11 TRANSPORT IN PLANTS
Translocation
11 TRANSPORT IN PLANTS
Long distance transport occurs through vascular system, xylem and phloem called translocation through mass flow. The direction of translocation may be unidirectional as in case of water and multidirectional as in minerals and organic solutes.
Means of transport (Short distance transport)
The transport of material into and out of the cells is carried out by number of methods. These are diffusion, facilitated diffusion and active transport.
Diffusion
In this system, the molecules move from a region of higher concentration to a region of lower concentration. This process requires no energy.
Factors affecting diffusion:
Permeability of membrane, Temperature, pressure, gradient of concentration and the size of substances.
Facilitated diffusion:
The diffusion of hydrophilic substances along the concentration gradient through fixed membrane transport protein without involving energy expenditure. For this the membrane possess aquaporins and ion channels. No ATP energy is utilized in this process.
Methods of Facilitated Diffusion:
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Symport:
- Two molecules cross the membrane in the same direction at the same time.
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Antiport:
- Two molecules move in opposite direction at the same time.
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Uniport:
- Single molecules moves across membrane independent of other molecules.
Porins:
The proteins that form huge pores in the outer membranes of the plastids, mitochondria and some bacteria which allow the small size molecules to pass through.
Aquaporins:
Proteins that facilitate diffusion of water molecules through/ across the plasma membrane of cell.
Facilitated Transport
Here, the system moves molecules from a region of higher concentration to a region of lower concentration with the help of a carrier, usually a protein. This process does not require any energy and hence is known as the passive process.
Active Transport
This mechanism transfers molecules from a region of lower to a region of higher concentration with the help of membrane proteins. This system is termed as active transport because it requires ATP to function.
Water Potential
Water potential is used by the plants to transport water to the leaves that help in carrying out photosynthesis. Solute potential and pressure potential are the two main components of water potential.
Solute potential is also known as osmotic potential and is negative in the plant cell. Pressure potential is positive in the plant cell. Higher the concentration of water in the system, greater will be the water potential.
Osmosis
Osmosis is the movement of molecules from a region of higher concentration to a region of lower concentration across a semi-permeable membrane until an equilibrium is reached.
The plant cell wall is freely permeable to substances in solution and water.
Osmosis is of two types:
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Endosmosis:
- This is the movement of water molecules enters into the cell when the cell is placed in a hypotonic solution.
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Exosmosis:
- This is the movement of water molecules out of the cell when the cell is placed in a hypertonic solution.
Isotonic:
If the surrounding solution balances the osmotic pressure of cytoplasm, the solution is called isotonic.
Hypotonic:
If the external solution is more dilute than cytoplasm, it is hypotonic. The cells swell up when placed in hypotonic solution.
Hypertonic:
If the external solution is more concentrated than cytoplasm, it is hypertonic. Cell will shrink in hypertonic solution.
Plasmolysis
Plasmolysis is the shrinkage of the cytoplasm of the cell away from its cell wall under the influence of hypertonic solution. The pressure of plasmolysis is usually reversible when the cell is placed in hypotonic solution.
Turgor pressure
The pressure builds up against the wall due to movement of water inside is called turgor pressure. It is responsible for enlargement and extension growth of cells.
Imbibition
Imbibition is a special type of diffusion when water is absorbed by solid colloids causing them to increase in volume. For example, absorption of water by seeds and dry woods. Imbibition is also a kind of diffusion because movement of water is from higher concentration to lower concentration.
Mass or bulk flow system
Long distance transport of water in plants takes place by mass or bulk flow system. It is the movement of substance in bulk from one point to another as a result of pressure difference between two points.
Absorption of water by plants
Water is absorbed along with mineral solutes by root hairs by diffusion. The absorbed water passes to deeper layer by two pathways.
Apoplast pathway
- It consists of non-living parts of plants body such as cell wall and intercellular spaces.
- There is little resistance in movement of water.
- It is faster.
- Metabolic state of root does not affect apoplast pathway.
Symplast pathway
- It consists of living parts of plant body such as protoplast connected to plasmodesmata.
- Some resistance occurs in the movement of water.
- It is slightly slower.
- Metabolic state of root directly affect symplast pathway.
