Active transport requires energy to move substances against their concentration gradient. In the context of calcium entering the cell, active transport would be required when the concentration of calcium inside the cell is lower than outside the cell, allowing calcium to move into the cell despite the concentration gradient.
Considering the options provided:
-
The cell is 20% calcium, and the cell's environment is 80% calcium. (Calcium would naturally move into the cell, but this is passive transport.)
-
The cell is 50% calcium, and the cell's environment is 50% calcium. (No net movement, equal concentrations.)
-
The cell is 40% calcium, and the cell's environment is 60% calcium. (Calcium would naturally move into the cell, but this is passive transport.)
-
The cell is 80% calcium, and the cell's environment is 20% calcium. (Calcium would need to move out of the cell, not in.)
From this analysis, none of the situations presented suggest active transport for calcium entry into the cell because active transport would be required when calcium is low inside the cell and high outside. Therefore, active transport would be appropriate if the cell has a much lower proportion of calcium than its environment, which is not represented in these options.
For an explicit scenario where calcium would need to enter via active transport, consider if the cell were, say, 10% calcium and the environment had 50% calcium.