Simple diffusion is the movement of molecules from a region of high concentration to a region of low concentration. No energy is needed as molecules are moving down a concentration gradient in a passive way.
Think about someone spraying a perfume bottle in the corner of a room. The perfume molecules are concentrated where the bottle has been sprayed but over time, the molecules will travel from the corner to the rest of the room where there are no perfume molecules. The same concept applies to molecules travelling across a cell membrane.
Molecule diffusion
The cell membrane is a partially permeable membrane which favours the passage of specific molecules for simple diffusion. Small, uncharged polar molecules can freely diffuse through the phospholipid bilayer without any assistance.
Oxygen and carbon dioxide
Oxygen and carbon dioxide are transported via simple diffusion during gaseous exchange. In the alveoli, there is a higher concentration of oxygen molecules than in the capillaries. Meanwhile, there is a higher concentration of carbon dioxide molecules in the capillaries than in the alveoli. Due to this concentration gradient, oxygen will diffuse into the capillaries and carbon dioxide will diffuse into the alveoli.
Fig. 1 - An illustration of gaseous exchange in the alveoli
Urea
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. By simple diffusion, urea will diffuse into the blood down its concentration gradient. This is because urea is highly polar, meaning that the cell membrane is permeable to these molecules.
Urea is made from the deamination (removal of an amine group) of amino acids. This urea needs to be excreted by the kidneys, hence why it diffuses into the bloodstream.
What is facilitated diffusion?
This type of diffusion follows all the rules of simple diffusion but here, membrane proteins are needed to transport the molecule across the phospholipid bilayer. Recall the cell membrane structure - the hydrophobic nonpolar core of tails makes the cell membrane impermeable to charged molecules, like ions. Therefore, these membrane proteins allow the transport of these charged molecules. Facilitated diffusion is still a passive process as the molecules are traveling down a concentration gradient without any energy expenditure.
Channel proteins
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)
Fig. 2 - An illustration of channel proteins embedded in a membrane
Carrier proteins
Carrier proteins are also transmembrane proteins, but these undergo a reversible conformational change in their protein shape to transport the molecules across the cell membrane. The process by which this happens is listed below:
Molecule binds to the binding site on the carrier protein.
The carrier protein undergoes a conformational change.
The molecule is shuttled from one side of the cell membrane to the other.
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.
Fig. 3 - An illustration of a carrier protein embedded in a membrane
Nerve impulses
Neurons carry nerve impulses along their axon. This is done through facilitated diffusion using channel proteins specific for sodium ions. They are termed voltage-gated sodium ion channels as they open in response to electrical signals.
The cell membrane of neurons have a resting membrane potential (-70mV) 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 enter the cell through the channel protein and this is called depolarization.
Glucose transport
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.
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.
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
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
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
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
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
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
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.
Adaptations for gas exchange in the lungs
So we have discussed the factors that affect how quickly molecules can diffuse across a membrane. A great example of how our body has adapted for efficient diffusion is the gaseous exchange that occurs between the capillaries and alveoli.
A steep concentration is constantly maintained due to ventilation and blood flow. Ventilation allows the continuous supply of oxygen to the alveoli, while the oxygen in the blood flows away. Meanwhile, ventilation takes carbon dioxide away and the blood flow supplies the carbon dioxide. This allows the gases to diffuse from a high concentration to a low concentration.
The diffusion distance for the gases is kept extremely small as the capillary endothelium and alveoli walls are one cell thick. Additionally, the capillaries are wrapped closely around the alveoli, meaning the gases do not have to travel far. These properties allow for a short diffusion distance and, therefore, a faster rate of diffusion.
A larger surface area is provided for by the many alveoli present in each lung. This enables more oxygen and carbon dioxide to diffuse across the surface at any given time, allowing for a faster rate of diffusion.
Adaptations for rapid transport in the ileum
Facilitated diffusion occurs in the epithelial cells of the ileum to absorb molecules like glucose. The ileum also has adaptations that help increase the rate of transport.
The epithelial cells 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.
The ileum is lined with a single layer of epithelial cells. This provides a short diffusion distance for transported molecules.
Cell Diffusion - Key takeaways
- Simple diffusion is the movement of molecules down their concentration gradient whereas facilitated diffusion is the movement of molecules down their concentration gradient using membrane proteins.
- Facilitated diffusion uses channel proteins and carrier proteins, which are both membrane proteins.
- The rate of diffusion is mainly determined by the concentration gradient, diffusion distance, temperature, surface area and molecular properties.
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