Primary Active Transport
In this type of membrane transport, the energy of high-energy ATP bonds is spent directly on the translocation of a specific substance. The main participants in the process are specialized enzymes — transport ATPases.
Main types of enzymatic pumps in cells:
- Sodium-potassium ATPase ($Na^+$, $K^+$-ATPase). Acting against the concentration gradient, it pumps three sodium ions out of the cytoplasm and pumps two potassium ions into the cell.
- Proton pumps ($H^+$-ATPases). Responsible for pumping hydrogen protons. They are necessary, in particular, for creating 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 level of calcium in the cytosol and form its intracellular stores inside mitochondria and the ER.
Mechanism of Action of Calcium ATPase
The operation of the enzymatic pump is a complex cyclic process accompanied by changes in the spatial structure of the transport protein. The cycle includes several stages:
- Binding. Two $Ca^{2+}$ ions attach to the active sites of the enzyme facing the cytosol.
- Activation. Ion binding alters the charge and conformation of the protein, sharply increasing its affinity for ATP molecules.
- Autophosphorylation. Hydrolysis of the ATP molecule occurs. The cleaved phosphate group binds to the enzyme itself.
- Translocation. Phosphate binding forces the protein to change shape: the channel closes from the cytosolic side and opens on the outer side of the membrane (or into the lumen of the organelle).
- Release. The affinity of the binding sites for calcium drops, and the ions leave the carrier, entering the extracellular environment.
- Dephosphorylation. With the participation of magnesium ions ($Mg^{2+}$), the enzyme releases inorganic phosphate.
- Return. Upon 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 spent directly on the transport of the target substance. Transport is carried out using the kinetic energy of the concentration gradient of helper molecules (most often sodium ions). This driving gradient is pre-established by primary active transport (e.g., the relentless work of $Na^+$, $K^+$-ATPase).
When a helper ion binds to a carrier protein, striving to move 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 its concentration gradient.
There are two main types of such membrane translocation:
- 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 the cell together with sodium ions.
- Active antiport. Molecules move in opposite directions. For example, the sodium-dependent exchanger in salivary gland cells uses the energy of incoming sodium to extrude calcium ions outward.
Factors of Carrier Protein Lability
For efficient functioning, transport proteins must possess high conformational lability — the ability to rapidly change 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 fatty acid unsaturation directly determines the phase state and microviscosity of the membrane.
- Electrical potential. Fluctuations in membrane charge affect 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.