Introduction
Long-distance transport of materials throughout a plant cannot be achieved by diffusion, as it is a prolonged process. Also, it holds over short distances for the movement of molecules. In organisms with complex structures, substances need to be moved over long distances. Sometimes, the location of uptake and storage are too far from each other such that diffusion or active transport may not be able to cope with. Long-distance transport systems are required to transport materials over long distances faster. Food, minerals, and water are generally transported through the Mass or Bulk flow system.
Mass or Bulk Flow System
➨It is the movement of materials in bulk from one place to another due to pressure differences between the two places
➨ The special characteristic of mass flow is that the material, whether in a suspension or in a solution, gets swept along at the same pace as in a flowing river
➨ Mass flow can be achieved either through,
- Positive Hydrostatic pressure gradient- When the pressure exerted is towards the gravity.
For example, Turgor in the plant cells, a garden hose.
- Negative Hydrostatic pressure gradient- When the pressure exerted is against the gravity.
For example, suction through a straw, pressure formed in xylem and cell wall
➨ Xylem translocates mainly Mineral salts, Water, Hormones and some organic nitrogen
➨ Phloem translocates mainly inorganic and organic solutes, basically from leaves to other plant parts
Transpiration Pull
Despite the absence of a heart or a circulatory system in plants, the upward flow of water through the xylem in plants can achieve fairly high rates, up to 15 metres per hour. How is this movement accomplished? A long standing question is, whether water is ‘pushed’ or ‘pulled’ through the plant. Most researchers agree that water is mainly ‘pulled’ through the plant, and that the driving force for this process is transpiration from the leaves. This is referred to as the cohesion-tension-transpiration pull model of water transport. But, what generates this transpirational pull? Water is transient in plants. Less than 1 per cent of the water reaching the leaves is used in photosynthesis and plant growth. Most of it is lost through the stomata in the leaves. This water loss is known as transpiration.
Absorption of Water by Plants
➨ Root hairs present on the tip of the roots absorb water and other minerals through the process called diffusion
➨ Once the root hairs take up the water, they need to pass it further down into deep root layers basically by two distinct pathways :
- Apoplast Pathway
- Symplast Pathway
Apoplast Pathway
➨Apoplast refers to the non protoplasmic components of a plant, including the cell wall and the intracellular spaces. This pathway does not provide any restrictions to water movement as water movement is through a Mass flow system. In apoplast, the water movement occurs by passive diffusion
➨ Mass flow of water arises due to the cohesive and adhesive properties of water
Symplast Pathway
➨ Symplast refers to the continuous arrangement of protoplasts of a plant, which are interconnected by plasmodesmata.In symplast, the water movement occurs by osmosis
➨ Surrounding cells are connected through cytoplasmic strands, which extend through plasmodesmata
➨ In this type of pathway, the water has to travel through the cell membrane, and thus the movement is relatively slower and down the potential gradient as usual
➨ Symplastic movements are aided by cytoplasmic streaming also
Note:
- Majorly, the flow of water in roots occurs via apoplast as the cortical cells are loosely packed and thus offer no barrier to water movement.
- Water movement through root layers is ultimately symplastic in the endodermis.
Root Pressure
➨ As numerous ions across 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 type of positive pressure is called Root Pressure and is responsible for transporting water up to small heights in the stem
➨ Root Pressure can only provide a moderate push in the overall process of water transport, and also, they do not play any major roles in water movement up tall trees
➨ The main work of the root pressure is to re-establish the chains of water molecules in the xylem, which frequently gets broken under tremendous pressure created by the process called, transpiration
➨ It is not responsible for the majority of the water transport as most of the plants meet their requirement through transpiration pull
Guttation
➨ We get to observe the effect of root pressure in over early morning or at night when the rate of evaporation is pretty low.
➨ Excess water gets collected in the form of small droplets over openings of veins near the tip of grass blades and leaves of many herbaceous parts of the plant. Such loss of water in the liquid phase is known as Guttation.
Conclusion
The movement of sugars in the phloem begins at the very source, where an active transport mechanism loads sugars into the sieve tube of the phloem. Loading of the phloem automatically sets up a water potential gradient which facilitates the bulk, i.e., mass movement in the phloem. As hydrostatic pressure within the sieve tube of the phloem increases, pressure flow starts, and the sap moves through the phloem. At the same time, the incoming sugars are transported actively out of the phloem and are transported in the form of complex carbohydrates. The high percentage of sugar decreases which decreases the total water potential and causes water to move by osmosis from the adjacent xylem into the phloem tubes, thereby increasing pressure. This increase in total water potential causes the bulk flow of phloem from source to sink