Читать книгу Understanding Anatomy and Physiology in Nursing - John Knight - Страница 26

Diffusion only allows movement of molecules from a high to low concentration; sometimes it is necessary to move molecules against their natural concentration gradients, from a low to a high concentration. Moving material against a concentration gradient requires energy. Fortunately, as we have seen above, cells hold a steady stockpile of energy in the form of the energy storage molecule ATP. Many molecules are continually transported across membranes against their natural concentration gradients, including electrolytes such as sodium (Na⁺) and potassium (K⁺) and amino acids. Since this process utilises channel proteins, it can be regarded as an ATP-powered form of facilitated diffusion and is termed active transport.

Active transport can be defined as:

The active movement of molecules against their natural concentration gradients using channel proteins and powered by the energy storage molecule ATP.

Good examples of active transport are the dedicated ion pumps that maintain the correct balance of ions across cell membranes (Figure 1.5). These pumps play a key role in generating electrical signals termed action potentials which are essential to the functioning of the nervous system (Chapter 6).

Figure 1.5 Active transport: sodium, potassium and calcium ion pumps