Transport in Plants

BIOLOGY TRANSPORT IN PLANTS

Transport in Plants

 In plants, materials such as gases, minerals, water, hormones and organic solutes need to be transported over short and long distances.  Short distance transport occurs through through diffusion and cytoplasmic streaming accompanied by active transport.  Long distance transport occurs through the xylem and phloem. This transport is called translocation.

Means of Transport

Facilitated Active Diffusion Diffusion Transport

Diffusion

 The movement of molecules or ions from the region of higher concentration to the region of lower concentration, until the molecules are evenly distributed throughout the available space is known as diffusion.

 The rate of diffusion gets affected by temperature, density of diffusing substances, medium in which diffusion is taking place, diffusion pressure gradient.

Characteristics of Diffusion  The diffusing molecules move randomly along the concentration gradient.  The direction of diffusion of one substance is independent of the movement of the other substance. www.topperlearning.com 2

BIOLOGY TRANSPORT IN PLANTS

 There is no energy expenditure.

Importance of Diffusion in Plants

 Diffusion helps in CO2 intake and O2 output in photosynthesis and CO2 output and O2 intake in respiration.  It is an effective means of transport of substances over very short distance.

Facilitated Diffusion

 The spontaneous passage of molecules or ions across a biological membrane mediated by specific transmembrane carrier proteins without spending metabolic energy is called facilitated diffusion.  Water soluble substances such as glucose, sodium ions and chloride ions are transported by this method.

Action of Transport of Proteins  The carrier protein acts as selective channels through which the molecules are transported across the membrane.  Large transporter proteins create huge pores in the outer membranes of plastids, mitochondria and bacteria through which variety of molecules are passed. These transporter proteins are called porins.  Aquaporins are the water channels through which massive amount of water diffuse into the cell.

Types of Facilitated Diffusion  Uniport: When a particular type of molecule moves across a membrane independent of the other molecule, the diffusion is called uniport.  Symport: When the two types of molecules move in the same direction at the same time, it is called symport.  Antiport: When the two types of molecules move in the opposite direction at the same time, it is called antiport.

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Active Transport

 The process of transport of materials across the biological membrane with the help of a mobile carrier protein involving expenditure of energy in the form of ATP is called active transport.  It is a kind of uphill transport against the concentration gradient and is faster than passive transport.  Carrier proteins on the cell membrane act as pumps to transport substances across the membrane.

Comparison of Different Transport Mechanisms

Property Simple Diffusion Facilitated Diffusion Active Transport 1. Requires special No Yes Yes membrane proteins 2. Uphill transport No No Yes 3. Requires ATP energy No No Yes 4. Movement of transport No No Yes proteins

Plant — Water Relations

 Water is an important constituent in living systems.  Water is essential to maintain the turgidity of cells, functioning of the protoplasm and regulation of constant body temperature.  Because water is important for all physiological activities of plants, it is important to understand plant– water relations concerned with some physiological processes in plants.

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Water Potential

 The difference between the free energy of water molecules in pure water and the energy of water in any other system is termed water potential.

 Water potential is denoted by the symbol psi ѱ/ ѱw.

 The water potential is expressed in pressure units such as pascals (Pa), bars or atmospheres.  Chemical potential of pure water at normal temperature and pressure is taken as zero.  Chemical potential of water in any other system such as a solution or in a cell will be less than zero, i.e. negative.  If there is a difference in the water potential between two regions, then the spontaneous movement of water will take place.  The amount by which the water potential is reduced as a result of the presence of solute is called the

solute potential or osmotic potential (ѱs).  The value of the solute potential is always negative. More the solute particles, the solute potential will

be more negative ѱs.

 Pressure potential (ѱp) is the positive pressure developed in a system because of osmotic entry of water into it.

 For a solution at atmospheric pressure, ѱw is equal to ѱs.

Osmosis

 Osmosis is the diffusion of solvent molecules from a region of higher concentration to a region of lower concentration through a semi-permeable membrane until equilibrium is reached.  Osmotic pressure is the pressure required to prevent the passage of pure water into an aqueous solution through a semi-permeable membrane, thereby preventing an increase in the volume of the solution. Osmotic pressure is usually measured in pascals, Pa.

