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.
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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:
- 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.
Internal structure of a root hair cell
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.
- Is next to the epidermis and has thickened walls at the corners which strengthen the stem.
- 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.