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Transportation in Plants

Transportation in Plants Synopsis

Synopsis

 

  • Transportation in plants is the process in which a substance is absorbed or synthesised in one part of the plant and is transported to the other part of the plant.
  • 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 diffusion and cytoplasmic streaming accompanied by active transport.
  • Long distance transport occurs through the xylem and phloem. This transport is called translocation.

 
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.

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.

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

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.
  • 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.

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.



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.

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.

Kinds of Transpiration 

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.



Xylem
  • It conducts water upwards in a plant. 
  • Xylem also provides strength to the stem and helps the plant to stand upright.
  • It is located in the centre of the plant body.
  • Xylem mainly consists of tracheids and vessels.