Translocation

Translocation is the movement of:

• Sucrose (the main transport sugar)
• Amino acids (raw materials for protein synthesis)

through the phloem, from sources to sinks.

Source: a tissue or organ that adds sugars to the phloem. Examples:

• A photosynthesising leaf (making sugar from CO₂).
• A potato tuber in SPRING (its starch is being broken down to glucose, then sucrose, for export).
• A storage organ being mobilised.

Sink: a tissue or organ that takes sugars OUT of the phloem to use or store. Examples:

• Growing tips (buds, root tips).
• Developing fruits (filling with sugar).
• Storage organs being filled (potato in summer, beetroot maturing).

Why sucrose, not glucose?

• Glucose is reactive — gets used up by respiration as it travels.
• Sucrose is more stable — survives the journey unreacted.
• At the sink, sucrose is split back into glucose + fructose for use.

Worked qualitative. Why does an apple get sweeter as it ripens?

• The apple is a SINK during ripening.
• Sugars travel from leaves → apple via phloem.
• Inside the apple, sucrose accumulates → sweeter taste.

Cambridge tip. Memorise the keyword phrase: "movement of sucrose and amino acids in the phloem from sources to sinks". Cambridge marks this almost word-for-word.

Sources and sinks aren't fixed — they SHIFT depending on the time of year and the plant's stage of life.

Same organ, different roles:

Organ	Spring	Summer
Potato tuber	SOURCE (starch breaks down → fuels new shoots)	SINK (filling with starch from leaves)
Tree trunk	SOURCE (stored sugars feeding new buds)	SINK (storing carbohydrate as bark and wood develops)
Underground bulb	SOURCE (feeding new shoots above ground)	SINK (refilling)

Direction of phloem flow:

• Spring: sugars flow UP from underground stores to feeding shoots.
• Summer: sugars flow DOWN from leaves to roots, fruits, storage organs.
• Late summer: sugars flow into seeds and fruits.

This bidirectional flow is one of the key differences between phloem (variable direction) and xylem (water always UP).

Worked qualitative. A daffodil bulb in March vs September.

• March: bulb is releasing stored sugars → SOURCE → flow upward, fueling green shoot growth.
• September: bulb is filling with sugars produced by summer leaves → SINK → flow downward into the bulb.

Cambridge tip. Cambridge often gives a scenario (a season + an organ) and asks you to identify "source or sink?". Remember: any organ that is RELEASING sugars is a source; any organ that is RECEIVING sugars is a sink.

Step 1 — loading. At the SOURCE, COMPANION CELLS pump sucrose INTO the phloem sieve tube using ACTIVE TRANSPORT (energy needed).

Step 2 — water follows. With sucrose in the sieve tube, the water potential there drops. Water from neighbouring xylem moves IN by osmosis → high pressure builds in the sieve tube at the source.

Step 3 — flow. High pressure at source + low pressure at sink (where sucrose is being removed) → sap flows from source to sink along the pressure gradient.

Step 4 — unloading. At the SINK, sucrose is removed (used or stored). Water leaves the sieve tube → low pressure here.

Why energy needed.

• Active loading at source uses ATP — that's why companion cells have so many MITOCHONDRIA.
• Without energy, no flow.

Worked qualitative. Why are companion cells right next to sieve tubes?

• They're like service vehicles — provide energy + control for the sieve tube (which itself is reduced, lacks a nucleus).
• Without companion cells, sieve tubes can't load sugar against the gradient.

Cambridge tip. At IGCSE level the focus is on source-to-sink and the role of phloem; full pressure-flow detail is for A-level. But knowing that ENERGY (active transport) is involved earns marks.