The plants absorb water and soluble mineral salts from the soil by their root system. This function of absorption is facilitated by the unicellular root hairs present on the roots. These unicellular root hairs enter in the interspaces of the soil particles, irregularly. These root hairs absorb the water found in the form of thin films around these soil particles.
Cell as a physiological unit Cells are the building blocks from which living things are made. A single cell may make up an entire organism, or groups of cells may be loosely organized and live together.
Scientists have known about cells for a long time. About two hundred years back biologists realised that ‘the cell is the basic unit of all living organisms, hence cell theory was proposed by two German scientists, ‘Schleiden and Schwann’.
The major ideas of cell theory are:
1. Living things are made up of cells and cell products.
2. Cells are very much alike in their structure and composition.
3. Cells have a set of functions that they carry out in order to stay alive.
4. New cells arise from old cells by cell reproduction.
However, plant cells are composed of cell wall and protoplast.
The term protoplast is generally used to refer collectively to the plasma membrane and protoplasm.
The protoplasm refers to the living contents of the cells and consists of cytoplasm and nucleus.
Generally, the plant cell possesses three compartments:
(i) Vacuole,
(ii) Protoplasm, and
(iii) Cell wall.
These compartments are separated from each other by plasma membranes, i.e., the tonoplast is found in between the vacuole and the protoplasm, while plasmalemma between the protoplasm and the cell wall. Plasmodesmata, connect the protoplasm of one cell to that of other cell.
The plasma membrane is selectively permeable. This means, it allows some materials to pass through and not others.
The plasma membrane is made up almost of protein and lipid molecules.
Protoplasm:
According to modem view, the protoplasm is a colourless, semi-transparent and viscous substance. It is considered as complex colloidal system of many phases.
However, it is well established that water is chief component of all physiologically active protoplasm and in such cases may constitute up to 90% of the protoplasm and in cases of hydrophytes even more. In dry seeds where the protoplasm is rather inactive, water may be less than 10% of the total protoplasm.
Importance of Water:
The water is the most important factor for the vital functions of the plants. The plants cannot survive in its absence. Water is a very important solvent, and usually consists most of the part of the protoplasm. Many biochemical reactions going on in the plants are catalysed by many enzymes, formed in the protoplasm. Usually 75% quantity of water is found in the cytoplasm.
Usually in the leaves, the quantity of water is 75%, whereas in the stems it is 60%. In several hydrophytes, e.g., algae, etc., this quantity exceeds up to 98%. In the same way, the quantity of water in the xerophytes is usually 60% or lesser than this.
In the dormant seeds, this quantity of water is only about 10%. This is, of course very much less for any vital function, and therefore, the germination of these seeds is only then possible, when they get sufficient moisture.
The cytoplasm can remain viable only in the presence of sufficient amount of water. In the scarcity of water, it dies. The water also plays an important role in the function of photosynthesis. The water is a good solvent; the minerals cannot be absorbed unless they are not soluble in water. The cell of the plants remains turgid by water and gives temporary mechanical support to the young plants.
The water found in the plants, is a very small part of the water absorbed by the plants, rest of the larger part of the water goes out from the surface of the plants, by a vital process, called transpiration.
Availability of the Water in the Soil:
About three forms of water are found in the soil. Just after the rains, because of gravitation some amount of water along with some mineral salts goes in the lower strata of the earth. This is called gravitational or free water and cannot be used by the plants.
Besides this, every particle of soil holds some imbibed water in it. This water is hold up in the soil particle with such a great imbibitions force, that it cannot be separated from it for the use of the plant.
This is called hygroscopic water, which also cannot be absorbed from the soil by the plants. Besides these each soil particle is surrounded by a loose film of water; this film is attracted by the capillary force to the soil particle, and such water film is called capillary water. Now it is to be noticed, that only this capillary water may be absorbed by the root hairs of the plants.
The mineral salts are also found in this water in soluble state, and are absorbed by the plants along with water. If soil lacks this capillary water, the plants very soon begin to wither and ultimately die.
Water Potential Ψ(Psi):
All plants and other living organisms require free energy to grow and reproduce.
In thermodynamics, free energy represents potential to do work. According to thermodynamic laws every component of a system possesses free energy capable of doing work under constant temperature conditions.
The potential energy of water is called water potential. Water potential is regarded as the tendency of water to leave a system. It is often used while explaining the direction in which water will flow from one cell to another, or from one part of the plant to another such as from soil to root, from root to leaves, from leaves to air, or from soil to air.
Water always moves from a region of higher water potential to that of lower water potential. It can also be said that the difference in water potential between two points is a measure of the amount of work, i.e., energy, needed to move water from one point to the other.
Osmotic movement of water involves certain work done and in fact the main driving force behind this movement is the difference between free energies of water on two sides of the selectively permeable membrane. For water free energy molecule is known as water potential (Ψw).
Water potential is measured in terms of pressure. Common measurement unit of water potential is Pascal, Pa; 1 Megapascal represents 10 bars, i.e., 1 Megapascal = 10 bars. One bar is close to one atmosphere of pressure, i.e., 1 bar = 0.987 atmosphere of pressure.
