Means of Transport
* Diffusion, facilitated diffusion and active transport are three means of transport in plants.
* Diffusion is a passive process; it requires no expenditure of energy.
* Diffusion facilitated by proteins without any energy expenditure is referred to as facilitated transport.
* Unlike diffusion, active transport requires energy to move molecules against the concentration quotient.
* Based on direction, short-distance transport in plants can be differentiated into three types-symport, antiport and uniport.
Imbibition and Water Potential
* Seeds absorb water by a special type if diffusion called imbibitions, where solids absorb water, causing an increase in their volume.
* The water potential gradient comprises two main components: solute potential and pressure potential.
* Pure water has the greatest possible water potential, whereas the water potential of all solutes is lower than that of pure water.
* The magnitude of the lowering of water potential due to the addition of solutes is called solute potential.
* Pressure potential, which is denoted by Ψp, is usually positive.
Osmosis
* The cell wall is freely permeable whereas the cell membrane is a semi-permeable membrane.
* The diffusion of water across a semi-permeable membrane like the cell membrane is referred to as osmosis.
* This potential of water molecules to move from a hypotonic solution to a hypertonic solution across semi-permeable membrane is called osmotic potential.
* The external pressure required to prevent the water from diffusing is known as osmotic pressure.
* Numerically, osmotic pressure is equal to osmotic potential. However, their signs differ.
Plasmolysis
* It is the solution that surrounds a plant cell that decides the direction of the water movement.
* The surrounding solution can be categorised into three types: isotonic, hypotonic and hypertonic.
* When a cell is placed in a hypertonic solution and water moves out of the plant cell, the cell membrane shrinks away from the cell wall. This phenomenon is called plasmolysis.
* When cells are placed on a hypotonic solution, cytoplasm builds turgor pressure against the cell wall.
* If the cell is placed in an isotonic solution, there is no net flow of water from either inside or outside the cell. The cells are said to be flaccid.
Long Distance Transport in plants
* Diffusion is a painfully slow process. Therefore, a mass or bulk flow system transports water, minerals and food across long distances in plants.
* The bulk flow system is a phenomenon that arises out of pressure differences between two points.
* The bulk flow system is conducted through the plants’s vascular tissues, xylem and phloem. The process is referred to as translocation.
* Xylem transports water, minerals and hormones upwards from the plant’s roots while phloem transfers organic or inorganic solutes from leaves to other parts of the plant.
How do Plants Absorb Water?
* Root hairs present on root tips absorb both water and minerals.
* After water is absorbed, it moves deeper into the root layers by either the apoplast pathway or symplast pathway.
* The apoplastic pathway is a continuous system of adjacent cell walls in a plant, interrupted only by waxy, suberised casparian strips in the root endodermis.
* The symplastic pathway is a system of interconnected protoplasms.
* Some plants have additional structures, like mycorrhiza, which help their roots in absorption.
Upward Water Movement in a Plant
* The upward movement of water in plants is made possible by a combination of root pressure and transpirational pull.
* When mineral ions from the soil are also pushed into the root vascular tissues along with water, the pressure inside the xylem increases. This positive pressure is known as root pressure.
* Transpiration pull is generated by loss of water from leaf stomata via transpiration.
* As Cohesion-tension helps in the transpirational pull, it is also known as the cohesiontension-transpirational pull model of water transport.
Transpiration
* Transpiration is the loss of water due to evaporation that occurs through pores in leaves called stomata.
* The opening and closing of stomata is directly impacted by the turgidity of guard cells.
* The transpiration pull for the ascent of water in xylem cells is made possible by three physical properties of water-cohesion, adhesion and surface tension.
* Transpiration not only creates this pull for the absorption and transport of water for photosynthesis; it also transports minerals from the soil to all parts of the plant.
Transport of Minerals
* Minerals can be absorbed from the soil both passively and by active transport.
* Plants can also remobilise minerals from older, dying parts to young, developing pars where they are required.
* Xylem and phloem exchange small amounts of the minerals they carry.
* Minerals are transported inside plants via both xylem and phloem elements.
Transport of Food
* Plant food is primarily sucrose.
* It is transported by vascular tissue called phloem, from a source to a sink.
* As simple experiment called girdling can be used to illustrate the role of phloem cells in trees.
* Phloem tissues are made up of sieve tube cells, which are long columns with holes in their end walls. These holes are called sieve plates.
* The phloem sap translocation from source to sink takes place by a mechanism called the pressure flow hypothesis.