Transportation in Plants

Plants take in CO₂, photosynthesize it into glucose having chemical energy and store it in leaves. Besides CO₂ plants need other raw materials for growth. These raw materials like nitrogen, phosphorous etc., are obtained from the soil by the roots. These raw materials need to be transported to each and every part of the plant, mainly to the leaves. If the distance between roots and leaves is small, then these raw materials can reach the leaves easily by the process of diffusion.

In case, distance between roots and leaves is more, then a proper system of transportation is required.

1.0Transpiration

The loss of water in the form of water vapours from the aerial parts of plant is known as transpiration.

Transpiration mainly occurs through stomata (about 80% to 90%) but it may also occur through cuticle (9%) and lenticels (1%).

Functions of Transpiration

1. It helps in absorption and upward movement of water.

2. It helps in temperature regulation. The plants are protected from the burning due to transpiration. Evaporation of water from leaf produces cooling effect. 

2.0Vascular System

The plant vascular system is a complicated network of conducting tissues that interconnects all organs and transports nutrients, water and minerals throughout the plant body. Transportation is done by a specialized vascular system which is present in the root, stem and leaves. Vascular system is made up of two types of vascular tissues :

1. Xylem

2. Phloem

In plants, the transport of materials can be divided into two parts -

(1) Transport of water and minerals through xylem tissue.

(2) Transport of food and other substances through phloem tissue.

(1) Transport of water and minerals [Ascent of Sap]

The plants require water and minerals for making food and performing other functions.

Plants absorb water from the soil through the roots. Thus, water has to be transported upwards to the other parts of the plant. This upward movement of water is called ascent of sap. It is called the sap as it contains many dissolved minerals. Ascent of sap involves root pressure and transpiration pull.

Ascent of sap is carried out by xylem tissue which consists of :-

(1) Xylem vessels

(2) Xylem tracheids

(3) Xylem fibers

(4) Xylem parenchyma

Xylem consists of four types of cells. Out of which only xylem parenchyma is living, rest all are non-living and lignified (lignin deposition). 

In flowering plants, xylem vessels and tracheids both conduct water and minerals upwards.

In non-flowering plants, vessels are absent, tracheids are the only conducting cells. 

In xylem tissue, vessels and tracheids of root, stem and leaves are interconnected to form a continuous system of water conducting channels reaching all parts of the plant.

3.0Absorption of Water and Minerals

The water and minerals are absorbed by land plants from the soil where it is present in the form of soil solution.

The main water and minerals absorbing organs are root hairs. The root hairs are directly in contact with the film of water in between the soil particles. Water gets into the root hairs by the process of osmosis. At the roots, cells in contact with soil actively take up ions (actively means by energy expenditure). This creates a difference in concentration of these ions between the root and the soil. Water, therefore, moves into the root from the soil.

Hence, there is steady movement of water into root xylem, creating a column of water that is steadily pushed upwards, from root cells (cortex to endodermis) to xylem vessels and tracheids. This is called lateral transportation.

Finally, the water and minerals in the vessels and tracheids move upward by the process called ascent of sap.

Root Pressure 

The pressure with which the water is pushed up in the xylem of the roots is called the root pressure. It is developed in the xylem due to metabolic activity of the root cells. Root pressure explains the transport of water to the leaves and other parts in the case of short plants like the herbaceous plants. However, in tall trees the root pressure is not sufficient to send the water up to the leaves. In these trees, transpirational pull together with root pressure transports water.

Transpiration pull and cohesion-tension theory

The main force responsible for upward movement of water is transpiration pull. It is generated in the leaves which pulls the water column filled in the xylem tracheids and vessels in the upward direction. 

Cohesion is the molecular attraction by which the particles of a body are united throughout the mass.

During day period, cohesion force (attractive force between water molecules) and transpiration pull helps in the upward movement of sap from roots to leaves.

The effect of root pressure in transport of water is more important at night. During the day when the stomata are open, the transpiration pull becomes the major driving force in the movement of water in xylem.

4.0Transportation of Food and Other Substances 

Sugar, amino acids and other substances are translocated from the site of synthesis to the site of storage through the phloem.

Transport of food from leaves to different parts of the plant is termed as translocation.

Translocation may be in an upward or downward direction depending on the need of the plant.

Translocation of food takes place in the sieve tubes with the help of adjacent companion cells of phloem tissue.

Phloem tissue consists of four components.

(1) Sieve tubes

(2) Companion cells

(3) Phloem parenchyma

(4) Phloem fibers

In phloem tissue, except for phloem fibers, all the cells are living. The mature sieve tube elements do not have nucleus.

Mechanism of Translocation

Translocation takes place with energy consumption in the form of ATP.

Sugar (sucrose) made in leaves is loaded into the sieve tubes of phloem by using energy from ATP.

It increases the osmotic pressure of the sieve tubes.

Water now enters into sieve tubes containing sugar by the process of osmosis.

Soluble material is then transferred from phloem tissue to other tissues which have less pressure than in the phloem.

Thus, according to plant requirements, the material is translocated from higher osmotic pressure to lower osmotic pressure areas.

For example, in the spring, sugar stored in root or stem tissue would be transported to the buds which need energy to grow.

Important Points:

• Heart in fishes is called venous heart because only deoxygenated blood pass through the heart.

• Vascular bundles consisting of xylem and phloem in the root, stem, leaf stalk and leaf veins are all continuous and form an unbroken system of tubes.

• Movement of water and minerals in plants is unidirectional i.e. from root to leaves only whereas translocation of food is bidirectional. 

• In anaemia, RBC count decreases.

• In the cold, WBC count increases.

• The left ventricle is the largest heart chamber.

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