Active Transport

Since the cell membrane is somewhat permeable to sodium ions, simple diffusion would result in a net movement of
sodium ions into the cell, until the concentrations on the two sides of the membrane became equal. Sodium actually
does diffuse into the cell rather freely, but as fast as it does so, the cell actively pumps it out again, against the
concentration difference.
The mechanism by which the cell pumps the sodium ions out is called active transport. Active transport requires the
expenditure of energy for the work done by the cell in moving molecules against a concentration gradient. Active
transport enables a cell to maintain a lower concentration of sodium inside the cell, and also enables a cell to
accumulate certain nutrient inside the cell at concentrations much higher than the extracellular concentrations.
The exact mechanism of active transport is not known. It has been proposed that a carrier molecule is involved,
which reacts chemically with the molecule that is to be actively transported. This forms a compound which is
soluble in the lipid portion of the membrane and the carrier compound then moves through the membrane against
the concentration gradient to the other side. The transported molecule is then released, and the carrier molecule
diffuses back to the other side of the membrane where it picks up another molecule. This process requires energy,
since work must done in transporting the molecule against a diffusion gradient. The energy is supplied in the form of
The carrier molecules are thought to be integral proteins; proteins which span the plasma membrane. These proteins
are specific for the molecules they transport.
Populating the inner membrane of the mitochondrion are many copies of a protein complex called an ATP synthase,
the enzyme that actually makes ATP! It works like an ion pump running in reverse. In the reverse of that process, an
ATP synthase uses the energy of an existing ion gradient to power ATP synthesis. The ion gradient that drives
oxidative phosphorylation is a proton (hydrogen ion) gradient; that is, the power source for the ATP syntheses is a
difference in the concentration of H+ on opposite sides of the inner mitochondrial membrane. We can also think of
this gradient as a difference in pH, since pH is a measure of H+ concentration.
The function of the electron transport chain is to generate and maintain an H+ gradient. The chain is an energy
converter that uses the exergonic flow of electrons to pump H+ across the membrane, from the matrix into the
intermembrane space. The H+ leak back across the membrane, diffusing down its gradient. But the ATP synthases
are the only patches of the membrane that are freely permeable to H+. The ions pass through a channel in an ATP
synthase, and the complex of proteins functions as a mill that harnesses the exergonic flow of H ' to drive the
phosphorylation of ATP Thus, an H+ gradient couples the redox reactions of the electron transport chain to ATP
synthesis. This coupling mechanism for oxidative phosphorylation is called chemiosmosis, a term that highlights the
relationship between chemical reactions and transport across the membrane. We have previously used the word
osmosis in discussing water transport, but here the word refers to the pushing of H+ across a membra!
Certain members of the electron transport chain must accept and release protons (H+) along with electrons,
while other carriers transport only electrons. Therefore, at certain steps along the chain, electron transfers cause H+
to be taken up and released back into he surrounding solution. The electron carriers are spatially arranged in the
membrane in such a way that H+ is accepted from the mitochondrial matrix and deposited - the intermembrane
space. The H+ gradient that results is referred to as a proton-motive force, emphasizing the capacity of the gradient
to perform work. The force drives H+ back across the membrane through the
specific H+ channels provided by ATP synthase complexes. How the ATP synthase uses the downhill H+ current to
attach inorganic phosphate to ADP is not yet known. The hydrogen ions may participate directly in the reaction, or
they may induce a conformation change of the ATP synthase that facilitates phosphorylation. Research has revealed
the general mechanism of energy coupling by chemiosmosis, but many details of the process are still uncertain. The
key feature of chemiosmosis is: It is an