If a cell has an increased need for a particular molecule already present in a higher concentration within a cell than in the extracellular fluid.
The correct answer and explanation is:
The correct answer is active transport.
Active transport is the process by which cells move molecules from areas of lower concentration to areas of higher concentration, which requires energy. This is in contrast to passive transport, which relies on diffusion and does not require energy.
In this case, when a cell has a higher concentration of a specific molecule than the extracellular fluid but still needs more of that molecule, active transport is used to pump the molecule into the cell. This process is driven by energy, typically from ATP, and involves specialized membrane proteins called pumps or transporters. One of the most well-known examples of active transport is the sodium-potassium pump, which helps maintain a high concentration of sodium ions outside the cell and a high concentration of potassium ions inside the cell.
Active transport is crucial in situations where molecules must be concentrated inside the cell for metabolic processes or other functions, even when they are already in a higher concentration inside the cell compared to the outside. Examples of this process include the uptake of nutrients such as glucose and amino acids, which are transported into cells even when they are more concentrated inside the cell than in the extracellular fluid.
The energy used in active transport is necessary because it works against the natural tendency of molecules to diffuse from areas of higher concentration to areas of lower concentration. By utilizing energy, cells can accumulate essential molecules even when it might not be thermodynamically favorable, thus maintaining their function and homeostasis.