Biological Membranes - A. N. Ogurtsov 2012

Structure and Functions of Biomembranes
Passive Transmembrane Transport
Transport of Substances Across the Membrane via Transporter Proteins

Only a few substances can diffuse across Cell/29.html">The Lipid Bilayer on their own (Figure 49).

The transport of most molecules across the membrane is mediated by specialized membrane transport Proteins. Water-soluble metabolites are carried across the membrane through the transmembrane domains of these proteins, which shield the metabolites from direct contact with the Hydrophobic core of the biomembrane.

Formally, these are classified into:

✵ ATP Pumps (Figure 51(a));

✵ Ion Channels (Figure 51(6));

✵ transporters or carriers (Figure 52).

ATP pumps, or simply pumps, are ATPases that harness the energy of ATP Hydrolysis to transport ions or small molecules across the membrane (Figure 51(a)).

The direction of transport is dictated by The Structure of the ATP pump and is independent of the metabolite concentration gradient across the membrane. Therefore, pumps function as active transporters, enabling Cells to establish concentration gradients by moving ions from a cellular compartment of low concentration to one of high concentration.

This type of transport is referred to as Active Transport, and the resulting ionic concentration gradient (electrochemical gradient) establishes the electrical Membrane Potential of the biomembrane.

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Figure 51 - Membrane Transport of substances: a - ATP pump, b - ion channel. Triangles indicate concentration gradients

Three types of ion pumps have been thoroughly studied:

1) Ca²⁺-ATPase;

2) Na⁺/K⁺-ATPase;

3) proton pump.

A typical ion transport rate through the membrane mediated by an ion pump ranges from 1 to 1000 ions per second.

Channel proteins (Figure 51(6)) facilitate the passive Transport of Molecules and ions across the biomembrane down their transmembrane concentration gradient.

Sometimes this channel-mediated process of substance diffusion across the membrane is referred to as Facilitated Diffusion via a fixed carrier.

Molecules and ions pass through the hydrophilic pore of the protein at a rate of 10⁷-10⁸ ions per second. Certain ion channels remain constantly open, whereas others feature protein "gates"—mobile domains that close the channel in response to specific external stimuli.

Transmembrane transport mediated by carriers or transporters is categorized into uniport, symport, and antiport systems (Figure 52).

Uniports transport a single type of molecule down its concentration gradient via facilitated diffusion. Glucose and Amino Acids are transported across The Plasma Membrane via such uniports. Consequently, uniport-mediated transport is classified as facilitated diffusion during passive membrane transport.

Figure 52 - Schemes of secondary active transmembrane transport: a - uniport, b - symport, c - antiport. Triangles indicate concentration gradients or electrical potentials across the biomembrane

Unlike uniports, antiports and symports couple the endergonic movement of a given molecule or ion against its concentration gradient with the exergonic movement of one or more different molecules or ions down their electrochemical concentration gradient.

Both of these transporter types are often collectively referred to as cotransporters, emphasizing their ability to move two Different types of molecules simultaneously.

Unlike ATP pumps, which derive energy for substance transport from ATP hydrolysis, cotransporters utilize energy previously stored in an electrochemical gradient. In this sense, this type of transport is also referred to as active transport; however, since the required Electrochemical Potential must be established beforehand (most commonly through the action of ion pumps), this mechanism is known as secondary active transport. The characteristic operating rate of a secondary active cotransporter is 10²-10⁴ molecules per second.

The operating cycle of both ATP pumps and secondary transporters involves a series of Conformational Changes in these transport proteins, during which the transported substances bind to the protein on one side of the membrane and, following conformational shifts, are released on the other side. Because such a cycle transfers between one and three molecules across the membrane, the transport rate of ATP pumps and transporters is relatively low (1—10,000 molecules per second).

Ion channels also undergo conformational switching between closed and open states, but when a channel is open, the ion flux through it is vastly higher, reaching up to 10^s ions per second.