What are porins? What role do they play in diffusion?

Explanation

**Step-by-Step Text Solution:**

1. **Definition of Porins**: Porins are a type of protein that form channels in the membranes of cells. They are specifically found in the outer membrane of organelles such as mitochondria and plastids, as well as in the membranes of some bacteria.

2. **Structure of Porins**: These proteins create hydrophilic (water-attracting) channels. This means that the channels allow water and other polar molecules to pass through easily.

3. **Function of Porins**: The primary role of porins is to facilitate the passage of large biomolecules and small-sized proteins across the membrane. They do this by allowing these substances to move according to their diffusion gradient, meaning they move from areas of higher concentration to areas of lower concentration.

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