In living beings, individual cells and organelles are separated from their environment by biomembranes. Material transport via the biomembrane is essential for the exchange of substances within the cell and with its environment.
If the transport across the biomembrane takes place without the provision of energy, this is referred to as passive mass transport. If membrane proteins such as protein channels or carrier proteins are used to help with this, diffusion is facilitated.
Diffusion definitions
Before you die relieved diffusion is explained in more detail, you should diffusion first be a concept. Since this article deals with the transport of substances across a biomembrane, diffusion will be explained to you using this example.
Due to physical laws, particles (usually molecules of gases and liquids) always strive for a concentration balance, so that the particles are homogeneously mixed.
This results in a particle movement from the place of high concentration to the place of lower concentration (along the concentration gradient). This is the movement of the particles. An additional provision of energy is not necessary. Therefore, it is passive transport.
There is a space which is separated into two sections by a biomembrane. In both sections, particles of the same type are present in different concentrations.
If the particles are able to cross the biomembrane, they strive for concentration equalization and migrate from the high-concentration section to the low-concentration section. This passive diffusion takes place until both sections have the same concentration of particles (concentration equalization).
Take a close look at Figure 1 again to better understand the process of diffusion.
Figure 1: Diffusion across a biomembrane
Depending on whether the particles are able to diffuse through the biomembrane without the help of channel proteins, a distinction is made between easier and easier diffusion. In both cases it is a passive transport.
are biomembranes semipermeable. That means they are semi-permeable. Certain molecules can diffuse through the membrane while other molecules are retained by the membrane.
You can find out more about the structure and properties of the biomembrane in a separate article. Come around!
Definition of simple diffusion
Simple diffusion is the simplest form of diffusion. If molecules are able to cross a biomembrane unaided, they can diffuse through the membrane without transmembrane proteins. In this case one speaks of simple diffusion.
the easy diffusion describes the passive transport (diffusion) of small particles, which due to their size and charge, the biomembrane without the help of Transmembrane proteins can happen (this includes, among other things, the particle movements of water molecules and gases such as oxygen and carbon dioxide). The particles migrate along the concentration gradient.
transmembrane proteins (also integral proteins) are proteins that are embedded in a biomembrane across the entire cross-section. These include channel proteins and carrier proteins (transport proteins), but also a whole range of proteins that are not involved in mass transport.
Definition of facilitated diffusion
Of course, larger molecules such as ions must also be able to cross a biomembrane. For this purpose, specific protein channels or so-called carrier proteins are embedded in the biomembrane. With the help of these channel proteins, larger molecules can now also diffuse through the cell membrane. One speaks of facilitated diffusion.
the facilitated diffusion describes passive transport (diffusion) across a biomembrane using transmembrane proteins. Transport proteins are always required for mass transport when particles cannot easily pass through the biomembrane due to their size and charge. The particles migrate along the concentration gradient.
Overview of mass transport
The figure below shows the different types of substance transport across the biomembrane. Here is an overview again:
- Easy diffusion (passive transport): transport of small molecules, without additional energy, along the concentration gradient.
- Facilitated Diffusion (passive transport): transport of larger molecules and ions, without additional energy, along the concentration gradient with the help of transmembrane proteins (channels and carriers).
- Active transport: Transport of molecules and ions, additional energy is required (e.g. ATP), against the concentration gradient
Figure 2: Active and passive mass transport
Facilitated diffusion through channels
Certain proteins are necessary for facilitated diffusion in order to transport large molecules and charged ions through the membrane. These integral proteins are stored in the membrane and are called carrier and channel proteins.
channel proteins are transmembrane transport proteins. They form channels in the cell membrane and enable the passive transport (facilitated diffusion) of specific molecules such as ions.
Properties of channel proteins
Channel proteins form hydrophilic (water-loving) channels through the hydrophobic (water-repellent) core of the membrane. This enables polar and charged particles to pass through the biomembrane, which would otherwise be prevented from passing by the hydrophobic core of the membrane.
