Transport in plants

Learning objectives

Draw low-power plan diagrams of a herbaceous dicotyledon stem, root and leaf in transverse section, and describe where the xylem and phloem are positioned in each organ.

Make labelled drawings of the conducting cells (xylem vessel elements, sieve tube elements and their companion cells) from slides and electron micrographs, and relate lignified walls, pits, sieve plates and the dense cytoplasm of companion cells to their functions.

Describe how water and dissolved mineral ions travel from the soil to the xylem by the apoplast pathway through cellulose walls and the symplast pathway through cytoplasm and plasmodesmata, and explain how the suberin of the Casparian strip in the endodermis blocks the apoplast.

Explain transpiration as water evaporating from mesophyll cell walls into the leaf air spaces and the vapour then diffusing out through open stomata, and explain the transpiration pull in the xylem by cohesion–tension and adhesion to cellulose, both due to hydrogen bonding.

Annotate transverse sections of xerophyte leaves such as marram grass, explaining how rolled leaves, sunken stomata in pits, hairs and thick waxy cuticles cut transpiration.

Explain translocation of sucrose and amino acids from sources to sinks, including loading by companion cells with proton pumps and cotransporter proteins and mass flow down a hydrostatic pressure gradient in the sieve tubes.