Transport In Plants - Session 2

Plants need water and nutrients to grow and thrive. These vital resources are in the soil surrounding their roots. But how do these nutrients and water reach other parts of the plants? The movement of water and solutes through plant tissues occurs through two main pathways. These are apoplast pathway and the symplast pathway. Let us understand these pathways.

The apoplast pathway is a complex system of intercellular spaces, extracellular matrix, and cell walls that allows for the movement of water, nutrients, and signaling molecules throughout the plant. Water uptake through the roots is one of the primary functions of the apoplast pathway. Water enters the root hairs and moves through the cell walls of the root cells. It then passes from one cell to another through the intercellular spaces. The apoplast pathway does not involve the crossing of any cell membranes. This makes it a faster route for transport.

The symplast pathway involves the movement of substances through the living cells of a plant. It relies on interconnected cytoplasmic strands called the symplast. Symplast is a continuous network of cytoplasm connected via plasmodesmata. Plasmodesmata are tiny channels that connect plant cells together. They act like little tunnels that allow substances to move between cells. These channels are found in the cell walls of plant cells.

Transpiration is a natural process in which plants release water vapor into the air through tiny openings on their plant-leaves called stomata. These stomata act like small pores that allow gases, including water vapor, to pass in and out of the plant. When the roots of plant absorb water from the soil, it travels up through the stem and into the plant-leaves. Once in the plant-leaves, the water molecules evaporate from the surface of the cells and escape through the stomata. This release of water vapors helps plants stay cool, maintain their shape, and transport nutrients throughout their tissues.

During transpiration, water loss from the plant-leaves creates a negative pressure gradient. This negative pressure gradient is also known as tension. This tension pulls water upward from the roots to the plant-leaves through the xylem vessels. The continuous column of water within the xylem helps maintain this flow. This upward movement of water is called the transpiration pull. It enables the transportation of essential minerals and nutrients from the roots to the rest of the plant cells. Additionally, as water evaporates from the plant-leaves, it helps regulate the temperature of the plant by making it cooler. It is similar to how sweating cools our bodies.

The cohesion tension theory explains the transport of water through xylem vessels. It is based on the properties of water and the cohesive forces between water molecules. According to the theory, water molecules are cohesive. Cohesive means they tend to stick together. This cohesion is due to hydrogen bonding between water molecules. This cohesive force creates a continuous column of water within the xylem vessels. Water molecules also stick to walls of xylem through hydrogen bonding. This process is called adhesion.

We know that plants undergo loss of water through transpiration. Transpiration creates a negative pressure or tension in the xylem. This tension, combined with the cohesive forces, pulls water upward from the roots to the plant-leaves. The cohesion tension theory explains that when water molecules evaporate from the surface of the leaf cells, it creates a pull on the water column in the xylem. This pull is transmitted through the cohesive forces from the leaf cells downward toward the root cells. As water molecules are lost from the plant-leaves, more water molecules are pulled up from the roots to replace them.

In plants, the source sink relationship refers to the movement of sugars from areas of production to areas of utilization or storage. The sugar is known as assimilate. The area of production of assimilate is called source. The area of utilization or storage of assimilate is called sink. Source sink relationship plays a vital role in the distribution of nutrients and energy throughout the plant.

Source tissues are regions where sugars are produced through photosynthesis. These source tissues have a high concentration of sugars. Through the process of photosynthesis, plants convert sunlight, carbon dioxide, and water into sugars. These sugars are primarily glucose. These sugars are then transported from the source tissues to other parts of the plant.

Sink tissues, on the other hand, are regions where sugars are actively consumed or stored, such as flowers, fruits, or developing roots. These sink tissues have a high necessity for energy and nutrients to support their growth, development, and reproductive processes.

The movement of sugars from source to sink occurs through a process called translocation. The transport of sugars primarily takes place through the phloem. Phloem is a specialized vascular tissue in plants. The phloem consists of sieve tubes, companion cells, and other supporting cells.

The movement of sugars in the phloem is facilitated by the pressure flow mechanism. Sugars are produced in the source tissues, such as mature plant-leaves, where photosynthesis occurs. Within the source tissues, sugars are actively transported into the companion cells. Companion cells are located next to the sieve tube elements. This active transport of sugars creates a high concentration of sugars in the companion cells.

The high concentration of sugars in the companion cells creates an osmotic gradient. We know that osmosis is the process by which water molecules move from an area of low solute concentration to an area of high solute concentration. In this case, water moves from adjacent cells or the xylem into the companion cells. The influx of water into the companion cells increases their turgor pressure. This increased pressure is transmitted to the sieve tube elements. This is because the cytoplasm of companion cells is connected to the sieve tubes via plasmodesmata.

The high turgor pressure in the sieve tubes at the source tissues creates a pressure gradient along the phloem tube, from source to sink. This pressure gradient drives the mass flow of sugars and water in the phloem. Now we know that source tissues have high pressure and sink tissues have low pressure. The sugars move from areas of high pressure to areas of lower pressure.

At the sink tissues the sugars are unloaded from the phloem. Sugars are unloaded first into companion cells through osmosis and diffusion. After that it moves to sink cells. The sugars are then used for growth, storage, or energy production.