Transport in Plants
The plants obtain their nutrients and water from their surroundings especially from water and soil. The transport in plants takes place through different means like diffusion, facilitated diffusion and active transport.
Diffusion is a passive and random movement of ions or molecules. In diffusion movement of substances takes place from higher concentration to lower concentration. It depends on various factors like gradient of concentration, pressure, temperature and permeability of the membrane.
The facilitated diffusion is the movement of substances across the membrane with the help of membrane proteins without the expenditure of energy (ATP). The protein forms a channel in the membrane which allows molecules to pass through.
The transport of molecule can be: symport, antiport and uniport.
The transport of molecules against the concentration gradient with the help of energy (ATP) is called active transport. The membrane proteins called pumps carry out Active transport.
Water is essential and plays an important role in different activities of plants. Water potential is the potential energy of water relative to pure water in reference conditions. The solute potential (Ψs) and pressure potential (Ψp) are the two main components that determine water potential.
Water potential of any solution can be represented as Ψw = Ψs + Ψp.
The osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration. The net direction and rate of osmosis depends on both the pressure gradient and concentration gradient.
The plasmolysis is the contraction of plant cell membrane away from its cell wall due to the loss of water through osmosis. The imbibition is defined as the swelling up of the cell colloids due to absorption of water or water vapour.
The long distance transport of substances in a plant cannot take place only by diffusion as it is a slow process. The water, minerals, and food are generally moved by a mass or bulk flow system.
There are two pathways through which the water moves deeper into the root layer: Apoplastic pathway and Symplastic pathway.
In plants water moves in upward direction against the gravity and is then transported to various parts of plant. This mechanism is described by root pressure and transpiration pull.
Transpiration is the loss of water from plants. It occurs through the stomata in the leaves during the exchange of gases like CO2 and O2. The process of transpiration depends upon various factors: light, humidity, temperature and wind speed.
Plants fulfill their their nutrient requirement from minerals and water in the soil.
Most of the minerals enter the root by active absorption while some minerals enter through passive absorption.
Once ions have reached the xylem, the transport of mineral ions to all parts of the plant is through the transpiration stream.
The mechanism used for the translocation of sugars from source to sink is called the pressure flow hypothesis.