Plants possess a sophisticated internal transport system, known as the vascular system, which allows for the efficient distribution of resources. Within this system, the phloem specializes in moving manufactured food, primarily sugars, from where they are produced to where they are needed for growth or storage. This ensures all parts of the plant receive the necessary energy and building blocks for survival.
The Phloem’s Structure
The phloem tissue is composed of specialized cells that facilitate transport. Two primary cell types are sieve tube elements and companion cells. Sieve tube elements are elongated cells that connect end-to-end, forming continuous tubes. These living cells lack a nucleus and most other major organelles at maturity, maximizing space for material movement. Their end walls, called sieve plates, have pores that allow passage between adjacent sieve tube elements.
Alongside each sieve tube element is a companion cell, which provides metabolic support. Companion cells contain numerous mitochondria, generating the energy (ATP) needed for active transport. They connect to sieve tube elements by small channels called plasmodesmata, allowing for substance exchange. Other cells, such as phloem parenchyma and phloem fibers, provide storage and structural support.
What Flows Through the Phloem
The primary sugar transported through the phloem is sucrose, a disaccharide made from one glucose and one fructose molecule. Sucrose is favored for long-distance transport because it is less reactive than glucose, preventing unintended chemical reactions in the phloem sap. Transporting sucrose, which contains more energy per molecule than glucose, is also more energy-efficient for the plant.
Beyond sucrose, the phloem also carries other dissolved organic compounds, including amino acids and various hormones that regulate plant growth and development. Some mineral ions, such as potassium and magnesium, can also be transported. These substances move from “source” areas, typically mature leaves, to “sink” areas, which are regions that consume or store these materials, such as roots, developing fruits, or growing tips.
How Transport Happens
The movement of substances through the phloem is best explained by the pressure-flow hypothesis. This theory proposes that a pressure gradient drives the bulk flow of phloem sap from source to sink. The process begins at a “source” where sugars, mainly sucrose, are actively loaded into the companion cells and then into the sieve tube elements. This active loading requires energy, often in the form of ATP, provided by the companion cells.
As sucrose concentration increases within the sieve tube elements at the source, the water potential inside these cells decreases. This causes water to move by osmosis from the adjacent xylem, which transports water, into the sieve tubes. The influx of water increases the internal hydrostatic pressure, or turgor pressure, within the sieve tube elements at the source. This elevated pressure then physically pushes the phloem sap, a sugary solution, along the sieve tubes towards areas of lower pressure, which are the “sinks”.
At the “sink” regions, such as roots or developing fruits, sugars are actively unloaded from the sieve tube elements for consumption or storage. As sugars are removed, the water potential within the sieve tubes at the sink increases, causing water to move out of the phloem and back into the xylem. This outflow of water reduces the turgor pressure at the sink, maintaining the pressure gradient that drives the continuous flow of sap from source to sink.
Why This Transport Matters
The transport of sucrose and other substances through the phloem is fundamental for a plant’s growth and survival. It allows non-photosynthetic parts of the plant, such as roots, stems, and developing flowers or fruits, to receive the energy and building blocks they need to grow and function. Without this supply, these parts would be unable to sustain themselves and would quickly cease to develop.
The phloem also plays a role in storing energy for future use. For example, excess sucrose produced during periods of high photosynthesis can be transported to storage organs like roots or tubers and converted into starch. This stored energy can then be mobilized when the plant needs it, such as during periods of dormancy or rapid growth. Furthermore, the phloem distributes signaling molecules, including hormones and small RNAs, throughout the plant, allowing for coordinated growth and responses to environmental changes. This intricate transport system ensures the plant can adapt and thrive in various conditions.