Question

 Select the incorrect statement. [NEET (Oct.) 2020]

(a) Transport of molecules in phloem can be bidirectional

(b) Movement of minerals in xylem is unidirectional

(c) Unloading of sucrose at sink does not involve the utilisation of ATP

(d) Elements most easily mobilised in plants from one region to another are phosphorus, sulphur, nitrogen and potassium

Answer/Explanation

Ans. (c)

Statement (c) is incorrect. It can be corrected as Unloading of sucrose at sink does require utilisation of ATP.
Long distance transport of sucrose from the source to sink is carried out by phloem. It is a bidirectional movement. Both loading of food at the source and unloading of the same at sink region are active processes, i.e. require energy in the form of ATP.

Question

 What is the direction of movement of sugars in phloem? [NEET (National) 2019]

(a) Upward

(b) Downward

(c) Bidirectional

(d) Non-multidirectional

Answer/Explanation

Ans. (c)

Sugars show bidirectional movements in phloem unlike unidirectional movement of water in xylem. In phloem, sugars move in both directions depending upon the source-sink relationship.
Initially leaves act as source of ‘food from where it moves to the sink (parts requiring food). Later, the food in sink is mobilised towards the growing buds of the plant.

Question

 The translocation of organic solutes in sieve tube members is supported by [CBSE AIPMT 2006]

(a) P-proteins

(b) mass flow involving a carrier and ATP

(c) cytoplasmic streaming

(d) root pressure and transpiration pull

Answer/Explanation

Ans. (b)

According to mass flow hypothesis the transport of organic solutes takes place from source to sink, this transport also depends on metabolic energy.
According to cytoplasmic streaming hypothesis (which was given by de Vries, 1885) the transport of organic solutes takes place by combination of diffusion and cytoplasmic streaming. Cytoplasmic streaming carry organic solutes from one and to the other end of sieve tube.
P-proteins has a role as defence against phloem feeding insects and sealing of damaged sieve tubes.

Question

 Loading of phloem is related to[CBSE AIPMT 2001]

(a) increases of sugar in phloem

(b) elongation of phloem cell

(c) separation of phloem parenchyma

(d) strengthening of phloem fibre

Answer/Explanation

Ans. (a)
Sugar synthesied in leaves (source) is ‘loaded’ in phloem through which it is transported to all other parts of plants where it is required (sinks).

Question

Translocation of carbohydrate nutrients usually occurs in the form of [CBSE AIPMT 1992]

(a) glucose

(b) maltose

(c) starch

(d) sucrose

Answer/Explanation

Ans. (d)

In plants, translocation, i.e. the movement of organic nutrients from the region of supply to the region of sink or utilisation occurs through phloem (sieve tube/sieve cells) tissue. Translocated organic nutrients constitute $10-26 \%$ carbohydrates (usually sucrose) and $1 \%$ nitrogenous components (mostly amino acids).

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