Active Transport Basics

Active Transport: Protein-Mediated Movement Up the Gradient

• Facilitated diffusion moves molecules down a concentration gradient, towards equilibrium. It's important but only accounts for movement down the gradient.
• Active transport moves solutes up a concentration gradient, away from equilibrium. This process requires energy.
• Diffusion is nondirectional, while active transport has inherent directionality.

The Coupling of Active Transport to an Energy Source

• Direct active transport couples to an exergonic chemical reaction, most often ATP hydrolysis. This reaction drives the transport of substances across the membrane.
• Indirect active transport couples the transport of a solute to the movement of an ion (e.g., protons) down its electrochemical gradient.

Direct Active Transport ATPases

• Four types of transport ATPases are identified: P-type (e.g., Na+/K+ pump), V-type ("vacuole" H+ pumps), F-type (also H+ pumps, ATP synthases), and ABC-type.
• These types differ in structure, mechanism, location, and roles.

V-Type ATPases

• V-type ATPases pump protons into organelles like vacuoles, vesicles, lysosomes, endosomes, and the Golgi complex.
• They consist of two multi-subunit components: an integral component embedded in the membrane and a peripheral component projecting out.

Direct Active Transport: The Na+/K+ Pump

• In a mammalian neuron, the potassium concentration inside is about 30 times higher than outside, and the sodium concentration inside is much lower (around 0.08 times) than outside.
• These electrochemical potentials are critical for coupled transport and nerve impulse transmission.

The Na+/K+ Pump is an Allosteric Protein

• The Na+/K+ pump exists in two conformational states (E1 and E2).
• E1 is open to the inside of the cell and has a high affinity for Na+ ions.
• E2 is open to the outside of the cell and has a high affinity for K+ ions.

Requirement for Energy

• Pumping Na+ and K+ ions against their concentration gradients requires energy.
• The Na+/K+ ATPase uses ATP hydrolysis to drive this transport.
• This pump is critical for the asymmetric distribution of ions across the cell membrane of animal cells.

Indirect Active Transport is Driven by Ion Gradients

• Indirect active transport (secondary active transport) doesn't directly use ATP hydrolysis.
• Movement of molecules up their electrochemical gradients is often coupled to the inward movement of Na+ (in animals) or protons (in plants, fungi, bacteria) which move down their gradient.

Symport Mechanisms of Indirect Active Transport

• Many cells pump sodium ions or protons out of the cell (e.g., Na+/K+ pump in animals).
• The resulting high extracellular concentration of Na+ facilitates the uptake of other substances (like sugars and amino acids), indirectly powering this process.

Indirect Active Transport: Sodium Symport Drives the Uptake of Glucose

• Glucose transport often happens via facilitated diffusion.
• In some cells, a Na+/glucose symporter drives glucose uptake even when glucose concentrations are lower outside the cell. This symporter uses the electrochemical gradient of Na+ to move glucose into the cell.

The Energetics of Transport

• Every transport event in a cell requires energy input.
• For uncharged solutes, only the concentration gradient matters.
• For charged solutes, both the concentration gradient and the electrical potential are involved.

For Uncharged Solutes

• The change in Gibbs free energy (ΔG) for transport of uncharged solutes depends solely on the concentration gradient.
• The formula for calculating inward transport is provided in the notes.

For Charged Solutes

• Transport of charged solutes involves the consideration of both the concentration gradient and the membrane potential (Vm).
• The membrane potential usually makes inward movement of cations more favorable.
• The formula for calculating the change in Gibbs free energy of transport for ions is given in the notes.

Examples (Lactose and Chloride Intake)

• The notes include specific examples concerning lactose and chloride ion uptake, demonstrating the energy requirements, and calculations.