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 During osmosis, water or solvent molecules move as follows:

From the region of To the region of Pure solvent Solution Dilute solution Concentrated solution High free energy of water molecules Low free energy of water molecules Higher water potential Lower water potential Higher diffusion pressure of water Lower diffusion pressure of water

 A solution whose concentration is more than that of the cell sap is known as a hypertonic solution.  A solution whose concentration is less than that of the cell sap is known as a hypotonic solution.  A solution whose concentration is equal to that of the cell sap is known as an isotonic solution.  Turgor pressure (TP) is the pressure developed in an osmotic system because of the entry of water which causes swelling of the system.  When a cell is placed in a hypotonic solution, water will enter the cell as the cell sap is more concentrated than the surrounding solution. This makes the cell turgid.  When a cell is placed in a hypertonic solution, water will diffuse out of the cell as the cell sap is less concentrated than the surrounding solution. This makes the cell flaccid i.e. the protoplast of the cell shrinks.  As the cell wall is rigid and less elastic, it cannot keep pace with the contraction of the plasma membrane.  The protoplast separates from the cell wall and assumes a spherical shape. This condition is called plasmolysis.  Plasmolysis is the withdrawal of protoplast of a plant cell from its cell wall because of excessive loss of water from the cell.

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Importance of Osmosis  Plants absorb water by osmosis.  Movement and distribution of water across cells occur through osmosis.  Rigidity of plant organs is maintained through osmosis.  Leaves become turgid and expand because of their osmotic pressure.  Opening and closing of stomata is affected by osmosis.

Imbibition

 Imbibition is the phenomenon of adsorption of water or any other liquid by solid particles of a substance without forming a solution.  The solid particles which adsorb water or any other liquid are called imbibants.  The liquid which is imbibed is known as imbibate.  The molecules of the imbibate are held in between or over the surface of solid substances through capillarity or by the force of adsorption.

Examples of Imbibition  A dry piece of wood placed in water swells and increases in volume.  Wooden doors and windows adsorb water in the rainy season and increase in their volume hence they are hard to open or close.

Importance of Imbibition to Plants  Imbibition causes swelling of seeds and results in breaking of the testa.  Imbibition is dominant in the initial stage of water absorption by roots.  Water enters the ovules which are ripening into seeds by the process of imbibition.  Imbibition pressure generated during the germination of seeds and spores is so enormous that it can break asphalt roads and concrete pavements.

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Transport of Water and Soil Water Relations

Bulk Flow System  Bulk flow system is a long distance transport system to move distances at faster rates.  Water, minerals and food are generally moved by a mass/bulk flow system.  A mass flow or bulk flow system is responsible for the movement of substances in bulk or en mass from the sites of production or adsorption to the sites of storage or consumption as a result of pressure differences between the two sites.  Bulk flow can be achieved through 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.  Xylem is associated with the translocation of mainly water, mineral salts, some organic nitrogen and hormones, from the roots to the aerial parts of plants.  The phloem translocates a variety of organic and inorganic solutes, mainly from the leaves to the other parts of plants.

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Mechanism of Water Absorption

 The water-absorbing structure of the plant is the root hair zone.  A root hair is a unicellular tubular propagation of the outer wall of the epiblema.

 When water is absorbed by the root hair and other epidermal cells, it moves centripetally across the cortex, endodermis, pericycle and finally enters the xylem.  Water moves from cell to cell along the concentration gradient.  Two possible pathways for the movement of water along the roots are the apoplast pathway and the symplast pathway.

Apoplast Pathway  The apoplast system includes interconnecting cell walls, intercellular spaces, cell walls of endodermis excluding Casparian strips, xylem tracheids and vessels.  In this pathway, water moves from the root hair to the xylem through the walls of intervening cells without crossing any membrane or cytoplasm. This movement of water occurs through bulk or mass flow.  The apoplast pathway does not provide any barrier to the movement of water. www.topperlearning.com 9

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 The apoplastic movement of water beyond the cortex is obstructed because of the presence of casparian strips in the endodermal cells.

Symplast Pathway  The cortical cells are living cells which remain interconnected with each other through the plasmodesmata traversing through their cell walls.  The water entering the cell sap of root hair moves into the underlying cortex cells through the plasmodesmata.  The endodermal cells of the root have a conspicuous waxy thickening, the casparian strip, in their cell walls which blocks the movement of water and minerals from one side to the other side via the cell wall route.  Therefore, water can reach up to the endodermis through the apoplast, but it moves through the endodermis by the symplast.

Role of Mycorrhiza  Mycorrhiza represents a mutualistic symbiotic association between the root system of higher plants and fungal hyphae.  The fungal hyphae form a network around the young root and invade the root cells.  The fungal hyphae penetrating into the root cells provide a direct passage for the transport of water, sugar and nitrogenous compounds into the root.

