Plant transport - Transpiration and Translocation

Transpiration definition

The loss of water vapour from leaves

How does transpiration work?

Water evaporates from the surface of mesophyll cells into air spaces and then diffuses out of the leaf via the stomata as water vapour.

Transpiration pull

When water is pulled upwards through xylem by suction because of the loss of water in the leaves.

Factors that affect rate of transpiration

• Temperature

• Wind speed

• Humidity

How does temperature affect rate of transpiration?

Higher temp, higher rate of transpiration, warm air outside has a lower conc. of water so water diffuses quicker/accepts more of it.

How does wind speed affect rate of transpiration?

Faster rate of transpiration in moving air because water vapour is carried away by the wind. Steep conc. gradient between air spaces and air outside is maintained.

Why does wilting occur?

Because when water is scarce, transport in the xylem decreases. Water lost to evaporation is faster than it can be taken up by the cell. The cells lose turgid pressure and become flaccid which causes wilting.

Translocation definition

Movement of sucrose and amino acids in the phloem from a source to sinks. (Some parts plants may acts as both at different times)

Sources

Parts of plants that release sucrose or amino acids. E.g region of production (leaves) or regions of storage (tubers)

Sinks

Parts of plants that RECIEVE sucrose and amino acids. E.g region of active growth (buds) or regions of storage (roots, stems, tubers).