Absorption Of Water Mechanism In-Plant | 2 Best Ways Of Water Absorption.
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In this article, we are going to learn Absorption Of Water Mechanism In Plant . in which we discuss how the plants absorb water from the soil (mechanism of absorption of water).
ABSORPTION OF WATER :
- Absorption is defined as the process by which one thing is absorbed or is absorbed by another.
- The uptake of water by the plant through the root is called absorption of water by plants.
- The plants take water from the soil through root hairs.
- Water is very essential for living and is also called the “liquid gold” of life.
- The water which is available for the plants for uptake from Soil is called capillary water.
- Plants absorb water from the soil in the form of a solution(water+mineral solutes).
- DIFFUSION is the net movement of anything (e.g. Atoms, ions, molecules) from a region of its higher concentration to a region of its lower concentration.
- The diffusion process is driven by a gradient of concentration. It is called ‘downhill transport‘
- Diffusion is a passive process i.e., it doesn’t involve the expenditure of energy.
- Diffusion is a slow process and is not dependent on a ‘living system’.
SITE OF ABSORPTION OF WATER :
- The function of water absorption is performed by the root of a plant.
- Root hairs are involved in the absorption process.
- Root hairs are present in the “Root hair zone” or “Zone of maturation” of the root.
- Water is absorbed along with mineral solutes, by the root hairs, purely by diffusion.
PATHWAY OF ABSORBED WATER :
- Once the water is absorbed by the root hairs into the plants, it can move deeper into root layers by two distinct pathways:
1) Apoplast pathway
. 2) Symplast pathway
APOPLAST WAY :
- Inside the plant, Apoplast is the space outside the plasma membrane within which material diffuses freely.
- The apoplast is the system of adjacent cell walls that is continuous throughout the plant, except at the Casparian strips of the endodermis in the root.
- The apoplastic movement of water occurs exclusively through the intercellular spaces and the cell walls and doesn’t involve the crossing of the cell membrane.
- It is gradient dependent and the Apoplast doesn’t provide any barrier to water movement and the movement is through mass flow.
- Most of the water flow in the root occurs via the apoplast.
- A Casparian strip is a band of waterproof tissue that is found on the side and the walls of the endodermis of the root.
- The strip prevents water from entering the pericycle except through the cytoplasm of the endodermal cells.
- The Casparian strip is made up of Suberin and sometimes lignin.
- The symplastic system is the system of interconnected protoplasts.
- Neighboring cells are connected through cytoplasmic strands that extend through plasmodesmata.
- During this movement, the water travels through the cells- their cytoplasm; and intercellularly moves through plasmodesmata.
- As the water has to enter the cells through the cell membrane, hence the movement is relatively slower.
- Movement is down the potential gradient.
MECHANISM OF WATER ABSORPTION :
- The water absorption in the plant occurs in two ways;
. 1) Active absorption
. 2) Passive absorption
- Active absorption refers to the absorption of water by roots with the help of ATP, generated by root respiration. As the root actively takes part in the process, it is called active absorption.
- According to Renner, active absorption takes place in low transpiring and well-watered plants.
- 4% of total water absorption is done by active absorption.
- Active absorption is absorption “by roots”.
There are 2 theories of active absorption
. 1) Osmotic theory
2) Non-Osmotic theory
OSMOSIS AND OSMOTIC PRESSURE:
- OSMOSIS: It is the movement of solvent in a solution from its higher concentration to its lower concentration through a semi-permeable membrane.
- Osmosis is a special type of diffusion, namely the diffusion of water molecules through a semi-permeable membrane.
- The net direction and the rate of osmosis depend on both the pressure gradient and concentration gradient.
- OSMOTIC PRESSURE(OP): The minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semi-permeable membrane.
- Water flows from low OP to high OP.
OSMOTIC THEORY OF WATER ABSORPTION:
- This theory was given by Atkins(1916) and Pristley(1923).
- According to this theory, the root cells behave as an ideal osmotic pressure system through which water moves from the soil solution to the root xylem along an increasing gradient of DPD.
NON OSMOTIC THEORY OF WATER ABSORPTION:
- This theory was given by Thimann(1951) and Kramer(1959).
- according to this theory, sometimes water is absorbed against a concentration gradient. This requires the expenditure of metabolic energy released from the respiration of root cells.
- This type of water absorption takes place when the OP of the soil solution is greater than the OP of the cell sap.
PASSIVE ABSORPTION :
- This mechanism is carried out without the utilization of metabolic energy.
- Here, only the root act as an organ of absorption or passage.
- It occurs in rapidly transpiring plants during the daytime, because of the opening of stomata and atmospheric conditions.
- The force of absorption of water is created at the leaf end i.e., transpiration pull.
- Passive absorption is absorption “through root”.
FACTORS AFFECTING ABSORPTION OF WATER IN PLANTS :
- Available Soil water: Dry soil decreases the rate of absorption. A decrease in Soil water reduces the rate of absorption.
- Soil temperature: Normal absorption takes place at a temperature of Soil between 20-35°C. An increase or decrease from this range inhibits absorption.
- Soil air: Absorption of water proceeds more rapidly in well-aerated soil.
- Soil concentration: The rate of absorption is inversely proportional to the concentration of minerals present in Soil.
- Transpiration: The rate of water absorption is directly proportional to the rate of transpiration.
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