At atmospheric pressure water potential of pure water is zero, and therefore, all solutions at atmospheric pressure have lower water potentials than water, i.e., they have a negative value.
Water potential Ψw is measured in relative quantity, and expressed as the difference between the potential of a solution in a given state and the potential of the same solution in a standard state.
Water potential is lowered by the addition of solutes and as water potential value is zero for pure water; all other water potential values will be negative. Thus, the movement of water will take place in osmotic or other systems from a region of higher water potential (i.e., less negative) to a region of lower water potential (i.e., more negative).
Water potential of any solution is influenced by three factors:
(i) Concentration,
(ii) Pressure and
(iii) Gravity.
This can be represented by following equation:
Ψw = Ψs + Ψp + Ψg
i.e., Ψs effect of solutes (i.e., solute potential or osmotic potential)
Ψp effect of pressure (i.e., pressure potential or hydrostatic pressure)
Ψg effect of gravity (i.e., gravity potential)
This means, pure water has a higher potential than the water inside a cell. In other words of pure water is zero, and therefore, water potential inside plant cells is negative.
Solute Potential Ψs:
Solutes present in a cell reduce the free energy of water, or the water potential.
Pressure Potential (Ψp) or Hydrostatic Pressure:
The positive hydrostatic pressure is called turgor pressure. The pressure potential for pure water in an open beaker is zero.
Gravity Ppotential (Ψg):
This term represents the effect of gravity on water potential. It depends on the height of water. If vertical height is less than five meters, the Ψg is negligible. In a plant cell only Ψs and Ψp are important, and considered, i.e., Ψw = Ψs + Ψp.
According to this equation when water moves into the cell from outside, the hydrostatic pressure, i.e., pressure potential (Ψp) increases, which results in an increased water potential (Ψw) of the cell, and the difference between the inside and outside Ψw (ΔΨw) is reduced.
On the other hand, when concentration of solute is increased in the cell, the solute potential (Ψs) is lowered, and therefore, water potential (Ψw) is decreased.
Thus, water moves into the cell from outside due to a water potential gradient. If a pressure is created on the cell, water moves out of it. Here, external pressure raises the water potential (Ψw) of the cell, and thus the difference in water potential (Ψw) inside and outside (ΔΨw) will be such that water will flow out of cell.
There are two basic factors that affect the water potential:
(i) Amount of solute and
(ii) External pressure.
(i) Amount of Solute:
Pure water at atmospheric pressure has a zero water potential. An addition of solutes lowers the water potential (Ψw), i.e., it becomes negative in value.
(ii) External Pressure:
Effect of pressure on water potential is just opposite to the effect of solutes, i.e., the increase in pressure increases the water potential (Ψw).
Absorption and Movement of Water:
The absorption of water and solutes from the soil seems to have been the first physiological processes to receive the attention of early writers on botanical subjects. According to Aristotle plant foods are complex substances which have been so prepared in the soil that they can be absorbed and used by the plant without any changes, hence without the necessity of excreting any water or by products, as do animals.
The earliest known attempt to explain absorption process was made by Andrea Cesalpino (1603). He, of course, accepted the Aristotle’s theory that plants absorb their food in solution from the soil.
The Pathway of Water through the Root:
Water enters the roots principally through the walls of the root and epidermal cell of the root tips. Absorption of water by individual root hair has been demonstrated experimentally. From the epidermal cells the water passes through successive rows of thin walled cortical cells and then through the cells of endodermis.
After passing through the endodermis water moves into xylem tissue ducts. Once in the xylem ducts its general direction of movement is upward. The xylem tissue is continuous from just back of the tips of roots, through the roots, into and through the stems, the petioles of leaves and ultimately terminates in the mesophyll of the leaf.
The xylem tissue through which the water moves thus a continuous system with the body of the plant.
Water in the root moves through two pathways:
(i) Apoplast pathway, and
(ii) Symplast pathway.
E. Munch of Germany introduced apoplast-symplast concept in 1932. He suggested that the interconnecting cell walls and intercellular spaces including the water-filled (or air-filled) xylem elements should be considered as a single system and called the apoplast.
The remaining part of the plant consists of the protoplasts of the cells. Evidence based upon electron micrograph now indicates that the cytoplasm connected from cell to cell via the plasmodesmata also constitutes an interconnected system. This system of interconnected protoplasm (excluding vacuoles) was termed by Munch the symplast.
Major proportion of water flow in the root cortex occurs via the apoplast, as cells of cortex are loosely packed, and therefore, cortex does not show any resistance.
However, apoplastic water movement beyond cortical region is blocked by casparian strip, present in endodermis. The casparian strip consists of a wax-like substance called suberin, which blocks water and solute movement through the cell wall of endodermis.
Thus, beyond endodermis, water is forced to move through cell membranes. Such movement of water through cell membrane is called transmembrane pathway. In this pathway water may also cross through the tonoplast surrounding the vacuole.
Once the water reaches root xylem, transpiration pull drives the water to move the leaves through the stem.