Cell Diffusion

What is diffusion in a cell?

Diffusion mechanism

In principle, all molecules will tend to reach their concentration equilibrium across the cell membrane, i.e. they will try to reach the same concentration on both sides of the cell membrane. Obviously, molecules don't have a mind of their own, so how can it be that they end up moving to eliminate their gradient?

Fig. 1. Simple diffusion diagram. Even though solute molecules will be moving from both sides, the net movement is from the high-concentration side to the low-concentration side, so the arrow is pointing in that direction.

This is the general principle of diffusion, but how does this apply to the cell?

Due to its lipid bilayer, the cell membrane is a semipermeable membrane. This means that it only allows molecules with certain characteristics to cross through it without the help of auxiliary proteins.

Fig. 2. Phospholipid structure. The lipid bilayer (i.e. the plasma membrane) consists of two layers of phospholipids facing opposite ways: the two hydrophobic tails are facing each other. This means that in the middle of the lipid bilayer there is a big section that does not allow charged molecules to move through.

In particular, the cell membrane only allows small, uncharged molecules to freely cross through the phospholipid bilayer without any assistance. All other molecules (big molecules, charged molecules) will require the intervention of proteins to cross through. Because of this, a cell can easily regulate the transport of molecules across a cell membrane by regulating the type and amount of auxiliary proteins it has on its plasma membrane. It cannot as easily regulate the molecules that cross the membrane where no proteins are involved.

Types of cell diffusion

Depending on if a molecule can freely diffuse across the cell membrane or if it needs protein assistance, we classify cell diffusion into two types:

• Simple diffusion
• Facilitated diffusion

Simple diffusion is the type of diffusion where no protein assistance is needed for molecules to cross the cell membrane. For example, oxygen molecules can cross the membrane without proteins.

Facilitated diffusion is the type of diffusion where proteins are needed for the molecule to flow down its gradient to the lower concentration side of the membrane. For example, all ions will need protein assistance to cross the membrane, because they are charged molecules and they will be repelled by the hydrophobic mid-section of the lipid bilayer.

There are two types of proteins that aid diffusion (i.e. that participate in facilitated diffusion): channel proteins and carrier proteins.

Channel proteins for facilitated diffusion

These proteins are transmembrane proteins, meaning they span the width of the phospholipid bilayer. As their name suggests, these proteins provide a hydrophilic 'channel' through which polar and charged molecules can pass through, such as ions.

Many of these channel proteins are gated channel proteins that can open or close. This is dependent on certain stimuli. This allows the channel proteins to regulate the passage of molecules. The main types of stimuli are listed:

• Voltage (voltage-gated channels)

• Mechanical pressure (mechanically-gated channels)

• Ligand binding (ligand-gated channels)

Carrier proteins for facilitated diffusion

Carrier proteins are also transmembrane proteins, but these do not open a channel for the molecules to pass through, but rather undergo a reversible conformational change in their protein shape to transport the molecules across the cell membrane.

The process by which the conformational change for carrier proteins happens is listed below:

1. The molecule binds to the binding site on the carrier protein.

2. The carrier protein undergoes a conformational change.

3. The molecule is shuttled from one side of the cell membrane to the other.

4. The carrier protein returns to its original conformation.

It is important to note that carrier proteins are involved in both passive transport and active transport. In passive transport, ATP is not needed as the carrier protein relies on the concentration gradient. In active transport, ATP is used as the carrier protein shuttles molecules against their concentration gradient.

What is the difference between osmosis and diffusion?

Osmosis and diffusion are two types of passive transport, but their similarities end there. The three most important differences between diffusion and osmosis are:

• Diffusion can happen with the molecules of the solute or of the solvent of a solution (solid, liquid or gas). Osmosis, however, only happens to the liquid solvent.
• For osmosis to take place, there needs to be a semipermeable membrane separating two solutions. In the case of diffusion, molecules naturally diffuse in any solution, regardless of the presence of a membrane or not. In the case of cellular diffusion, there is a membrane, but molecules also diffuse when mixing two drinks, for example.
• In diffusion, molecules move down their gradient (from the region of high concentration to the region of low concentration). In osmosis, the solvent moves from a region of high potential to one of lower potential. High water potential just means that there are more water molecules in a solution compared to another, connected one. Usually, this means that water moves from a region of low solute concentration to one of high concentration, i.e. in the opposite direction to what the solute would travel via diffusion.

Let's summarise the differences between diffusion and osmosis in a table:

Table 1. Differences between diffusion and osmosis

What factors affect the rate of diffusion?

Certain factors will affect the rate at which substances will diffuse. Below are the main factors you need to know:

• Concentration gradient

• Distance

• Temperature

• Surface area

• Molecular properties

Concentration gradient and rate of diffusion

This is defined as the difference in the concentration of a molecule in two separate regions. The greater the difference in concentration, the faster the rate of diffusion. This is because if one region contains more molecules at any given time, these molecules will move to the other region more rapidly.

