Diffusion

How do molecules move across the plasma membrane without the use of energy? In this article, we will discuss molecular diffusion: what it is, what are some examples, what factors affect the rate of diffusion, and how it is distinguished from osmosis. We will also touch on facilitated diffusion and the types of transport proteins involved.

What is molecular diffusion in biology?

Molecules are in constant motion, and as a result, they have thermal energy that causes them to move towards any available space, a process called molecular diffusion (or simply diffusion). They move in the direction of the concentration gradient: from a region of higher concentration to a region of lower concentration until the two have reached dynamic equilibrium.

The diffusion of molecules across the plasma membrane is considered a form of passive transport, meaning it does not require energy. However, due to the selective permeability of the plasma membrane, not all molecules can passively diffuse through it. The size, charge, or other properties of the molecules may prevent them from passing through the plasma membrane.

The process of molecular diffusion is illustrated in Figure 1 below.

Figure 1. This simple diagram shows how molecules (the red dots) move from an area of higher to lower concentration. Over time, diffusion has led the two areas to reach equilibrium. Source: Diffusion.en.jpg: derivative work: Quasar Jarosz, Public domain, via Wikimedia Commons.

What are some examples of diffusion?

Consider someone opening a bottle of perfume in the middle of a crowded room. The perfume has the maximum concentration in the bottle and the lowest concentration toward the room's walls. The perfume diffuses away from the bottle, and as the perfume spreads, more people will smell it.

The same process occurs when substances move across the plasma membrane through diffusion: the difference in concentration between the cytosol (the fluid contained in the cell) and the extracellular fluid (the fluid outside the cell) causes molecules to move towards the region with lower concentration. If the characteristics of the molecule allow it to flow freely through the membrane (small, hydrophobic, nonpolar), it will easily cross towards the area with lower concentration until equilibrium is met.

What factors affect the rate of diffusion?

Diffusion happens at different rates depending on several factors:

• Concentration gradient: The higher the concentration difference, the faster the diffusion. As the distribution of substances approaches equilibrium, the diffusion rate slows.
• Solvent density: The higher the density of the solvent, the lower the rate of diffusion. This is because molecules move slower through a denser medium. On the other hand, the lower the density of the solvent, the faster the diffusion.
• Molecular mass: Heavier molecules travel more slowly, so they diffuse more slowly. On the other hand, lighter molecules travel faster, so they diffuse faster.
• Temperature: Higher temperatures increase thermal energy which leads to an increase in the rate of movement of the molecules, thereby increasing the rate of diffusion. The opposite is true of lower temperatures.

Osmosis vs diffusion: what are their similarities and differences?

Osmosis and diffusion are both passive transport processes that occur when two solutions of different concentrations equalize through the movement of particles from an area of higher to lower concentration.

However, osmosis and diffusion are considered two different processes. A key distinction is that in diffusion, both solvent and solute molecules move, but in osmosis, only solvent molecules move across the membrane.

Osmosis is a specific type of diffusion wherein solute molecules on each side of a semi-permeable membrane reach equilibrium, not by moving themselves, but rather through the movement of water toward the region of higher solute concentration. This tends to happen when solute particles cannot pass through a barrier themselves.

For example, Figure 2 shows how osmosis takes place in a U-shaped tube with two sections separated by a dialysis membrane. When sugar is added to water on one side of the membrane, osmosis occurs through the membrane. The water level rises where the sugar was added and falls on the other.

Figure 2. This diagram demonstrates osmosis using a U-shaped tube with two sections separated by a dialysis membrane. Source: KDS4444, CC0, via Wikimedia Commons.

What is facilitated diffusion?

Facilitated diffusion is a type of diffusion in which molecules or ions that cannot cross the phospholipid bilayer of the plasma membrane are diffused across the membrane with the help of transport proteins.

Facilitated diffusion is a form of passive transport: while it speeds up the movement of molecules through the plasma membrane, it does not change the direction of the movement. Molecules still move in the direction of the concentration gradient, and thus the process doesn't require chemical energy directly.

There are two types of transport proteins: channel proteins and carrier proteins. We will discuss each of these briefly in the following section.

Channel proteins

Channel proteins create hydrophilic channels for ions and other small molecules to pass through via diffusion. Channel proteins tend to selectively allow the passage of specific substances.

Figure 3. This diagram shows how channel proteins facilitate diffusion by creating hydrophilic passageways. Source: OpenStax, CC BY 3.0, via Wikimedia Commons.

On the other hand, some ion channel proteins (channel proteins that transport ions) act like gated channels that open or close in response to certain stimuli.

Carrier proteins

Carrier proteins bind to solute molecules and change shape in a way that enables them to carry them across the membrane. Carrier proteins tend to be more selective than channel proteins.

Figure 4. This diagram shows how carrier proteins facilitate diffusion by changing their shape according to the specific solute being transported. Source: OpenStax, CC BY 3.0, via Wikimedia Commons.