Wednesday, 11 September 2019

Transport System in Plants

The Transport System in Plants

The Transport System in Plants


Plants need to move or transport various types of substances over short (i.e., within the cell, across the membrane and cell to cell) or long distances (i.e., from root to leaves). The long-distance transport that occurs through vascular tissues, i.e., xylem and phloem are called translocation.

MEANS OF TRANSPORT

Passage of materials into and out of the cells, i.e., short-distance transport is carried out by a number of methods – diffusion, facilitated diffusion, active transport.
• Diffusion is the movement of materials along the concentration gradient. of lower concentration. It is slow and passive,  i.e ., no energy expenditure takes place.
• Facilitated diffusion: The passive absorption of hydrophilic substances, mediated by a carrier, is called facilitated diffusion. It takes a long concentration gradient and no energy is utilized. The movement is facilitated through fixed membrane transport proteins.
• The transport proteins allow the passage of selected ions and other polar molecules. They may be of two types - carrier proteins and channel proteins.
– Carrier proteins bind the particular solute to be transported and deliver it to the other side of the membrane.
– A molecule, when moves across the membrane through carrier protein independently of another molecule, the process is called uniport. Some carrier proteins allow transport only if two types of molecules move together, this process is called cotransport. In cotransport, when both the molecules cross the membrane in the same direction at the same time, it is called symport and when it is in the opposite direction, the process is called antiport.
– Channel proteins allow diffusion of the solutes of an appropriate size. They are of two types – ion channels and porins.
– Ion channels allow specific ions and they are often gated-voltage gated, mechanical and ligand-gated.
– Porins are large protein pores which allow even small-sized proteins to pass through. They are present in the outer membranes of plastids, mitochondria and some bacteria.
• Active transport involves movable carrier proteins called pumps which employ ATP energy for transport across the membrane.
It is an uphill process, i.e., against the concentration gradient and is faster than passive transport.

PLANT- WATER RELATIONSHIP

• Water is essential for all physiological activities of the plants. It is often a limiting factor for plant growth and productivity, both in agriculture and natural ecosystems. Water absorption in plants depends on various methods and factors.

Water Potential
• Water potential (yw ) is the difference between the free energy of water molecules in pure water and the free energy of water in any other system e.g., water in a solution or in a plant cell or tissue. At atmospheric pressure, pure water has zero water potential. The presence of solute reduces the free energy of water and thus decreases the water potential (negative value). Water always moves from the area of high water potential or high free energy to the area of low water potential or low free energy.

Water potential in a plant
cell or tissue can be written as yw = y m + ys + y p were,
– y m = Matric potential, y s = Solute potential and y p = Pressure potential
– Matric potential (y m ) is influenced by the presence of the matrix. It is not significant in osmosis, so often disregarded, simplifying equation as yw = y s + y p.
– Solute potential (osmotic potential) (ys ) is the amount by which the water potential is reduced as a result of the presence of the solute. It always has a negative value. The more the solute molecules, the lower is yw.
– Hydrostatic pressure (pressure potential) (y p ) is the pressure which develops in an osmotic system due to osmotic entry or exit of water from it. A positive pressure develops in a plant cell or system due to entry of water into it, which is called turgor pressure (TP). The cell wall, being elastic, presses the protoplast with an equal and opposite force. This force exerted by the cell wall over the protoplast is called wall pressure (WP).

Osmosis
• Osmosis is a process by which molecules of a solvent pass through a semipermeable membrane from a region of higher concentration to the region of lower concentration. The osmotic entry of water into a cell is termed as endosmosis whereas the osmotic withdrawal of water from a cell is called as exosmosis.
• Osmotic pressure (OP) is the pressure which needs to be applied to an OP is numerically equal to osmotic
potential or solute potential (ys ), but the osmotic potential has a negative value, while osmotic pressure has a positive value.
• Diffusion pressure deficit (DPD) is the reduction in the diffusion pressure of water in a solution over its pure state. It is also known as suction pressure. DPD = OP – WP (= TP). Osmosis is used for preserving fruits and meats, though the process is quite different for the two. In case of fruit, osmosis is used to dehydrate it, whereas, in case of preservation of meat, osmosis draws salt into it, thus preventing the intrusion of bacteria.
• The behaviour of plant cells with regard to water movement depends on the surrounding solution that may be of three types:
– A hypotonic solution has lower osmotic concentration and hence lower osmotic pressure as compared to another solution.
– The two solutions with the same osmotic concentration or osmotic pressure are termed as isotonic solutions.

Plasmolysis
•The first stage of plasmolysis is called limiting plasmolysis. The cell is called flaccid. The extra hypertonic external solution continues to withdraw water from the central vacuole by exosmosis. Central vacuole shrinks causing shrinkage of protoplast from the cell wall and causes incipient plasmolysis. At this stage, the hypertonic solution enters the cell in between the protoplast and the cell wall. Due to continued exosmosis protoplast shrinks further and withdraws from cell wall except at one or a few points, known as evident plasmolysis. Immediately after the plasmolysis and is due to endosmosis.

Imbibition
• It is adsorption of water by the solid particles of a substance that causes an enormous increase in size and volume. The absorbents that take part in imbibition are called imbibing, e.g., seeds, dry wood, etc. The water which gets adsorbed in this process is called imbibing.
• Amount of imbibition depends upon - 
(i) water potential gradient between adsorbent and water and
(ii) affinity of the adsorbent for water.

