Enzymes are necessary for many cellular reactions because they speed up the reactions by lowering the activation energy required, making the reactions occur fast enough to meet the needs of the cell. Without enzymes, these reactions would be too slow to sustain life processes. This explains why the reactions would otherwise not proceed sufficiently fast under normal biological conditions.

Specifically, enzymes act as biological catalysts that increase reaction rates by over a million-fold, allowing vital cellular processes to happen in fractions of a second instead of years. Therefore, the best explanation among common options given is that the reactions would be too slow to meet the needs of the cell if enzymes were missing.

Summary of why enzymes are necessary:

• Enzymes lower the activation energy for reactions.
• This increases reaction speed enough to support cellular functions.
• Without enzymes, many cellular reactions would be too slow to sustain life.
• Enzymes are reusable and highly specific to substrates in the cell.

This is why enzymes are essential for metabolism, DNA replication, digestion, and many other cellular activities.

The best concise explanation is:
The reactions are too slow to meet the needs of the cell if enzymes are missing.