28.2 DIFFUSION VS. BULK TRANSPORT

How does oxygen get from the air in your lungs into your bloodstream? How does atmospheric carbon dioxide get into leaves? How does ammonia get from seawater into the cells of planktonic algae? The answer to all three questions is the same: by diffusion. But oxygen absorbed by your lungs doesn’t reach your toes by diffusion alone—it is transported actively, and in bulk, by blood pumped through your circulatory system. Bulk transport of oxygen, nutrients, and signaling molecules, at rates and across distances far larger than can be achieved by diffusion alone, lies at the functional heart of complex multicellularity.

28.2.1 Diffusion is effective only over short distances.

Diffusion is the random motion of molecules, with net movement occurring when there are areas of higher and lower concentration of the molecules (Chapter 5). Because diffusion supplies key molecules for metabolism, it exerts a strong constraint on the size, shape, and function of cells. Diffusion is effective only over small distances and so it strictly limits the size and shape of bacterial cells, as discussed in Chapter 26. Diffusion also constrains how eukaryotic organisms function.

Oxygen provides a good example. Most eukaryotes require oxygen for respiration. If a cell or tissue must rely on diffusion for its oxygen supply, its thickness is limited by the concentration difference in oxygen between the cell or tissue and its environment. The concentration difference is dependent on the amount of oxygen in the environment and the rate at which oxygen is used for respiration inside the organism. In shallow water that is in direct contact with the atmosphere, diffusion permits animals to reach thicknesses of about 1 mm to 1 cm, depending on their metabolic rate. Of course, a quick swim along the seacoast will reveal many animals much larger than this. And, obviously, you are larger than this. How do you and other large animals get enough oxygen to all of your cells to enable you to survive?

28.2.2 Animals achieve large size by circumventing limits imposed by diffusion.

Sponges can reach overall dimensions of a meter or more, but they actually consist of only a few types of cell that line a dense network of pores and canals and so remain in close contact with circulating seawater (Fig. 28.5a). The large size of a sponge is therefore achieved without placing metabolically active cells at any great distance from their environment. Likewise, in jellyfish, active metabolism is confined to thin tissues that line the body. Essentially, a large flat surface is folded up to produce a three-dimensional structure (Fig. 28.5b). The jellyfish’s bell-shaped body is often thicker than the metabolically active tissue, but its massive interior is filled by materials that are not metabolically active. This material constitutes the mesoglea, the jellyfish’s “jelly.” The mesoglea provides structural support but does not require much oxygen.

Figure 28.5: Circumventing limits imposed by diffusion. (a) Sponges can attain a large size because the many canals in their bodies ensure that all cells are in close proximity to the environment. (b) Jellyfish also have thin layers of metabolically active tissue, but their familiar bell can be relatively thick because it is packed with metabolically inert molecules (the mesoglea, or “jelly”).

But what about us? How does the human body circumvent the constraints of diffusion? Our lungs gather the oxygen we need for respiration, but the lung is a prime example of diffusion in action, not a means of avoiding it. Because lung tissues have a very high ratio of surface area to volume, oxygen can diffuse efficiently from the air you breathe into lung tissue (Chapter 39). A great deal of oxygen can be taken in this way, but how does it get from the lungs to our brains or toes? The distances are far too large for diffusion to be effective. The answer is that oxygen binds to molecules of hemoglobin in red blood cells and is carried through the bloodstream to distant sites of respiration. We circumvent diffusion by actively transporting oxygen through our bodies. More generally, the circumvention of diffusion is what makes complex multicellularity possible.

28.2.3 Complex multicellular organisms have structures specialized for bulk transport.

We have seen how humans circumvent the limitations of oxygen diffusion. Many other animals similarly rely on the active circulation of fluids to transport oxygen and other essential molecules, including food and molecular signals, across distances far larger than those that could be traversed by diffusion alone. Indeed, without a mechanism like bulk transport, animals could not have achieved the range of size, shape, and function familiar to us.

Figure 28.6: Bulk transport. (a) The circulatory system in animals and (b) the vascular system in plants allow these organisms to get around the size limits of diffusion.

Bulk transport is the means by which molecules move through organisms at rates beyond those possible by diffusion across a concentration gradient (Fig. 28.6). In animals, the active pumping of blood through the circulatory system supplies oxygen to tissues that may be more than a meter distant from the lungs (Fig. 28.6a). Bulk transport carries the organic molecules required for respiration large distances from the intestinal cells that absorb these molecules from the digestive tract. (Again, the molecules are transported actively in the bloodstream.) Endocrine signaling molecules such as hormones (Chapter 9) also move rapidly through the body by means of the blood and other fluids. Bulk transport is key to complex multicellularity in humans and most other animals.

Complex organisms other than animals also rely on bulk transport. A redwood tree must transport water upward from its roots to leaves that may be 100 meters above the soil. If plants relied on diffusion to transport water, they would be only a few millimeters tall. How, then, do they move water? Plants move water by bulk transport through a system of specialized tissues powered by the evaporation of water from leaf surfaces (Fig. 28.6b; Chapter 29). Vascular plants also have specialized tissues for the transport of nutrients and signaling molecules upward and downward through roots, stems, and leaves. Thus, like animals, plants rely on specialized tissues to transport essential materials over long distances at rates many times faster than could be accomplished by diffusion alone.

The circumvention of diffusion can be seen in other groups characterized by complex multicellularity. Fungi transport nutrients through networks of filaments that may be meters long, relying on osmosis to pump materials from sites of absorption to sites of metabolism (Chapter 34). The giant kelps have an internal network of tubular cells that transports molecules through a body that can be tens of meters long. In general, when some cells within an organism are buried within tissues, far from the external environment, bulk transport is required to supply those cells with molecules needed for metabolism.