Transport in Plants
Plants perform the process of transportation in different ways in both short distance as well as long distances.
- The long-distance transport occurs mainly through vascular tissues (i.e., xylem, and phloem) and the process is called translocation.
- Over small distances, materials move through diffusion, cytoplasmic streaming, and active transport.
- Absorption of water in plants: The water is absorbed through roots and transported to the tip of the growing stem. The process involves diffusion, osmosis, and water potential gradient. diffusion
Means of Transport in Plants
There are three important means of Transport in Plants:
- Facilitated diffusion
- Active transport
Diffusion can be defined as the movement of molecules or ions of a solute or solvent from the region of its higher concentration to the region of its lower concentration.
The molecules are diffused from a region of high partial pressure to a region of low partial pressure as a result of their inherent kinetic energy. The molecules continue to move till an equilibrium is reached. It is a slow process and is mostly observed in liquids and gases.
Diffusion is very important for transport in plants since it is the only means for gaseous movement within the plant body. Diffusion rates are affected by the gradient of concentration, the permeability of the membrane separating them, temperature, and pressure.
Facts to know about diffusion:
- The movement by diffusion is passive.
- It occurs down the concentration gradient
- The process of diffusion does not require special carrier molecules.
- The process is not sensitive to inhibitors.
- No energy expenditure takes place.
- Not selective
- In diffusion, molecules move in a random fashion.
- It is a slow process and is not dependent on a ‘living system’.
Substances that possess a hydrophilic moiety, find it difficult to pass through the membrane, the movement of the substances has to be facilitated. Membrane proteins provide sites at which such molecules cross the membrane. They do not set up a concentration gradient: a concentration gradient must already be present for molecules to diffuse even if facilitated by the proteins. This process is called facilitated diffusion. (NCERT)
- Facilitated diffusion cannot cause the net transport of molecules from a low to a high concentration – this would require the input of energy.
- The transport rate reaches a maximum when all of the protein transporters are being used.
- Facilitated diffusion can show a saturation effect.
- Facilitated diffusion is along the concentration gradient through specific sites present in the cell membranes without the cell spending any energy on the movement. Smaller particles will show a higher rate of facilitated diffusion than larger particles.
- There are certain protein transporters present in the cell membrane which allow the passage of substances.
- These transporters consist of a specific configuration suitable for particular particles.
- Protein transporters often form channels and aquaporins. Ion channels are specific for different ions, e.g., K+, Cl–, NO–3, PO3-4. Water channels or aquaporins are specialized to allow the passage of water through them.
- Porins are protein-lined pores found in the outer membranes of mitochondria.
Passive symport and Antiport:
Symport: Some carriers or transport proteins allow diffusion if only two types of molecules move together. If the movement of particles is in the same direction, it is called symport.
Antiport: If the movement of solute is in the opposite direction to each other, it is antiport.
Uniport: When a molecule moves across a membrane independently of another one, it is called a uniport.
Active Transport in Plants:
Active transport can be defined as a mode of transport that involves the expenditure of energy, to transport and pump molecules against concentration gradient. Important features of active transport in plants:
- Active transport is carried out by specific membrane proteins.
- Since active transport occurs against the concentration gradient, the carrier proteins involved are called pumps. Pumps can transport substances from a low concentration to a high concentration (uphill transport).
- The transport rate becomes maximum when all the protein transporters are being used or saturated.
Plant Water Relations
Water plays an important role in all living organisms. It is essential for all physiological activities of plants and plays a vital role in all organisms. It is a medium in which protoplasm forms a crysto-colloidal complex.
- Protoplasm: Water in cells in which different molecules are dissolved and several particles suspended.
The distribution of water within a plant varies as watermelon consists of 90%, Dry seeds-10 to 15%, and herbaceous plants consist of 85 to 90% of their fresh weight. Woody parts have less amount of water with compare to their softer parts.
Terrestrial plants require a greater amount of days as they lose lots of water through evaporation from leaves, by the process called transpiration. For example, maize absorbs 3 lit. water a day. Mustard plants absorb water equal to their own weight in about 5 hours. Due to this reason, water is very often the limiting factor for plant growth and agricultural yield.
