Active Transport

FIELDS OF STUDY: Biochemistry; Molecular Biology; Genetics

ABSTRACT

The process of active transport is defined, and its importance in biochemical processes is elaborated. Active transport is an essential feature of the biochemistry of living systems and helps maintain the necessary concentrations of various biochemical components and electrolytes for the proper functioning of cellular metabolism.

The Mechanics of Active Transport

In living cells, biochemical processes transport materials necessary for a properly functioning metabolism through cell membranes. Passive transport does not require an input of energy to move materials across cell walls because it operates in the same direction as the concentration gradient, moving the materials from an area of high pressure to one of low pressure. Active transport can be thought of as a "shuttle service" for ions and other polar materials that cannot pass through a cell membrane by diffusion, a kind of passive transport. Instead, those entities must be physically transported across membranes by various mechanisms collectively termed pumps. A pump is a type of mediated transport system that functions to conduct ions, amino acids, glucose, and other polar compounds through the nonionic lipid bilayer, the highly nonpolar material that makes up the cell wall. Pumps always work against the concentration gradient to move materials out of regions of low concentration and into regions of higher concentration, using energy derived from biochemical reactions. The transported material is subsequently used in other biochemical reactions that return the energy used during transport.

Cell Walls and Lipid Bilayers

Long-chain fatty acids are organic molecules whose molecular structure consists of a single hydrocarbon chain terminated by a carboxylic acid functional group (−COOH). The carboxyl group is highly polar and hydrophilic, while the hydrocarbon moiety, or portion, of the molecule is very nonpolar and hydrophobic. Carboxylic acids are converted to esters by enzyme-mediated reactions with alcohols. In an ester, the carboxyl functional group retains the highly polar character that it had in its free carboxylic acid form, giving the long-chain esters, called lipids, a polar-nonpolar structure similar to that of the free carboxylic acids. When carboxylic acids are esterified with glycerol, which has three hydroxyl (−OH) functional groups, the resulting triesters are called triglycerides. Lipids and triglycerides are the principal forms in which long-chain fatty acids are found in biological systems.

The hydrocarbon chains and the carboxyl-based portions of fatty acids and their esters do not interact with each other due to their different hydrophilicities—that is, the degrees to which they attract and interact with water and other polar molecules—but they are quite capable of interacting with the corresponding portions of other molecules. The hydrocarbon chains associate preferentially with each other, as do the carboxyl portions. The basic structure of the lipid bilayer results from the hydrocarbon portions of the acids of two layers of such molecules intermingling and essentially dissolving each other. The carboxyl functions on the other ends of the hydrocarbon chains thus form two hydrophilic surfaces, one on either side of the very hydrophobic interior layer. The resulting structure is a lipid bilayer.

The walls of all animal cells are formed of lipid bilayers, allowing them to interact with water-based fluids while isolating the sensitive materials and processes that take place within each cell. The fluid inside of each cell is also water based, which necessitates some means of transporting vital polar materials from the exterior of the cell to the interior and moving extraneous materials and metabolites in the opposite direction for elimination. This movement is accomplished by active transport.

Functions of Active-Transport Systems

Active-transport systems serve a variety of functions in the biochemistry of living systems. Their principal function is to allow the organism to extract "fuels" and other essential materials for use in the metabolic functions that occur within cells. This is a very important function, and the nature of active transport allows cells to retain a relatively high concentration of such materials even when their concentrations outside of the cell are quite low. A second important function of active-transport systems is to regulate and maintain the organism’s metabolic steady state, a balanced state in which the material and energy that the organism removes from its environment through living functions is equal to the energy and materials that it returns to the environment through those same functions. The biochemical processes of metabolism use energy and materials taken from the environment. Anabolic processes remove materials from the environment and use energy from reactions involving those materials to build and support the life of the organism. Catabolic processes remove used materials from the organism and return them to the environment, releasing the energy stored in those materials.

