In this article we will discuss about the Transport of Ions and Other Molecules through Biological Cell Membrane.
The living organisms can be resolved into organs, glands, tissues, cells and organelles. It is very interesting in biology to know how solutes and water get into and out of cells and organelles. Most attention is to be paid to erythrocytes and mitochondrion. The cell membrane is a complex lipoprotein structure.
Some channels are continuously open, whereas others are gated i.e. they have gates that open or close. Some are gated by alterations in membrane potential (voltage gated) whereas others are opened or closed when they bind a ligand (ligand gated).
The ligand is often external (neurotransmitter or hormone) or internal (intracellular Ca++, cAMP). Other transport proteins are carriers that bind ions and other molecules and then change their configuration, moving the bound molecule from one side of the cell membrane to the other.
Molecules move from areas of high concentration to areas of low concentration (down their chemical gradient). Cations move to negatively charged areas whereas anions move to positively charged areas (down their electrical gradient), ligand gated channel.
Some of the carrier proteins are called uniports because they transport only one substance. Others are called symports because transport requires the binding of more than one substance to the transport protein and the substances are transported across the membrane together.
Example:
In the intestinal mucosa that is responsible for the cotransport by facilitated diffusion of Na+ and glucose from the intestinal lumen into mucosal cells. Other transporters are called anti-ports because they exchange one substance for another. Example: Na+– K+ ATPase.
Na-K+ ATPase:
It catalyses the hydrolysis of ATP to ADP and uses the energy to extrude 3Na+ from the cell and take 2K+ into the cell for each mole of ATP hydrolysed. The pump is said to have a coupling ratio of 3/2. Its activity is inhibited by ouabain and related to digitalis glycosides used in the treatment of heart failure.
Na+-K+ ATPase is a heterodimer made up of α and β subunit.
Na+ and K+ transport occurs through a subunit.
β subunit is a glycoprotein.
Substances passing through the lipid bilayer of the cell membrane by simple diffusion are:
1. All lipid soluble substances.
2. Lipid soluble gases mainly CO2, O2 and N2.
3. Water—though not lipid soluble—passes because of small molecular size and high kinetic energy.
Substances passing through protein channels of cell membrane by simple diffusion are:
1. Ions mainly Na+, K+and Ca++.
2. Water molecules.
A. Passive Diffusion:
1. Some solutes pass through cell membrane by simple diffusion with the concentration gradient.
This can be expressed by the modification of Fick’s law:
ds/dt = PA(C0 – Ci)
where, P = the permeability coefficient.
A = area of membranes.
C0 and Ci = the concentration of solution outside and inside the membrane, respectively.
ds/dt = rate of movement of solute.
2. Lipid-soluble solutes pass more readily through cell membranes than lipid-insoluble solutes. Because the cell membrane consists of small water-filled pores of radius about 0.4 nm. through which water- soluble solute of suitable molecular size pass, surrounded by lipid areas through which lipid-soluble solutes penetrate.
3. Water diffuses through the cell pores from a solution of low concentration to a solution of high concentration and this “bulk flow” of liquid across the membrane will speed up molecules diffusing in the direction of the flow and slow down those moving in the opposite direction. This “drag” effect is a second force acting in passive diffusion.
4. The third force which may operate is an electric potential across the membrane. Many cell membranes can maintain potential difference between their inside and outside and the potential gradient acts as a driving force for passive transport across the cell. The membrane acts as a passive barrier.
B. Facilitated Transfer:
1. Some compounds, e.g., sugar, amino acids, pass through membranes at a greater rate than expectations. This is because of the effect of a carrier.
2. The carrier in the membrane combines with the substance to be transported and in some way ferried through the membrane and released on the other side.
3. In case of enzymic reactions, there is a “saturation effect”. The rate of transport of the solute increases when the carrier, enzyme, is saturated. This type is sometimes termed “catalysed diffusion”.
4. Another mechanism is that the substance to be transferred is converted into another which will penetrate the membrane more easily, e.g., the mitochondrial membrane is impermeable to acyl coenzyme A derivatives. The acyl group is transferred to carnitine to form acyl carnitine derivative which can pass through the membrane. The acyl coenzyme A derivative is then reformed on the other side of the membrane.
Fatty acids can also be transferred into and out of mitochondria.
Acetyl-CoA within the mitochondria can be transferred to oxaloacetate to yield citrate to which the mitochondrial membrane is permeable. The citrate passes out into the cytoplasm where it is split enzymically to give acetyl-CoA again.
C. Pinocytosis:
1. The cell membrane forms pockets or invaginations which can draw materials on the outside towards the cell interior.
2. The vesicles extend into the cell where they are pinched off and finally release their contents into the cell by some unknown way.
3. This process occurs in the foetal and newborn animals and helps the absorption of intact protein from the gut.
D. Transport of Ions:
1. The membrane itself contains polar groups and is, therefore, electrically charged.
2. The transport of most ions occur more slowly than the non- electrolytes. But H+, OH− penetrate all cell membranes easily. The red cell is easily penetrated by Cl− and HCO−3.
3. In the case of ions, especially, Na+ and K+, the permeability is very small. The high concentration of K+ and low concentration of Na+ which are often found in cells are maintained by special mechanism which involve the expenditure of energy.
E. Active Transport:
1. The process by which solutes can often pass through membranes against their concentration gradient requires energy. This process is termed active transport.
2. Active transport is involved in the absorption from the small intestine of glucose and galactose, amino acids and other substances important to the body.
3. An active transport device which forces Na+ out and K+ in has been referred to as the “Sodium Pump”.
4. The mechanism requires a carrier which can exist in two forms with different affinities for Na+ and K+. ATPase is involved in it (see active transport of glucose).