What is facilitated diffusion?
Facilitated diffusion is a type of passive transport like osmosis, but in this type of transport, molecules move from a high concentration region to a low concentration region with the help of a membrane protein and it is also a type of diffusion where there is a movement along a concentration gradient.
It is a selective process, which means that only certain molecules and ions can pass through the membrane and other molecules are prevented from passing through the membrane. Those molecules and ions that are allowed to pass through or diffuse across the membrane are aided by the electric charge and pH of the concentration.
Definition of facilitated diffusion
Facilitated diffusion is defined as a type of passive diffusion that involves the passive movement of molecules along a concentration gradient that is guided by the presence of another molecule – typically an integral membrane protein that forms a pore or channel for the molecules to move from one side of the membrane to the other side of the membrane.
Because of the presence of a concentration gradient, molecules can diffuse into (or out of) the cell by moving down the concentration gradient. However, the molecules are charged or polar, and they are unable to cross the phospholipid portion of the membrane on their own hence the need for membrane proteins to help them cross.
The work of the membrane proteins that facilitate the transport of these molecules is to protect the molecules from the hydrophobic core of the membrane thereby, allowing their movements to areas of lesser concentration. In order to help accomplish the diffusion process in cells, facilitated diffusion requires little energy but not in the form of adenosine triphosphate (ATP) or guanosine triphosphate (GTP) because the materials are transported along their concentration gradient.
- Channel protein
- Carrier protein
Channel protein functions by stretching the membrane and forming hydrophilic tunnels across the membrane thereby, permitting targeted molecules to pass through via diffusion. Channels are highly selective and will only transport one type of molecule (or a few closely related molecules). The movement of polar and charged compounds via a channel protein allows them to bypass the hydrophobic core of the plasma membrane, which would otherwise slow or block their entrance into the cell. For example, aquaporins are a type of channel proteins and work by permitting water to quickly cross the membrane and are found in plant cells, red blood cells, and certain parts of the kidney where they function by minimizing the quantity of water lost as urine.
Some channel proteins are always open, but others are closed or gated, which means they can open or close in response to a specific signal (like an electrical signal or the binding of a molecule). In the membranes of cells that transmit electrical signals, such as nerve and muscle cells, there are gated ion channels for sodium, potassium, and calcium ions. The expansion and contraction of these channels, as well as the resulting changes in ion levels within the cell, are important in electrical transmission across membranes (in nerve cells) and muscle contraction (in muscle cells).
Carrier protein is another type of transmembrane protein involved in facilitated diffusion transport. They have the capability to change shape in order to transport a target molecule from one side of the membrane to the other side of the membrane.
Carrier proteins, like channel proteins, are generally selective to one or a few substances. The carrier proteins that participate in facilitated diffusion simply permit hydrophilic molecules to move down a concentration gradient that already exists rather than acting as pumps.
Materials are transported at different rates by channel and carrier proteins. Channel proteins, in a broad sense, transport molecules much faster than carrier proteins. This is because channel proteins are simple tunnels unlike carrier proteins, that need to change shape and reset each time a molecule is moved. Channel protein may facilitate diffusion at tens of millions of molecules per second, whereas a carrier protein may work at a thousand or so molecules per second.
Functions of transmembrane protein
- Serves to connect and join two cells.
- Helps to pinpoint metabolic pathways.
- They are in charge of facilitated diffusion and active transport
- They may serve as markers for cellular identification.
- Serve as cytoskeleton and extracellular matrix attachment points
- They act as peptide hormone receptors.
Facilitated diffusion examples
- Glucose transport
- Gas Transport
- Ion transport
The above listed are some examples of facilitated diffusion which will be explained subsequently;
Glucose transport is an example of facilitated diffusion. Due to the nature of glucose being a large polar molecule, it cannot pass through the membrane’s lipid bilayer. As a result, it requires carriers known as glucose transporters to pass through the membrane’s lipid bilayer. The epithelial cells of the small intestine, for example, actively transport glucose molecules after digestion of dietary carbohydrates and these molecules will then enter the bloodstream through facilitated diffusion. In this example, it is observed that facilitated diffusion is used to transport glucose that is taken from the bloodstream into the cells.
In this facilitated diffusion example, oxygen diffuses because of a higher concentration of saturated oxygen pressure on one side of the membrane. The transport of oxygen in the blood is facilitated via the carrier protein in the red blood cells known as hemoglobin, whereas that of the diffusion of oxygen in the red skeletal muscle cells is through the help of the carrier protein called myoglobin. Both of these membrane proteins (hemoglobin and myoglobin) have an affinity to oxygen.
It is also observed that carbon monoxide and carbon dioxide have a similar mechanism to that of the oxygen transport process in their own transport across the body.
Because of the charge that ions carry, despite being small molecules, they cannot diffuse through the lipid bilayer of biological membranes without help. This results in the need for facilitated diffusion to transport them along their concentration gradient through membrane proteins that can provide a passageway for ions such as potassium ions, sodium ions, and calcium ions. These membrane proteins are known as ion channels. These channels can move ions down their concentration gradient at a very fast rate, often at a rate of 106 ions per second or more, without the use of chemical energy. These transmembrane proteins permit ions and solutes to be transported selectively across the plasma membrane.
These proteins aid in the movement of water across the lipid bilayer. They accomplish this task by allowing water to pass through the pores of the channel into and out of the cell while preventing the movements of ions and solutes at the same time.
