Xylem and phloem

Xylem and phloem are the two transport tissues in a plant. Xylem carries water and mineral ions up from the roots to the stem and leaves, through hollow tubes of dead cells strengthened with lignin. Phloem carries dissolved sugars made in the leaves to the rest of the plant, up and down, through tubes of living cells. The movement in the xylem is part of transpiration, and the movement in the phloem is called translocation.

Xylem and phloem: the differences

Together, xylem and phloem form vascular bundles, the veins you can see in a leaf. AQA's GCSE Biology specification treats the roots, stem and leaves as a plant organ system for transport, with xylem and phloem as its transport tissues.

Xylem: structure and function

Xylem's job is to move water and dissolved mineral ions from the roots to every leaf. Its structure suits that job:

• Dead cells with no contents leave a clear channel for water to flow.
• No end walls between the cells, so they join into long continuous tubes.
• Lignin in the walls makes them strong and waterproof, stops the tubes collapsing, and helps hold the plant up.

Water enters the root hair cells by osmosis, crosses the root and enters the xylem. It travels up the stem and into the leaves, where it evaporates and leaves through the stomata. This continuous flow of water up the plant is the transpiration stream. See specialised cells for how root hair, xylem and phloem cells are adapted.

Phloem function and structure

Phloem's function is to carry the sugars made in photosynthesis from the leaves to where the plant needs them: growing shoots and roots for immediate use, and storage organs such as potatoes, where sugar is stored as starch. AQA describes this as transport for immediate use or storage. Because the sugar can be needed above or below the leaves, phloem carries it both ways.

• Tubes of elongated living cells joined end to end.
• Pores in the end walls let the cell sap flow from one cell to the next. These perforated walls are called sieve plates, and the cells are called sieve tube elements.
• Companion cells beside each sieve tube are packed with mitochondria and supply the energy the sieve tubes need.

For AQA GCSE, you need the tubes of elongated cells and the pores in their end walls; the detailed structure and how the sugars are moved aren't required. At A-level you go further: sugars are loaded into the phloem at a source, such as a leaf, and flow to a sink, such as a root, by mass flow.

Transpiration and translocation

Transpiration is the loss of water vapour from the leaves, mostly through the stomata. As water evaporates from the leaf, more is pulled up the xylem to replace it, so transpiration keeps the stream moving. Stomata are tiny pores, mostly on the underside of the leaf, opened and closed by a pair of guard cells. They control gas exchange and water loss: open stomata let carbon dioxide in for photosynthesis, but also let water vapour out.

Factors that affect transpiration rate

AQA's specification asks you to explain the effect of four factors on the rate of transpiration. Any one of them answers "name a factor that affects transpiration rate", but exam questions usually want the explanation too.

Measuring transpiration with a potometer

A potometer measures how fast a cut shoot takes up water, which is very close to its rate of transpiration. An air bubble in a thin tube moves along as the shoot draws water up. AQA lists measuring the rate of transpiration by the uptake of water as a practical opportunity, and expects you to use compound measures like a rate.

Example: the bubble moves 18 mm in 6 minutes. The rate of water uptake is 18 ÷ 6 = 3 mm per minute. If the tube has a radius of 0.5 mm, the volume taken up each minute is π × 0.5² × 3 = 2.36 mm³ per minute. To test a factor, change only that factor, such as switching on a fan for air movement, and keep everything else the same.

Common mistakes

• Swapping the two. Phloem and food both start with an f sound, so phloem carries the food (sugars), and xylem carries the water.
• Saying phloem carries glucose. Sugars travel mainly as sucrose; "dissolved sugars" is the safe phrase.
• Saying xylem cells are alive. Mature xylem is dead and hollow.
• Saying phloem only goes down. It carries sugars up to growing shoots too.
• Saying humidity speeds up transpiration. More humid air slows it down.

To test plant transport under time pressure, try today's GCSE Biology Higher or A-level Biology: Year 1 short timed paper. Each paper mixes topics.

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