Transport of Na⁺ and Ca²⁺ across the plasma membrane is an example of

Correct — A, antiport. Membrane carriers are classified by how many solutes they move and in which direction: a uniporter carries one solute one way, a symporter carries two solutes together in the same direction, and an antiporter carries two solutes in opposite directions across the membrane. Sodium and calcium are handled by exactly this last arrangement — the sodium-calcium exchanger of the plasma membrane lets sodium ions run inward down their steep electrochemical gradient and uses that energy to drive calcium ions out. Because the two ions cross in opposite directions on the same protein, the transport is an antiport.

• (b)symport — In a symport the two solutes move the same way. The sodium-glucose cotransporter of the intestine, which carries sodium and glucose inward together, is the standard example — but sodium and calcium go opposite ways, not together.
• (c)uniport — A uniport moves a single kind of molecule, with no second solute coupled to it. Naming two ions in the stem is by itself enough to rule this out.
• (d)aquaporins — Aquaporins are water channels. They are not a category of coupled solute transport at all, and they carry neither sodium nor calcium.

Carrier-mediated transport is named by direction and number. A uniport moves one solute; a symport moves two in the same direction; an antiport moves two in opposite directions. Symport and antiport are together called cotransport, and both are the usual machinery of secondary active transport: the cell first spends ATP to build a steep sodium gradient, then lets sodium fall back in through a coupled carrier and captures that energy to move a second solute. The sodium-calcium exchanger is a textbook antiporter, and the sodium-potassium pump moves the two ions in opposite directions as well.

This item can be answered without knowing any particular transporter, because the vocabulary itself does the work. Two ions are named, which removes uniport; aquaporin is a water channel, which removes option (d); and the remaining choice is only between the same direction and opposite directions. Calcium is kept extremely low inside the cytosol while sodium is kept low inside and high outside, so the two ions cannot be travelling the same way — sodium must be coming in and calcium going out.

• Uniport moves one solute; symport moves two the same way; antiport moves two in opposite directions.
• Symport and antiport together are called cotransport.
• The sodium-calcium exchanger uses the inward sodium gradient to expel calcium, so it is an antiporter.
• Cytosolic free calcium is held about ten thousand times lower than the concentration outside the cell.
• Aquaporins are channel proteins for water and belong to no cotransport class.

• Reading 'two ions' as automatically meaning symport.
• Choosing aquaporin because it is the only word in the list that names a real protein.
• Assuming every coupled transport is primary active transport; most cotransport is secondary and burns no ATP directly.

Pure terminology, but of the kind that can be reasoned out — the number of solutes named in the stem already eliminates two options.

• practice — not a real PYQ

  A carrier protein that moves two different solutes across a membrane in the same direction is called a

  • (a)uniporter
  • (b)symporter
  • (c)antiporter
  • (d)channel

  Answer(b) symporter — the sodium-glucose cotransporter of the small intestine is the standard example.

• practice — not a real PYQ

  Secondary active transport differs from primary active transport in that it

  • (a)moves solutes only down their gradients
  • (b)does not use ATP directly but draws on an ion gradient built by a pump
  • (c)requires no membrane protein
  • (d)occurs only in plant cells

  Answer(b) does not use ATP directly but draws on an ion gradient built by a pump — the ATP was spent earlier, by the pump that created the gradient.