of the solute particles.

Osmotic potential is the lowering of free energy of water in a system due to the presence of solute particles. Osmotic pressure is the hydrostatic pressure developed in a solution when it is separated from pure solvent by semipermeable membrane in a rigid vessel. Osmotic potential occurs in a confined system or an open system whereas osmotic pressure develops only in a confined system. The value of osmotic potential is negative whereas that of osmotic pressure is positive.

1) limiting plasmolysis- gradual loss of water decreases turgor pressure or pressure potential to zero. Protoplast is in contact with cell wall but does not press it.

2) incipient plasmolysis- turgor pressure decreases i.e., becomes negative, contracting protoplast withdraws from cell wall. Contraction initially takes place from the corner.

3) evident plasmolysis- turgor pressure becomes more negative. Protoplast becomes nearly rounded. Cell cannot remain alive for long in this stage.

When water flows into the cell and out of the cell and are in equilibrium, the cell are said to be flaccid. Plasmolysis is usually reversible. When the cells are placed in a hypotonic solution water diffuses into the cell causing the cytoplasm to build up a pressure against the wall, that is called turgor pressure. The pressure exerted by the protoplast due to entry of water against the rigid walls is called pressure potential. Because of the rigidity of the cell wall, the cell does not rupture. This turgor pressure is ultimately responsible for enlargement and extension growth of cells.

Imbibition is a special type of diffusion when water is absorbed by solids-colloids causing them to enormously increase in volume. Absorption of water by seeds and dry wood are the example of imbibition. The pressure that is produced by the swelling of wood had been used by Prehistoric man to split rocks and boulders. 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. Water potential gradient between the absorbent and the liquid imbibed is essential for imbibition. In addition, for any substance to imbibe any liquid, affinity between the adsorbent and the liquid is also a prerequisite.

Imbibition plays a great role in the germination of seed, rupturing of seed coat and emergence of seedlings is due to higher imbibition pressure developed by seed kernel. Among plant imbibants, phycocolloids are the best imbibants. Imbibition has three important features:- volume change, production of heat and development of pressure. Magnitude of imbibition pressure is:-

Phycocolloids>pectin>protein>starch>cellulose.

Water and Minerals, and food are generally moved by a mask or bulk flow system. Mass flow is the movement of substances in bulk or en masse from one point to another as a result of pressure differences between the two points. It is a characteristic of mass flow that substances, whether in solution or in suspension, are swept along at the same pace, as in a flowing river. 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 tissue of plants is called translocation.

Xylem and phloem. 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.

Meyer called it diffusion pressure deficit and Renner called it suction pressure. Pure water has maximum diffusion pressure. There is always a difference between the diffusion pressure of solvent and its solution. The difference between the diffusion pressure of a solution and a pure solvent, when both are subjected to the same atmospheric pressure is called diffusion pressure deficit. Its value is always positive for a cell. DPD = osmotic pressure - turgor pressure

When a cell is flaccid, DPD = OP. Entry of water into cell causes turgor pressure. When cell is target country of water would stop i.e., when increasing turgor pressure becomes equal to decreasing osmotic pressure OP - TP = 0. Thus, DPD = 0.

Roots absorb most of the water. The responsibility of absorption of water and Minerals is more specifically the function of the root hairs that are present in millions at the tips of roots. Root hairs are thin walled cylinder extension of root epidermal cells that greatly increase the surface area for absorption. Water is absorbed along with general solids, by the root hairs, purely by diffusion. Once water is absorbed by the root hairs, it can move deeper into root layers by two distinct Pathways it can move deeper into root layers by two distinct Pathways:-

1) apoplast pathway-consists of non-living parts.

2) symplast pathway- consists of living parts.

Most of the water flow in the roots occurs via the apoplast since the cortical cells are loosely packed and hands over no resistance to water movement. The inner boundary of the cortex, 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 all regions that are not suberised, into the cells proper through the membranes. The water then moves through the simplest and again crosses a membrane to reach the cells of the xylem. The movement of water through the root layers is ultimately symplastic in the endodermis.

