# Transport in Plants

> Xylem and phloem, how water climbs a tree by transpiration, how sugar moves by translocation, and the factors that change the rate.

From HSC Biology on WhizzNotes, free study notes for NSW students: https://whizznotes.com/biology/transport-in-plants/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: Water to the top of a mountain ash
The mountain ash trees of Victoria's forests can grow over 90 metres tall. They are the tallest flowering plants on Earth. On a hot day, one big tree can lift hundreds of litres of water from its roots to its highest leaves.

There is no heart, no pump and no muscle. The tree does it with tiny tubes, sticky water and the sun. Every leaf losing water at the top pulls the next drop up from below, like a very long chain being drawn up a well.

**In short:** Plants move materials in two kinds of tubes. Xylem carries water and minerals up from the roots, pulled by transpiration (water evaporating from the leaves), and phloem carries sugars from where they are made to where they are used, by translocation.

## Starting point: what you already know

- Water moves by osmosis. Minerals can be taken in by active transport (see The Cell Membrane and Transport).
- Leaves make glucose by photosynthesis. Stomata let gases in and water vapour out (see Nutrient and Gas Exchange).
- Plants have roots, stems and leaves.

## Two transport tissues

Plants have **vascular tissue** made of two kinds of tubes, usually found together in **vascular bundles**.

| | Xylem | Phloem |
|---|---|---|
| Carries | Water and mineral ions | Sugars (mainly sucrose) and amino acids |
| Direction | **Up** only, from roots to leaves | **Up and down**, from where sugar is made or stored to where it is needed |
| Cells | Dead, hollow tubes with no end walls | Living **sieve tubes** with **companion cells** |
| Walls | Thick and strengthened with **lignin** | Thin |
| Process | **Transpiration** | **Translocation** |

Lignin in xylem walls also makes wood strong. A tree trunk is mostly old xylem.

### Exam tip: A memory hook
**X**ylem goes **up**, like the shape of an X pointing up. **Ph**loem carries **ph**ood (food).

## Transpiration: how water moves up

**Transpiration** is the loss of water vapour from the leaves, mostly through the stomata. It powers the movement of water up the plant.

1. Water **evaporates** from cells inside the leaf and diffuses out through the stomata.
2. This loss **pulls** water out of the xylem in the leaf.
3. Water molecules stick to each other (**cohesion**) and to the xylem walls (**adhesion**), so a continuous column of water is pulled up the stem.
4. Water enters the root hairs from the soil by **osmosis** to replace it.

This is called the **transpiration stream**, and the idea is the **cohesion-tension theory**.

*Diagram: A plant with arrows showing water entering the roots, moving up the stem in xylem and leaving the leaves as vapour, and sugar moving from leaves to roots in phloem*
*Xylem carries water up from the roots. Phloem (orange) carries sugar from the leaves to where it is needed, such as the roots.*

## Factors that change the rate of transpiration

| Factor | Effect | Why |
|---|---|---|
| Higher temperature | Faster | Water evaporates faster |
| Stronger wind | Faster | Moist air is blown away from the leaf |
| Higher humidity | **Slower** | Air outside is already moist, so the gradient is smaller |
| Brighter light | Faster | Stomata open for photosynthesis |

Transpiration can be measured with a **potometer**: a cut shoot sealed into a water-filled tube. As the shoot takes up water, a bubble in the tube moves, and its speed shows the rate.

## Translocation: how sugar moves

Sugar moves in the **phloem** from a **source** (where it is made or released, like a leaf) to a **sink** (where it is used or stored, like a growing root, a fruit or a potato tuber). In spring, stored starch in roots can become a source and send sugar up to new buds.

**Mind map**

```text
Transport in plants
  Xylem
    Water and minerals
    Upward only
    Dead lignified tubes
  Transpiration
    Evaporation from leaves
    Cohesion and adhesion
    Pulls water up
  Rate
    Heat and wind: faster
    Humidity: slower
    Potometer measures it
  Phloem
    Sugars and amino acids
    Living sieve tubes
    Source to sink
```

### Year 11: The pressure flow model of translocation
How does sugar move in the phloem? The **pressure flow** (mass flow) model explains it.

1. At the **source**, companion cells use **active transport** to load sucrose into the sieve tubes.
2. This makes the sap very concentrated, so water enters from the nearby xylem by **osmosis**.
3. Pressure builds up at the source end.
4. At the **sink**, sucrose is removed and used or stored, so water leaves and pressure falls.
5. Sap flows from high pressure to low pressure, carrying the sugar.

Evidence comes from **aphids**, small insects that push their mouthparts into a single sieve tube. If the aphid is cut away, sap keeps flowing out of the mouthpart under pressure. Scientists collect and test this sap. **Radioactive tracers** (carbon-14 in $\mathrm{CO_2}$) also show sugar moving from leaves to roots and fruits.

### Year 12: Plants, water and disease
Plant pathogens in Module 7 often attack the vascular tissue. **Myrtle rust** and **Panama disease** of bananas (a fungus that blocks the xylem) cause plants to wilt even in wet soil, because water can no longer move up. Understanding plant transport helps farmers spot these diseases early.

## Quick summary

- Xylem carries water and minerals up from the roots. Phloem carries sugars from sources to sinks.
- Transpiration is the loss of water vapour from leaves. It pulls water up through the xylem.
- Cohesion and adhesion keep the water column unbroken.
- Heat, wind and light speed up transpiration. Humidity slows it down.
- Translocation moves sugar in the phloem from source to sink.

## Quiz

Try these questions from the chapter quiz.

1. Which tissue carries water up a plant?
   - A. Xylem
   - B. Phloem
   - C. Epidermis
   - D. Stomata
   - Answer: A. Xylem. Xylem carries water and minerals upward.

2. Which tissue carries sugars?
   - A. Xylem
   - B. Phloem
   - C. Root hairs
   - D. Cuticle
   - Answer: B. Phloem. Phloem carries sugars from sources to sinks.

3. Transpiration is
   - A. the movement of sugar in phloem
   - B. the loss of water vapour from leaves
   - C. the uptake of minerals by roots
   - D. the making of glucose
   - Answer: B. the loss of water vapour from leaves. Water evaporates and leaves through stomata.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/biology/transport-in-plants/#quiz