How to Answer Translocation and Xylem Exam Questions

Translocation is the movement of dissolved sugars (sucrose) and amino acids around a plant in the phloem, from where they are made or stored (the sources) to where they are used or stored (the sinks). Xylem is the other transport tissue: it carries water and mineral ions upwards from the roots. Most marks in this topic are lost by mixing the two up, so this guide sets out what each tissue does, how its structure fits the job, and the wording that scores.

This guide is about exam answers. If you want the topic explained from the start, with diagrams and a comparison table, read the translocation revision notes and the xylem and phloem revision notes first.

Translocation: the definition examiners credit

Plants make food in their leaves by photosynthesis, but the rest of the plant needs it too. Translocation is how that food gets there. The sugar is moved as sucrose, dissolved in water, along with amino acids, through the phloem.

The key words are source and sink. A source is a part of the plant that releases sucrose into the phloem, such as a photosynthesising leaf. A sink is a part that takes sucrose out of the phloem, such as a growing shoot tip, a developing fruit, a root or a storage organ filling up.

Because the direction depends on where the sources and sinks are, translocation can go up or down the plant, and in different phloem tubes at the same time. That is the main contrast with xylem, where water moves in one direction only: upwards.

Xylem: structure and the transpiration stream

Xylem vessels carry water and mineral ions from the roots to the stems and leaves. The water is pulled up by the transpiration stream: water evaporates from the leaves through the stomata, and the water behind it is drawn up the xylem to replace it. Water molecules stick together, so the water moves up as a continuous column.

Each feature of a xylem vessel has a purpose:

• Dead, hollow cells. There is no cytoplasm or nucleus in the way, so water flows freely.
• No cross walls. The end walls between cells have broken down, so the cells form one long continuous tube. If you have searched “cross walls in xylem”, the answer is that there are none, and that is the point.
• Walls strengthened with lignin. Lignin is a tough, waterproof substance. It stops the vessel collapsing inwards under the low pressure of the pulled water column, and it helps support the plant.
• Narrow tubes. Water can be drawn up a narrow column without it breaking.

Phloem: structure and why the cells are living

Phloem is made of two cell types working as a pair, and unlike xylem it is made of living cells.

• Sieve tube elements are the transport cells. They are joined end to end, and the end walls have holes in them called sieve plates, which let sap move from cell to cell. They have very little cytoplasm and no nucleus, which leaves room for the sap to flow.
• Companion cells sit beside each sieve tube element. They have a nucleus and many mitochondria. They keep the sieve tube element alive and provide the energy needed to load sucrose into the phloem.

That energy point matters: translocation needs energy from respiration, which is why the phloem needs living cells with plenty of mitochondria beside them.

Xylem and phloem compared

A “compare xylem and phloem” question wants a point about both in each sentence. Learn them as pairs:

• Substance carried: xylem carries water and mineral ions; phloem carries sucrose and amino acids.
• Direction: xylem, upwards only, from roots to leaves; phloem, up and down, from sources to sinks.
• Cells: xylem is dead and hollow with no cross walls; phloem is living, with sieve plates and companion cells.
• Walls: xylem has lignin; phloem does not.
• Energy: water moves up the xylem because of evaporation from the leaves; translocation in the phloem needs energy from respiration.

Root xylem and phloem: where they sit

In a root, the xylem and phloem run through the middle in a central core. The xylem forms a star shape in the centre, with the phloem lying between its arms. This arrangement gives the root strength to resist being pulled, and it puts the xylem in position to receive the water that has crossed the root from the root hair cells. In the stem, the two tissues sit together in vascular bundles, and the same bundles carry on into the veins of the leaf.

Answers that score and answers that lose marks

These are the slips examiners see again and again.

• “Phloem carries water.” Water moves in the xylem. The phloem carries sucrose and amino acids, which happen to be dissolved in water. Write “sucrose and amino acids”.
• “Glucose” instead of sucrose. Glucose is made in photosynthesis, but sucrose is what is transported. Writing glucose is one of the most common ways to lose the mark.
• “Xylem carries water down as well.” Xylem is upwards only. Phloem goes both ways.
• “Xylem is made of living cells.” Mature xylem vessels are dead. The living partners in the phloem are the companion cells.
• “Sugar goes from the leaves to the roots.” That is only one direction. Say “from sources to sinks”, then give an example of each.

For a structure-and-function question, always link the feature to the job: “no cross walls, so water flows in a continuous column” scores; “no cross walls” alone does not.

What A-Level adds

At A-Level, movement in the phloem is explained by the mass flow hypothesis: sucrose is loaded into the sieve tubes at the source, water follows by osmosis, and the resulting pressure pushes the sap towards the sink, where sucrose is unloaded. You do not need this at GCSE or IGCSE, where the source-to-sink description is enough.

Practise it until the wording is automatic

This is a topic where precise vocabulary decides the marks, so it rewards practice against a mark scheme. Revise with the IGCSE Cambridge xylem and phloem notes and translocation notes, learn the key terms with the translocation flashcards, then test yourself on the translocation worksheet. On GCSE, start with AQA plant organisation and transport or Edexcel transport in plants. Every written answer on Exaim is marked against the board’s mark scheme, so you see exactly which words earned the mark and which did not. It also helps to know your command words, since “compare” and “explain” ask for different things.