Long Distance Transport in Xylem and Phloem for NEET

Transport over long distances in plants helps transfer organic solutes between the xylem and phloem. This takes place through extensive exchange processes. The transfer of the nutrients between xylem and phloem is very important for mineral nutrition in plants.

Diffusion and active transport are not the best options for the transport of water and minerals from the roots to the leaves and vice-versa because diffusion is a slow process. These processes are only permitted to be used for short distances such as one cell to the adjacent cell or in situations such as this.

So, to transport substances at a faster and more efficient rate, long-distance transport mechanisms had to be developed. Thus, in vascular plants, the xylem and phloem evolved to facilitate and accelerate substance transport.

Transport over Long Distances in Plants

• Water and food (nutrients) are transported through two main tissues in plants: xylem and phloem.

• Long-distance transport is critical for shoot nutrition and removing toxic elements from leaf tissues.

• Long distance transport of water in the xylem occurs by the gradient in hydrostatic pressure, also called root pressure, and by the gradient in the water potential.

• Long distance transport in phloem is through the sieve tube cells of phloem, which are living cells.

• The long-distance transport of water, as well as other inorganic and organic solutes, takes place from roots to shoots, where the stems play a pivotal role.

• Xylem to phloem transfer, which takes place in the stem, is influenced by the rate of volume flow in the xylem.

Xylem

Xylem is a transport tissue observed in vascular plants with phloem. The pivotal function of the xylem is the transportation of water and nutrients from the roots to the other parts of plants like shoots and leaves. It also provides support to the plant. The word “Xylem” was coined by scientist Carl Nageli in the year 1858.

Types of Xylem

Broadly, the xylem is classified into two types: primary and secondary. Although they have the same function, they are classified based on their growth.

• Primary Xylem: It is formed due to the primary growth of the plant. It can be seen in the tips of shoots, roots, and flower buds. It assists in the growth of the plants and for the roots to grow long. It is known as primary xylem as it occurs first in the growing season.

• Secondary Xylem: It is formed due to the secondary growth of the plant. Its main function is to make the plant wider as time goes by. It happens every year after the primary growth takes place. It forms dark rings around the trees, which enables us to determine the age of the tree. Conifers and angiosperms are two groups where we can observe secondary xylem.

Cells of Xylem

The xylem cells are classified into four types. They are described below.

• Tracheids: They are the primary cells of the xylem. They have an elongated shape with a tube-like structure, tapering at the end. The majority of the cell wall of tracheids is perforated with pits. The secondary thickening in tracheids have various patterns like spiral thickening, annular thickening, reticulate thickening, pitted thickening, and scalar form thickening.

• Vessels: They are also referred to as trachea, which come under the second category of the xylem cells. They consist of short and tube-like cells. They include segments or vessel elements.

• Xylem Fibres: These dead cells contain a central lumen and lignified walls. It provides mechanical support to the plant and helps to transport water from the roots to a different part of the plant.

• Xylem Parenchyma: These are the only living cells in the xylem that contain starch and fat. Xylem parenchyma can help in the transport of water over short distances.

Phloem

Phloem is a transport tissue, which is seen in vascular plants. It helps in the transportation of soluble organic compounds. Phloem is also known as the food conducting tissue formed by living cells that use energy in the form of ATP and turgor pressure to transport organic compounds to the organs of the plants like buds, roots, flowers, and fruits.

Cells of Phloem

The phloem cells are classified into five types. They are described below.

• Sieve Elements: The sieve elements of the phloem are narrow and elongated cells that are joined together to form the sieve tube of the phloem. The sieve elements are considered highly specialized cells seen in plants. Once they mature, there lack a nucleus and organelles like Golgi bodies, ribosomes, and cytosol. This is to increase the space in the cell for translocation.

• Sieve Plates: The sieve plates of phloem are located between the connections of sieve member cells or are also modified as plasmodesmata. They are large and thin and have pores that help exchange materials between the sieve element cells.

• Companion Cells: A sieve element cell is always associated with a companion cell in angiosperms. In gymnosperms, the companion cell is replaced by an albuminous cell or Strasburger cell. These cells have a nucleus present, which is filled with dense cytoplasm. The cytoplasm contains numerous mitochondria and ribosomes. Due to the presence of these organelles, the companion cells perform many metabolic activities and other cellular functions. The companion cells and sieve cells are joined together through plasmodesmata.

• Phloem Parenchyma: The phloem parenchyma consists of many cells that make up the filler of the plant tissues. The parenchyma is made up of thin and flexible walls, which are made of cellulose. Its main function is the storage of proteins, fats, and starch. In some plants, it also stores resins and tannins.

• Phloem Sclerenchyma: The phloem sclerenchyma is the most important tissue present in the phloem. It provides strength, stiffness and support to the plant. Phloem sclerenchyma consists of sclereids and fibres. Both are dead when they mature and have a secondary thick cell wall.

Mass Flow or Bulk Flow

A mass flow or bulk flow system is the transfer of bulk material from a place of adsorption or production to a region of consumption or storage due to the pressure difference between the two regions. Mass flow occurs via a positive hydrostatic or negative hydrostatic gradient.

Apoplast and Symplast Pathway

Apoplast Pathway

• In the apoplast pathway, water is transported from the root hairs across the cell walls of intervening cells to the xylem.

• This pathway occurs in the cortex and ends when it reaches the endodermis due to the presence of a band of impermeable matrices called Casparian strips.

• The apoplast route facilitates the transportation of water and solutes across a tissue or organ.

• It is composed of non-living parts.

• Here the movement of water occurs by passive diffusion.

• It doesn’t show resistance to the water movement.

Apoplast and Symplast Pathway

Symplast Pathway

• In the symplast pathway, water moves between the cytoplasm and vacuole, through the plasma membrane and plasmodesmata, and across the cortex of plant cells.

• This route is slower than the apoplast route.

• It is composed of living parts.

• Here the movement of water occurs by osmosis.

• It shows some resistance to water movement.

Conclusion

The vascular tissues of a plant transport nutrients throughout the plant in the same way that the circulatory system transports nutrients throughout the human body. While blood is the primary nutrient solvent in humans, water is the primary nutrient solvent in plants. Animals, on the other hand, use blood pressure to transport nutrients throughout the body, whereas plants use gravity and the cohesive properties of water. Plants lack the ability to actively transport water to their individual cells.

Capillary action, on the other hand, allows water to flow upward against gravity. Plant vascular tissue is classified into two types: xylem, which transports water, and phloem, which transports organic molecules such as glucose.

Apoplasts and symplasts are the two routes that plants use to transport water from root hair cells to the root xylem. Apoplasts include non-living parts of plants, such as cell walls and intracellular spaces. Symplasts contain living parts of the plant, such as the protoplasm.