While the xylem provides a unidirectional "bottom-up" flow of water and minerals, the plant's survival depends equally on the distribution of the chemical energy it manufactures. Translocation is the physiological process by which the products of photosynthesis and other organic nutrients are moved through the plant. Unlike transpiration, translocation is a multidirectional and energy-dependent process. The IGCSE 0654 Extended syllabus requires a clear definition of translocation and an understanding of the dynamic relationship between sources and sinks.

Translocation is formally defined as the movement of sucrose and amino acids in the phloem from regions of production (sources) to regions of utilization or storage (sinks).

The Phloem Tissue

The phloem is a living vascular tissue consisting of sieve tube elements and companion cells.

• Sieve Tubes: Form a continuous pipeline for the transport of phloem sap.
• Companion Cells: Provide the metabolic energy (ATP) required to actively load sucrose into the sieve tubes.

The direction of flow in the phloem is dictated by the relative positions of supply and demand, which shift throughout the plant's life cycle.

1. Sources: The Providers

A source is any part of the plant that releases sucrose or amino acids into the phloem.

• Primary Example: Mature leaves during the growing season, which produce an excess of sugars via photosynthesis.
• Secondary Example: Storage organs (like tubers or bulbs) at the beginning of a new growing season, when they break down stored starch into sucrose to power new growth.

2. Sinks: The Consumers

A sink is any part of the plant that uses or stores sucrose or amino acids.

• Growth Sinks: Actively dividing tissues such as root tips, shoot tips (apical meristems), and developing flowers or fruits.
• Storage Sinks: Tissues where nutrients are converted into insoluble forms for long-term use, such as developing potato tubers, seeds, or the cortex of the root.

A critical concept in plant physiology is that the status of an organ as a "source" or "sink" is not fixed.

• In Summer: The leaves are the sources, and the roots/tubers are the sinks (storing energy).
• In Spring: The roots/tubers become the sources (releasing stored energy), and the new leaf buds are the sinks (using energy to expand).

Translocation is the "logistics network" of the plant, ensuring that every cell—from the deepest root to the highest flower—receives the chemical energy and building blocks it needs. By mastering the source-sink model, we can understand how plants adapt their internal nutrient flow to survive dormant periods and maximize reproductive success.