9700 · 7.2
Transport mechanisms — FAQ
Frequently asked questions for 9700 Transport mechanisms. Direct answers first, then deeper explanation — then practise with marking.
What is the difference between the apoplast and symplast pathways in Biology 9700?
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.
Why does transpiration pull water up the xylem without the plant using energy?
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.
Is translocation in the phloem active or passive?
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.