Distance and rate of diffusion

The smaller the diffusion distance, the faster the rate of diffusion. This is because your molecules do not have to travel as far to get to the other region.

Temperature and rate of diffusion

Recall that diffusion relies on the random movement of particles due to kinetic energy. At higher temperatures, molecules will have more kinetic energy. Therefore, the higher the temperature, the faster the rate of diffusion.

Surface area and rate of diffusion

The larger the surface area, the faster the rate of infusion. This is because at any given time, more molecules can diffuse across the surface.

Molecular properties and rate of diffusion

Cell membranes are permeable to small, uncharged nonpolar molecules. This includes oxygen and urea. However, the cell membrane is impermeable to larger, charged polar molecules. This includes glucose and amino acids.

Membrane proteins and rate of diffusion

Facilitated diffusion relies on the presence of membrane proteins. Some cell membranes will have an increased number of these membrane proteins to increase the rate of facilitated diffusion.

Examples of diffusion in biology

There are numerous examples of diffusion in biology. From cellular gas exchange to bigger processes like the absorption of nutrients in the digestive system, all of these need of the basic process of cell diffusion. Some types of cells have even developed special features to increase their surface for diffusion and osmotic exchange.

Oxygen and carbon dioxide diffusion

Oxygen and carbon dioxide are transported via simple diffusion during gaseous exchange. In the alveoli of the lungs there is a higher concentration of oxygen molecules than in the capillaries that irrigate that same organ. Therefore, oxygen will tend to flow from the alveoli into the blood.

Meanwhile, there is a higher concentration of carbon dioxide molecules in the capillaries than in the alveoli. Due to this concentration gradient, carbon dioxide will diffuse into the alveoli and exit the body through normal breathing.

Urea diffusion

The waste product urea (from the breakdown of amino acids) is made in the liver, and there is, therefore, a higher concentration of urea in liver cells than in the blood.

Urea is a highly polar molecule and therefore, it can't diffuse through the cell membrane on its own. Urea diffuses into the blood via facilitated diffusion. This allows cells to regulate urea transport so that not all cells absorb urea.

Nerve impulses and diffusion

Neurons carry nerve impulses along their axon. Nerve impulses are just differences in the cell membrane's potential, or the concentration of positive ions on each side of the membrane. This is done through facilitated diffusion using channel proteins specific for sodium ions (Na⁺). They are termed voltage-gated sodium ion channels as they open in response to electrical signals.

The cell membrane of neurons has a specific resting membrane potential (-70 mV) and a stimulus, such as mechanical pressure, can trigger this membrane potential to become less negative. This change in membrane potential causes the voltage-gated sodium ion channels to open. Sodium ions then enter the cell through the channel protein because their concentration inside the cell is lower than the concentration outside the cell. This process is called depolarisation.

Glucose transport by facilitated diffusion

Glucose is a large and highly polar molecule and therefore cannot diffuse across the phospholipid bilayer by itself. The transport of glucose into a cell relies on facilitated diffusion by carrier proteins called glucose transporter proteins (GLUTs). Note that glucose transport via GLUTs is always passive, although there are other methods of transporting glucose across the membrane that are not passive.

Let's take a look at glucose entering red blood cells. There are many GLUTs distributed in the red blood cell membrane as these cells rely entirely on glycolysis to make ATP. There is a higher concentration of glucose in the blood than in the red blood cell. The GLUTs use this concentration gradient to transport the glucose into the red blood cell without the need for ATP.

Adaptations for rapid glucose transport in the ileum

As mentioned before, some cells that specialise in absorbing or excreting molecules, such as the cells of the alveoli or those of the ileum, have developed adaptations to improve the transport of substances across their membranes.

Facilitated diffusion occurs in the epithelial cells of the ileum to absorb molecules like glucose. Because of the importance of this process, epithelial cells have adapted to increase the rate of diffusion.

Fig. 6. Glucose transport in the ileum. As you can see, there are also passive glucose transporters in the ileum, but there is another system too: the sodium/glucose cotransporter. Although this carrier protein doesn't directly use ATP to transport glucose into the cell, it uses the energy derived from transporting sodium down its gradient (into the cell). This sodium gradient is maintained by the Na/K ATPase pump, which does use ATP to export sodium and import potassium into the cell.

Epithelial cells of the illeum contain microvilli which make up the brush border of the ileum. Microvilli are finger-like projections that increase the surface area for transport. There is also an increased density of carrier proteins embedded in the epithelial cells. This means more molecules can be transported at any given time.

A steep concentration gradient between the ileum and the blood is maintained by continuous blood flow. Glucose moves into the blood by facilitated diffusion down its concentration gradient and due to continuous blood flow, the glucose is being constantly removed. This increases the rate of facilitated diffusion.

Additionally, the ileum is lined with a single layer of epithelial cells. This provides a short diffusion distance for transported molecules.

Overall, the ileum has evolved to increase the diffusion of molecules like glucose from the lumen of the intestines to the blood.