11. TRANSPORT IN PLANTS
Diffusion is a slow and short distance movement. E.g. movement of molecules across a typical plant cell (about 50 μm) takes about 2.5 s.
Long distance transport systems are necessary to move substances faster across long distances.
Movement of substances in bulk (en masse) from one point to another due to pressure differences between two points is called Mass (bulk) flow. E.g. movement of water, minerals and food.
In mass flow, substances (in solution or in suspension) are swept along at the same pace as in a flowing river. But in diffusion, different substances move independently depending on their concentration gradients.
Bulk flow is achieved either through a +ve hydrostatic pressure gradient (e.g. a garden hose) or a -ve hydrostatic pressure gradient (e.g. suction through a straw).
Bulk movement of substances in long distance through the conducting tissues (xylem & phloem) is called translocation.
Absorption of Water by Plants
Absorption of water and minerals occurs by diffusion through millions of root hairs present at the root tips.
Root hairs increase the surface area for absorption.
The absorbed water is moved deeper into root layers by 2 pathways:
- Apoplast pathway
- Symplast pathway
It is a system of adjacent cell walls that is continuous except at the casparian strips of endodermis in the roots.
It occurs exclusively through the intercellular spaces and cells walls. It does not cross the cell membrane.
Water movement through apoplast is dependent on the gradient and occurs through mass flow.
The apoplast does not provide any barrier to water movement.
As water evaporates into the intercellular spaces or the atmosphere, tension develops in the continuous stream of water in the apoplast. Hence mass flow of water occurs due to adhesive and cohesive properties of water.
2. Symplast pathway:
It is the system of interconnected protoplasts.
Here, water travels through cytoplasm; intercellular movement is through the plasmodesmata (junction between neighbouring cells through which cytoplasmic strands extend).
Water has to enter the cells through cell membrane; hence the movement is slower. Movement is down a potential gradient.
Symplastic movement may be aided by cytoplasmic streaming. E.g. In Hydrilla leaf, movement of chloroplast due to cytoplasmic streaming is easily visible.
Most of the water flow in the roots occurs via the apoplast since the cortical cells are loosely packed. So, water can move without resistance. However, the endodermis is impervious to water due to the casparian strip (a band of suberised matrix). So water molecules are directed to non-suberised wall regions. The water then moves through the symplast and again crosses a membrane to reach the xylem.
The water movement through the root layers is ultimately symplastic in the endodermis. This is the only way water and solutes can enter the vascular cylinder.
In young roots, water enters directly into the xylem vessels and tracheids. These are non-living conduits and so are parts of the apoplast.
Some plants have additional structures for water and mineral absorption. E.g. mycorrhiza is a symbiotic association of a fungus with a root system. The fungal filaments form a network around the young root or they penetrate root cells. The hyphae absorb mineral ions & water from soil. The roots provide sugars & N compounds to mycorrhizae. Some plants have an obligate association with the mycorrhizae. E.g. Pinus seeds cannot germinate and establish without mycorrhizae.
Water Movement up a Plant
Water moves up a stem against gravity. So it needs energy.
As ions from the soil are actively transported into the vascular tissues of the roots, water follows (its potential gradient) and increases the pressure inside the xylem. This positive pressure is called root pressure.
It helps to push up water to small heights in the stem.
Experiment to prove existence of root pressure:
During early morning, having atmospheric moisture, cut a soft plant stem horizontally near the base. Drops of solution ooze out of the cut stem. This is due to the positive root pressure.
At night and early morning evaporation is low. So excess water collects as droplets around special openings of veins near the tip of grass blades, and leaves of many herbaceous parts. Such water loss in liquid phase is called guttation.
Root pressure can only provide a modest push in the water transport. It has no major role in water movement up tall trees. Greatest role of root pressure is to re-establish continuous chains of water molecules in the xylem which often break under the tensions created by transpiration.
In most plants, majority of water transport occurs by transpiration pull.
In plants, the water flow upward through the xylem achieves high rates (up to 15 m /hr).
Water is mainly pulled through the plant due to transpiration pull. It is a driving force due to transpiration. This is known as cohesion-tension-transpiration pull model of water transport.
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