Vascular system: xylem and phloem
Plants, unlike single-celled organisms, are large and complex, requiring specialized long-distance transport systems to move essential materials throughout their bodies, as simple diffusion is insufficient for their size and low surface area to volume ratio. This transport system is known as the vascular system, which is made up of two primary tissues: xylem and phloem. These systems collectively ensure that substances absorbed or produced in one part of the plant can reach all other cells where they are needed or stored.
It is important to note that, unlike mammalian circulatory systems which transport gases, plant vascular systems do not transport gases like carbon dioxide and oxygen. Instead, these gases diffuse through air spaces within the plant's stems, roots, and leaves, which is sufficient due to plants' lower metabolic energy demands compared to animals.
Xylem
Function and Transport: The xylem tissue primarily transports water and inorganic ions (mineral salts). Water moves from the roots up to the leaves. This movement is unidirectional. Xylem also provides structural support to the plant due to its lignified walls.
Structure: Xylem vessels are very long, tube-like structures formed from dead cells (vessel elements) joined end to end. They have no end walls, creating an uninterrupted tube for water flow. Their walls are thickened with lignin, a woody substance that provides strength and makes them impermeable to water. Pits in the walls, where there is no lignin, allow water and mineral ions to move into and out of the vessels and into surrounding tissues. Xylem vessels also have a narrow diameter, which contributes to adhesion and helps prevent air locks.
Mechanism of Transport (Cohesion-Tension Theory): Water movement up the plant is primarily a passive process driven by transpiration.
Transpiration: Water evaporates from the leaves (specifically from the wet cell walls of mesophyll cells into the air spaces) and then diffuses out of the leaf through specialized pores called stomata. This creates a water potential gradient from the soil (higher water potential) to the atmosphere (lower water potential).
Tension/Suction: The evaporation of water from the leaves creates a tension (suction) at the top of the xylem column, which pulls more water into the leaf. This is known as the transpirational pull.
Cohesion and Adhesion: Water molecules are cohesive (they stick together due to hydrogen bonds), so when some molecules are pulled into the leaf, others follow, creating a continuous column of water in the xylem. Water molecules are also adhesive (attracted to the hydrophilic cellulose and lignin in the xylem walls), which prevents the water column from pulling away from the walls and helps maintain its integrity against gravity and tension.
Mass Flow: The movement of water and dissolved mineral ions up through xylem vessels is by mass flow, meaning all the water molecules and dissolved solutes move together.
Root Pressure: Water is continuously taken in by the roots by osmosis and "pushed" up the xylem, contributing to the pressure difference. Water enters root hairs by osmosis, moving from the soil (higher water potential) across the root cortex to the xylem (lower water potential). The Casparian strip in the endodermis blocks the apoplast pathway, forcing water into the symplast pathway and allowing the plant to control mineral ion uptake.
Phloem
Function and Transport: Phloem tissue transports organic substances (assimilates), primarily sucrose (converted from glucose produced in photosynthesis) and amino acids, to where they are needed in the plant. These substances are moved from "sources" (where they are produced or released from storage, e.g., mature leaves or sprouting tubers) to "sinks" (where they are used for growth, metabolism, or stored, e.g., growing points, fruits, roots, or developing tubers). Phloem transport is bidirectional, meaning substances can move both up and down the plant in different sieve tubes simultaneously.
Structure: Phloem tissue consists of sieve tube elements and companion cells.
Sieve tube elements are living cells joined end to end to form continuous tubes. They have a thin layer of cytoplasm and typically lack a nucleus and few other organelles to reduce resistance to flow. Their end walls are perforated, forming sieve plates with pores that allow solutes to pass through.
Companion cells are associated with each sieve tube element and carry out the living functions for the sieve cells, including providing the energy (ATP) needed for active transport of solutes. They have a dense cytoplasm, a nucleus, and numerous mitochondria. Plasmodesmata connect the cytoplasm of companion cells to the sieve tube elements, allowing the flow of substances. Phloem walls are not lignified.
Mechanism of Transport (Mass Flow Hypothesis): Phloem sap moves by mass flow. This is an energy-requiring (active) process, meaning the plant uses ATP to create the necessary pressure differences.
Loading at Source: At the source (e.g., leaves), solutes (sucrose) are actively loaded from companion cells into the sieve tubes of the phloem. This involves proton pumps (pumping H+ ions out of the companion cell into the cell wall, creating a high H+ concentration) and co-transporter proteins (linking the movement of H+ ions back into the cell with sucrose transport).
Water Entry and High Pressure: The active loading of sucrose lowers the water potential inside the sieve tubes, causing water to enter from the xylem and companion cells by osmosis. This influx of water creates a high hydrostatic pressure inside the sieve tubes at the source end.
Mass Flow: This pressure gradient (from high pressure at the source to lower pressure at the sink) pushes solutes along the sieve tubes towards the sink.
Unloading at Sink: At the sink, solutes are removed from the phloem to be used or stored. This removal increases the water potential inside the sieve tubes, causing water to leave by osmosis, which lowers the pressure.
Distribution within the Plant: Xylem and phloem are found together in vascular bundles.
In stems, vascular bundles are typically arranged in a ring near the outside, with phloem on the outside (closer to the epidermis) and xylem on the inside (closer to the center).
In roots, the vascular tissue is in a single, central stele, with the xylem forming an 'X' shape in the center surrounded by phloem bundles.
In leaves, vascular bundles form the midrib and veins, with xylem typically on the upper side (closer to the upper epidermis) and phloem on the lower side.
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