Adaptations of phloem sieve tubes and companion cells for translocation of sap
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Question
What is the phloem?
Answer
The plant transport tissue that carries **dissolved sugar (sucrose)** around the plant; its conducting cells are living **sieve tubes**.
Question
What is translocation?
Answer
The movement of **dissolved sugar** through the phloem from a **source** to a **sink**.
Question
What is a source in translocation?
Answer
Any part that **makes or releases** sugar — usually a photosynthesising **leaf** (but also a store being broken down).
Question
What is a sink in translocation?
Answer
Any part that **uses or stores** sugar — for example a **growing root**, a **fruit**, or a store being built up.
Question
In which direction can translocation occur?
Answer
**Either up or down** the plant — it always runs from a source to a sink, wherever those are.
Question
How is sugar loaded into the phloem at the source?
Answer
By **active transport** (against its gradient), which uses **ATP**.
Question
Which cell supplies the energy to load sugar into the sieve tube?
Answer
The **companion cell** — it is packed with **mitochondria** and keeps the sieve tube alive.
Question
After sugar is loaded, what makes the sap move?
Answer
Water **follows by osmosis**, raising the **pressure**, which pushes the sap by **bulk flow** to the sink.
Question
What is bulk flow?
Answer
The **mass movement** of the sugary sap along the sieve tubes, driven by the **pressure difference** between source and sink.
Question
Name two structural features of a sieve tube cell.
Answer
**Sieve plates with pores** (sap flows between cells) and **little cytoplasm / no nucleus** at maturity (a clear channel); a **companion cell** sits alongside.
Question
Why is phloem described as living tissue?
Answer
Its sieve tubes are kept alive by **companion cells**, and translocation **needs energy** — it stops if the cells are killed. (Xylem is dead.)
Question
Give two differences between phloem and xylem.
Answer
Phloem carries **sugar**, is **living**, and is **two-way**; xylem carries **water**, is **dead**, and is **one-way** (roots to leaves).
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