Some Important Terms Related to Plant Water Relation in Transport in Plants
Water Potential (Ψw)
Water potential is a concept fundamental to understanding water movement. It is the decrease in chemical potential per unit molal volume of water present in a system over its pressure state at the same temperature and pressure. Solute potential and pressure potential are the two main components that determine water potential.
Water potential is the sum of three forces: Solute potential + pressure potential
Ψw = Ψs + Ψp
The greater the concentration of water in a system, the greater kinetic energy or ‘water potential’. Pure water will have the greatest water potential. The Greek symbol used for water potential is ‘psi or Ψw’, and it is expressed in pressure units such as pascals (Pa).
Solute potential (Ψs): All solutions consist of a lower water potential than pure water, the magnitude of this lowering due to a dissolution of a solute is called solute potential or Ψs. It is always negative. The more solute molecules, the lower the Ψs.
For a solution at atmospheric pressure Ψw (Water potential) = Ψs (Solute potential)
Pressure potential (Ψp): Pressure potential is a positive pressure that develops in a system due to the osmotic entry of water into it. Pressure potential is also called hydrostatic pressure or turgor pressure.
- As the pressure potential rises, it neutralizes the solute potential and therefore raises the water potential.
Osmosis is the net diffusion of water molecules from a dilute solution to a concentrated solution when the two are separated by a semipermeable membrane. If the solution is separated from its pure solvent, the molecules of solvent move from pure solvent to the solution.
- Osmosis is a term used to refer especially to the diffusion of water across a differentially or selectively permeable membrane.
- Osmosis occurs spontaneously in response to a driving force.
- The net direction and rate of osmosis depend on both the pressure gradient and concentration gradient.
- Endosmosis: The process when water enters the cell sap.
- Exosmosis: The process by which water moves outside the cell.
Permeability: A body is permeable to a substance if it allows the passage of the substance through it. Some membranes are semipermeable as they allow the solvent but not the solute to pass through them. In the case of an impermeable membrane neither can pass through. The term osmosis is used to refer specifically to the diffusion of water across a differentially or selectively permeable membrane. So, osmosis is the net diffusion of water molecules from a dilute solution to a concentrated solution when the two are separated by a semipermeable membrane.
- The net direction and rate of osmosis depend on both the pressure gradient and concentration gradient.
- Until the equilibrium is reached, the water moves from its region of higher concentration to the region of lower concentration.
- At equilibrium, the two chambers possess nearly the same water potential.
Reverse Osmosis (Hyper filtration)
A process that allows the removal of water molecules from various contaminants through a semipermeable membrane. The process requires a driving force i.e. pressure from a pump to push the fluid through the membrane as water molecules move from a concentrated solution to a dilute solution.
Hypertonic Solution: When a solution gains water or solvent from some other specified solution by osmosis across a semipermeable membrane.
- The external solution is more concentrated.
- Cells swell in hypotonic solutions and shrink in hypertonic ones.
Hypotonic Solution: If the solution loses water or solvent to other specified solutions by osmosis across a semipermeable membrane called a hypotonic solution.
- 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, called turgor pressure.
- Turgor pressure is responsible for the enlargement and extension growth of cells.
- The pressure exerted by the protoplast due to the entry of water against the rigid walls is called Pressure potential Ψp.
Isotonic Solution: If a solution neither gains nor loses water by osmosis during the separation through semipermeable from a specified solution. So, in an isotonic solution, there is no net flow of water toward the inside or outside.
- If the external solution balances the osmotic pressure of the cytoplasm it is said to be isotonic.
- Flaccid: When water flows into the cell and out of the cell in equilibrium, the cells are termed flaccid.
Importance of Osmosis in Transport in Plants
- The roots of plants absorb water through osmosis.
- Cells absorb or lose water on the basis of their osmotic relations.
- Cells maintain their turgidity through proper osmotic solutes concentration and osmotic absorption of water.
- Cells enlarge only in response to the entry of water into them.
- The organs of plants like flowers, fleshy fruits, leaves, etc maintain their shape and stretched form by osmosis.
- Due to turgidity, the young roots are able to penetrate the soil.
- Maintaining high osmotic concentration, the plants protect themselves from desiccation or excessive loss of water.
