Membrane Transport II: Active Transport and Transport Energetics

Membrane Transport II: Active Transport and Transport Energetics

Key Concepts of Active Transport

Active transport is a fundamental process in biochemistry, enabling cells to move substances against their electrochemical gradients. This process is energetically unfavorable (endergonic) and requires coupling to a source of free energy.

• Active transport occurs against an electrochemical gradient and is endergonic.

• Requires a source of free energy, typically by coupling to a favorable process.

• Primary active transport uses ATP hydrolysis as its energy source.

• Secondary active transport uses the electrochemical potential from another favorable transport process.

• Transporters can be classified as uniports (single substrate), symports (two substrates in same direction), or antiports (two substrates in opposite directions).

Types of Membrane Transporters

Transporters are proteins that facilitate the movement of molecules across biological membranes. They are classified based on the direction and number of substrates transported.

• Uniport: Transports a single type of molecule.

• Symport: Transports two molecules in the same direction.

• Antiport: Transports two molecules in opposite directions.

Thermodynamics of Membrane Transport

The energetics of transport depend on both chemical and electrical gradients. The free energy change for transport is given by:

• Equation: $\Delta G_t = RT \ln \left( \frac{C_2}{C_1} \right) + ZF \Delta \psi$ where:

  • $R$ = 8.314 J/(mol·K) (gas constant)

  • $T$ = temperature in Kelvin

  • $C_1$, $C_2$ = concentrations on each side of the membrane

  • $Z$ = charge of the molecule

  • $F$ = 96.5 kJ/(mol·V) (Faraday constant)

  • $\Delta \psi$ = electrical potential difference (Volts)

• Be careful to define the direction of transport and the sign of the transmembrane potential.

Example: Transport of an Uncharged Solute

• For an uncharged solute ($Z=0$), only the chemical gradient matters.

• Calculation: $\Delta G_t = RT \ln \left( \frac{C_2}{C_1} \right)$ If $C_1 = 2.0 \times 10^{-4}$ M, $C_2 = 1.0 \times 10^{-3}$ M, $T = 310$ K: $\Delta G_t = (8.314 \times 310) \ln(5) = 4.1$ kJ/mol (positive, unfavorable)

Example: Transport of an Ion

• For a charged solute ($Z \neq 0$), both chemical and electrical gradients contribute.

• Calculation: $\Delta G_t = RT \ln \left( \frac{C_2}{C_1} \right) + ZF \Delta \psi$ If $Z=+1$, $\Delta \psi = -0.075$ V: $\Delta G_t = 4.1$ kJ/mol $- 7.2$ kJ/mol $= -3.1$ kJ/mol (negative, favorable)

Primary Active Transport

Primary active transport directly uses chemical energy, usually from ATP hydrolysis, to move ions against their gradients.

• P-type ATPases are a major class of primary active transporters.

• Examples include Ca2+ ATPase (SERCA pump), Na+/K+ ATPase, H+ ATPase, and H+/K+ ATPase.

• Mechanism involves phosphorylation and conformational changes to transport ions.

Worked Example: ATP-Driven Ion Transport

• Problem: Calculate the maximum Na+ gradient achievable by an ATP-dependent transporter given $[ATP] = 3$ mM, $[ADP] = 1$ mM, $[P_i] = 0.5$ mM, and $\Delta G_{ATP}$ for hydrolysis = -30 kJ/mol.

• Steps:

  1. Calculate free energy from ATP hydrolysis: $ATP \rightarrow ADP + P_i$

  2. Set $\Delta G_t + \Delta G_{ATP} = 0$ to find the maximum gradient.

SERCA Ca2+ Pump

• Active transporter, P-type ATPase, uniport.

• Pumps Ca2+ back into the sarcoplasmic reticulum in muscle cells after contraction.

• Mechanism involves binding of Ca2+, phosphorylation, conformational change, and release of Ca2+ into the lumen.

Na+/K+ ATPase

• Active cotransporter, antiport.

• Maintains Na+ and K+ gradients in animal cells.

• Pumps 3 Na+ out and 2 K+ in, creating a net negative membrane potential.

• Essential for electrical signaling in neurons and resting potential.

• Consumes ~25% of total energy in a resting human.

• Mechanism involves sequential binding, phosphorylation, and release of ions.

• Dephosphorylation step can be inhibited by cardiotonic steroids (e.g., digitalis), blocking transport.

Secondary Active Transport

Secondary active transport couples the movement of one molecule against its gradient to the movement of another molecule down its gradient, often using ion gradients established by primary active transport.

• Commonly involves symport or antiport mechanisms.

• Examples: Lactose permease (symport of lactose and H+), Na+-glucose symporter (symport of Na+ and glucose).

Worked Example: Ion-Gradient Driven Transport

• Problem: What is the highest ratio of concentration of substrate S (inside/outside) that can be established by coupling to Na+ gradient?

• Steps:

  1. Determine free energy available from Na+ gradient.

  2. Set $\Delta G_S + \Delta G_{Na^+} = 0$ to find the maximum gradient for S.

Lactose Permease

• Best-studied secondary active transporter (from E. coli).

• Symport: lactose and H+ both move into the cell.

• Lactose moves against its concentration gradient, coupled to H+ moving down its gradient.

• H+ gradient is produced by a primary transporter.

• Conformational changes allow cotransport ("rocking banana" model).

Na+-Glucose Symporter

• Located in intestinal epithelial cells.

• Uptake of glucose is coupled to Na+ moving down its gradient.

• Na+ gradient is maintained by Na+/K+ ATPase (primary active transport).

• Glucose is then delivered to blood via facilitated diffusion (GLUT2).

Summary Table: Transport Types and Examples

Type	Passive	Active
	Simple Diffusion	Facilitated Diffusion	Primary	Secondary
Polarity of molecule	Nonpolar	Polar	Polar	Polar
Energetics	Favorable	Favorable	Unfavorable	Unfavorable
Made favorable by coupling to:	N/A	N/A	Hydrolysis of ATP	Transport of a different ion or molecule down its electrochemical gradient
Example (and uniport/symport/antiport class)	Diffusion of oxygen	GLUT1 (uniport)	SERCA pump (uniport)	Lactose permease (symport)
		Cl--HCO3- exchanger (antiport)	Na+/K+ ATPase (antiport)	Na+-glucose symporter

Additional info:

• Active transport is essential for maintaining cellular homeostasis, electrical signaling, and nutrient uptake.

• Transporters are generally slower than ion channels due to conformational changes required for substrate movement.

• Inhibition of key steps (e.g., dephosphorylation in Na+/K+ ATPase) can have significant physiological effects, such as those exploited by cardiac drugs.