What are companion cells? Give their functions. In your answers, provide examples by comparing them to the sieve cells

Answer 1

Companion cells are specialized cells that are closely associated with sieve tube elements in angiosperms (flowering plants). They are derived from the same mother cell and are connected through numerous plasmodesmata, which allow for communication and material exchange between the two cell types.

Functions of companion cells:
1. Assimilate loading: Companion cells play a vital role in the loading of assimilates, such as sugars and amino acids, into the sieve tube elements. They actively transport these molecules from surrounding tissues, using ATP energy, and then transfer them to sieve cells for long-distance transport through the phloem.
- For example, in the process called phloem loading, sucrose molecules are actively transported into companion cells from source tissues (e.g., leaves) and then move through plasmodesmata into sieve tube elements for transport to other plant parts.

2. Metabolic support: Companion cells provide metabolic support to the sieve tube elements, which lack several organelles to maintain their high efficiency as conduits for long-distance transport. They assist in cellular functions, energy production, and protein synthesis required for the sieve tube elements' survival and proper functioning.
- In contrast, sieve cells, which are found in gymnosperms and non-flowering vascular plants, do not have companion cells and thus lack this metabolic support, showing a less efficient phloem transport system.

3. Maintaining the structure and function of sieve tubes: Companion cells help regulate the development and maintenance of sieve tubes. They control the differentiation and specialization of sieve elements, ensuring their proper size, shape, and successful integration into the phloem system.
- As an analogy, companion cells can be considered as the "support staff" for sieve tube elements, helping them carry out their transport duties effectively, similar to how a professional athlete's support team helps maximize their performance.

Overall, companion cells play crucial roles in the phloem function by facilitating assimilate loading, providing metabolic support, and maintaining the structure and function of sieve tubes, which enables efficient long-distance transport of sugars and other organic molecules throughout the plant.