Why is diffusion insufficient to meet the oxygen requirements of multi- cellular organisms like humans?

Explanation

**Step-by-Step Solution:**

1. **Understanding Diffusion**: Diffusion is the process by which molecules move from an area of higher concentration to an area of lower concentration. In simple terms, it allows substances like oxygen to pass through membranes and reach cells.

**Hint**: Think about how oxygen moves in and out of cells through concentration gradients.

2. **Single-Celled vs. Multi-Celled Organisms**: Single-celled organisms, like bacteria, have a large surface area relative to their volume. This allows them to efficiently absorb oxygen directly from their environment through diffusion.

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