Transpiration and translocation

Last time

Before we start: last time, in Teeth, the digestive system and bile, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Pulled from the top

Water does not get pushed up a tree. It gets pulled. As water evaporates from the leaves, it pulls a column of water up the xylem behind it. That loss is transpiration. And sugars travel the other way, in the phloem, by translocation.

Transpiration

Transpiration is the loss of water vapour from leaves. Water evaporates from the surfaces of the mesophyll cells into the air spaces. Then the water vapour diffuses out through the stomata.

The transpiration stream

Water molecules stick together, so as water leaves the leaf, it pulls more water up the xylem behind it. This creates a tension that pulls a continuous column of water up from the roots. This flow, from roots to leaves, is the transpiration stream.

Why transpiration helps

It carries water and mineral ions up to the leaves. Evaporation cools the leaves on hot days. Water keeps cells turgid, supporting the leaves and stems.

Four factors

Higher temperature: water molecules have more kinetic energy, so they evaporate and diffuse faster. Lower humidity: the air is drier, so the water vapour gradient between leaf and air is steeper. More wind: moist air is blown away from the leaf, keeping the gradient steep. Brighter light: the stomata open wider.

Factors in a table

Here are the four factors. The fastest transpiration is hot, dry, windy and bright. The slowest is cold, humid, still and dark.

Measuring it

A potometer measures the rate of water uptake by a cut shoot, which is close to the rate of transpiration. As the shoot takes up water, an air bubble moves along a scale. Measure how far it moves in a set time. Change one factor, for example with a fan for wind, and keep the others constant.

Wilting

If a plant loses water faster than it takes it in, its cells lose turgor and become flaccid. The leaves and stems droop: the plant wilts. Wilting also closes the stomata, which reduces further water loss.

Translocation

Translocation is the movement of sucrose and amino acids in the phloem, from sources to sinks. A source is where they are made or released, such as photosynthesising leaves. A sink is where they are used or stored, such as roots, growing buds and fruits. A part can change roles: a potato store is a sink in summer, and a source in spring when it sprouts.