NEET BIOLOGY

Detailed explanation-1: -Sieve tubes are considered living cells without a nucleus because the nucleus of companion cells controls their functional activities. Companion cells, which unite with sieve tubes to form the sieve element-companion cell complex, are related to sieve-tube members.

Detailed explanation-2: -The unique features of sieve tube members are that they have no cell nucleus, ribosomes or vacuoles. The main function of the sieve tube is transportation of carbohydrates, primarily sucrose in the plant (e.g., from the leaves to the fruits and roots).

Detailed explanation-3: -Sieve tubes are elongated tubular conducting channels of phloem. Each sieve tube is formed of several cells called sieve tube members, sieve tube cells or sieve elements. Sieve tube members are placed end to end. Their end walls have many small pores or sieve pits.

Detailed explanation-4: -In plant cells, the nucleus is absent in the sieve tubes. Sieve tubes cells are conducting cells of the phloem and do not contain nucleus and ribosomes.

Detailed explanation-5: -The phloem is composed of sieve tubes, companion cells, phloem parenchyma and fibres. The sieve tubes are the tube-like cells without the nucleus. These cells have lignified cell wall and perforations for the conduction of food.