Explain how water and mineral ions are transported through the plant by…

The idea

Water is absorbed by root hair cells and moves up the xylem as a continuous column. Transpiration is the loss of water vapour from leaves, mainly through stomata; evaporation from leaf cells creates tension that pulls the column upward, and mineral ions dissolved in the water travel with it. Stomata are pores in the lower epidermis, each bounded by two guard cells. When guard cells take in water they swell and the pore opens; when they lose water the pore closes. Open stomata allow carbon dioxide in for photosynthesis and water vapour out; closing reduces water loss in hot or dry conditions.

What earns the marks

• Water is taken up by root hair cells and mineral ions are absorbed with it, then carried in the xylem.
• Transpiration is loss of water vapour from the leaf, chiefly through stomata, driven by evaporation from leaf cells.
• Evaporation creates tension that pulls water up the xylem in a continuous column, carrying dissolved mineral ions.
• Stomata are pores bounded by guard cells; guard cells swelling opens the pore and losing water closes it.
• Open stomata allow carbon dioxide in and water vapour out; closure reduces water loss.
• Xylem tissue transports water and mineral ions upward; it is not the site of water vapour loss.

Common mistakes and how to put them right

• Confusing xylem with phloem.

  Put it right: Xylem carries water and mineral ions upward, phloem carries sucrose; correct the transport tissue named.

• Saying stomata are cells.

  Put it right: Stomata are pores, and the guard cells are the cells that open and close them.

• Claiming guard cells open the pore by losing water.

  Put it right: It is water uptake that swells guard cells and opens the pore.

• Thinking transpiration happens mainly through the roots.

  Put it right: Water is lost as vapour mainly from leaves through stomata.

Exam tips

• Read the question stem to see whether it asks about the pore, the guard cells or the whole transport pathway before choosing.
• Eliminate options that swap xylem and phloem roles, since this is the most common distractor.
• Check the direction of water movement in each option: uptake at roots, upward in xylem, loss at leaves.