That nutrients which  are allowed to pass through the cell membrane technically fuel to  the cell.
In animals, the process of diffusion is involved in the transfer of materials within the cells like transfer of food. When they breathe,  the oxygen is passed from lungs to all cells of the body through diffusion. Similarly, when cell excretes carbon dioxide then it is again diffused from cells to lungs through this process.
In plants, it is essential for the exchange of gases. Uptake of minerals is also affected by the process of diffusion.Also, it helps in removing of excess water through the process of transpiration.
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