Transpiration and stomata – GCSE Combined Science Biology

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A big tree can move hundreds of litres of water from its roots to its leaves in a day. It has no heart or pump. This topic is about how a plant does it.

It matters because plants need that water to stay upright, to photosynthesise and to stay cool. It also explains why plants wilt on hot days.

By the end you will be able to describe how water moves through a plant. You will explain how stomata open and close. You will say what speeds up water loss, and work out a rate from a potometer.

The transpiration stream

Water enters a root hair cell by osmosis. Osmosis is water moving from a dilute solution to a more concentrated one, through a membrane. Then the water moves across the root and into the xylem. Xylem is the tissue that carries water up.

In the leaf, water evaporates from the cells inside and diffuses out through the stomata. Diffuses means spreads out. This loss of water vapour is called transpiration.

As water leaves the leaf, more water is pulled up the xylem to replace it. Think of drinking through a straw. This flow from roots to leaves is the transpiration stream.

Why transpiration is useful

The water that is pulled up does several jobs. It carries mineral ions from the soil up to the leaves.

It keeps the cells turgid, which means firm and full of water. This supports the plant. It also gives the leaves water for photosynthesis. The evaporation of water cools the leaf on a hot day, like sweat cooling your skin.

So transpiration is not wasted water. It is the cost a plant pays for having open stomata.

Stomata and guard cells

Stomata are small holes, mostly on the underside of leaves. Carbon dioxide goes in for photosynthesis. Oxygen and water vapour go out.

Each stoma is surrounded by two guard cells. Think of two long balloons. When they fill with water they become turgid and bend apart, so the stoma opens. When they lose water they become flaccid, which means soft and limp. The stoma closes.

Stomata are usually open in the light and closed in the dark. They also close if the plant is short of water.

Temperature and light

Several things change how fast a plant loses water. Think of washing on a line. It dries fastest when it is hot, dry and windy.

A higher temperature makes water evaporate and diffuse faster. So transpiration speeds up.

More light opens the stomata wider, so more water vapour can escape. Transpiration speeds up. If there is a risk of wilting, a plant can close its stomata. This saves water but stops carbon dioxide getting in, so photosynthesis slows. That is the trade-off.

Humidity and wind

Humidity is the amount of water vapour in the air. Water vapour diffuses from where there is lots to where there is less. This difference is called the concentration gradient.

When the air is dry, the gradient is steeper. Water vapour leaves the leaf faster, so transpiration speeds up. When the air is humid, the gradient is smaller, so transpiration slows down.

Moving air blows away the humid air near the leaf. This keeps the gradient steep, so transpiration speeds up.

Measuring with a potometer

A potometer is a tube of water attached to a cut shoot. As the shoot takes up water, an air bubble moves along a scale. The distance moved in a set time shows the rate of water uptake. This is almost equal to the rate of transpiration.

Rate of uptake = distance moved by the bubble ÷ time. If the bubble moves 30 mm in 5 minutes, the rate is 30 ÷ 5 = 6 mm per minute.

To compare conditions, change only one factor, such as adding a fan. Cut the shoot under water so no air gets into the xylem. Seal the joints so air does not leak in.

Recap

Water goes from the root hair into the xylem, up to the leaf, then evaporates and leaves by the stomata.

Guard cells open the stomata when they are turgid.

Transpiration rises with higher temperature, lower humidity, more wind and more light.

Rate of uptake = distance ÷ time.