Transport in Plants

Imagine your kitchen is on the top floor of a tall building, but the only tap is in the basement. You would need a way to carry water up — and a way to carry meals back down. A plant has exactly this problem.

A plant takes in water and minerals at its roots, deep in the soil. But it makes its food (sugars) in its leaves, high up where the sunlight is. Nothing can simply stay where it starts.

So a plant needs a transport system — a set of tubes that moves substances from where they enter to where they are needed:

• Water and minerals must travel up from roots to leaves.
• Sugars made in the leaves must travel outwards and down to every other part.

Unlike you, a plant has no heart to pump things around. Instead it uses clever physical ideas — surface area, evaporation and pressure — to keep substances moving. Over the next sections you will meet the two tube systems that make this happen: xylem and phloem.

Roots do two jobs: they anchor the plant firmly in the ground, and they absorb water and dissolved minerals from the soil.

The real heroes of absorption are tiny cells near the root tips called root hair cells. Each one has a long, thin projection — a "hair" — that pokes out between the soil grains.

Why a hair shape? It is the big idea of biology again: structure suited to function. The thin projection gives the cell a very large surface area in contact with the soil. More surface touching more soil means more water can be absorbed, and faster.

Roots also absorb dissolved minerals, such as nitrates, which the plant needs to grow healthy leaves and stems.

Once water is inside the root, it needs to travel up — sometimes many metres up a tall tree. The plant uses a system of tubes called xylem.

Xylem tubes are made of dead, hollow cells joined end to end, with their end walls gone — so they form long, continuous straws running from the roots, up the stem, and into every leaf.

Two important facts to remember about xylem:

• It carries water and dissolved minerals.
• It carries them in one direction only — upwards, from roots to leaves.

The water travelling up the xylem is not just for drinking — it carries minerals to the leaves, keeps cells firm, and is a raw material for photosynthesis.

If xylem is a straw, what does the "sucking"? The answer is transpiration.

Leaves have tiny holes called stomata. Water inside the leaf turns into water vapour and escapes through these holes into the air — this loss of water vapour is called transpiration.

Here is the clever part. As water leaves the top of the plant, it pulls more water up behind it, all the way from the roots. This continuous flow of water up the plant is called the transpiration stream.

Transpiration speeds up when it is hot, dry, windy or bright, because water evaporates faster in those conditions. On a hot day a large tree can move hundreds of litres of water from soil to sky.

Water travelling up is only half the story. The leaves make sugars in photosynthesis — and those sugars must reach every other part of the plant: the growing tips, the flowers, the roots.

This is the job of phloem, the plant's second set of tubes. Phloem carries dissolved sugars (sometimes called sap) away from the leaves. Moving sugars around the plant in this way is called translocation.

There are two big differences between phloem and xylem:

• Phloem carries sugars; xylem carries water and minerals.
• Phloem can carry sugars in either direction — up or down — to wherever they are needed. Xylem only goes up.

Where do the sugars go? Some travel to growing parts so the plant can build new cells. Some travel to storage organs, such as roots, where they are stored for later — this is why a potato or a carrot tastes sweet and starchy.

Think of phloem as the plant's delivery service: it takes food from the "kitchen" (the leaves) and drops it off wherever a parcel is needed.

Understanding transport in plants opens up several later topics:

• Photosynthesis — xylem delivers the water that leaves use to make sugars; phloem carries those sugars away.
• Plant growth and crops — farmers think about water supply and minerals because they affect transport and yield.
• Human transport systems — your circulatory system also moves substances around a body. Comparing the two helps both make sense.
• Ecosystems and the water cycle — transpiration returns huge amounts of water to the air, feeding clouds and rainfall.

When a later topic mentions water, minerals or sugars moving inside a plant, picture the two sets of tubes — xylem and phloem — and you will already be halfway to the answer.