Channel proteins form passageways that are usually permeable only to a specific type of particle. This ensures a regulated and specific transport of substances across the cell membrane. Based on selectivity, channel proteins in selective channel proteins and non-selective channel proteins divided.
While some protein channels are always open, some channel proteins can switch between an open and closed state through conformational changes. As a rule, binding by a messenger substance (e.g. neurotransmitter) leads to a change in the conformation of a channel protein. Based on the mechanism of conformational change, channel proteins are classified as follows:
- Voltage-gated channel proteins (Conformation change with change in tension of the biomembrane)
- Chemically controlled channel proteins (Conformational change by binding a messenger substance)
- Mechanically controlled channel proteins (conformational change by mechanical stimuli)
- Temperature-gated channel proteins (conformational change due to temperature changes)
- Light-gated channel proteins (Conformational change when irradiated with certain wavelengths)
Examples of channel proteins
Using two typical examples of channel proteins, the functionality and importance of channel proteins and facilitated diffusion should be explained to you.
Sodium Ion Channels (Na+)
Sodium ion channels are protein channels which, by forming a pore, enormously increase the permeability of a cell membrane for Na+ ions. Other ions are unable to pass through sodium ion channels.
Depending on the type, sodium ion channels can be voltage-gated or opened by binding neurotransmitters. They play a crucial role in neuronal conduction via action potentials. During excitation, the opening of sodium channels triggers an influx of sodium ions into the cell.
The strong influx of sodium ions leads to a change in the voltage of the cell and thus generates an action potential, which enables excitation conduction. The influx of ions is based on facilitated diffusion.
For a deeper insight into conduction conduction, check out the Conduction Conduction Learning Set.
aquaporins
Aquaporins (also known as water channels) are channel proteins for water molecules. Aquaporins are needed when it is important that as much water as possible has to cross the membrane quickly.
Aquaporins are important when the water balance of cells needs to be regulated. In plant cells, they are involved in regulating the internal pressure of the cells (tugor). In mammals, for example, they play a role in the regulation of water in the red blood cells, kidney and eye.
Carrier-mediated facilitated diffusion
carrier proteinsalso called carrier proteins, change shape (conformational change) to transport a large molecule or charged ion from one side of the membrane to the other side.
carrier proteins are transmembrane transport proteins. They form locks in the cell membrane and enable the passive transport (facilitated diffusion) of specific molecules such as sugar molecules.
The difference to protein channels is that the diffusing molecules bind to the carrier protein and are then released through a conformational change on the other side of the membrane.
Properties of carrier proteins
Carrier proteins are used for passive transport along the concentration gradient (facilitated diffusion).
Large molecules and ions are transported through the biomembrane by a change in conformation. The conformational change is usually in response to the binding of their target molecule, which change moves the molecule to the opposite side.
Due to a specific binding site, carrier proteins as well as channel proteins are substrate-specific. This means that only a certain type of molecule, more precisely a group of structurally related molecules, can pass through the cell membrane through the carrier proteins.
Due to the necessary conformational change, carrier-mediated diffusion is slower than channel-mediated diffusion. Carrier proteins operate at a rate of approximately 1000 molecules per second, while channel proteins can be expected to transport tens of millions of molecules per second.
Carrier proteins can also serve to transport molecules against the concentration gradient by providing energy (active transport). As a rule, transmembrane proteins, which are involved in active transport, are called ATPases or pump designated.
Carrier proteins and pumps can transport single molecules, but also coupled multiple molecules. Depending on the type of transport, a distinction is made between the following:
- uniport: A molecule is transported in across the biomembrane.
- symport: Two molecules are transported coupled in the same direction across the cell membrane.
- antiport: Two molecules are transported in opposite directions.
Symport and antiport are usually about secondary active transport. This means that one of the two molecules is transported with the concentration gradient and the other molecule against the concentration gradient. The energy required for active transport is obtained from transport along the concentration gradient.
Example of carrier proteins
An important family of carrier proteins are the glucose transporters….