Solute Transport across Membranes:-Active Transport Results in Solute Movement against a Concentration or Electrochemical Gradient

In passive transport, the transported species always moves down its electrochemical gradient and is not ac cumulated above the equilibrium concentration. Active transport, by contrast, results in the accumulation of a solute above the equilibrium point. Active transport is thermodynamically unfavorable (endergonic) and takes place only when coupled (directly or indirectly) to an exergonic process such as the absorption of sunlight, an oxidation reaction, the breakdown of ATP, or the concomitant flow of some other chemical species down its electrochemical gradient. In primary active trans port, solute accumulation is coupled directly to an exergonic chemical reaction, such as conversion of ATP to ADP+Pi (Fig. 11–35). Secondary active transport occurs when endergonic (uphill) transport of one solute is coupled to the exergonic (downhill) flow of a different solute that was originally pumped uphill by primary active transport.

The amount of energy needed for the transport of a solute against a gradient can be calculated from the initial concentration gradient. The general equation for the free-energy change in the chemical process that con verts S to P is

ΔG=ΔG+RTln [P]/[S]

where R is the gas constant, 8.315 J/mol K, and T is the absolute temperature. When the “reaction” is simply

FIGURE 11–35 Two types of active transport. (a) In primary active transport, the energy released by ATP hydrolysis drives solute move ment against an electrochemical gradient. (b) In secondary active transport, a gradient of ion X (often Na⁺) has been established by primary active transport. Movement of X down its electrochemical gradient now provides the energy to drive cotransport of a second solute (S) against its electrochemical gradient.

transport of a solute from a region where its concentration is C₁ to a region where its concentration is C₂, no bonds are made or broken and the standard free-en ergy change, ΔG⁰, is zero. The free-energy change for transport, ΔG_t, is then

If there is a tenfold difference in concentration between two compartments, the cost of moving 1 mol of an un charged solute at 25 ⁰C across a membrane separating the compartments is therefore

 ΔG_t (8.315 J/mol K) (298 K) (ln 10/1) =5,700 J/mol=5.7 kJ/mol

Equation 11–2 holds for all uncharged solutes. When the solute is an ion, its movement without an accompanying counterion results in the endergonic separation of positive and negative charges, producing an electrical potential; such a transport process is said to be electrogenic. The energetic cost of moving an ion depends on the electrochemical potential, the sum of the chemical and electrical gradients:

where Z is the charge on the ion, is the Faraday constant (96,480 J/V mol), and ΔΨis the transmembrane electrical potential (in volts). Eukaryotic cells typically have electrical potentials across their plasma mem branes of about 0.05 to 0.1 V (with the inside negative relative to the outside), so the second term of Equation 11–3 can make a significant contribution to the total free-energy change for transporting an ion. Most cells maintain more than tenfold differences in ion concentrations across their plasma or intracellular membranes, and for many cells and tissues active transport is therefore a major energy-consuming process. The mechanism of active transport is of fundamental importance in biology. the formation of ATP in mitochondria and chloroplasts occurs by a mechanism that is essentially ATP-driven ion transport operating in reverse. The energy made available by the spontaneous flow of protons across a membrane is calculable from Equation 11–3; remember that ΔG for flow down an electrochemical gradient has a negative value, and ΔG for transport of ions against an electrochemical gradient has a positive value.

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مواضيع ذات صلة

Solute Transport across Membranes:- Ion-Selective Channels Allow Rapid Movement of Ions across Membranes

Solute Transport across Membranes:- ABC Transporters Use ATP to Drive the Active Transport of a Wide Variety of Substrates

Solute Transport across Membranes:- P-Type Ca2+ Pumps Maintain a Low Concentration of Calcium in the Cytosol

Solute Transport across Membranes:-The Chloride-Bicarbonate Exchanger Catalyzes Electroneutral Cotransport of Anions across the Plasma Membrane

Solute Transport across Membranes:- The Glucose Transporter of Erythrocytes Mediates Passive Transport

Solute Transport across Membranes:- Transporters Can Be Grouped into Superfamilies Based on Their Structures

Solute Transport across Membranes:- Passive Transport Is Facilitated by Membrane Proteins

Membrane Dynamics:- Membrane Fusion Is Central to Many Biological Processes

The Composition and Architecture of Membranes:- Sphingolipids and Cholesterol Cluster Together in Membrane Rafts

The Composition and Architecture of Membranes:- Peripheral Membrane Proteins Are Easily Solubilized

The Composition and Architecture of Membranes:- A Lipid Bilayer Is the Basic Structural Element of Membranes

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins:- Cooperative Ligand Binding Can Be Described