Discussion
The schematic depicts an unexpanded plant cell within a hypocotyl with the arrangement of cellulose microfibrils marked on its cell wall.
[The schematic shows a cube-shaped cell whose side walls carry cellulose microfibrils running horizontally, around the cell, with randomly oriented fibrils on its top face.]
Which one of the following shapes would most likely result from the expansion of this cell if the pattern of the cellulose fibrils does not change?
Answer: A
The expanded cell is (A), the tall, vertically elongated block.
Cellulose microfibrils are stiff in tension along their length. When they are wrapped around the cell horizontally, like hoops on a barrel, they resist any widening of the cell, but the wall can still yield parallel to the cell's axis, between the hoops. Turgor-driven expansion is therefore channelled into elongation along the axis, perpendicular to the microfibrils, which is how hypocotyl cells grow long.
(B), a widened cell, would need the hoops to stretch. (C), an unchanged cube, would mean no expansion at all. (D), a sphere, is the shape expected when microfibrils are laid down randomly, so the wall yields equally in every direction.
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