Phloem tissues are mostly responsible for transport of

Correct — D, food. A vascular plant has two conducting tissues with opposite jobs. Xylem carries water and the minerals dissolved in it upward from the roots. Phloem carries the food that the leaves have made — sucrose above all, along with amino acids — away to wherever it is needed or stored, whether that is a growing shoot tip, a developing fruit, a root or a tuber. Because those destinations can lie above or below the leaf, phloem transport runs in both directions, unlike the one-way upward flow in xylem.

• (a)water — Water is the xylem's cargo, drawn up mainly by transpiration pull. Some water does move with the sugar in phloem, but it is the solvent, not the material being transported.
• (b)oxygen — Plants have no tissue dedicated to carrying gases. Oxygen and carbon dioxide diffuse in and out through stomata and lenticels and move through the intercellular spaces.
• (c)minerals — Mineral ions absorbed by the roots travel upward in the xylem, dissolved in the transpiration stream, not in the phloem.

Phloem is a complex tissue built from sieve tubes, companion cells, phloem parenchyma and phloem fibres. Sieve tubes are living cells joined end to end through perforated sieve plates, and they lose their nucleus at maturity, which is why each depends on a companion cell alongside it. The movement of food through them is called translocation.

The contrast with xylem is what makes this pair memorable. Xylem transport is essentially passive — the plant spends no energy on it, the water being pulled up as it evaporates from the leaves, and the flow goes one way only, from root to shoot. Phloem transport costs energy: sugar is loaded into the sieve tube using ATP, water then enters osmotically, and the pressure so built up drives the contents towards a region where the sugar is being consumed or stored. That is the pressure-flow explanation, and it accounts for the fact that food can travel down to a root or up to a fruit as the season demands. A practical illustration is ringing a tree — stripping a band of bark, and with it the phloem, starves the root while leaving the water supply intact.

• Phloem translocates the products of photosynthesis, chiefly sucrose, together with amino acids.
• Its elements are sieve tubes, companion cells, phloem parenchyma and phloem fibres.
• Phloem transport runs in both directions, since the destination may lie above or below the leaf.
• Xylem carries water and dissolved minerals upward only, driven largely by transpiration pull and without expenditure of energy.
• Loading sugar into a sieve tube uses ATP, and the osmotic entry of water that follows creates the pressure which drives the flow.

• Assigning minerals to phloem because they are 'nutrients'; minerals from the soil travel in xylem.
• Expecting a plant to have a transport tissue for oxygen on the analogy of animal blood.
• Thinking of translocation as upward only, on the model of the xylem's one-way flow.

Asked as a one-line function question in which every wrong option is something a plant genuinely moves, but by a different route.

• practice — not a real PYQ

  The upward movement of water in tall trees is chiefly explained by

  • (a)root pressure alone
  • (b)transpiration pull with cohesion and adhesion
  • (c)active pumping by the xylem vessels
  • (d)capillary action alone

  Answer(b) transpiration pull with cohesion and adhesion — evaporation at the leaves pulls a continuous column of water upward.

• practice — not a real PYQ

  Which one of the following is a living component of phloem?

  • (a)Vessel
  • (b)Tracheid
  • (c)Companion cell
  • (d)Xylem fibre

  Answer(c) Companion cell — it supports the enucleate sieve tube alongside it.