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Water Movement Up a Plant

 The upward conduction of water in the form of a dilute solution of mineral ions from the roots through the stem to the aerial parts of plants is called the ascent of sap.  Several kinds of forces are required in order to maintain the continuous passage of sap through the plant.

Root Pressure  Root pressure is the hydrostatic pressure developed in the roots because of the continued inward movement of water through cell-to-cell osmosis which helps in the movement of ascent of cell sap upwards through the stem.  Root pressure develops because of active absorption which depends on the active accumulation of solute in xylem sap.  Root pressure usually develops during the night when absorption is maximum and transpiration is minimum.

 Root pressure can be inhibited by using cyanide, lack of O2 and low temperature.

Transpiration Pull  Continuous loss of water from the leaves due to transpiration makes the mesophyll cells absorb water from the adjacent internal mesophyll cells and compensate for the loss of water. This loss causes a water deficit in the xylem.  Rapid transpiration develops a pull or tension in the xylem called the transpiration pull.  Transpiration pull is called negative pressure because it develops from the aerial parts and progresses to the underground parts of plants.

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Cohesion-Adhesion Forces  A force of mutual attraction present between the water molecules is called cohesive force.  Water molecules are also attached with the walls of the vessels and tracheids through a force called adhesive force.  Cohesive and adhesive forces work together to maintain the continuity in between water and the cell wall.  The forces are responsible for maintaining unbroken continuity of the water column from the roots to the leaves.  This water column is pulled upwards continuously without breaking, from the roots to the leaves by transpiration.

Transpiration

 The loss of water in the form of vapour from the aerial parts of the plant is called transpiration.  Factors such as temperature, light, humidity, wind speed affect the rate of transpiration.

Structure of Stomata  Stomata are tiny apertures found on the epidermis of leaves and young green stems.  Each stoma is surrounded by two epidermal bean-shaped guard cells.  Guard cells are bordered by one or more modified epidermal cells called subsidiary or accessory cells.

Mechanism of Opening and Closing of Stomata  The surfaces of spongy mesophyll cells in the leaf are exposed to intercellular spaces.  These cells give out a thin film of water. Water from this film evaporates.  Water vapour formed saturates the air in the intercellular spaces, diffuses into the connecting intercellular spaces and reaches the sub-stomatal space. Finally, it escapes in the atmosphere through the air.  This creates the pull in plant tissues.  Water is absorbed by the roots rises through the stem and reaches the tissues of leaves.

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Uptake of Mineral Ions

 The minerals are mainly absorbed by the roots which are in direct contact with the soil solution.  Some mechanisms of absorption require energy and some do not.  Mineral ions are absorbed by both passive and active transport.  Passive transport does not require energy, however; the active transport requires energy.  Specific proteins in the membranes of root hair cells actively pump ions from the soil into the cytoplasm of the epidermal cells.  The endodermal cells have many transport proteins embedded in their plasma membrane which allow the passage of some solutes but not the others.  Transport proteins of endodermal cells are control points which decide the quantity and the types of solutes which reach the xylem.

Translocation of Mineral Ions

 The absorbed mineral ions move laterally from the epiblema to the xylem through the cortex, endodermis and pericycle, from where they are transported upwards through the xylem along with the transpiration system.  They are also remobilised from older dying leaves to the young leaves.  Before the leaf fall in deciduous plants, minerals are sent to the other parts of the plant.

Phloem Transport: Flow From Source to Sink

 The organic compounds such as glucose and sucrose produced during photosynthesis are translocated from the green cells to the non-green parts of plants through the phloem tissue.  Translocation occurs through the phloem in the upward, downward and radial directions from the leaves to the storage organs.  Translocation requires the expenditure of metabolic energy. www.topperlearning.com 14

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 Factors such as temperature, light, metabolic inhibitors, mineral deficiencies and hormones affect the rate of translocation in phloem.

Mass Flow Hypothesis

 Sucrose from leaves moves into the companion cells of the phloem and from there it moves into the sieve tubes by active transport.  This creates the hypertonic condition in the phloem which draws more water from the xylem into the phloem.  As the osmotic pressure inside the phloem increases sucrose and other minerals moves into the areas of lower osmotic pressure i.e. sink.  Once the sucrose moves out of the phloem, the sap in the phloem becomes hypotonic and water moves to xylem.  Entry of water in the phloem facilitates the mass movement of nutrients to the sink.

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Girdling Experiment  Girdling or ringing experiment proves that organic solutes are translocated through phloem.  In this experiment the region of the bark containing phloem is removed. As a result, the food material accumulates at the edge of the ring, and with time, this region swells into a ridge.