Dear Medical Aspirants, This lesson starts with a discussion on Transport in plant in Series of lectures of Plant Physiology for NEET 2022 Exam.
Dr. Prahlad Sharma shares his various preparation strategies along with examples that will help you to crack the exam.
LONG DISTANCE TRANSPORT OF WATER
Water, minerals and food are generally moved by a mass or bulk flow system.
Mass flow is the movement of substances in bulk from one point to another as a result of pressure differences between the two points.
In mass flow, the substances are swept along at the same pace.
Bulk flow can be achieved either through a positive hydrostatic pressure gradient or a negative hydrostatic pressure gradient.
The bulk movement of substances through the conducting or vascular tissues of plants is called translocation.
The higher plants have highly specialised vascular tissues called xylem and phloem.
Xylem is associated with translocation of mainly water, mineral salts, some organic nitrogen and hormones, from roots to the aerial parts of the plants.
The phloem translocates a variety of organic and inorganic solutes from the leaves to other parts of the plants.
ABSORPTION OF WATER BY PLANTS
Water is absorbed along with mineral solutes, by the root hairs, purely by diffusion.
Root hairs are thin-walled slender extensions of root epidermal cells that greatly increase the surface area for absorption.
Once water is absorbed by the root hairs, it can move deeper into root layers by two distinct pathways:
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 roots.
The apoplastic movement of water occurs through the intercellular spaces and the walls of the cells.
The apoplast does not provide any barrier to water movement and water movement is through mass flow.
As water evaporates into the intercellular spaces or the atmosphere, tension develop in the continuous stream of water in the apoplast, hence mass flow of water occurs due to the adhesive and cohesive properties of water.
The symplastic system is the system of interconnected protoplasts.
Neighbouring cells are connected through cytoplasmic strands that extend through the structure called plasmodesmata.
During symplastic movement, the water travels through the cytoplasm and intercellular movement is through the plasmodesmata.
The inner boundary of the cortex, the endodermis is impervious to water because of a band of suberised matrix called the casparian strip.
Water molecules are unable to penetrate the layer, so they are directed to wall region and the water then moves through the symplast and again crosses a membrane to reach the cells of the xylem.
Once inside the xylem, water is again free to move between cells as well as through them.
Some plants have additional structures associated with them that help in water and mineral absorption.
A mycorrhiza is a symbiotic association of a fungus with a root system.
The fungus provides minerals and water to the roots, in turn the roots provide sugars and N-containing compounds to the mycorrhizae.
Some plants have an obligate association with the mycorrhizae. For example- Pinus seeds cannot germinate and establish without the presence of mycorrhizae.
As various 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 and this positive pressure is called root pressure.
Loss of water in its liquid phase is known as guttation.
Root pressure provide a modest push in the overall process of water transport and re-establish the continuous chains of water molecules in the xylem which often break under the enormous tensions created by transpiration.
Root pressure does not account for the majority of water transport and hence most plants meet their need by transpiratory pull.
Water is mainly ‘pulled’ through the plant, and that the driving force for this process is transpiration from the leaves, which is referred to as the cohesion-tension-transpiration pull model of water transport.
Most of it is lost through the stomata in the leaves and this water loss is known as transpiration.