Moving substances through plants

By UK GCSE Biology · independent study · myAudience

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Moving substances through plants

Xylem transports water and mineral ions from roots toward leaves. Evaporation from leaf surfaces contributes to transpiration, helping maintain the upward movement of water. Phloem translocates dissolved sugars between sources and tissues using or storing them.

Translocation moves dissolved sugars through phloem from a source to a sink, such as from a photosynthesizing leaf to a growing root. Sources and sinks can change with season and development. A potometer estimates water uptake, not photosynthesis directly, and uptake is not perfectly identical to transpiration because some water is used or stored in the plant.

Water enters living root cells, travels through the root and rises through supported xylem vessels. Their mature conducting elements lack living contents, whereas phloem uses living sieve tubes and associated companion cells for sugar transport. A source exports sugar and a sink uses or stores it, so transport can occur in different directions in different tubes. Compare transpiration measurements as water uptake per unit time and keep other conditions constant.

Worked example: Why might greater air movement increase transpiration when other conditions stay suitable?

Air movement removes moist air near the leaf

This can maintain a water-vapor gradient

Answer: Evaporation and diffusion of water vapor may increase.

Check your understanding: Does phloem carry only water from roots?

Answer: No.

Why: Its principal role in this comparison is transport of dissolved sugars between sources and sinks.