4.2. Transport of substances across membranes

Uniport is transport of a substance in one direction depending on the gradient of concentration.

Symport is transport of two substances in one direction through one carrier.

Antiport is movement of two substances in different directions through a single carrier.

Symport and antiport are cotransport systems.

Passive transport:

1. Simple (passive) diffusion. Small neutral molecules such as Н₂О, СО₂, О₂, NH₃, urea, ethanol, hydrophobic organic compounds with low molecular mass (steroid hormones, benzene) diffuse through the membrane without the involvement of specific mechanisms. Transfer of substances is carried by the concentration gradient and at a low rate.

2. Facilitated diffusion

Lipid bilayer is almost impermeable for larger polar molecules (glucose, amino acids), as well as for ions, as its interior is hydrophobic. These substances are transported across the membrane on the gradient of concentration, but with the participation of membrane proteins.

2a. Transfer through ion channels. Transmembrane transport of several ions (Са²⁺, Na⁺, K⁺, C1⁻) occurs through ion channels. These are intrinsic proteins of membrane. They form a transmembrane hydrophilic (water-filled) channel. The selectivity of channels to ions is determined by the presence of specific center in protein canal for ion linking. Channels can be either closed or open. Signal to change the state of the channel may be a hormone or other signaling molecule. The example of controlled channels is Ca-channels.

2b. Transfer by transmembrane carrier protein (translocase). Each substance or group of similar substances has its own carrier. Transferred substance joins to the translocase. Then carrier changes its conformation. The substance is released from the other side of the membrane. Since there is no hydrophobic barrier, this mechanism is called facilitated diffusion. Example is facilitated diffusion (uniport) of glucose in erythrocytes with GLUT-1. Molecule of glucose links to translocase on the outer surface of the plasma membrane. Conformational change occurs, and the center of the carrier, occupied by glucose, is open inside the cell. As a result of conformational changes a carrier loses affinity for glucose, and the molecule is released in the cell cytosol. Separation of glucose causes a conformational change of the carrier protein, and it returns to the original conformation.

Passive transport does not require energy.

Fig. 16. a - types of transport, b - passive and active transport:

1 - passive diffusion, 2 - diffusion through the channel, 3 - diffusion through the carrier, 4 - the active transport, 5 - secondary-active transport.

Active transport of substances occurs against the concentration gradient and it is associated with the supply of metabolic energy. In this way, there is transfer of many mineral ions from the extracellular fluid into the cell or in the opposite direction, the transfer of amino acids from the intestinal lumen into the cells of the intestine, transport of glucose from the primary urine through the kidney tubules cells in the blood. The main source of energy for active transport is ATP. Therefore, as a rule, these systems are ATPases.

The example is Na⁺,K⁺-pump. There is an integral plasma membrane protein sodium-potassium ATPase. It moves potassium ions into the cell, and sodium ions from the cell.

ATPase attaches inside of the membrane three Na⁺ ions. These ions alter the conformation of the active site of ATPase, and it hydrolyzes one molecule of ATP and attaches phosphate. The released energy is used to change the conformation of ATPase, after which three sodium ions are on the outer side of the membrane, and the phosphate ion is replaced by 2 K⁺ ions from the outside. Then, the carrier conformation changes to the original, and K⁺ ions are inside membrane. Here, K⁺ ions are split off, and the carrier is again ready for use.

The work of Na⁺,K⁺-pump creates not only the difference in concentration, but difference in charges. On the outer side of the membrane a positive charge is created, and negative charge is inside. It is necessary for the nerve impulse transmittion.

Secondary active transport. It is cotransport system. The gradient of a substance is used to transport the other. Carrier in this case has specific binding sites for both substances. The substance is transported against its concentration gradient by symport or antiport. Symport and antiport can occur due to the energy of the concentration gradient of Na⁺ ions, produced by Na⁺,K⁺-pump. In this way there is, for example, amino acid absorption from the intestine and the glucose from the primary urine and intestines.

An example of secondary-active symport is transport of glucose and sodium ions, secondary-active antiport - Ca, Na-active transport.

The secondary active transport has probably the greatest value for transfer of carbohydrates, amino acids and other metabolites in comparison with other mechanisms.

There are special mechanisms for the transport of proteins, polysaccharides, and nucleic acids through the membrane. These are endocytosis (phagocytosis and pinocytosis) and exocytosis.

Violation of the transport functions is due to more than 20 “transport diseases”, including renal glucosuria, cystinuria, malabsorption of glucose, galactose, and vitamin B₁₂.