Scheme 5.1. Synthetic routes to TPE-based compounds
1.2. Transportation across the biological membrane 1. Different modes of transportations
Figure 1.2. Different modes of transportations.
The cells require the raw materials from their surroundings for the desired biosynthesis and energy productions within the living organism. Simultaneously waste or by-products generated from these biosynthesis or energy productions should be released
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to its environment to maintain a proper balance within the cells. The requirement of these raw materials is supplied by numerous elegant protein machinery which resites within the cell membrane. Although few non-polar ions could easily pass through the membrane for polar and charged species, a membrane protein is essential.8 These proteins are selective for particular ions and mainly transport these ions across the phospholipid cell membrane.
In most cases, the membrane proteins facilitate the diffusion of the ions down its concentrations gradient. Still, transport often needs to be facilitated against the concentration gradient, electric charge, or both, in which cases the ions or solutes must be pumped in a sequential process that requires the energy which is coming from the hydrolysis of ATP or supplied in the form of the movement of another solute down its electrochemical gradient with sufficient energy to carry out another solutes or ions up its gradient.9 Ions could also move across the membrane via channels formed by specific proteins. The ionophore could also carry out small molecules that could mask the charge of the ions or solutes and facilitate them to diffuse through the membrane.10
1.2.2. Active and Passive Transport
The transportation of the ions or solutes could be assisted by two processes- the active process where the ions move from lower concentration to higher concentrations gradient with the help of energy in the form of ATP.9 On the other hand, passive transport deals with the movement of the ions from higher concentration gradient to lower concentrations gradient without any energy requirement.9 This passive transport is facilitated by numerous membrane proteins. To transport through the phospholipid membrane, the charged or polar solutes have to give up the interaction energy in their hydrated cells and then could diffuse the membrane where they are poorly soluble within it. During this transportation process, initially, the energy was peeled from its hydrated cells to move the water-soluble ions or solutes into the hydrophobic membrane. The numerous noncovalent interaction between the transporting molecules or proteins compensates this energy loss which is again regained as ions or solutes leave the membrane on the other side and is rehydrated again. Transmembrane passage of the ions or molecules is really a high energy state compared to the enzyme-catalyzed chemical reactions. For both cases, the activation barrier must be overcome to facilitate this process.
The activation energy to translocate the polar solute across the membrane is so large that
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the hydrophobic membrane is virtually impermeable to polar and charge species over time relevant to cell growth and division. The membrane proteins machinery lowers the activation energy by providing an alternative pathway to the ions through the bilayer. The charge and polar substrate are bound by numerous transporter with stereo-chemical specificity through multiple non-covalent interactions. The negative free energy change for these noncovalent interactions actually compensates for the loss of water from the hydrated cells, thereby lowering the free energy change for transmembrane passage. This magnitude of lowering the free energy change for channel-forming proteins is much more than the carrier kind of transporters. The channel offers alternative pathways for a specific substrate to transport across the bilayer without dissolve in the bilayer, which actually further lowers the free energy change for transmembrane diffusion.11
Figure 1.3. Energy changes during the passage of hydrophilic solute across the hydrophobic membrane.
1.2.3. Transport pathway and mechanism
The transmembrane transport assisted by the numerous transporter molecules generally follow the following pathways
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Uniport: The ions or solutes are transported in a single direction. Glucose transporter (GLUT) protein is a well-known example of a uniport mechanism that is found in erythrocytes.
Symport: This is a co-transport pathway where the two different species of chemical entity are transported in the same direction. Na/glucose transporter proteins are examples of this kind of symport pathway.
Antiport: This one is also a co-transport process where the two different chemical species are being transported in opposite directions. Na+/H+ antiporter proteins channels are a well-known example of this category.
All these processes could be active and passive depending upon the electrochemical gradient, which is already mentioned in detail. Most of the transporter molecules perform all these transportation either a channel mechanism or a carrier mechanism. Channel basically provides an alternative pathway by providing a hydrophilic pathway through which the transporter could avoid the hydrophobic lipid interaction. Whereas the mobile carrier is typically ionophore or small organic molecules that translocate within the membrane where it binds with the ions at the polar leaflet of the membrane and carry them across the hydrophobic membrane interior by simple diffusion and then release them on the opposite leaflet of the membrane in response to the electrochemical gradient.12
Figure 1.4. Different types of transportations pathway
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