Cells and organisation · GCSE Biology
Plant transport
Compare xylem and phloem for GCSE Biology: transpiration stream, translocation of sucrose, and how root hair cells link water and mineral uptake to both tissues.
Xylem: water and minerals up, dead hollow tubes, transpiration pull. Phloem: sucrose both ways, living sieve tubes, translocation. Root hairs: osmosis for water, active transport for ions.
The important bits
What you need to know
- 1
Plants need transport systems because they have a small surface area to volume ratio and roots may be far from leaves.
- 2
Xylem transports water and dissolved mineral ions from roots to stems and leaves in the transpiration stream. Xylem vessels are dead cells with lignin-strengthened walls and no end walls, forming continuous pipes.
- 3
Phloem transports dissolved sugars (mainly sucrose) by translocation. It can move both up and down the plant to sources (where sugar is made or stored) and sinks (where sugar is used or stored).
- 4
Phloem sieve tubes have perforated end plates; companion cells provide energy and control for loading sucrose. Phloem tissue is living.
- 5
Root hair cells increase surface area for uptake. Water enters by osmosis; mineral ions such as nitrate enter by active transport, requiring mitochondria for respiration.
- 6
Water and ions move into the xylem in the root, then upward with transpiration. Sugars made in photosynthesising leaves are loaded into phloem and delivered to roots, fruits and growing tips.
- 7
In spring, stored starch in roots may be converted to sucrose and moved upward in phloem. In summer, sucrose flows from leaves to roots for storage.
- 8
Ringing experiments (removing a ring of bark including phloem) show swelling above the cut — evidence that phloem carries sugars downward. Dying leaves above a xylem cut show water transport in xylem.
Quotations worth analysing
Short evidence. Real method.
“Translocation is the movement of dissolved sugars through the phloem.”
Name sucrose and phloem. Do not use translocation for water — that is the transpiration stream in xylem.
“Xylem vessels are adapted to transport water: lignin support, hollow lumen, no end walls.”
Each feature supports a continuous column of water under tension. “Strong tubes” alone is insufficient.
“Root hair cells absorb water by osmosis and mineral ions by active transport.”
Never swap the two processes. Ions are often more concentrated inside the cell than in soil water.
Go deeper
Xylem versus phloem — keep the table straight
Xylem: substance water + minerals; direction upward (mostly); tissue dead; driven by transpiration pull and cohesion. Phloem: substance sucrose (+ amino acids); direction up and down; tissue living; driven by loading at source and unloading at sink (pressure flow is the Higher-tier detail, but GCSE wants source/sink language). A classic trap is “phloem carries water to the leaves” — that is xylem. Another is drawing arrows only upward in phloem. In autumn, leaves export sucrose before abscission; in a potato tuber, phloem unloads sucrose for storage as starch.
Go deeper
How root hairs connect to both pathways
The same root hair that takes in water by osmosis also pumps nitrate in by active transport. Nitrate travels in xylem water to leaves for amino acid synthesis. Sucrose from photosynthesis in the leaf enters phloem and can return to the root to fuel growth and storage. Without mineral ions, the plant cannot make proteins even if water is plentiful — linking organisation to biochemistry. Exam questions often pair a root hair diagram with “explain adaptations”: large surface area, thin wall, many mitochondria (ions), no chloroplasts (underground).
Go deeper
Evidence and application questions
Girdling (phloem removed) causes a sugar swell above the ring and root starvation below. A xylem-only cut wilts the shoot because transpiration continues but uptake is blocked. Mass-flow hypotheses are not always examined, but source–sink ideas are: growing fruit, meristems and storage organs are sinks; mature photosynthetic leaves are sources. When herbicides are applied to leaves, some systemic types travel in phloem to roots — a real-world link if asked about why phloem is bidirectional.
See the idea in action
A student removes a ring of bark from a tree trunk, including phloem but not xylem. Explain what happens (3 marks). Sugars made in the leaves cannot pass down through the removed phloem to the roots. Sugars accumulate above the ring, causing swelling. Roots below the ring are starved of sugar and may die, even though xylem still carries water upward and the leaves do not immediately wilt.
Exam technique
Turn knowledge into marks
In comparison answers, use rows: what is moved, direction, living or dead, driving force. Root hair questions always split water (osmosis) from minerals (active transport, mitochondria).
Common mistakes
Do not give these marks away
- 01
Saying phloem transports water or xylem transports sucrose.
- 02
Claiming phloem only moves substances upward.
- 03
Describing mineral ion uptake as osmosis or diffusion from dilute soil into concentrated cell sap.
Which tissue transports sucrose by translocation?
AXylem
BPhloem
CEpidermis
DCambium
Show the answer
Phloem. Phloem moves dissolved sugars such as sucrose to sources and sinks. Xylem carries water and mineral ions in the transpiration stream.
Quick questions
If this is the bit you searched
What is the difference between xylem and phloem?
Xylem carries water and minerals upward in dead vessels; phloem carries sucrose up and down in living sieve tubes.
What is translocation?
The transport of dissolved sugars through the phloem from sources (e.g. leaves) to sinks (e.g. roots, fruits).
Why do root hair cells have many mitochondria?
To transfer energy from respiration for active transport of mineral ions into the cell.
What evidence shows phloem transports sugars?
Removing a ring of bark (phloem) causes sugar to accumulate above the cut and roots below to be starved.