Role of the Xylem & Phloem

A large plant cannot rely on substances simply spreading from cell to cell — that would be far too slow to reach a tall stem or a wide network of roots. Instead, flowering plants have two transport tissues that carry substances quickly over long distances:

• Xylem — transports water and mineral ions.
• Phloem — transports sucrose (a sugar) and amino acids.

Both tissues run all the way through the plant — from the roots, up the stem, and into the leaves — a bit like two separate sets of pipes laid alongside each other.

A simple memory hook:

• Xylem → carries water ("xylem = water").
• Phloem → carries food (ph sounds like f → sucrose and amino acids).

Exam tip. A very common question simply asks "Name the tissue that transports water" or "Name the tissue that transports sugars." Make sure you can match each tissue to what it carries without hesitating.

The xylem transports water and dissolved mineral ions (such as nitrate ions) from the roots to the rest of the plant — up the stem and out to the leaves.

Three things to remember about xylem transport:

• What it carries: water and mineral ions.
• Where from / where to: from the roots → upwards to the stem and leaves.
• Direction: one direction only — always upwards.

This upward flow is called the transpiration stream. Water evaporates from the leaves (transpiration), which "pulls" more water up the xylem to replace it — rather like sucking water up a straw. Because the pull comes from the leaves at the top, water can only travel up.

Exam tip. The two facts examiners reward most are (1) xylem carries water and mineral ions and (2) it moves them in one direction, upwards, from the roots. Do not write that xylem carries sugars — that is the phloem's job.

The phloem transports sucrose and amino acids between the leaves and other parts of the plant. This movement is called translocation.

Three things to remember about phloem transport:

• What it carries: sucrose (made from the glucose produced in photosynthesis) and amino acids.
• Where from / where to: mainly from the leaves (where sugar is made) to wherever it is needed or stored.
• Direction: two directions — up or down.

Why two directions? Sugar made in the leaves may need to travel down to the roots for storage, or up to a growing shoot or developing fruit. Because different parts of the plant need food at different times, phloem must be able to carry substances either way — unlike xylem, which only ever flows up.

Worked through:

• In summer, leaves make lots of sugar → phloem carries sucrose down to store in the roots.
• In spring, new shoots grow before many leaves open → phloem carries stored sugar up to the shoots.

Exam tip. Translocation = the transport of sucrose and amino acids in the phloem, in either direction. The marks are: correct substances (sucrose + amino acids) and the "up or down" point.

The way each tissue is built matches the job it does.

Xylem — dead, hollow tubes:

• Xylem is made of dead cells joined end to end.
• The cells have no contents and no end walls, forming long hollow tubes.
• Water can flow up these empty tubes without obstruction, which is exactly what is needed for a fast, continuous transpiration stream.
• The walls are strengthened (with lignin), which also helps support the plant.

Phloem — living cells:

• Phloem is made of living cells.
• Moving sucrose and amino acids around the plant requires energy, so the cells must be alive (energy comes from respiration).
• Because phloem cells are alive, substances can be loaded in and moved in either direction — this is why translocation can go up or down.

Exam tip. The classic contrast is xylem dead vs phloem living. Link each to its job: dead hollow tubes = unobstructed water flow; living cells = energy needed to move food in either direction.

It helps to learn the two tissues as a direct comparison. If you can reproduce this table, you can answer almost any question on this subtopic.

Feature	Xylem	Phloem
What it transports	Water and mineral ions	Sucrose and amino acids
Direction	One way — upwards	Two ways — up or down
From → to	Roots → stem and leaves	Leaves ↔ rest of plant
Name of the process	Transpiration stream	Translocation
Living or dead?	Dead (hollow tubes)	Living cells

A few quick reminders to lock it in:

• Both tissues run continuously from root to leaf.
• Xylem flow is driven by transpiration (water lost from leaves), so it can only go up.
• Phloem flow uses energy, so substances can be carried either way to wherever they are needed.

Exam tip. If a question gives 4 marks for "compare xylem and phloem", give paired points — e.g. "Xylem carries water, whereas phloem carries sucrose." Comparison marks need both sides of the pair.