Correlate the structural adaptations of sieve tubes and companion cells to the transport of organic matter

Vascular Tissues

4.1

Vascular Tissues

Introduction to Vascular Tissues

Plants are multicellular organisms which are formed from cells that need water and nutrients.
The large and tall size of the plants creates a total surface area that is insufficient for the plant to absorb its basic needs from the surroundings.
In order to overcome this problem, plants have vascular tissues to transport water, mineral salts and nutrients to all the cells.
Plants which have a transport system are known as vascular plants.
Non-vascular plants like alga and moss on the other hand, do not have any transport system.
Transport in plants is carried out by vascular tissues which consist of:
- Xylem tissue: transport water and mineral salts from roots to shoots
- Phloem tissue: transport photosynthetic product and other organic substances from shoots to other parts.
- The xylem tissue contains xylem vessel and tracheid as the main transport vessels,whereas the phloem tissue contains sieve tube and companion cell as the main transport vessels.

The Differences between Xylem Tissue and Phloem Tissue

Aspect	Xylem Tissues
Main cell type	Xylem vessel & tracheid.
Lignified wall	Lignified in the form of annular, spiral, scalariform, reticular and pit.
Condition of the cell after mature	Death cells and cell contents degenerated. Cells form a long and hollow vessel. Have plate or nnote for the flow of water & mineral. Have pits for the glow of water and minerals laterally.
Materials transported	Water, minerals, salt & phytojhormones.
Main function	Transport water and mineral in plants. Provide mechanical support to plants.

Aspect	Phloem Tissues
Main cell type	Sieve elemetns & companion cell.
Lignified wall	Unlignified.
Condition of the cell after mature	Living vells with degenerated organelles. Cells form a long tube filled with cytoplasm. Have sieve tubes for the flow of organic substance.
Materials transported	Sucrose, amino acid, organic acid, phytohormones & water.
Main function	Transport sucrose and other organic substances.

Structural Adaptations of Xylem Vessels and Tracheids to Transport Water and Mineral Salts

Xylem Vessel

Xylem vessel consists of dead cells at maturity which do not have cytoplasm.
These cells are arranged longitudinally from end to end to form a continuous tube to allow water flow from the roots to the leaves.
The walls of the xylem vessel have uneven lignin thickening to:
- Give strength to xylem vessels to prevent them collapsing due to the tension force and pressure changes when water moves through it.
- Prevent the plant from being bent.

Tracheid

The cell wall of tracheid also has lignin thickening and pits to allow water movement to adjacent cells.

Structural Adaptations of Sieve Tubes and Companion Cells to the Transport of Organic Substances

Phloem tissues transport and distribute dissolved organic compounds such as sucrose, amino acids and plant hormones to all parts of the plant.
Phloem is a living cell because it has cytoplasm
Adapatation of sieve tubes and companion cells to their functions:
- Sieve tubes do not have nuclei, ribosomes or vacuoles. This allows sucrose molecules to pass through sieve tubes easily.
- On both ends of the sieve tube, there is a sieve plate that has pores through which organic compounds can flow from one sieve tube to the next.
- Companion cells contain mitochondria to provide energy in the form of ATP to transport sucrose from the leaf to the sieve tube through active transport.