Final answer:
Amino acids and sucrose are the pair of substances transported in the phloem of plants, with the phloem serving as a conduit for distribution of these essential organic compounds from sources to sinks within the plant.
Step-by-step explanation:
The pair of substances transported in the phloem of plants are amino acids and sucrose. Phloem is a type of vascular tissue responsible for transporting organic compounds from the sites of photosynthesis (sources) to other parts of the plant that need them (sinks). This transport includes the sugars, particularly sucrose, which is the product of photosynthesis, and various organic nitrogen compounds, including amino acids.
These substances are moved throughout the plant in the form of an aqueous solution known as phloem sap, which can contain up to 30 percent sugar. Sucrose is actively transported into phloem sieve-tube elements (STEs) and is then translocated from source to sink, providing the energy required for growth or stored for later use. Amino acids, which are important for protein synthesis, are also transported via the phloem to various parts of the plant.