Final answer:
Sieve-tube cells are phloem cells in plants responsible for the transport of sugars and amino acids. They have rigid cell walls and are connected by perforated sieve plates that allow substance flow. Companion cells, which are packed with organelles, provide metabolic support to the sieve-tube cells.
Step-by-step explanation:
Structure and Function of Sieve Tube Cells
The phloem tissue is an essential part of the vascular system of plants, comprising several cell types, including sieve-tube cells, companion cells, phloem parenchyma, and phloem fibers. Sieve-tube cells are specialized cells arranged end to end, forming long sieve tubes responsible for transporting organic substances like sugars and amino acids throughout the plant. These cells are characterized by their rigid cell walls which help maintain the plant's structure.
Each sieve-tube cell is connected to its neighbor by sieve plates, which are perforated to allow the phloem sap to flow between them. The rigidity of the cell wall is crucial as it withstands the internal pressure generated by the flow of sap. Sieve-tube cells lack a nucleus and various other organelles at maturity, which allows for a more efficient flow of substances.
Adjacent to the sieve-tube cells are companion cells that perform essential metabolic functions and provide the energy needed for the transport of substances. These companion cells are densely packed with ribosomes and mitochondria, which are absent in the sieve-tube cells. Lateral sieve areas connect the sieve-tube elements to the companion cells, ensuring proper communication and transport between them.