In the constant bustle and jostling of molecules inside our bodies everything tends to spread out.

Sometimes the fact that substances spread out works well for us, like in the case of oxygen and carbon dioxide. But cells also spend a lot of effort collecting particles that spread out and putting them back into their compartments. Like our homes, cells get messy as they live their lives. One of their chores is to put stuff back in its place after it spreads out.

Why this is important… Membranes form a barrier to some substances, but it is important that others get a free pass. This is especially true of oxygen and carbon dioxide, which pass freely through membranes in cells lining the lungs. Both are vital to releasing energy from food molecules in our cells.

Foundations

Molecules that are chemically similar to the cell membrane can move through without the use of protein channels. But the membrane is a barrier to molecules that are unlike the membrane, like polar molecules and ions.

At a small scale, differences in concentration cause molecules to “want” to move somewhere. They don’t have a destination, they just spread out. This simple fact of life drives a lot of how cells work.

To build a mental picture you might watch this short video clip – Diffusion of Food Dye in Hot and Cold Water. It shows the process of diffusion in action – molecules spreading out. In this example, food dye is placed in a beaker. The dye in a warm beaker (A) diffuses faster than in the cold beaker (B).

Key terms

Diffusion – process of a substance spreading out within a fluid (verb: diffuse)

Simple diffusion – spreading out of a substance across cell membranes without the need for special channels through the membrane (one of the mechanisms of membrane transport)

Details

Oxygen and carbon dioxide have no problem moving through cell membranes. Look back at the chemical structure of O₂ and CO₂ – both are very symmetrical molecules so they can’t form polar covalent bonds. That means they are nonpolar and can “get along” with the phospholipid bilayer. They are also very small, which helps them slip right through the phospholipid bilayer.

Stop & Think #5: Given the nonpolar nature of chemicals like gasoline and paint thinner, how would they move around the body? How might they disrupt cell membranes?

Gradients drive the movement of oxygen and carbon dioxide. Just like heat spreads out to cool down the body, molecules naturally want to spread out (diffuse). All molecules, especially in liquid or gas states, are constantly moving around like people mingling at a party, except molecules move randomly. If you release a chemical into this environment, it gets jostled around causing it to spread out. The warmer the environment, the faster the movement. Once they are spread out, the jostling continues, but the net movement ends. That is, it no longer spreads out, it just moves around like the mingling guests at a party.

Put these two ideas together and you have the concept of simple diffusion. Molecules that can move freely through the cell membrane will spread out across it. The cell membrane does not act like a barrier in this case – the molecules move through like it wasn’t even there.

Stop & Think #6: Explain the process of diffusion in your own words. Then come up with your own analogy for how it works (similar to the party analogy above).

Diagram of molecules diffusing across cell membrane over time. Each frame shows molecules more spread out.

FIGURE 4-8. Simple diffusion. Time series showing particles spreading out across a membrane.
("Simple diffusion" by Mariana Ruiz Villarreal, LadyofHats is in the Public Domain, CC0)

Breathing takes advantage of simple diffusion.

Oxygen moves from air spaces of the lungs into the blood. Blood carries it to body cells where it diffuses into the cells that have used up their oxygen.
As cells produce carbon dioxide, it builds up, then diffuses into the blood. It is carried to the lungs and diffuses into air spaces to be exhaled.

The rate of diffusion is affected by a variety of factors. In the body, the key factors affecting this are:

Difference in concentrations. The more different the concentrations (steeper the gradient) in two areas, the faster diffusion will occur.
Temperature. Higher temperatures cause molecules to move faster, speeding up diffusion.
Size of particles diffusing. Smaller particles move more quickly than large particles.
Thickness of the membrane. Thicker membranes slow down diffusion. All cell membranes are the same thickness, but sometimes a layer of fluid builds up around the membrane and slows diffusion.

VIDEO 4d: Simple diffusion

After watching this video, you should be able to:

describe simple diffusion and what makes particles spread out
describe the cell membrane as a fluid mosaic and selectively permeable
predict what substances will pass through the cell membrane by simple diffusion
explain why cells are small and how this relates to diffusion

Oxygen and carbon dioxide move down their concentration gradients. Looking at the concentration of oxygen and carbon dioxide in lungs versus the blood (Table 4-2), we see why carbon dioxide moves out of the blood and oxygen moves into the blood.

Table 4-2. Concentrations of oxygen and carbon dioxide in the lungs. The concentrations are reported in partial pressures, which are measured in millimeters of mercury (mm Hg).

	Blood in lung tissue	Air in alveoli of lungs
Oxygen	40	104
Carbon dioxide	46	40

One problem: diffusion is a very slow process. It takes oxygen several seconds to move even a tenth of a millimeter by diffusion! But a breath only lasts about 2 seconds. How could we possibly move enough oxygen into the bloodstream if diffusion is such a slow process?

Surface area is the solution! The lungs are not just two balloons. The bronchioles branch repeatedly and end in very small air sacs called alveoli – 150 million of them! This brings the total membrane surface in contact with air to about 70 square meters – about the floor area of an average classroom or lab. With air spread out over such a large area, lots of oxygen can move into the blood. And since the membranes are very thin (thousandths of a millimeter) there is still plenty of time for oxygen to diffuse into the blood.

Just for fun: Why is garlic breath so hard to get rid of? Even if you brush your teeth or chew a breath mint, the garlic odor seems to come back. This is because the aromatic molecules of garlic easily pass through cell membranes. They pass from your stomach into your blood, then from your blood into your lungs. The breath odor is not the lingering garlic in your mouth, it is the garlic molecules in your blood!

Once oxygen is in the blood, it diffuses into the cells. Oxygen levels in the blood coming from the lungs remains fairly high. But the oxygen levels in the body cells is often low because cells keep using up oxygen. Therefore, oxygen flows from the blood into the cells, down its gradient. The blood returning to the lungs has been depleted of oxygen so it is ready for more oxygen to diffuse in from the next breath.

Stop & Think #7: Trace the path of an oxygen molecule from the air to a cell inside the body. Try the same thing for carbon dioxide, but from cell to exhaled breath.

Oxygen and carbon dioxide levels in the blood are kept steady by controlling our rate of breathing. Breathing is mostly controlled automatically. The inner part of our brains (the brainstem) monitors blood concentrations of oxygen and carbon dioxide, as well as blood pH. It adjusts our breathing rate to keep these levels constant. For example, when we work harder our cells create more carbon dioxide which accumulates in the blood. This rise in CO₂ causes us to breathe faster, increasing the delivery of CO₂into the lungs to get rid of the excess. This is another example of homeostasis.

Learning objectives

Explain simple diffusion and identify examples of it in the body
Explain the concept of selective permeability as it applies to cell membranes
Describe what types of substances can move freely through the cell membrane
Describe how environmental conditions affect the rate of diffusion.

Stop & Think Questions Explained

5. Since those solvents are nonpolar, they would not mix well with the water in the body. However, they could easily dissolve the phospholipids and cholesterol of the cell membranes. This would disintegrate the cells.

6. Your description should include individual objects that gradually spread out. Any analogy you choose is fine – the more familiar it is to you, the more it helps you to understand. Use an analogy, though, not an example.

7. Oxygen diffuses into the bloodstream (mostly into red blood cells). It is carried around the entire body where it diffuses out of blood, across the membranes of the blood vessels and cell.