INTRODUCTION TO TRANSPORTATION

Biology

INTRODUCTION TO TRANSPORTATION

  • Transport is the movement of substances within an organism.
  • All living cells require oxygen and food for various metabolic processes.
  • These substances must be transported to the cells.
  • Metabolic processes in the cells produce excretory products which should be eliminated before they accumulate.
  • The excretory products should be transported to sites of excretion.
  • Organisms like amoeba are unicellular.
  • They have a large surface area to volume ratio.
  • The body is in contact with the environment.
  • Diffusion is adequate to transport substances across the cell membrane and within the organism.
  • Large multi-cellular organisms have complex structure where cells are far from each other hence diffusion alone cannot meet the demand for supply and removal of substances.
  • Therefore an elaborate transport system is necessary.

Transport in plants

  • Simple plants such as mosses and liverworts lack specialized transport system.
  • Higher plants have specialized transport systems known as the vascular bundle.
  • Xylem transports water and mineral salts .
  • Phloem transports dissolved food substances like sugars.

Internal structure of roots and root hairs

The main functions of roots are:

  • Anchorage
  • Storage
  • Gaseous exchange.
  • The outermost layer in a root is the piliferous layer.
  • This is a special epidermis of young roots whose cells give rise to root hairs.
  • Root hairs are microscopic outgrowths of epidermal cells.
  • They are found just behind the root tip,
  • They are one cell thick for efficient absorption of substances.
  • They are numerous and elongated providing a large surface area for absorption of water and mineral salts.
  • Root hairs penetrate the soil and make close contact with it.
  • Below the piliferous layer is the cortex.
  • This is made up of loosely packed, thin walled parenchyma cells.
  • Water molecules pass through this tissue to reach the vascular bundles.
  • In some young plant stems, cortex cells contain chloroplasts.
  • The endodermis (starch sheath) is a single layer of cells with starch grains.
  • The endodermis has a casparian strip which has an impervious deposit controlling the entry of water and mineral salts into xylem vessels.
  • Pericyc1e forms a layer next to the endodermis.
  • Next to the pericycle is the vascular tissue.
  • In the Dicotyledonous root, xylem forms a star shape in the centre, with phloem in between the arms.
  • It has no pith. In monocotyledonous root, xylem alternates with phloem and there is a pith in the centre.
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Internal structure of a root hair cell

The Stem

The main functions of the stem are:

  • Support and exposure of leaves and flowers to the environment,
  • Conducting water and mineral salts
  • Conducting manufactured food from leaves to other parts of the plant.
  • In monocotyledonous stems, vascular bundles are scattered all over the stem, while in dicotyledonous stems vascular bundles are arranged in a ring.
  • Vascular bundles are continuous from root to stems and leaves.
  • The epidermis forms a single layer of cells enclosing other tissues.
  • The outer walls of the cells have waxy cuticle to prevent excessive loss of water.
  • The cortex is a layer next to the epidermis.
  • It has collenchyma, parenchyma and schlerenchyma cells.

Collenchyma

  • Is next to the epidermis and has thickened walls at the corners which strengthen the stem.

Parenchyma

  • Cells are irregular in shape, thin walled and loosely arranged hence creating intercellular spaces filled with air.
  • They are packing tissues and food storage areas. Sclerenchyma
  • Cells are closely connected to vascular bundles.
  • These cells are thickened by deposition of lignin and they provide support to plants. Pith
  • Is the central region having parenchyma cells.

Absorption of Water and Mineral Salts Absorption of Water

  • Root hair cell has solutes in the vacuole and hence a higher osmotic pressure than the surrounding soil water solution.
  • Water moves into the root hair cells by osmosis along a concentration gradient.
  • This makes the sap in the root hair cell to have a lower osmotic pressure than the surrounding cells.
  • Therefore water moves from root hair cells into the surrounding cortex cells by osmosis.
  • The process continues until the water gets into the xylem vessels.

Uptake of Mineral Salts

  • If the concentration of mineral salts in solution is greater than its concentration in root hair cell, the mineral salts enter the root hair cell by diffusion.
  • If the concentration of mineral salts in the root hair cells is greater than in the soil water, the mineral salts enter the root hairs by active transport.
  • Most minerals are absorbed in this way.
  • Mineral salts move from cell to cell by active transport until they reach the xylem vessel.
  • Once inside the xylem vessels, mineral salts are transported in solution as the water moves up due to root pressure, capillary attraction and cohesion and adhesion forces.

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