Question

Which of the following helps in maintenance of the pressure gradient in sieve tubes?

• A. Albuminous cells
• B. Sieve cells
• C. Phloem parenchyma
• D. Companion cells

Answer 1

The Correct Option is D

The question asks about the structure that helps maintain the pressure gradient in sieve tubes, a component of the phloem tissue in plants. Let's review each option and determine which is most suitable:

1. Albuminous cells: These cells are related to sieve cells in gymnosperms and are involved in supporting the function of sieve cells, but they are not directly responsible for maintaining the pressure gradient in sieve tubes.
2. Sieve cells: Sieve cells and sieve tube elements are the conductive cells in the phloem that transport nutrients. While they are involved in transport, the maintenance of pressure gradients is not their primary function.
3. Phloem parenchyma: These cells are involved in the storage and lateral transport of nutrients but do not directly maintain the pressure gradient in the sieve tubes.
4. Companion cells: These cells are closely associated with sieve tube elements and play a crucial role in the loading and unloading of sugars into the sieve tubes. They actively pump organic solutes into the sieve tubes, generating an osmotic pressure difference that creates the pressure gradient necessary for the transport of substances through the phloem.

Conclusion: The correct answer is Companion cells. Companion cells maintain the pressure gradient by loading sugars into sieve tubes, which lowers water potential and causes water to flow into the tubes, generating pressure. This pressure drives the flow of nutrients through the phloem.