The phloem is one of the two main vascular tissues in plants, forming an extensive transportation network throughout the organism. This living tissue acts as the internal distribution system, ensuring all parts of the plant receive the organic compounds necessary for survival, growth, and reproduction. Found alongside the xylem, which transports water and minerals, the phloem is responsible for the long-distance movement of substances to sites where they are needed or stored. Its proper functioning is fundamental to the plant’s ability to maintain its structure and coordinate developmental processes.
The Primary Role in Plants
The primary function of the phloem is the translocation of photosynthates, which are soluble organic compounds produced during photosynthesis, mainly the sugar sucrose. Sucrose is the plant’s main form of energy currency, and the phloem transports it from photosynthetic organs, typically mature leaves, to all non-photosynthetic parts. This movement ensures that tissues unable to produce their own food, such as roots, developing fruits, and growing buds, are supplied with the energy required for metabolic activities and cell construction.
Sucrose is the most abundant substance in the phloem sap, but the tissue also serves as a conduit for a variety of other molecules. Amino acids, the building blocks of proteins, are transported to growing regions where protein synthesis is rapid. The phloem also carries signaling molecules, including plant hormones and specific messenger RNA (mRNA) sequences. This complex fluid composition coordinates growth, development, and environmental responses across large distances in the plant body.
Specialized Cells for Transport
The phloem tissue is composed of several cell types, but transport occurs within the sieve tube elements, the main conducting cells in flowering plants. These cells are arranged end-to-end, forming continuous channels for material flow. To maximize space for sap flow, a mature sieve tube element lacks a nucleus and many other organelles, such as ribosomes and a large central vacuole, resulting in a highly reduced internal structure.
Adjacent to each sieve tube element is a companion cell, which provides the necessary metabolic support. The companion cell retains a dense cytoplasm, a nucleus, and numerous mitochondria, performing the regulatory and energetic functions the sieve tube element cannot manage alone. The two cells are connected by specialized junctions called plasmodesmata, which facilitate the exchange of substances, including the active loading of sugars into the sieve tube.
The Process of Nutrient Movement
The movement of sap through the phloem is explained by the pressure-flow hypothesis, which relies on a gradient of hydrostatic pressure to drive the bulk flow of the sugar solution. The process begins with active loading, where sucrose is transported, often against a concentration gradient and requiring metabolic energy, from the producing cells into the sieve tube elements at the source. This movement of sugar significantly increases the solute concentration within the sieve tube.
The increased solute concentration causes the water potential within the sieve tube to decrease, prompting water to move by osmosis from the neighboring xylem tissue into the phloem. This influx of water creates a substantial increase in hydrostatic pressure, or turgor pressure, within the phloem at the source end. This high-pressure area then drives the mass flow of the phloem sap down the tube toward the sink, where the pressure is lower.
At the sink end, the sugars are actively unloaded from the sieve tube elements into the surrounding sink cells, which are either consuming the sugar for energy or converting it into an insoluble storage form like starch. The removal of the sugar from the sieve tube increases the water potential at the sink. Consequently, water moves back out of the phloem and often returns to the adjacent xylem tissue, which lowers the hydrostatic pressure at the sink. The continuous difference in pressure established between the high-pressure source and the low-pressure sink sustains the steady, directional flow of the phloem sap throughout the plant.
Understanding Sources and Sinks
The phloem sap always flows from a sugar-supplying source to a sugar-consuming or storage sink. A source is any part of the plant that produces or releases sugars in excess of its own needs, such as a mature, photosynthesizing leaf. Storage organs, such as potato tubers or beetroots, can also act as sources when they break down their stored starch back into sucrose for export to other plant parts.
A sink is any part of the plant that requires a net import of sugars for growth, metabolism, or long-term storage. Typical sinks include non-photosynthetic organs like roots, developing fruits and seeds, and young, expanding leaves that have not yet matured enough to produce their own photosynthates. The relationship between sources and sinks is dynamic, changing based on the plant’s developmental stage and the time of year. For instance, a storage root is a sink during the summer as it accumulates starch, but it becomes a source the following spring when it remobilizes that stored sugar to fuel the growth of new shoots and leaves.