Final answer:
Companion cells are specialized for the transport of sugar between the apoplast and symplast, providing metabolic support and energy for the phloem's sieve-tube elements that carry sugars and amino acids.
Step-by-step explanation:
Cells that are specialized for the transport of sugar between apoplast and symplast are companion cells. These cells work alongside the phloem's sieve-tube elements (STEs), which are arranged end to end to form a long sieve tube, responsible for the transport of organic substances like sugars and amino acids.
Companion cells are essential for providing metabolic support to the STEs, which have reduced cytoplasmic contents and lack certain organelles due to their specialized function in transport. The companion cells possess more ribosomes and mitochondria compared to STEs and are crucial for the production of energy necessary for the active transport of sugars into the phloem cells.