Saturation kinetics refers to the situation where the reaction rate:

• A. Increases linearly with substrate concentration indefinitely
• B. Decreases with higher substrate concentration
• C. Approaches a maximum as substrate concentration increases
• D. Is unaffected by substrate concentration

Answer: (C)

The essential idea is that the rate becomes limited by the finite number of active sites available for reaction. At low substrate levels, increasing substrate provides more molecules to react, so the rate rises. But once all active sites are occupied, adding more substrate cannot speed things up, so the rate levels off and approaches a maximum value (Vmax). This is the hallmark of saturation kinetics, often described by Michaelis-Menten behavior where V = Vmax [S] / (Km + [S]); as substrate concentration becomes very large, V approaches Vmax. Therefore, the rate approaches a maximum as substrate concentration increases.

The other descriptions don’t fit: the rate doesn’t keep increasing without bound because sites get saturated; it doesn’t necessarily decrease with more substrate, and it isn’t independent of substrate concentration.