What kind of molecules can and cannot pass
easily across a lipid bilayer?
Structure of a lipid bilayer
A lipid bilayer is a membrane mainly composed of lipid molecules, usually a phospholipids (See Figure 1). Phospholipids are formed from 3 components :- (1) 2 fatty acids tails – these are hydrophobic; (2) a negatively-charged hydrophilic phosphate group; and (3) a glycerol backbone.
The bilayer structure is favourable energetically because the hydrophobic fatty acid tails cluster together to exclude water, where the hydrophilic head groups (consisting of the phosphate group and glycerol backbone) are on the two surfaces of the membrane, which allows them to have contact with the surrounding water. This structure also allows the membrane to be polarised, which plays an important role in transport across the membrane. When a molecule has both a hydrophilic and hydrophobic nature, it is said to be amphiphatic. (1)
In addition to phospholipids, the bilayer membrane consists of a number of different other molecules:-
Cholesterol – aids in fluidity, as it disrupts the orderly packing of the phospholipids. •
Channel proteins, which allow specific molecules to pass through them. •
Carbohydrates, which are found either attached to a protein (forming a glycoprotein) or a lipid (forming a glycolipid). Glycolipids on the cell surface act as markers for cellular recognition, whilst glycoproteins are important in immune cell recognition. (1)
What kind of molecules can and cannot pass easily across a lipid bilayer?
Figure 1 clearly shows the different molecules that can and cannot transport easily across the plasma membrane. All lipid soluble membranes (hydrophobic molecules and small uncharged molecules) can pass easily through the membrane in both directions. The movement of these molecules is dependent only on their concentrations inside and outside the cell, and these molecules are transported due to the effects of concentration gradients via simple diffusion. (2)
On the other hand, large uncharged polar molecules, e.g. glucose, need the help of a carrier mediated transporters (for glucose, these are the GLUTs, as there are 5 different glucose transporters in the body, found in varying locations, some of which overlap for specific functions) in order to pass through the barrier. Charged ions also require the help of an integral transport protein in order to be transported through the plasma membrane. (2)
Glucose transport across a membrane is faster than simple diffusion, because of this carrier mediation by an integral membrane transporter protein. GLUTs have 12 trans-membrane domains, which are found in all 5 of the GLUTs. Figure 2 shows the 5 different types and their locations. In the GLUTs both the amino and carboxyl terminals are exposed on the cytoplasm side of the membrane. Binding of glucose to the active site of the GLUT causes a conformational change, and releases glucose to the other side of the membrane. (3) Each of the different GLUTs bind with different degrees of efficiency. The GLUTs tend to be recycled, and expression can be increased or decreased according to need. (2) Increased expression of the transporter is stimulated by Insulin (see Figure 3). Transport via the GLUTs is an example of passive facilitated diffusion. (4)
Like glucose, charged ions also require an integral membrane protein in order to be transported across the lipid bilayer membrane, however, unlike the GLUTs, which are of the passive facilitated diffusion type, ion transport is an example of active transport, i.e. it requires energy. These integral membrane proteins used to transport ions are often referred to as “ion pumps”. These pumps require energy in order transport the ions against their concentration gradient, and this is either gained by ATP or by using the concentration gradient of another ion to facilitate the transport. The transport proteins that use the gradient system are called...
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