S.J. Singer and G.L. Nicholson (1972) introduced the ........ to explain the structure of cell membrane.

Explanation

**Step-by-Step Solution:**

1. **Identify the Scientists:** The question mentions S.J. Singer and G.L. Nicholson, who are the key figures in this context.

2. **Determine the Year:** The year given in the question is 1972, which is significant as it marks the introduction of a specific model related to cell membranes.

3. **Understand the Context:** The question is asking for a specific term or model that these scientists introduced to explain the structure of the cell membrane.

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