Casperian strip
The inner boundary of cortex, endodermis is impervious to water due to suberised matrix called Casperian strip. Water molecules are directed through wall regions that are not suberised.
Water flows through the different layers of roots to reach the xylem tissues as follows:
Mycorrhiza
A mycorrhiza is the symbiotic association between a fungus and angiospermic roots. The fungal filaments form a network around the young root to have large surface area that help to absorb mineral ions and water from the soil. The fungus provides minerals and waters and roots in turn provide organic and nitrogen containing compounds.
Vital force theory
Vital force theory was forwarded by J.C. Bose in 1923. This theory believes that the innermost cortical cells of the root absorb water from the outer side and pump the same into xylem channels.
Pressure theory
Root pressure theory was forwarded by Priestley in 1916. Root pressure is positive pressure that develops in the xylem sap of the root of plants. It can be responsible for pushing up water to small heights in plants.
Guttation
Loss of water in liquid phase by herbaceous plants from the tips of leaf blades is known as guttation.
Theory of Capillarity
Water rises in tubes of small diameters, kept in vessels having water due to force of surface tension. Similarly, water rises in the walls of xylem channels due to adhesion and cohesion. This theory is called Theory of Capillarity.
Tension theory
Cohesion Tension theory was put forwarded by Dixon and Joly in 1894. According to this theory water is mostly pulled due to driving force of transpiration from the leaves. The water molecules remain attached with one another by cohesion force. The water molecule does not break in vessels and tracheid due to adhesive force between their walls and water molecules. Because of tension created by transpiration, the water column of plant is pulled up passively from roots to great heights.
Transpiration is the loss of water in the form of water vapour from aerial parts of plants. The following purpose is fulfilled by transpiration.
- Creates transpirational pull for absorption and transport in plants.
- Supplies water for photosynthesis.
- Transport minerals and salts from soil to other parts of plant.
- Cool the leaves and maintain their shape and size.
Photosynthesis is limited by available water. C4 plants are twice as efficient as C3 plants in term of fixing carbon. Although C4 plants uses half as much water as C3 plants for the same amount of CO2 fixed.
Uptake and transport of mineral nutrients
- Most of the minerals enter the roots by active absorption into the cytoplasm of epidermal cells because.
- Minerals are present in the soil as charged particles (ions) which cannot move across cell membranes.
- The concentration of ions in soil is usually lower than concentration in roots.
- Active absorption needs energy in form of ATP. Active uptake of ions is also responsible for water potential gradient in roots.
- Transport proteins of epidermal cells are control point where quantity and type of solutes that reach the xylem is adjusted.
- The ions that reaches to xylem by active or passive transport moves further upward along with transpirational pull.
- The chief sink of mineral elements are growing region of plants like apical meristem, young leaves, growing flower and fruit, and the storage organs.
- Minerals are frequently remobilized from older senescing part of the plants to young growing parts of plant.
- The elements most readily mobilized include phosphorus, Sulphur, nitrogen and potassium. The element like calcium is not mobilized as it is the structural components of plant body.
Phloem transport
Flow from Source to Sink
- Food (sucrose) is transported by phloem from source to sink. The part of plant that synthesize the food is called source and part where food is used or stored is called sink.
- The source and sink can be reversed by the plants depending upon the season or plant’s need. So, the direction of movement in the phloem is bi-directional.
- Phloem sap is mainly water and sucrose, but other sugars, hormones and amino acids are also translocated through it.
Pressure flow or Mass flow hypothesis
- It is the most accepted theory for the translocation of sugar from source to sink. Glucose is prepared at source by photosynthesis which is converted into disaccharides (sucrose). Sucrose moves into companion cells and then into sieve tube cells by active transport.
- Loading of phloem at source creates a water potential gradient that facilitates the mass movement in the phloem.
- Sieve tube cells of phloem forms a long column with holes in their wall called sieve plates. Cytoplasmic strands pass through the hole in the sieve plates to form continuous filament. Hydrostatic pressure developed in sieve tube cells moves the sap in the phloem.
- At sink, incoming sugar is actively moved out of the phloem as complex carbohydrates. The loss of solute produces a high-water potential in the phloem and water passes out and returning into xylem.