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Transport in Plants

Transport In Plants

Transpiration
The loss of water in the form of water vapours from the leaves and aerial parts of plant is called transpiration.

Types of Transpiration

  • Stomatal transpiration- Occurs through stomata
  • Cuticular transpiration- Occurs through surface of stem and leaves
  • Lenticular transpiration- Occurs through lenticels

Occurs mainly through openings called stomata.

Stomata are the minute openings found in the epidermal layer of the leaves. A stoma is surrounded by two bean shaped guard cells which regulate its opening and closing. 

Stomatal Transpiration

In plants, water is absorbed through the roots, This absorbed water has to be transported throughout the plant's body for various physiological functions. It rises up in the stem through xylem and reaches the tissues of leaves through veins. The mesophyll cells of the leaves have their surfaces exposed to the intercellular spaces. Some amount of water forms a thin layer over these surfaces. The water from this film gets evaporated and form water vapours. These water vapours can diffuse through the intercellular space and reach the sub stomatal space and finally escape through stomata.

Regulation of Stomatal Transpiration

  • Stomata:

    • Open in the day and close during the night

    • Also contribute in the exchange of O2 and CO2

    • Opening and closing of stomata is influenced by the turgidity of the guard cells.

  • Inner walls of the guard cells (towards stomatal opening): Thick and elastic

  • When turgidity increases within two guard cells flanking each stomatal pore, the thin outer walls bulge out, and the inner walls assume a crescent shape.

  • Radial orientation of microfibrils in the cell wall of the guard cells makes it easier for the stoma to open.

  • When turgidity decreases within the guard cells, the inner walls regain their original shape, the guard cells become flaccid and stoma closes.

  • Based on the distribution of stomata, 2 types of leaves:

    • Dorsiventral: More number of stomata on the lower surface of leaves; found in dicots.

    • Isobilateral: Equal number of stomata on both sides of leaves; found in monocots.

Differences between Evaporation and Transpiration
 
Evaporation Transpiration
Loss of water from surface of water bodies in the form of vapours Loss of water from aerial parts of plants in the form of vapours
A fast process A slow process
A physical change controlled by temperature and pressure A partly physical and vital process controlled by internal and external factors of leaves

Transport in Plants

  • Short distance transport: By diffusion and cytoplasmic streaming, supplemented by active transport

  • Long distance transport: Translocation (occurs through vascular system)

  • Transport of water and minerals through the xylem: Unidirectional

  • Transport of organic and mineral nutrients through the phloem: Multidirectional

Need of Water and Minerals for Plants

  • Water is an essential requirement for sustaining life of all the living organisms, including plants. In plants, water is needed for four main purposes:
    • Photosynthesis : Water is used as raw material for photosynthesis.
    • Transpiration : To maintain the temperature of plants, water is needed.
    • Transportation : Various substances are transported inside the plants through water.
    • Mechanical stiffness : Water provides turgidity to plant tissues.
  • Need of Minerals : Minerals are needed as nutrients for the plants, 
    • They act as important constituents of the cell and its organelles.
    • They are required for the synthesis of a variety of compounds and enzymes inside the cell.

Means of Transport

Three means of transport in plants:

  • Diffusion

  • Facilitated Diffusion

  • Active Transport

Diffusion

  • An important means of transport inside the plant body

  • Movement of molecules in a random manner, across the concentration gradient

  • Slower process, involving no expenditure of energy

  • Not dependent on the living system

  • Depends upon:

    • Concentration gradient

    • Permeability of the membrane

    • Temperature

    • Pressure

Facilitated Transport

  • Diffusion depends upo…

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