Movement of water is apoplastic. Water is absorbed by the root hair due to diffusion pressure deficit gradient produced by transpiration that develops in the leaf. Root simply acts as a passage or channel for water movement. Water absorption from soil and its involved movement is osmotic pressure dependent. Passage of water from living cells to xylem channel requires accumulation of solute in xylem which is an energy dependent process. Hence, pumping of water in xylem channel is active. This creates a positive pressure in xylem called root pressure.

1) Passive absorption (through roots)

Force for passive absorption lies in shoot. Transpiration pull plays a great role. Negative pressure developed in xylem. Rate of absorption is high. Responsible for 96% of total absorption. Occurs in rapidly transpiring plants. It is apoplastic.

2) Active absorption (by roots)

Force for active absorption develops in root. Osmotic pressure and energy plays the major role. Positive pressure developed in phloem. Rate of absorption is low. Only 4% of water uptake. Occurs in slowly transpiring ones. It is symplastic.

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 from a much larger volume of soil that perhaps a root cannot do. The fungus provides minerals and water to the roots, in turn the roots provide sugars and nitrogen containing compounds to the mycorrhiza. Pinus seeds can not germinate and establish without the presence of mycorrhiza.

As various ions from the soil are actively transported into the vascular tissues of the roots, water follows and increases the pressure inside the xylem. This positive pressure is called root pressure, and can be responsible for pushing up water to small heights in the stem. Effects of root pressure is also observable at night and early morning when evaporation is low and excess water collects in the form of droplets around special openings of veins near the tip of glass blades, and leaves of many herbaceous parts. Such water loss in its liquid phase is known as guttation. The greatest contribution of root pressure maybe to re-establish the continuous chains of water molecules in the xylem which often break under the enormous tensions created by transpiration. Root pressure does not account for the majority of water transport; most plants meet their needs by transpiratory pull.

Most researchers agreed that water is mainly pulled through the plant, that the driving force for this process is transpiration from the leaves. It was given by Dixon and Jolly.

Transpiration is the 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 the pores called stomata. Transpiration is affected by several external factors: temperature, light, humidity, wind speed. Plant factors that affect transpiration include number and distribution of stomata, percent of open stomata, water status of the plant, canopy structure etc. The transpiration driven ascent of xylem sap depends mainly on the following physical properties of water:

1) Cohesion- mutual attraction between water molecules

2) Adhesion- attraction of water molecules to polar surfaces such as the surface of tracheary elements

3) Surface tension- water molecules are attracted to each other in the liquid phase more than to water in the gas phase.

Normally stomata are open in the daytime and closed during the night. The immediate cause of the opening or closing of the stomata is the change in the turgidity of guard cells. The inner wall of each guard cell, towards the pore or stomatal aperture, is thick and elastic. Usually the lower surface of dorsiventral (often dicotyledonous) leaf has a greater number of stomata while in an isobilateral (often monocotyledonous) leaf, they are about equal on both surfaces.

1) Stomatal transpiration- occurs through stomata, responsible for 50 to 97% of total transpiration.

2) Cuticular transpiration- water vapours lost directly from outer walls of epidermal cells through the cuticle; about 3 to 10% of total transpiration.

3) Lenticular transpiration- lenticles are aerating pores in the cork of woody stems, twigs, fruits etc. Water vapours are lost through these openings. About 0.1% of total water loss.

4) Bark transpiration- it occurs through the bark of woody stem; responsible for 1% of total transpiration.

Guard cells are bordered by one or more modified epidermal cells called subsidiary cells or accessory cells. In monocots, guard cells are ellipsoidal or dumbbell shaped called graminaceous stomata are poaceous stomata. Stomata functions as turgor operated valves. Potometer is the device for measuring the role of transpiration.

Tensile strength is an ability to resist a pulling force and capillarity is the ability to rise in thin tubes. Cohesion, adhesion and surface tension give water high tensile strength and high capillarity. In plants, capillarity is aided by the small diameter of the tracheary elements- the tracheids and the vessel elements.

