Transportation in Plants

Transportation in Plants Synopsis

Synopsis

Transportation in plants is the process in which a substance is absorbed or synthesised in one part of the plant and is transported to the other part of the plant.
In plants, materials such as gases, minerals, water, hormones and organic solutes need to be transported over short and long distances.
Short distance transport occurs through diffusion and cytoplasmic streaming accompanied by active transport.
Long distance transport occurs through the xylem and phloem. This transport is called translocation.

Diffusion

The movement of molecules or ions from the region of higher concentration to the region of lower concentration, until the molecules are evenly distributed throughout the available space is known as diffusion.

Importance of Diffusion in Plants

Diffusion helps in CO₂ intake and O₂ output in photosynthesis and CO₂ output and O₂ intake in respiration.
It is an effective means of transport of substances over very short distance.

Facilitated Diffusion

The spontaneous passage of molecules or ions across a biological membrane mediated by specific transmembrane carrier proteins without spending metabolic energy is called facilitated diffusion.
Water soluble substances such as glucose, sodium ions and chloride ions are transported by this method.

Active Transport

The process of transport of materials across the biological membrane with the help of a mobile carrier protein involving expenditure of energy in the form of ATP is called active transport.
It is a kind of uphill transport against the concentration gradient and is faster than passive transport.
Carrier proteins on the cell membrane act as pumps to transport substances across the membrane.

Comparison of Different Transport Mechanisms

Bulk Flow System

Bulk flow system is a long distance transport system to move distances at faster rates.
Water, minerals and food are generally moved by a mass/bulk flow system.
A mass flow or bulk flow system is responsible for the movement of substances in bulk or en mass from the sites of production or adsorption to the sites of storage or consumption as a result of pressure differences between the two sites.
Xylem is associated with the translocation of mainly water, mineral salts, some organic nitrogen and hormones, from the roots to the aerial parts of plants.
The phloem translocates a variety of organic and inorganic solutes, mainly from the leaves to the other parts of plants.

Mechanism of Water Absorption

The water-absorbing structure of the plant is the root hair zone.
A root hair is a unicellular tubular propagation of the outer wall of the epiblema.
When water is absorbed by the root hair and other epidermal cells, it moves centripetally across the cortex, endodermis, pericycle and finally enters the xylem.
Water moves from cell to cell along the concentration gradient.

Water Movement Up a Plant

The upward conduction of water in the form of a dilute solution of mineral ions from the roots through the stem to the aerial parts of plants is called the ascent of sap.
Several kinds of forces are required in order to maintain the continuous passage of sap through the plant.

Root Pressure

Root pressure is the hydrostatic pressure developed in the roots because of the continued inward movement of water through cell-to-cell osmosis which helps in the movement of ascent of cell sap upwards through the stem.
Root pressure develops because of active absorption which depends on the active accumulation of solute in xylem sap.
Root pressure usually develops during the night when absorption is maximum and transpiration is minimum.
Root pressure can be inhibited by using cyanide, lack of O₂ and low temperature.

Transpiration Pull

Continuous loss of water from the leaves due to transpiration makes the mesophyll cells absorb water from the adjacent internal mesophyll cells and compensate for the loss of water. This loss causes a water deficit in the xylem.
Rapid transpiration develops a pull or tension in the xylem called the transpiration pull.
Transpiration pull is called negative pressure because it develops from the aerial parts and progresses to the underground parts of plants.

Cohesion-Adhesion Forces

A force of mutual attraction present between the water molecules is called cohesive force.
Water molecules are also attached with the walls of the vessels and tracheids through a force called adhesive force.
Cohesive and adhesive forces work together to maintain the continuity in between water and the cell wall.
The forces are responsible for maintaining unbroken continuity of the water column from the roots to the leaves.
This water column is pulled upwards continuously without breaking, from the roots to the leaves by transpiration.

Transpiration

The loss of water in the form of vapour from the aerial parts of the plant is called transpiration.
Factors such as temperature, light, humidity, wind speed affect the rate of transpiration.

Structure of Stomata

Stomata are tiny apertures found on the epidermis of leaves and young green stems.
Each stoma is surrounded by two epidermal bean-shaped guard cells.
Guard cells are bordered by one or more modified epidermal cells called subsidiary or accessory cells.

Kinds of Transpiration

Mechanism of Opening and Closing of Stomata

The surfaces of spongy mesophyll cells in the leaf are exposed to intercellular spaces.
These cells give out a thin film of water. Water from this film evaporates.
Water vapour formed saturates the air in the intercellular spaces, diffuses into the connecting intercellular spaces and reaches the sub-stomatal space. Finally, it escapes in the atmosphere through the air.
This creates the pull in plant tissues.
Water is absorbed by the roots rises through the stem and reaches the tissues of leaves.

Xylem

It conducts water upwards in a plant.
Xylem also provides strength to the stem and helps the plant to stand upright.
It is located in the centre of the plant body.
Xylem mainly consists of tracheids and vessels.
Xylem vessels do not contain cytoplasm or nuclei.
Vessels are made of cellulose and lignin.
Tracheids are long, thin, spindle-shaped cells with pits in their walls.

Mechanism of Transport of Water and Minerals

Water enters the root hair through osmosis, and mineral ions enter the root cells by active transport.
Both water and minerals move upward from cell to cell through the cortex of the root by osmosis.
From the cortex, water and minerals are brought to the xylem.
The sap which contains water and dissolved minerals move upwards from the root cells to xylem. The upward movement of sap is called the ascent of sap.
The xylem vessels of the roots are in continuation with the xylem vessels of the stem.
Due to difference in concentration of water in roots and soil, a column of water is created which ensures that water reaches to the parts located at a height.

Transpiration is the loss of water in the form of water vapour from the aerial parts of a plant.
It occurs through openings called stomata.
Water loss through evaporation lowers the concentration of water inside mesophyll cells. Due to this, water enters mesophyll cells from neighbouring xylem vessels through osmosis.
As water evaporates from the leaves, a suction force is created. This force helps to draw more water up through the stem which causes the roots to absorb more water from the soil.
Higher the rate of transpiration, greater the rate of absorption of water and solutes from the soil.
Transpiration also helps in maintaining the temperature of the plant body.

Phloem

It conducts manufactured food from the leaves to different parts of the plant.
The food in the phloem can move in the upward and downward directions.
Phloem mainly consists of sieve tube cells and companion cells.

Sieve tubes are living cells of the phloem. They contain cytoplasm but no nucleus.
The end walls of cells form sieve plates.
Sieve plates have small pores in them which allow food to pass through the phloem.
Each sieve tube cell has a companion cell next to it.

Mechanism of Transport of Water and Minerals

Food synthesised during photosynthesis is loaded into sieve tubes by utilising ATP.
The presence of food inside the phloem develops the concentration gradient for water. Thus, water enters the phloem by osmosis.
Osmosis develops high pressure inside the phloem which transports the food from the phloem to plant parts where the food concentration is less.
This process is called translocation.
In spring, the sugar stored in root or stem tissues is transported to the buds.

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Transportation in Plants