Mini-Lesson

Transport in plants

This mini-lesson walks you through Cambridge IGCSE Transport in plants: the two transport tissues xylem & phloem, water uptake by root hair cells, the transpiration stream and the factors affecting it, and translocation.

Work through each screen, answer the questions as you go (some wordy, some calculations) and collect ⭐ stars. Watch for the Key term flags. Press Start when you're ready.

Transport · the two tissues

Xylem & phloem

Plants have two transport tissues, both running through the roots, stem and leaves:

Xylem — carries water and dissolved mineral ions from the roots up to the leaves. This is one-way only. Xylem vessels are made of dead, hollow cells strengthened with lignin.
Phloem — carries sucrose (sugars) and amino acids from the leaves to the rest of the plant (this is called translocation). Phloem can move things both ways and is made of living cells.

In a stem, xylem is towards the inside and phloem towards the outside of each vascular bundle.

Don't mix them up: xylem carries water up (think "xtra water"); phloem carries food (sugars) — the word starts with a 'ph' but sounds like an 'f'.

Quick check

Which tissue carries what?

?Which statement correctly describes what xylem transports and in which direction?

Transport · water uptake

Water uptake by root hair cells

Water enters the plant through root hair cells near the root tips. Each root hair cell has a long, thin extension that gives a large surface area for absorbing water:

The soil water is more dilute than the cell contents, so water enters by osmosis.

Soil water is a dilute solution (high water potential); the cell sap is more concentrated.
So water moves in by osmosis — across the partially permeable membrane, dilute → concentrated.
Mineral ions are usually less concentrated in the soil, so they are taken in by active transport (needs energy).

Key contrast: water enters by osmosis (no energy). Mineral ions enter by active transport (uses energy from respiration).

Sort it

Xylem, phloem, or both?

Tap a statement, then tap the tissue it describes. (Some are true of both.)

🔁 Both tissues

💧 Xylem only

🍬 Phloem only

Transport · transpiration

Transpiration

Transpiration is the loss of water vapour from a plant's leaves. Water evaporates from the surfaces of the mesophyll cells and diffuses out through the stomata (tiny pores, mostly on the underside of the leaf).

Evaporation from the leaves pulls a continuous column of water up the xylem.

Factors that increase the rate of transpiration:

Higher temperature — more evaporation.
Higher light intensity — stomata open wider.
Lower humidity — a steeper water-vapour gradient out of the leaf.
More air movement (wind) — carries the vapour away, keeping the gradient steep.

Quick check

Which change speeds it up?

?A plant is moved into windier, drier air. What happens to its rate of transpiration, and why?

Calculate

Your turn — rate of water loss

1Using a potometer, a shoot takes up 4.5 cm³ of water in 30 minutes. Calculate the mean rate of water uptake in cm³ per minute.

cm³/min

Hint: rate = volume ÷ time = 4.5 ÷ 30.

Calculate

Your turn — comparing conditions

2In still air a shoot loses 2.0 cm³ of water per hour. In moving air it loses 3.0 cm³ per hour. Calculate the percentage increase in water loss caused by the wind.

%

Hint: % increase = (3.0 − 2.0) ÷ 2.0 × 100.

Key term · translocation

Translocation

Translocation is the movement of sucrose and amino acids in the phloem, from sources to sinks:

A source is a part that makes or releases sugars — e.g. a leaf doing photosynthesis, or a storage organ in spring.
A sink is a part that uses or stores sugars — e.g. a growing root, a developing fruit, or a storage organ in summer.

The direction of translocation can change: the same organ can be a source at one time of year and a sink at another.

Compare: transpiration (xylem) is one-way, water, no energy; translocation (phloem) can go either way, sugars, and needs energy.

Quick check

Source or sink?

?In summer, a leaf carries out lots of photosynthesis. In terms of translocation, what is this leaf acting as?

Match it

Match each idea to its term

Tap a description on the left, then its matching term on the right.

Description

Term

Quick check

How do minerals get in?

?Mineral ions are often at a lower concentration in the soil than inside a root hair cell. How are they taken up?

Recap

The big ideas to know

Xylem: dead, lignified cells; water + minerals; roots → leaves; one way

Phloem: living cells; sucrose + amino acids; both directions

Root hair cells: large surface area; water in by osmosis; minerals by active transport

Transpiration: water vapour lost through stomata; ↑ by heat, light, wind, low humidity

Transpiration stream: evaporation pulls water up the xylem

Translocation: sugars in phloem, from source to sink

You've covered the whole of Cambridge IGCSE Transport in plants. Press Finish to see your score.

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