Absorption of water in plants
• Absorption of water in land plants mainly occurs from the soil through roots which are often extensive and grow rapidly into the soil. Plants have developed a mass or bulk flow system that operates through the development  of pressure difference between the source and sink.
• There are two different pathways of a water passage from root hair to xylem inside the root-apoplast and symplast. It provides the least resistance to the movement of water. It is interrupted by Water does not enter cell vacuoles and moves through plasmodesmata. This type of movement is aided by cytoplasmic streaming of individual cells. It is, however slower than apoplastic movement.
• Absorbed water and minerals together constitute the sap. The upward movement of sap from the root towards the top of the plant is known as the ascent of sap. There are many theories to explain the ascent of sap.
• Root pressure theory was introduced by Priestly in 1916. Root pressure is a positive hydrostatic pressure which develops in the xylem sap of roots. water follows and thus pressure inside the xylem is increased. Root pressure can push up water to small heights only.
• Root pressure is maximum during the rainy season in the tropical region and during the spring season in the temperate zone.
• Root pressure is absent under conditions of starvation, low temperature, drought and reduced availability of oxygen. Hence, this theory can account for the absence of sap only in the herbaceous plants. In tall trees, the magnitude of pressure developed due to root pressure is too small to push the water to the apical region.
• The most widely accepted theory for the ascent of sap is transpiration pull theory or cohesion-tension theory. It was proposed by Dixon and Jolly in 1894. According to this theory, there is a continuous column of water from the root through the stem and into the leaves. There is another force called adhesion force between the walls of tracheary elements and water molecules. The column of water does not fall down under the impact of gravity because forces such as cohesion, tension and adhesion keep the water in place.

• Intercellular spaces present amongst mesophyll cells of the leaves are always saturated with water vapours and connected to the outside air through stomata. Outside air has lower water potential than the moist air present inside the leaf, thus water vapours diffuse out of the leaves. As a result, the turgor pressure of mesophyll cells decreases and the diffusion pressure deficit (DPD) increases. Now, these cells take water from adjoining cells and the turgor pressure of those adjoining cells decreases. This process is repeated and ultimately water is absorbed from nearest xylem vessels of the leaf. As there is a continuous water column inside the xylem elements, a tension or transpiration pull is transmitted down to the root, resulting in the passive upward movement of water.

Transpiration
• Transpiration is the process of water loss in the form of water vapour from the aerial parts of the plant. It may be stomatal, cuticular and lenticular type. Most of the transpiration (50-97%) occurs through the foliar surface,  i.e ., stomatal transpiration.

• Stomata is a pore or aperture, found in the epidermis of leaves, enclosed by two guard cells and facilitates gaseous exchange. Usually, the lower surface of a dorsiventral leaf has a greater number of stomata while in an isobilateral leaf, stomata are about equal in number on both surfaces.

• Opening and closing of stomata are governed by a change in O.P. or turgidity of guard cells. Different theories about the mechanism of stomatal movements have been proposed. Malate or K + ion pump hypothesis is the most accepted one which was initially given by Fujino (1959) and later modified by Levitt (1974). It is represented in the given flow chart. Factors affecting transpiration rate are temperature, light, humidity, wind speed (external factors) and leaf area, leaf structure, root/shoot ratio mucilage and solutes (internal factors).

• Transpiration (i) controls the rate of absorption of water from the soil, (ii) controls the ascent of sap; (iii) responsible for the cooling of leaves, hence regulates the plant temperature, (iv) protects the leaves from heat injury. It also has some adverse effects like wilting in plants, limiting photosynthesis, etc.

Guttation

•All plants do not show guttation. In general, guttation occurs when transpiration rate is very low as compared to rate of water absorption. Due to this, root pressure is developed and water is pushed out through specialised pores at vein endings called hydathodes.

UPTAKE AND TRANSPORT OF MINERALS


• All minerals cannot be passively absorbed by the roots, unlike water. It is because of two factors - (i) minerals are present in the soil as charged particles or ions which cannot move across cell membranes and 
(ii) the concentration of minerals in the soil Is most minerals must enter the root epidermal cell through active transport. their further transport up the stem to all parts of the plants is through the transpiration stream.

PHLOEM TRANSPORT


• In plants, food (primarily sucrose) is transported by the vascular tissue phloem from the source to a sink. Transport of organic solutes from one part of the plant to the other through phloem sieve tubes is called translocation.

• Food is mostly synthesised in the leaves and also in young stems. The leaves, therefore, serve as the ‘source’. The synthesised food is translocated to the growing regions and also to the storage organs of the plant. So, these regions serve as ‘sink’. Since the source-sink relationship is variable, the direction of movement in the phloem can be upwards or downwards, i.e., bidirectional. This contrasts with that of the xylem where the movement is always unidirectional, i.e., upwards.

• The most accepted mechanism for the translocation of sugars from source to sink is called the pressure-flow hypothesis. As glucose is prepared at the source i.e., in leaves (by photosynthesis), it is converted to sucrose (a disaccharide).

• The sucrose is then moved into companion cells and then into the sieve tube cells by active transport.

• This process of loading at the source produces a hypertonic condition in the phloem.  Water in the adjacent xylem moves into the phloem by osmosis. As osmotic pressure builds up, the phloem sap moves to areas of lower pressure.

• At the sink, osmotic pressure is kept low by converting soluble organic substances into an insoluble form. Again active transport is necessary to move the sucrose out of the phloem sap and into the cells which will use the sucrose converting it into energy, starch or cellulose.

• As sugars are removed from the sieve tubes, the osmotic pressure of the phloem decreases and water moves out of the phloem.


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