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Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002.

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Biochemistry. 5th edition.

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Section 13.1The Transport of Molecules Across a Membrane May Be Active or Passive

Before we consider the specifics of membrane-protein function, we will consider some general principles of membrane transport. Two factors determine whether a molecule will cross a membrane: (1) the permeability of the molecule in a lipid bilayer and (2) the availability of an energy source.

13.1.1. Many Molecules Require Protein Transporters to Cross Membranes

As discussed in Chapter 12, some molecules can pass through cell membranes because they dissolve in the lipid bilayer. Such molecules are called lipophilic molecules. The steroid hormones provide a physiological example. These cholesterol relatives can pass through a membrane in their path of movement, but what determines the direction in which they will move? Such molecules will pass through a membrane located down their concentration gradient in a process called simple diffusion. In accord with the Second Law of Thermodynamics, molecules spontaneously move from a region of higher concentration to one of lower concentration. Thus, in this case, an entropy increase powers transport across the membrane.

Matters become more complicated when the molecule is highly polar. For example, sodium ions are present at 143 mM outside the cell and 14 mM inside the cell, yet sodium does not freely enter the cell because the positively charged ion cannot pass through the hydrophobic membrane interior. In some circumstances, as during a nerve impulse (Section 13.5.3), sodium ions must enter the cell. How are they able to do so? Sodium ions pass through specific channels in the hydrophobic barrier formed by membrane proteins. This means of crossing the membrane is called facilitated diffusion, because the diffusion across the membrane is facilitated by the channel. It is also called passive transport, because the energy driving the ion movement originates from the ion gradient itself, without any contribution by the transport system. Channels, like enzymes, display substrate specificity.

How is the sodium gradient established in the first place? In this case, sodium must move, or be pumped, against a concentration gradient. Because moving the ion from a low concentration to a higher concentration results in a decrease in entropy, it requires an input of free energy. Protein transporters embedded in the membrane are capable of using an energy source to move the molecule up a concentration gradient. Because an input of energy from another source is required, this means of crossing the membrane is called active transport.

13.1.2. Free Energy Stored in Concentration Gradients Can Be Quantified

An unequal distribution of molecules is an energy-rich condition because free energy is minimized when all concentrations are equal. Consequently, to attain such an unequal distribution of molecules, called a concentration gradient, requires an input of free energy. In fact, the creation of a concentration gradient is the result of active transport. Can we quantify the amount of energy required to generate a concentration gradient (Figure 13.2)? Consider an uncharged solute molecule. The free-energy change in transporting this species from side 1, where it is present at a concentration of c1, to side 2, where it is present at concentration c2, is

Image ch13e1.jpg

Figure 13.2. Free Energy and Transport.

Figure 13.2

Free Energy and Transport. The free-energy change in transporting (A) an uncharged solute from a compartment at concentration c1 to one at c2 and (B) a singly charged species across a membrane to the side having the same charge as that of the transported (more...)

For a charged species, the unequal distribution across the membrane generates an electrical potential that also must be considered because the ions will be repelled by the like charges. The sum of the concentration and electrical terms is called the electrochemical potential. The free-energy change is then given by

Image ch13e2.jpg

in which Z is the electrical charge of the transported species, ΔV is the potential in volts across the membrane, and F is the faraday [23.1 kcal V-1 mol-1 (96.5 kJ V-1 mol-1)].

A transport process must be active when ΔG is positive, whereas it can be passive when ΔG is negative. For example, consider the transport of an uncharged molecule from c1 = 10-3 M to c2 = 10-1 M.

Image ch13e3.jpg

At 25°C (298 K), ΔG is + 2.7 kcal mol-1 (+11.3 kJ mol-1), indicating that this transport process requires an input of free energy. It could be driven, for example, by the hydrolysis of ATP, which yields -12 kcal mol-1 (-50.2 kJ mol-1) under typical cellular conditions. If ΔG is negative, the transport process can occur spontaneously without free-energy input.

Ion gradients are important energy storage forms in all biological systems. For instance, bacteriorhodopsin (Section 12.5.2) generates a proton gradient at the expense of light energy, an example of active transport. The energy of the proton gradient in turn can be converted into chemical energy in the form of ATP. This example illustrates the use of membranes and membrane proteins to transform energy: from light energy into an ion gradient into chemical energy.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2002, W. H. Freeman and Company.
Bookshelf ID: NBK22454

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