It creates transpiration pull for absorption and transport of plants. It supplies water for photosynthesis. Transports minerals from the soil to all parts of the plant. It cools leaf surfaces, sometimes 10 to 15 degrees, by evaporative cooling. It maintains the shape and structure of the plants by keeping cells turgid.

An actively photosynthesizing plant has an insatiable need for water. Photosynthesis is limited by available water which can be swiftly depleted by transpiration. The humidity of rainforests is largely due to this vast cycling of water from root to leaf to atmosphere and back to the soil. The evolution of the C4 photosynthetic system is probably one of the Strategies for maximizing the availability of CO2 while minimising water loss. C4 plants are twice as efficient as C3 plants in terms of fixing carbon. However, a C4 plant loses only half as much water as the C3 plant for the same amount of CO2 fixed.

Plants obtain carbon and oxygen from carbon dioxide. It obtain hydrogen from water and nutrients from minerals in soil.

A) External factors

1) light- blue light (more) and red light are effective, constituting its action spectrum.

2) relative humidity is inversely proportional to rate of transpiration.

3) temperature is directly proportional to rate of transpiration.

4) wind blow increases rate of transpiration.

5) available soil water increases transpiration.

B) Internal factors-

1) root shoot ratio is directly proportional to rate of transpiration. Short plants have high root shoot ratio.

2) structure of leaf- thick cuticle, waxy coating, thick-walled hypodermis, sunken stomata reduce the rate of transpiration.

3) number and distribution of stomata, number of open stomata, plant water status, canopy structure also affect the transpiration.

Unlike water, all minerals cannot be possibly absorbed by the roots. Two factors account for this: (I) minerals are present in the soil as charged particles ions which cannot move across cell membranes and (ii) the concentration of minerals in the soil is usually lower than the concentration of minerals in the root. Therefore, 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, and therefore for the uptake of water by osmosis. Some ions also move into the epidermal cells passively. Ions are absorbed from the soil by both passive and active transport. Specific proteins in the membranes of root hair cells actively pump ions from the soil into the cytoplasm of the epidermal cells. Transport proteins of endodermal cells are control points, where a plant adjust the quantity and types of solutes that reach the xylem. Note that the root endodermis, because of the layer of suberin, has the ability to actively transport ions in one direction only.

Further transport up the stem to all parts of the plant is through the transpiration stream. The chief sinks for the mineral elements are the growing regions of the plant, such as the apical and Lateral Meristem, young leaves, developing flowers, fruits and seeds, and the storage organs. Unloading of mineral ions occurs at the fine vein endings through diffusion and active uptake by these cells. Mineral ions are frequently immobilized, particularly from older, senescing parts. Older dying leaves export much of their mineral content to younger leaves. Similarly, before leaf fall in deciduous plants, mineral are removed to other parts. 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, much of it is carried in the organic form as amino acids and related compounds. Small amount of phosphorus and sulphur are carried as organic compounds. In addition, small amount of exchange of material does take place between xylem and phloem. Mineral elements are translocated through xylem in both inorganic and organic form.

Food, primarily sucrose, is transported by the vascular tissue of phloem from a source to a sink. Source is understood to be that part of plant which synthesizes the food i.e., the leaf, and sink, the part that needs or stores the food. Source and sink may be reversed depending on the season, or the plant's needs. Sugar stored in roots maybe mobilized to become a source of food in the Early Spring when the buds of trees, act as sink; they need energy for growth and development of the photosynthetic Apparatus. The movement in the phloem is bidirectional which contrast with that of the xylem where the movement is always unidirectional. Unlike one way flow of water in transpiration, food in phloem SAP can be transported in any required direction so long as there is a source of sugar and a sink able to use, Store or remove the sugar. Phloem SAP is mainly water and sucrose, but other Sugars, hormones and amino acids are also transported or translocated through phloem.