Simple diffusion Vs Facilitated diffusion
|Simple diffusion||Facilitated diffusion|
|A type of passive transport that is used in transporting molecules and gases across the body.||A type of passive transport used to transport molecules and ions.|
|Small nonpolar molecules (such as oxygen and carbon dioxide) easily diffuse across the plasma membrane.||Polar molecules (such as glucose and amino acids), larger ions (such as sodium ions and chloride ions), and large nonpolar molecules (such as retinol) use facilitated diffusion across the plasma membrane.|
|Substances move from a high-concentration area or region to a low-concentration area or region.||Substances move from a high-concentration area or region to a low-concentration area or region.|
|Does not necessitate the use of chemical energy, such as ATP or GTP.||Does not necessitate the use of chemical energy, such as ATP or GTP.|
|There is no need for transport proteins.||Transport proteins are required.|
|The rate is generally slower but more straightforward because it does not rely on the ability of membrane proteins to bind with substances for transport.||The rate is generally faster, but it is influenced by factors such as temperature and the types of membrane proteins involved, and thus may be influenced by membrane protein inhibitors.|
Facilitated diffusion Vs Active transport
|Facilitated diffusion||Active transport|
|Facilitated diffusion occurs along a concentration gradient.||It occurs against the direction of the concentration gradient.|
|It is a passive method that does not require any energy.||Active transport requires energy to transport materials.|
|In transport, it employs both gated channel proteins and carrier proteins.||It uses carrier proteins hence the need for energy in the form of ATP or electrochemical energy.|
|It is used primarily for large, polar molecules that, due to their hydrophilicity, cannot cross the phospholipid bilayer (polar molecules).||It is used to transport molecules against their concentration gradient.|
It is important to note that active transport is generally concerned with the collection of high molecular concentrations required by the cell, such as ions, glucose, and amino acids. For example, active transport is demonstrated by the absorption of glucose in human intestines and the absorption of mineral ions into plant root hair cells.
Importance of facilitated diffusion
- This type of transport is required in living organisms to regulate what enters and exits the cell especially when it comes to large or polar molecules.
- Facilitated diffusion in biological processes is essential for keeping homeostatic optimal levels of molecules and ions inside the cell. Cellular transport, including facilitated diffusion, is driven by the unequal distribution of substances between the intracellular fluid and the extracellular fluid. The movement between these two regions is an attempt to achieve balance.
- Large molecules such as glucose and amino acids, for example, play critical roles in cell functions. Glucose is an essential nutrient, and amino acids are involved in a variety of cell processes, including cell division. Facilitated diffusion allows molecules to pass through cell membranes and membranes of organelles such as the nucleus, allowing these processes to take place.
- Facilitated diffusion is used to transport the following; oxygen, carbon dioxide, ions (K+, Na+, Ca2+), water, amino acids, glucose, and, nucleic acids.
Factors Influencing Diffusion
- Concentration gradient
- Carrier protein capacity
- The number of carrier protein sites
The main factor behind fluid diffusion is plainly the possibility of Brownian motion. Every molecule exhibits some degree of erratic, random movement, which is largely determined by temperature. The energy of these molecules increases as the temperature rises.
When a substance gradually spread to other regions when there is a high concentration of the same substance in a specific region. The movement of the substance results in the random movement of the molecules across all the regions and some are bound to move outwards, into a region with a low concentration. For example, When an individual enters a room with a strong scent, the odorous molecules diffuse outwards from the skin or clothes to the sensory receptors that are found in the nose. When these receptors are activated in the nose, they allow people to perceive the randomly moving molecules. This example indicated the need for a concentration gradient for molecules to diffuse from a region of high concentration to a region of low concentration.
Carrier protein capacity
The percentage of binding between the substance to be transferred and the protein, as well as the transfer speed, influence the rate of diffusion.
The number of carrier protein sites
The greater the number of sites, the greater the diffusion capacity and the faster the diffusion.
Facilitated diffusion across membranes
Diffusion occurs all over the biosphere. It can be seen in the movement of air and water, and it is an essential force that drives global weather patterns. The presence of lipid-based membranes within living systems creates compartments that allow for the selective concentration of water-soluble substances. For example, mitochondrial membranes can separate the organelle into two distinct regions: the inner matrix and the inter-membranous space. Each of these sub-compartments has a distinct composition and function from the adjacent spaces. Order is generated in this way by nearly every unit of the living world, from organelles within a cell to entire organ systems and organisms. This, however, implies that ions, small molecules, proteins, and other solutes have different concentrations across lipid bilayers.
Furthermore, polar, charged, or hydrophilic molecules cannot pass through biological membranes. While this is useful for maintaining the integrity of each compartment, molecules must also be able to move across membranes along their concentration gradient when necessary.
Diffusion of gases
The movement of oxygen and carbon dioxide in actively respiring tissues and cells is an excellent example of this. These cells require oxygen and glucose for energy, while carbon dioxide must be removed and expelled from the body. There is no direct involvement of ATP or other energy required in the diffusion of the molecules because each of these molecules is moving from a high concentration region to a low concentration region. They must, however, cross multiple lipid bilayers – from mitochondrial membranes to the cell’s plasma membrane, and then the lipid bilayers of endothelial cells lining blood capillaries, red blood cell membranes, and finally the membranes of cells forming the alveolar sacs in the lungs.
Does facilitated diffusion require energy?
Facilitated diffusion does not require energy, the reason being that the movement of the molecules is from a region of higher concentration to a region of lower concentration.
Is facilitated diffusion active or passive?
Facilitated diffusion is a type of passive transport like osmosis because it does not require energy. The reason such type of diffusion is not active is that the molecules transported are not moving against the concentration gradient.