Active transport of substances across the membrane

Primary active transport

In this type of membrane transport, the energy of high-energy ATP bonds is expended directly on moving a specific substance. The main participants in the process are specialized enzymes — transport ATPases.

Main types of enzyme pumps in cells:

• Sodium-potassium ATPase ($Na^+$, $K^+$-ATPase). Acting against the concentration gradient, it pumps three sodium ions out of the cytoplasm and two potassium ions into the cell.
• Proton pumps ($H^+$-ATPases). Responsible for pumping hydrogen protons. They are necessary, in particular, for establishing a specific acidic environment inside lysosomes.
• Calcium pumps ($Ca^{2+}$-ATPases). Located both in the plasma membrane and in the membranes of the endoplasmic reticulum (ER). Their main function is to maintain an extremely low calcium level in the cytosol and to build up intracellular stores within the mitochondria and ER.

Mechanism of action of the calcium ATPase

The operation of the enzyme pump is a complex cyclic process accompanied by changes in the spatial structure of the transport protein. The cycle includes several stages:

1. Binding. Two $Ca^{2+}$ ions attach to the active sites of the enzyme facing the cytosol.
2. Activation. The attachment of ions alters the charge and conformation of the protein, which sharply increases its affinity for ATP molecules.
3. Autophosphorylation. Hydrolysis of the ATP molecule occurs. The cleaved phosphate group binds to the enzyme itself.
4. Translocation. Phosphate attachment forces the protein to change its shape: the channel closes on the cytosolic side and opens on the outer side of the membrane (or into the lumen of the organelle).
5. Release. The affinity of the binding sites for calcium drops, and the ions leave the carrier, passing into the extracellular environment.
6. Dephosphorylation. With the participation of magnesium ions ($Mg^{2+}$), the enzyme cleaves off inorganic phosphate.
7. Return. After losing the phosphate, the protein loses its affinity for magnesium, assumes its original shape, and becomes ready for a new transport cycle.

Secondary active transport

Unlike primary transport, in this case, ATP energy is not expended directly on moving the target substance. Transport is driven by the kinetic energy of the concentration gradient of helper molecules (most commonly sodium ions). This driving gradient is pre-established by primary active transport (e.g., the relentless activity of the $Na^+$, $K^+$-ATPase).

When a helper ion binds to a carrier protein while moving down its gradient, it alters the protein's conformation. This increases the carrier's affinity for the target compound, allowing it to capture and transport it against the concentration gradient.

There are two main types of such membrane transport:

• Active symport. Both substances move strictly in the same direction. A classic example is the absorption of glucose molecules in intestinal cells, where glucose enters alongside sodium ions.
• Active antiport. Molecules move in opposite directions. For example, the sodium-dependent exchanger in salivary gland cells uses the energy of sodium entering the cell to extrude calcium ions outward.

Factors affecting carrier protein lability

For efficient operation, transport proteins must possess high conformational lability — the ability to rapidly alter their spatial structure. This ability directly depends on the physicochemical properties of the membrane in which they are embedded:

• Cholesterol proportion. Cholesterol molecules restrict the mobility of fatty acid chains, thereby regulating the overall fluidity and viscosity of the lipid bilayer.
• Lipid composition. The degree of unsaturation of fatty acids directly determines the phase state and microviscosity of the membrane.
• Electrical potential. Fluctuations in membrane charge affect the charged regions of transport proteins.
• Presence of specific ligands. The attachment of hormones, substrates, or neurotransmitters triggers allosteric changes in the structure of the carrier protein.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Facilitated diffusion and the function of calcium channels in the endoplasmic reticulum
• Mechanisms of action of the insulin-dependent glucose transporter (GLUT-4)
• Endocytosis and exocytosis as variants of vesicular transport
• Passive symport and passive antiport of substances

Other topics in «Structure and Functions of Biological Membranes»

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