Ordinary Companion Cells:
1. Function: Ordinary companion cells play a crucial role in the transport of sugars and other organic compounds from the source to the sink tissues of a plant. They assist in the loading and unloading of assimilates in the phloem.
2. Structure: Ordinary companion cells are closely associated with sieve tube elements in the phloem. They have thin cell walls with numerous plasmodesmata (openings) that connect them to the sieve tube elements. These plasmodesmata facilitate the exchange of materials between the two types of cells.
3. Nucleus and Organelles: Ordinary companion cells possess a large nucleus and are metabolically active. They contain all the usual organelles found in plant cells, such as mitochondria, endoplasmic reticulum, and Golgi apparatus.
4. Specialized Functions: Ordinary companion cells are responsible for providing metabolic support to sieve tube elements. They supply the energy and biomolecules required for maintaining the osmotic pressure in the phloem, as well as for the synthesis and transport of substances like proteins, amino acids, hormones, and signaling molecules.
5. Genetic Coherence: Ordinary companion cells are directly connected to the germ cells, such as seeds or pollen grains, from which they originate. They share the same genetic makeup as these germ cells and are, therefore, able to provide the necessary instructions for synthesis and transport of specific molecules in the phloem.
Intermediate Companion Cells:
1. Role: Intermediate companion cells are an intermediate stage between ordinary companion cells and sieve tube elements during development. They are responsible for the division and differentiation of ordinary companion cells into mature sieve tube elements.
2. Morphology: Intermediate companion cells have larger cell sizes and longer life spans compared to ordinary companion cells. They possess dense cytoplasm, numerous mitochondria, and specialized structures called plasmodesmal transfer plates that aid in the transfer of materials between intermediate companion cells and sieve tube elements.
3. Differentiation: Intermediate companion cells undergo structural and functional transformations as they mature into sieve tube elements. They gradually lose their nucleus, ribosomes, and other organelles, becoming highly specialized for long-distance transport.
4. Transport Function: Intermediate companion cells provide a conduit for the efficient transfer of sugars and other assimilates from source to sink tissues. They actively transport essential molecules into sieve tube elements and help maintain the flow of phloem sap.
5. Translocation Support: Intermediate companion cells support the process of translocation by assisting in the maintenance of osmotic pressure and regulating the composition of phloem sap. They help in the synthesis and mobilization of energy-rich molecules required for long-distance transport in the phloem.
In summary, ordinary companion cells are responsible for actively transporting sugars and metabolites, while intermediate companion cells aid in the development and differentiation of sieve tube elements. Both cell types have critical roles in maintaining phloem function and enabling efficient movement of nutrients throughout the plant.
Discuss characteristics of ordinary companion cells and intermediate companion cells
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