Transport mechanisms — common mistakes

For an annotated drawing, each label must do two things: name the feature and state its effect on water loss — "sunken stomata: trap humid air, reduce the water potential gradient, so less diffusion of water vapour". A feature on its own scores nothing; "reduces transpiration" on its own scores nothing.

Two words decide most 7.2 marks: osmosis for every movement of water across a membrane (root hair, symplast, endodermis, into and out of sieve tubes) and mass flow for movement along a tube down a pressure gradient (xylem and phloem). Never write that sucrose "diffuses" along the phloem, and never say water is "sucked" up the xylem — it is pulled by the tension that evaporation creates.

The apoplast pathway runs through the cellulose cell walls and the spaces between cells, so water never crosses a membrane and moves quickly by diffusion or mass flow. The symplast pathway runs through the cytoplasm and from cell to cell through plasmodesmata, so water crosses partially permeable membranes and moves by osmosis. At the endodermis the Casparian strip of suberin blocks the apoplast, forcing all water into the symplast.

Evaporation from the mesophyll cells lowers the hydrostatic pressure at the top of the xylem. Because water molecules cohere by hydrogen bonds, that tension is transmitted down the whole column, and adhesion to the cellulose walls stops the column pulling away. The plant spends no ATP — the energy comes from the sun that evaporates the water.

Both. Loading sucrose into the sieve tubes is active: companion cells use ATP to pump out hydrogen ions and a cotransporter brings sucrose in with the returning H⁺. The movement along the sieve tube is then mass flow down a hydrostatic pressure gradient, which needs no energy at the tube itself — the pressure comes from water entering by osmosis at the source and leaving at the sink.