**Question:** Following occurs in living cells only:

A. Enzymatic reactions
B. Electrical impulses
C. Protein synthesis
D. Respiration

**Core Concept:**

Living cells are the basic structural and functional units of all organisms, comprised of organelles and macromolecules. Enzymatic reactions, electrical impulses, protein synthesis, and cellular respiration are essential processes that occur within these cells.

**Why the Correct Answer is Right:**

Enzymatic reactions are catalyzed by enzymes, which are proteins that speed up chemical reactions within cells. They are specific and reversible, which makes them crucial for various cellular processes.

**Why Each Wrong Option is Incorrect:**

B. Electrical impulses (also known as action potentials) are generated by specialized cells like neurons and are essential for communication within the nervous system. While they occur within cells, they are not confined to living cells only.

C. Protein synthesis, also known as transcription and translation, is the process by which cells produce proteins based on genetic information. It occurs within cells but is not specific to living cells only as it also occurs in non-living systems like prokaryotic cells and viruses.

D. Respiration is the process by which cells convert nutrients into energy. It occurs within cells, particularly in mitochondria, and is essential for cellular function. Although cellular respiration is essential in living cells, it can also occur in non-living cells like certain bacteria, although not in the same context.

**Clinical Pearls:**

1. Enzymatic reactions are highly specific and reversible processes that occur within cells, making them essential for cellular function.
2. Electrical impulses are vital for communication, but they occur in living cells and not exclusively within them.
3. Protein synthesis is a crucial cellular process, but it can also occur in non-living systems.
4. Respiration is essential for cellular energy production, but it can also happen in some non-living cells like bacteria under specific conditions.

**Correct Answer:** D. Respiration (mitochondrial respiration)

In summary, while all four options are associated with living cells, option D, cellular respiration, occurs in mitochondria of eukaryotic cells and certain bacteria, making it the correct answer. The other options are specific to living cells but occur in other non-living systems as well.