When a potassium ion (K) moves from the soil into the vacuole of a cell on the surface of a root, it must pass through several cellular structures. Which of the following correctly describes the order in which these structures will be encountered by the ion?

A) plasma membrane --> primary cell wall --> cytoplasm --> tonoplast
B) secondary cell wall --> plasma membrane --> primary cell wall --> cytoplasm --> tonoplast
C) primary cell wall --> plasma membrane --> cytoplasm --> tonoplast
D) primary cell wal --> plasma membrane --> tonoplast --> cytoplasm±vacuole
E) tonoplast --> primary cell wall --> plasma membrane --> cytoplasm

First, the outermost layer of the cell is the cell wall. Plant cells may have a primary and a secondary wall, but the latter is not always present. The primary wall is always located outwards (and in the case of having a secondary wall, it will be located between the primary wall and the plasma membrane).

Then, inside the cell wall, we will find the plasma membrane (also called plasmalemma).

When crossing the membrane, the ion will be in the cytoplasm of the cell and will be directed towards the vacuole, which is surrounded by its membrane called tonoplast. The vacuole is an organelle that has no definite shape, although it is always surrounded by the tonoplast, and it contains different substances such as water and enzymes.