Active transport maintains a constant optimal amount of various inorganic elements within the living cells of an organism. Potassium ions, for example, are essential to the proper functioning of many intracellular processes. An active-transport system produces potassium-ion channels in cell walls of nerves and muscles, including the cardiac muscles. Potassium ions are delivered into the cytoplasm of the cell via these channels to replace ejected sodium ions, thus maintaining a constant ionic concentration within the cell. The system maintains a relatively high concentration of potassium ions in most aerobic cells, between 100 and 150 millimolars (mM), whether they are plant, animal, or microbial in nature and regardless of the concentration outside of the cells. (A 1 mM solution has a concentration of 0.001 moles per liter.) The potassium ions that are pumped into the cell also serve to maintain the electric potential across the cell membrane, a factor that affects the free-energy change in reactions involved in active-transport systems.

Active Transport in Action

The transfer of ions across a membrane or against a concentration gradient by active transport is accompanied by a free-energy change (ΔG) that can be calculated by one of two equations. The first equation represents the free-energy change for the transfer of neutral materials against a concentration gradient. This is described by the equation

where R is the gas constant

T is the absolute temperature in kelvins, ln is the natural logarithm function, and c₁ and c₂ are concentrations on either side of the membrane in molars, or moles per liter (M), with c₂ being greater than c₁.

The second equation, which represents the free-energy change for the transfer of electrically charged materials, needs to account for the charge on the material being transported and the difference in electric potential across the membrane. The latter is determined by the neutral nature of the lipid bilayer, which causes it to act as a capacitor, or energy-storage device, and the presence of charge as maintained by the potassium ions in the cytosol. The free-energy expression for the transport of charged species across a cell membrane is given by the equation

where Z is the charge on the ion, F is the Faraday constant (96,485.3365 coulombs per mole, the electric charge on one mole of electrons), and ΔΨ is the difference in electric potential across the membrane in volts.

ATP and Active Transport

The energy used in active-transport systems is obtained through enzyme-mediated reactions of adenosine triphosphate (ATP). ATP molecules consist of a molecule of the nucleobase adenine that is bonded to a molecule of ribose sugar, which in turn is bonded to a triphosphate ion. A magnesium ion coordinates and stabilizes the second and third segments of the triphosphate moiety. Energy is derived from the structure by the enzymatic cleavage of the third phosphate segment from the triphosphate moiety, transforming the molecule into adensosine diphosphate (ADP), and it is restored by concatenating, or joining, a third phosphate ion to ADP to re-form ATP.

The function of muscle cells depends on the active transport of calcium ions and sodium ions, a process termed the calcium ion pump or Ca²⁺ pump. The calcium ion pump works in an organelle of muscle cells called the sarcoplasmic reticulum and is powered by ATP hydrolysis reactions mediated by the enzyme calcium adenosine triphosphatase. This process is critical to the contraction and relaxation of muscle fibers, especially heart muscles. The sarcoplasmic reticulum is a cell structure that stores and releases calcium ions to aid in this contraction and relaxation. In muscle cells, the rapid release of calcium ions from the sarcoplasmic reticulum into the cytosol, the cellular fluid outside of the organelles, triggers contraction of the muscle, while rapid removal of calcium ions from the cytosol and back into the sarcoplasmic reticulum triggers relaxation of the muscle. The normal concentration of free calcium ions in the cytosol is between 0.1 and 0.2 micromolar (μM, or 10⁻⁶ moles per liter), increasing when the muscle contracts and returning to the normal value when it relaxes.

PRINCIPAL TERMS

• adenosine triphosphate (ATP): a molecule consisting of adenine, ribose, and a triphosphate chain that is used to transfer the energy needed to carry out numerous cellular processes.
• cell membrane: a biological membrane that forms a semipermeable barrier separating the interior of a cell from the exterior.
• concentration gradient: the gradual change in the concentration of solutes in a solution across a specific distance.
• diffusion: the process by which different particles, such as atoms and molecules, gradually become intermingled due to random motion caused by thermal energy.
• passive transport: the passage of materials through a membrane with no input of energy required.

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