Question

P-Protein is found in
A. Collenchyma
B. Parenchyma
C. Xylem
D. Sieve tube

Answer 1

Hint: P-protein stands for Phloem protein. P-protein is a type of protein that is present in the translocating sieve elements of phloem. P-protein can occur in a variety of forms depending on the plant species.

Complete answer:
• In most of the angiosperms, the phloem contains proteinaceous structures that deposit within differentiating sieve elements and remain present in translocating sieve elements. These proteinaceous structures are together known as P-proteins.
• P-proteins can occur in filamentous form or crystalline form. This depends on the type of plant species.
• P-proteins are formed when the sieve tubes or elements are young. When the sieve tubes or elements reach maturity, these proteins become active and start performing translocation of materials.
• Inside a sieve element, P-protein is found lining its internal wall.
• The main function of a sieve tube is conduction or translocation of food. P- Protein can form a gel and function as a puncture repairing substance by forming plugs at the damaged sites of sieve tubes preventing leakage of translocating material.
• Sieve tube is present in conducting tissue of plants. Plant has two conducting tissues –xylem and phloem. Xylem is involved in conduction of water/minerals and phloem in conduction of food. However, xylem has vessel elements for conducting water. Hence, there is no sieve tube in xylem. Sieve tube is present only in phloem where it helps in conduction of food.
• Further, the sieve tube is composed of living cells, unlike the mature vessel elements in xylem which are composed of dead cells.
• Sieve tube is absent in collenchyma and parenchyma as they are non-conducting tissues. The function of collenchyma is to provide support to plants, while parenchyma is mainly involved in storage of food.

Hence, the correct answer is option (D).

Note: Sieve tube is found in phloem. It is the conducting element of phloem. It helps in conduction or translocation of food in plants. Sieve tube is made up of elongated cells which are interconnected to each other via sieve plates.