Root hairs increase contact with soil and the area available for uptake. Water can enter root cells by osmosis, while mineral ions can be taken up by energy-dependent transport against electrochemical gradients. Xylem carries water and mineral nutrients towards transpiring shoots. Evaporation from leaves creates tension; cohesion between water molecules helps maintain the water column. Root pressure can also contribute under some conditions.
Phloem translocates dissolved organic products, especially sucrose, from sources to sinks. A mature photosynthesising leaf can be a source; growing roots, fruits and storage organs can be sinks. The same organ can change roles over time, so phloem movement is not simply always downward. Plants can convert transported sugars to starch for storage in organs such as tubers.
Example 1
Trace water from soil to the atmosphere using three structures.
Water enters roots, travels through xylem, and leaves as vapour mainly through stomata.
Example 2
A growing fruit receives sucrose from a mature leaf. Identify source, sink and tissue.
The mature leaf is the source, fruit the sink, and phloem the transport tissue.
Watch out: Phloem transports from sources to sinks; their positions determine direction and may be above or below one another.