What are the limitations of diffusion in multicellular organisms?

Diffusion in multicellular organisms is limited by distance, time, size of molecules, and the concentration gradient.

Diffusion is a passive process where molecules move from an area of high concentration to an area of low concentration until equilibrium is reached. However, in multicellular organisms, this process has several limitations. The first limitation is distance. In large organisms, cells may be located far from the source of the molecules they need, making diffusion inefficient. This is because the rate of diffusion decreases as the distance increases. For instance, nutrients from food would take a long time to reach all cells in a large organism through diffusion alone.

Another limitation is the size of the molecules. Larger molecules move slower than smaller ones, making their diffusion less efficient. For example, proteins and other large molecules cannot easily diffuse across cell membranes, limiting their distribution within the organism. This is why cells have developed specialised transport systems, such as the circulatory system in animals, to overcome this limitation.

The concentration gradient also affects the rate of diffusion. The steeper the gradient, the faster the rate of diffusion. However, in multicellular organisms, maintaining a steep concentration gradient can be challenging. This is because as molecules diffuse, the concentration gradient decreases, slowing down the rate of diffusion. Cells must therefore constantly work to maintain a steep gradient, which requires energy.

Lastly, diffusion is a slow process, which can limit its effectiveness in multicellular organisms. This is particularly true for organisms that have high metabolic rates and therefore require a rapid supply of nutrients and removal of waste products. To overcome this, organisms have developed efficient transport systems, such as the circulatory system in animals and the xylem and phloem in plants, which can transport substances much faster than diffusion alone.