Plasmolysis is the process in which cells lose water in a hypertonic solution. Plasmolysis occurs when water moves out of the cell and the cell membrane of a plant cell shrinks away from its cell wall.
- The water is drawn out of the cell through diffusion into the extracellular fluid causing the protoplast to shrink away from the wall. The cell is said to be plasmolyzed.
- If a plasmolyzed cell is again placed back in pure water or hypotonic solution, de-plasmolysis occurs.
- plasmolysis is used for the preservation of fish, dry fruits, pickles, jams, etc in excess of salt or sugar.
- The plasmolytic method is applied to determine the osmotic pressure of a cell in the laboratory.
Imbibitin is a special type of diffusion when water is absorbed by solid particles causing them to increase in volume. for example, the absorption of water by seeds and dry wood. The seedling emerges out of the soil into the open by the pressure due to imbibition. Imbibition is also diffusion since water movement is along a concentration gradient.
- The solid particles that absorb water are called imbibants.
- The liquid that is imbibed is termed imbibate.
- The water potential gradient between the absorbent and the liquid imbibed is essential for imbibition.
Long Distance Transport of water
In large and complex organisms, often substances have to be moved long distances. Long-distance transport can’t occur by diffusion or active transport special long-distance transport systems are necessary so as to move substances along long distances and at a much faster rate. Water and minerals, and food are generally moved by a mass or bulk flow system.
Mass flow or Bulk flow is the movement of substances in bulk from one region to another due to pressure difference between two points.
- The pressure difference is created by the development of either a positive hydrostatic pressure gradient or a negative hydrostatic pressure gradient.
- The bulk movement of substances through the conducting or vascular tissues of plants is called translocation.
- The higher plants possess highly vascular tissues i.e. xylem and phloem. Xylem is associated with the translocation of water, mineral salts, some organic nitrogen, hormones, etc from roots to the aerial parts of plants.
- Floem translocates a variety of organic and inorganic solutes, mainly from the leaves to the other parts of the plants.
Water Absorbing System in Transport in Plants
The main absorbing system of plants comprises the roots. Root hairs that are present in millions at the tip of roots perform the function of absorption of water from the soil. Root hairs are thin-walled slender extensions of root epidermal cells that greatly increase the surface area for absorption.
Water is absorbed by root hairs by diffusion, and after that, the water moves from the soil to the xylem through a passageway made of root hair cell ⇒ cortex ⇒ endodermis ⇒ pericycle ⇒ xylem parenchyma.
There is two possible pathways (soil to xylem) Transport in Plants :
- Apoplast Pathway
- Symplast Pathway
The apoplast is the system of adjacent cell walls that is continuous in the plant, except at the casparain strips of the endodermis in the 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.
- The apoplastic 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 intracellular 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.
Water moves in the following path: Soil → Cell wall of root hairs → walls of cortical cells → endodermis → pericycle → xylem parenchyma → xylem channels.
In the Symplast pathway, the living part of the plant is included and made up of interconnected protoplasts of the neighboring cells. The cells of the cortex are living and remain connected through plasmodesmata traversing the cell walls.
- All cells of the cortex are bounded by a continuous selectively permeable membrane.
- The cell membrane of one cell is connected to the cell membrane of the adjacent cell through plasmodesmata so water moves from one cell to another through the symplast pathway.
- Thus due to the presence of the Casparian strip, water reaches up to the endodermis through the apoplast but through the endodermis by symplast.
- Movement is down a potential gradient.
- Symplatsic movement may be aided by cytoplasmic streaming. ex. in the Hydra leaf, the movement of chloroplast is due to streaming.
Some plants have additional structures associated with them that help in water absorption.
- A mycorrhiza is a symbiotic association of fungus with a root system.
- The fungal hyphae have a large surface area and extend into the soil for a sufficient distance. They absorb both water and minerals from the soil and pass the same to plant roots.
- The roots provide sugars and N-containing compounds to the mycorrhizae.
- In many cases mycorrhizal association is obligate. For example, Pinus seeds, Pinus seeds cannot germinate, and establish without the presence of mycorrhizae.
Water Movement up a Plant
The water with dissolved minerals termed sap is absorbed mainly by roots and moved upward to all parts of the plants through the stem, and the movement is called the ‘ascent of sap’. The upward movement ‘ascent of sap’ occurs mainly through the xylem.
