Who proposed the cell theory and when?

Explanation

### Step-by-Step Solution:

1. **Identify the Scientists**: The cell theory was proposed by two scientists: Matthias Schleiden and Theodor Schwann.

2. **Determine the Year of Proposal**: The cell theory was proposed in the year 1839.

3. **Understand the Cell Theory**: The cell theory states that:
- All living organisms are composed of cells.
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