The survival of higher plant life depends on an intricate, long-distance transport system known as the vasculature. This network runs throughout the plant body and is composed of two primary tissues: the xylem and the phloem. These systems distribute necessary resources from where they are acquired or produced to every cell that needs them. They enable plants to achieve significant height and maintain complex biological processes.
Distinct Roles in Plant Survival
The fundamental difference between the two transport tissues lies in the materials they carry and the purpose of the transport. Xylem is primarily tasked with conveying water and dissolved inorganic nutrients, or minerals, from the soil up to the rest of the plant body. This upward movement hydrates the plant and supplies the raw materials required for photosynthesis in the leaves. The xylem also provides significant mechanical support, allowing the plant to stand upright and reach for sunlight.
In contrast, the phloem’s primary function is to distribute the plant’s energy, mainly in the form of sugars like sucrose. These organic nutrients are manufactured during photosynthesis in the leaves, known as “source” tissues. The phloem delivers this nourishment to all non-photosynthetic parts of the plant, such as roots, developing fruits, and growing shoot tips, collectively known as “sink” tissues. This system ensures that energy is effectively allocated to growth, metabolism, and storage throughout the organism.
Specialized Cellular Structures
The contrasting functions of the xylem and phloem are enabled by their different cellular compositions. The functional components of the xylem, namely the tracheids and vessel elements, are non-living cells at maturity. These cells die after forming, leaving behind hollow tubes with no internal contents or end walls, creating a continuous pipeline for water flow. Their cell walls are thickened with lignin, which provides mechanical support and prevents the vessels from collapsing under the negative pressure created during water transport.
The conducting cells of the phloem, called sieve tube elements, are uniquely living cells, though they lack a nucleus and most other organelles at maturity. They are arranged end-to-end to form the sieve tube, and the walls separating them are perforated with pores, resembling a sieve, which facilitates the passage of the sugary sap. Alongside each sieve tube element is a companion cell, which retains a full complement of organelles, including a nucleus. The companion cell performs the metabolic functions for both itself and its associated sieve tube element, including the active movement of sugars into and out of the sieve tube.
Mechanics of Transport
The physical processes that drive movement through these two tissues are entirely different, reflecting their specialization. Water and minerals move through the xylem primarily through a passive process known as the cohesion-tension theory. This movement does not require the plant to expend metabolic energy, as the force is generated externally by the evaporation of water from the leaves, a process called transpiration. As water vapor leaves the stomata, it creates a negative pressure, or tension, that pulls the entire column of water up from the roots. The flow of xylem sap is unidirectional, moving only upward from the roots to the leaves.
Conversely, the transport of sugars through the phloem, known as translocation, is an active process that requires metabolic energy (ATP). This movement is explained by the pressure-flow hypothesis, which relies on the active loading of sugars into the sieve tube elements at the source tissues. This high sugar concentration creates a low water potential, causing water to flow into the phloem by osmosis and generating positive pressure. This pressure gradient pushes the sap toward the sink tissues, where the sugars are actively unloaded, and the water returns to the xylem. Phloem transport is bidirectional, moving organic nutrients both up and down the plant depending on the location of the source and sink tissues.