Theories of Ascent of Sap
Root Pressure Theory:
The root pressure theory was proposed by Priestly in 1916. This is a positive pressure found in xylem channels due to the metabolic activities of roots. Root pressure is caused due to diffusion pressure gradient and is maintained by the activity of living cells.
- Root pressure is maximum during the early morning of spring and rainy season when droplets of water ooze out from the leaf tips of grasses and leaf margins of many herbaceous plants. This loss of excess water is called guttation.
- Root pressure decreases as the day advances.
- Reduced aeration, lack of water (drought), and low or high temperature inhibit root pressure.
- Root pressure only provides a modest push in the overall process of water transport.
- Root pressure can be measured by a ‘manometer’.
Transpiration Pull and Cohesion of Water Theory:
The ‘transpiration pull and cohesion of water theory’ was originally proposed by Dixon and Joly (1894) and further improved by Dixon in 1914. According to this theory, transpiration creates a pull over the water column which is lifted upwards like a rope and is not broken due to the presence of strong cohesion force among its molecules.
Transpiration is the loss of water in plants in the form of vapor. It mainly occurs through stomata. Besides the loss of water vapors in transpiration, the exchange of oxygen and carbon dioxide in leaves also occurs through stomata.
Mechanism of transpiration Transport in Plants:
- Leaves absorb light and infrared radiation from the surrounding during the day time and they absorb heat from surrounding air during night time.
- The temperature increases during the daytime due to radiant energy and heat during the night, which causes evaporation of water from the cells of leaves.
- During vaporization, the turgid cells of the parenchyma (chlorenchyma) lose water into the intercellular spaces. As a result, the intercellular space gets saturated with water vapor.
- The immediate cause of the opening or closing of stomata is a change in the turgidity of the guard cells.
- The water vapor diffuses out from the intercellular spaces into the dry dry atmosphere mainly through the opening between the guard cells of the stomata. A very small amount of water is lost from the leaf surface through the cuticle.
- The loss of water vapor continues till the stomatal apertures are open. Therefore, transpiration is regulated by the opening and closing of stomata.
The transpiration-driven ascent of xylem sap depends mainly on the following physical properties:
- Cohesion: the 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.
Transpiration and Photosynthesis – A Compromise
The process of photosynthesis requires water, minerals, turgid cells, and cooler temperature. All of these requirements are fulfilled by transpiration. This process creates a pull for water and mineral absorption.
Transpiration performs many functions in transport in plants:
- It creates a transpiration pull for the absorption and transport of plants.
- Supplies water for photosynthesis.
- Transport minerals from the soil to all parts of the plant.
- Cols leaf surfaces, sometimes 10 to 15 degrees by evaporative cooling.
- Maintains the shape and structure of the plants by keeping cells turgid.
In the process of photosynthesis, a slight deficiency of water due to excessive transpiration reduces the rate of photosynthesis.
The ratio between the amount of water that transpired and the amount of dry matter formed is called the transpiration ratio. The value indicates the water requirement of the plant. It is about 600 in most mesophytic plants.
C4 plants transpire approx 300 g of water for every gram of dry matter. C4 plants are twice efficient as C3 plants in terms of fixing carbon dioxide (making sugar). C4 plants lose only half as much water as C3 plants for the same amount of CO2 fixed.
Plants with CAM – crassulacean acid metabolism have the lowest demand for water as they keep their stomata closed during the day.
Uptake and Transport of Mineral Nutrients
Plants get carbon and most of their O2 from CO2 present in the atmosphere, and the remaining nutritional requirements are obtained from water and minerals in the soil.
Uptake of Mineral Ions:
All minerals cannot be passively absorbed by roots because of these factors:
- Minerals are present in the soil as charged particles that cannot move across cell membranes.
- 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.
Translocation of Mineral Ions:
The solute absorbed by roots passes through the cortex, endodermis, Pericyle, xylem parenchyma, and xylem channels. The movement can happen through apoplast, symplast, or mixed pathways.
The vascular tissue phloem transports food (sucrose). In phloem, the movement can be bi-directional movement. Phloem sap is mainly water and sucrose, hormones, and amino acid are transported or translocated through the phloem.
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