Glossary: Transport in Plants

This chapter's terms in context

Chapter 19 covers four terms describing how water and dissolved substances move through a plant: root hair cell, transpiration, transpiration stream and translocation. A root hair cell is a specialised epidermal cell found near the tip of a root, extended into a long, thin, hair-like projection that greatly increases the surface area in contact with soil water, allowing it to absorb water by osmosis (down a water potential gradient) and mineral ions by active transport (against a concentration gradient, using energy from the large number of mitochondria this cell contains). Because a root hair cell must carry out both a passive process (osmosis for water) and an active one (active transport for minerals) at the same time, examiners frequently test whether a candidate can name the correct process for the correct substance rather than describing water and mineral uptake as though they happened by the same mechanism.

Transpiration is the loss of water vapour from a plant's aerial surfaces, mainly through the stomata of the leaves, and although it appears to be a simple loss of water, it is in fact the driving force behind the upward movement of water through the whole plant: as water evaporates from the leaf's mesophyll cells and diffuses out through open stomata, it creates a region of lower water potential inside the leaf, which pulls more water upward through the xylem to replace it, a continuous column of water and mineral salts known as the transpiration stream. This pulling mechanism, called transpiration pull or the cohesion-tension mechanism, relies on the strong cohesive forces between water molecules that allow the whole column to be pulled upward as a continuous unit without breaking, from the root hair cells all the way to the leaf.

Because the rate of transpiration is controlled largely by environmental conditions, higher temperature, lower humidity, greater wind speed and higher light intensity (which causes more stomata to open) all increase the rate of transpiration, this chapter connects directly back to the structure of the stoma and guard cell covered in the previous chapter, and examiners frequently combine the two by asking a candidate to explain how a stated environmental change would affect both stomatal aperture and the resulting transpiration rate together.

Translocation, in contrast to the one-directional, physical-forces-driven transpiration stream in the xylem, is the active, living transport of dissolved organic food, mainly sucrose, through the phloem from a source to a sink, and it does not depend on transpiration at all, it is a separate transport system running in living cells rather than dead ones. A common structural confusion in this chapter is treating transpiration stream and translocation as two names for the same overall transport process, when they occur in different tissues (xylem versus phloem), involve different substances (water and minerals versus dissolved food), and are driven by entirely different mechanisms (physical pulling forces versus an active, energy-requiring loading and unloading process).

Paper 3 investigations on this chapter typically use a potometer to measure the rate of water uptake by a leafy shoot under different conditions, and a correctly reasoned answer must explain that the potometer measures water uptake, which is used as an indirect estimate of transpiration rate, rather than measuring transpiration itself directly.

A related exam scenario places a leafy shoot and a leafless shoot of the same size in identical potometer set-ups, and asks a candidate to predict and explain the difference in water uptake between them: because stomata, the main exit route for water vapour during transpiration, are located almost entirely on leaves, the leafless shoot loses far less water and therefore draws up far less water through its xylem, directly demonstrating that transpiration through the leaves is what drives the bulk of water movement through the whole plant rather than the roots pushing water upward on their own.