Fig. 3.1 shows a transverse section through the stem of a young dicotyledonous plant. The vascular bundles contain both xylem and phloem tissue. The phloem ...

This question is about translocation, the transport of food in the phloem, and how phloem differs from xylem. Because the figure only shows the arrangement of xylem and phloem in the vascular bundles, base your answers on named tissues and functions rather than on any measurement.

(a) Translocation is the movement of sucrose and amino acids [1] through the phloem, from sources to sinks [1]. A full definition names both the substances and the direction.

(b) The tissue that carries out translocation is the phloem [1].

(c) Two substances transported in the phloem are any two of sucrose, amino acids or other soluble sugars [2]. Note that sugar travels as sucrose, not as glucose.

(d) A source is a part that makes or releases food/sucrose, such as a leaf [1], while a sink is a part that uses or stores food, such as a root or fruit [1].

(e) A growing root tip is a sink because it is growing and dividing rapidly [1], so it uses sugars for respiration and to build new cells [1]. Since it cannot photosynthesise underground, food must be delivered to it rather than made there [1].

(f) Two structural differences are any two of: phloem cells are living whereas xylem vessels are dead [1]; phloem has sieve plates and companion cells, whereas xylem has lignified walls and no cross walls [1].

(g) Translocation needs energy because it is an active process [1]: energy is required to load sucrose into the phloem [1], and this energy comes from respiration in the companion cells [1]. This is why phloem, unlike xylem, must be made of living cells.

Exam tip: "source to sink" is the phrase examiners look for; always name the transported substances as sucrose and amino acids, not simply "food".