Question

1. Explain why an ion that enters a cell through a symport is moving through the symplastic route.
2. If an ion is moving through the apoplastic route, what will happen to it when it reaches the endodermis?

Answer 1

Symplastic movement of molecules occur via cytoplasm. Movement of molecules occurs by plasmodesmata. It does not require ATP.

Apoplastic movement of molecules occur via cell wall. It requires ATP as energy source.

Symport is movement of two molecules across plasma membrane against concentration gradient. It requires energy in the form of ATP.

Symport occurs via symplastic route because of following reasons -

It is the movement of molecules against concentration gradient. So, if it will occur via apoplast, that is, via cell wall, it will lead to differential H+ ion concentration across cell wall. In cytoplasm, it will also cause a change in pH, but that would be balanced by other molecules present in cytoplasm and also excess H+ ions would be pumped out easily.

Also, cotransport of 2 molecules would occur easily in fluid filled cytoplasm, rather than stiff cell wall.

Next question -

If movement occurs by apoplast in a cell, then it will stop at endodermis due to presence of casparian strips. These strips are present in radial manner in endodermis and are impermeable to water movement.

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